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Global Training – The finest automotive learning Cars, Trucks, Vans · Overall Vehicle - Engine Combustion · Go Participant Document T0298F Issue date 09.05.2012

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Page 1: Global Training – The finest automotive learning Cars ... · Global Training – The finest automotive learning Cars, Trucks, Vans · Overall Vehicle - Engine Combustion · Go Participant

Global Training – The finest automotive learning

Cars, Trucks, Vans · Overall Vehicle - Engine Combustion · Go Participant Document

T0298F Issue date 09.05.2012

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This document is intended for training purposes only. The exercises performed in the course cannot simply be implemented in practice without regard to various considerations. Country-specific laws, regulations and specifications must always be observed.

The training documents are not subject to the ongoing update service. When working at the vehicle, always use the most up-to-date workshop aids (e.g. EPC net, WIS net, DAS, special tools) provided by the manufacturer for the vehicle in question.

Printed in Germany

© 2012 Copyright Daimler AG

Publisher: Global Training

This document, including all its parts, is protected under the laws of copyright. Any further processing or use requires the previous written consent of Daimler AG. This applies in particular to reproduction, distribution, alteration, translation, microfilming and storage and/or processing in electronic systems, including databases and online services.

Note: The term "employee" always refers to both female and male members of staff.

1511 3162 - 1st edition 09.05.2012 76

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Contents

T0298F <> Participant Document I

Contents

1 Orientation .................................................................................................................2 1.1 Welcome and introduction.................................................................................................... 2

2 Introduction to mixture formation..............................................................................3 2.1 Mixture formation .............................................................................................................. ... 3

3 Mixture formation in spark ignition engines...............................................................6 3.1 Mixture formation in spark ignition engines.......................................................................... 6 3.2 Injection systems in spark ignition engines .......................................................................... 8 3.3 Direct injection ............................................................................................................... .... 15 3.4 Exercises on mixture formation .......................................................................................... 17

4 Ignition in spark ignition engines .............................................................................22 4.1 Ignition in spark ignition engines ........................................................................................ 22 4.2 Student exercise on ignition in spark ignition engines ........................................................ 32

5 Exhaust treatment in spark ignition engines............................................................36 5.1 Exhaust treatment in spark ignition engines....................................................................... 36 5.2 Student exercise on exhaust treatment in spark ignition engines....................................... 41

6 Mixture formation in diesel engines.........................................................................43 6.1 Mixture formation in diesel engines.................................................................................... 43 6.2 Student exercise work on mixture formation in diesel engines........................................... 56

7 Exhaust treatment in diesel engines ........................................................................59 7.1 Exhaust treatment in diesel engines ................................................................................... 59

8 Increasing output by forced induction .....................................................................61 8.1 Increasing output by forced induction ................................................................................ 61 8.2 Student exercise on increasing output................................................................................ 67

9 Power and torque.....................................................................................................69 9.1 Power and torque ............................................................................................................... 69 9.2 Student exercise work on power and torque ...................................................................... 70

10 Exhaust system........................................................................................................71 10.1 Exhaust system................................................................................................................. .. 71

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1 Orientation 1.1 Welcome and introduction

2 T0298F <> Participant Document

1 Orientation

1.1 Welcome and introduction

Welcome to the self pace training course T0298F.

During the self paced training course T0298F you will obtain an overview of all basic topics on

engine combustion.

You'll learn about these topics by completing student exercises.

We wish you success in the course, and hope this self paced training will

prove to be informative!

Fundamentals of engine combustion

Whether combustion occurs by the diesel or the spark-ignition principle, there must always be

mixture formation, compression, ignition and combustion. This unit of the course introduces the

combustion process and the systems involved. It enumerates alternative methods of boosting

the power of internal combustion engines and clarifies issues of exhaust production and

treatment. These topics are covered separately according to the spark ignition and diesel

engine combustion principles.

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2 Introduction to mixture formation 2.1 Mixture formation

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2 Introduction to mixture formation

2.1 Mixture formation

The combustible mixture

In order for the fuel to burn completely a certain quantity of oxygen or air is required. The blend

of fuel and air is termed the mixture. We will examine the fuel first.

Simple and reliable formation of a combustible fuel/air mixture which ignites readily and quickly and burns without residues.

Low weight and small space for the energy unit and for the fuel tank.

Reliable power transmission.

Immediate readiness for operation.

Convenient and safe accommodation and carriage in the vehicle.

Fuel

Fuels and their accommodation in the vehicle must satisfy certain requirements with regard to

suitability for combustion and transportation:

Fuel types: There are three types: solid, liquid and gaseous fuels. Today, however, only liquid

fuels are significant and to a limited extent gaseous fuels. The requirements listed above are

best satisfied with liquid fuels. The most common liquid fuels are gasoline and diesel. These

consist of different hydrocarbons from the same origin (crude oil). Hydrocarbons are

compounds of carbon (C) and hydrogen (H2). The manufacturing process begins with the

distillation of crude oil into its constituent parts. Further processes refine the components to

produce gasoline and diesel.

In addition there are other fuels based on regenerative (reusable) energy sources. Some of the

following so-called alternative fuels are already in use; others are in discussion:

Methane, obtained in part from animal by-products

Methanol, obtained from wood biomass

Ethanol, obtained from sugarcane biomass

Vegetable oil, obtained from rapeseed biomass

Hydrogen, obtained from nuclear or solar energy

The use of alternative fuels requires modifications to the engine, the vehicle and the fuel

distribution infrastructure. Their use in the near future can only be expected in certain niche

sectors. In the medium term methanol appears to be the most promising alternative, while the

chances of hydrogen technology may only appear in the long term.

Properties of fuels

The main property of fuels, and the reason they are used for combustion, is that they store

chemical bond energy. The internal combustion engine converts the chemical energy into

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mechanical energy (power). The fuel's chemical energy is expressed as its so-called net heating

value in joules per kilogram (J/Kg).

In addition, fuels have certain properties with regard to their readiness to ignite under high

pressures and at high temperatures. This readiness is referred to as the combustibility. The

measure for the combustibility is the octane number (spark ignition engine) or the cetane

number (diesel engine).

Spark ignition fuel (gasoline) should have a low combustibility so that residual mixture does not

detonate by itself. As a reminder: The spark ignition engine ignites the fuel in a controlled

manner by means of a spark.

The octane number therefore indicates the resistance of gasoline to the occurrence of

undesirable autoignition. Undesired autoignition is also referred to as combustion knocking.

Engine knock is feared because it can cause irreparable damage to the engine. Knocking

mostly occurs at low engine speeds in full load mode (acceleration knock) or at high engine

speeds (high-speed knocking).

The higher the octane number, the more resistant to knocking and the higher the quality of the

fuel. At filling stations the octane number is stated with the letters RON. RON stands for

Research Octane Number and is an international standard. In contrast to gasoline, diesel fuel

must have a high combustibility. As a reminder: The diesel principle is based on autoignition.

The key figure for the combustibility of diesel is the cetane number. Diesel fuel also possesses

another property. Diesel becomes viscous and therefore unusable at low temperatures. This

viscosity begins to occur at approx. –24°C.

The table below lists the octane and cetane numbers for common fuels.

Fuel Octane/cetane numbers

Gasoline 91

Premium 95

Super Plus 98-100

Diesel 50

Air requirement

The air requirement is the volume of air that is required for the complete combustion of a fuel.

It is also referred to as the stoichiometric air requirement.

In engine combustion the fuel/air mixture usually varies from the stoichiometric mixture ratio

to a greater or lesser extent. The ratio of the actual air mass mL to the stoichiometric air mass

is called the air/fuel ratio .

Three mixture states are differentiated in engine operation:

A mixture with air deficiency has a < 1 and is referred to as a “rich mixture”.

A mixture with a stoichiometric mixture ratio has a = 1 (ratio of fuel : air = 1 : 14.8) and is termed the stoichiometric mixture.

A mixture with excess air has a > 1 and is known as a “lean” mixture.

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Nearly all spark ignition engines today are equipped with a regulated catalytic converter. This

must be operated with an approximately stoichiometric mixture.

Diesel engines are always operated with excess air in order to avoid excessive soot formation.

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3 Mixture formation in spark ignition engines

3.1 Mixture formation in spark ignition engines

Spark ignition fuels are conditioned outside the cylinder with the aid of a carburetor (not

covered here because no longer state of the art), by injection into the intake manifold or by

direct injection into the combustion chamber.

The pictures illustrate intake manifold injection and direct injection.

Mixture formation and metering must fulfill the following tasks:

The formation of a gaseous fuel/air mixture from finely distributed fuel.

The accurate metering of the fuel for the desired air/fuel ratio.

The adjustment of the mixture volume by flow restrictors in order to regulate the power output.

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1. Injection nozzle

2. Spark plug

3. Intake manifold

4. Exhaust port

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The mixture formation system is involved not only in optimizing combustion in the context of

the stoichiometric air ratio, but also is influencing other critical operating variables:

Enabling quick starting.

Enabling stable idling.

Operating economically in the partial-load range.

Reacting rapidly to load changes, e.g. acceleration.

Metering the fuel so that the pollutant exhaust emissions remain as low as possible in all operating states.

Enriching the mixture with fuel in order to obtain the maximum power output.

Operating regardless of atmospheric conditions.

Mixture composition: The mixture composition has a great influence on the main operating

variables. It is described by the fuel/air mixing ratio or the air/fuel ratio.

The air/fuel ratio range in which spark ignition engines can be operated is defined by the rich

and lean misfire limits or by corresponding ignition limits.

Air/fuel ratio range of a spark ignition engine: 0.5-0.7 < < 1.3-1.7

The mixture ratio basically depends on:

Engine temperature

Engine speed

Engine load

The following engine operating states require a rich mixture:

Cold start

Warm-up

Acceleration (full load)

Full power (full load)

Full load is the state where the vehicle is driven with the throttle valve wide open. The engine

speed is immaterial.

When the engine is warm and the engine load is low (so-called partial load), a lean mixture is

used to drive economically.

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3.2 Injection systems in spark ignition engines

Mixture formation systems in spark ignition engines – Intake manifold injection

In spark ignition engine injection, a pump delivers the fuel and injection valves inject it either in

front of each cylinder individually (multi-point injection) or centrally in the zone of highest air

speed (single-point injection). In the case of single-point injection, the fuel is injected by only

one injection valve (single point); in multi-point injection by one injection valve per cylinder

(multiple points).

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Essentially, intake manifold injection is either continuous or discontinuous.

Continuously operated systems are mechanical systems that inject fuel constantly (continuously). •

Discontinuous systems are regulated by electromagnetic valves. These are actuated on demand by an electronic control unit and therefore do not always inject fuel (discontinuous).

Certain control systems are required in order the adjust the fuel stream to suit the air flow to

achieve the desired air/fuel ratio. These are the mixture formation systems. They must be able

to measure the air volume or mass as well as control the allocation of the fuel quantity

according to the measured air volume. Other components of the mixture formation systems

include the injection valves.

On-demand mixture formation can be imagined as follows: The driver demands a certain power

output from the engine via the accelerator pedal position. The accelerator pedal is linked to the

throttle valve in the engine. The throttle valve controls the load state in the mixture formation

system. When more power is required, the throttle valve opens further. This allows a greater

flow of air into the combustion chambers. This air volume is measured. With the greater

measured air volume a greater quantity of fuel can be injected, and the engine receives more

power.

We will now discuss exactly how these mixture formation systems operate.

Mechanically controlled continuous single fuel injection (K-Jetronic)

In the so-called “K-Jetronic” from Bosch the fuel is injected continuously and metered by

measuring the intake air volume.

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The opening of the throttle valve also causes the baffle plate to open. This happens because of

the intake vacuum: When the throttle valve is open, the engine can draw in a large air volume,

pulling the baffle plate far into the air funnel.

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The deflection of the baffle plate is directly proportional to the air volume. In the fuel distributor

the control plunger alters the cross section of rectangular slots according to the deflection of

the baffle plate. In this way the fuel quantity to the injection nozzles is metered. The number of

slots is the same as the number of cylinders in the engine. The control plunger is connected to

the baffle plate by a linkage. The pressure differential valve ensures that the pressure across

the metering slots is constant. The injection valves atomize the fuel very finely even at low flow

rates and open at a fuel pressure of approx. 3.3 bar.

An electric pump (fuel pump) supplies the system with a pressure that is kept constant at

about 5.7 bar by the pressure regulator. A pressure reservoir maintains the pressure for a short

time after the engine is switched off. This is necessary to facilitate restarting when the engine is

hot.

The basic setting of the air/fuel ratio is set at idle by an adjusting screw and then remains

practically constant throughout the entire partial-load range. The mixture is enriched for wide

open throttle by changing the contour of the air funnel.

For cold starts (cold start enrichment = rich mixture) a special cold start valve is used. The

warm-up compensator (control pressure regulator) and ab auxiliary air valve are used for warm-

up. The warm-up compensator regulates the control pressure acting on the control plunger

according to temperature and regulates the additional fuel volume in warm-up by increasing or

reducing the control pressure.

Electromechanically controlled continuous single fuel injection (KE-Jetronic)

The K-Jetronic was developed to produce the KE-Jetronic by adding an electronic control (the

basic system is the same as the K-Jetronic). This allows intelligent functions to be implemented,

such as regulating to = 1 for catalytic converter cleaning or warm-up enrichment. The

advantage of the KE-Jetronic over purely electronic systems is its limp-home ability. The basic

system continues to operate mechanically if the electronic system fails.

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Electronically controlled single fuel injection (L-Jetronic)

Advances in semiconductor technology have made it possible to control the injection quantity

electronically. This provides a number of correction capabilities to improve fuel economy,

running characteristics and exhaust emissions. In the L-Jetronic system the air volume drawn

in by the engine is measured directly and the quantity of fuel to be injected is calculated

electronically.

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Injection valves

The fuel is injected by electromagnetically actuated injection valves.

The injection valves consist of a nozzle needle with solenoid armature and a valve body with

magnet coil. When de-energized, a spring presses the nozzle needle into its seat. When excited

by electrical pulses from the control unit, the nozzle needle lifts approx. 0.1 mm off its seat and

fuel can escape through an annular gap. The fuel supply comes from the side or the rear end of

the valve.

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The fuel is delivered from the tank via an electric pump and a fuel filter. The fuel filter holds

back impurities and must be replaced in line with the maintenance intervals.

All the injection valves in an engine are actuated with pulses by the control unit simultaneously.

They therefore inject fuel in different phases of the operating cycle of each cylinder.

In order to create approximately identical conditions in all cylinders, the injection quantity is

split into 2 halves and injected twice per operating cycle of each cylinder (intermittent injection

in groups). The resulting displacement of part of the fuel in front of the closed intake valve

improves mixture formation.

The control unit calculates the injected fuel quantity from the pulse duration of the valves. The

control pulses are triggered by the contact breaker in the distributor.

A cold start facility is also integrated. A finely atomizing starting valve guarantees a dependable

cold start.

A bank of contacts at the throttle valve shuts of the fuel supply completely in deceleration

mode above the engine idle speed and also controls wide open throttle enrichment.

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Electronically controlled fuel injection (LH-Jetronic)

The basic system is the same as the L-Jetronic. One of the advances in this system is a change

in the way the air volume is measured. Instead of measuring the air volume with a pressure

governing flap, the air mass is measured by a hot wire mass air flow sensor, or after 1987 by

the hot film MAF sensor. A thin, heated platinum wore or a platinum film resistor is cooled by

the mass air flow. This changes its electrical resistance. The change is registered in an amplifier

and the current through the wire or film resistor is increased until there is a constant

temperature in the hot wire or film resistor. The current required for this is a measure of the

intake air mass.

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Another advance is the cold start enrichment. This is now integrated in the control unit and is

achieved by lengthening the injection period or increasing the injection frequency. This does

away with the cold start valve and the thermo-time switch

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Motronic engine control (ME)

The Motronic features a single control unit to provide complete electronic control of the engine

timing. The Motronic is available in a wide variety of variants. The table below provides an

overview of the Motronic variants and their installation in the various Mercedes engines:

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ME version

Engine Models Market launch

Remarks

ME 1.0 119

120

129/140

210/129

140

September 1995

New introduction

ME 2.0 112

113

129/163

202/208

210/215

220

March 1997 New introduction

ME 2.1 104

111

129/140

170/202

208/210

August 1996 New introduction

ME 2.7 137 220/215 January 2000 New introduction

ME 2.7.1 275

285

215/220

230/240

01.10.2002 Replaces ME 2.7

ME 2.8 112

113

163/170

202/203

208/209

210/211

215/220

230

June 2000 Replaces ME 2.0

ME 2.8.1 112

113

170/203

209/211

215/220

230

March and October 2001

As ME 2.8,

but with specific AMG elements

The modern generation of Motronic comprises the following functions:

Electronically controlled injection system with mass air flow sensor.

Electronically controlled ignition system.

Idle speed control: As the idle speed drops and fluctuates when consumers are switched on (radiator fan, A/C compressor, etc.) during idling, the Motronic works to counteract this, in order to achieve a stable idle speed.

Lambda control.

Knock control: At each cylinder there is a knock sensor (microphone) that detects combustion knock. As soon as knocking occurs, the sensor reports it to the Motronic. The firing point is then retarded. If this stops the knocking, then the ignition timing is gradually advanced again in stages.

Electronic boost pressure control (only in combination with a turbocharger).

Control of the exhaust gas recirculation.

Camshaft timing.

Control of the variable-geometry intake manifolds. In some vehicles the length and geometry of the intake ports can be changed in order to improve torque according to the operating point.

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Cruise control.

Adaptive adjustment of the opening speed of the throttle valve to suit the driver's driving style (ME 2.8).

Pressure sensor in the intake manifold for altitude detection (the air pressure is less at altitude) (ME 2.8).

The Motronic also assists the control units of the other vehicle systems. For example, it is

linked to the control unit of the automatic transmission. During a gearshift the torque must be

reduced in order to protect the transmission and to improve comfort. The link to the ABS

control unit allows the acceleration skid control (ASR) to operate to increase driving safety.

Other links exist in vehicles with ESP between the vehicle dynamics control unit and the

Motronic. These enable specific drive and braking intervention to be performed.

Block diagram of the ME Motronic

Input (sensor) Processing Output (actuators)

Engine rpm / inductive sensor Main relay

Ignition TDC of first cylinder / Hall sensor

Fuel pump relay / fuel pump

Air mass / mass air flow sensor Injection valve

Throttle valve position / throttle valve potentiometer

Electronic accelerator actuator motor

Engine temperature / engine NTC

Fuel tank vent valve

Residual oxygen upstream of CAT / oxygen sensor I

Exhaust gas recirculation valve

Intake manifold pressure / pressure sensor

Shutoff valve

Pressure differential / pressure sensor

Oxygen sensor I heater

Residual oxygen downstream of CAT / oxygen sensor II

Oxygen sensor II heater

Ignition TDC of first cylinder / Hall sensor

Secondary air valve

Accelerator pedal position / accelerator pedal potentiometer

Control unit

Basic adjustment via characteristics map

Startup control

Post-start, wide open throttle, acceleration enrichment

Deceleration fuel shutoff

Rpm limitation

Lambda control

Idle speed control

Tank ventilation system

Exhaust gas recirculation

Torque control

Electronic accelerator function

Vehicle speed regulation

Load change regulation

Secondary air injection

EOBD II

CAN bus system Secondary air injection pump

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3.3 Direct injection

Spark ignition direct injection

This is a relatively new method of internal mixture formation, which was introduced by

Mitsubishi in 1997. In direct injection systems the fuel is injected directly into the combustion

chamber by electromagnetically operated injection valves. One injection valve is allocated to

each cylinder. The mixture is formed inside the cylinder. When operating, the engine draws in

only air, and not the fuel/air mixture as in conventional injection systems. This is one

advantage of the new system: Fuel can no longer condense in the intake manifold and produce

high exhaust emission values. With external mixture formation the fuel/air mixture is generally

homogeneous (finely and evenly mixed) throughout the combustion chamber in the

stoichiometric ratio. Mixture formation in the combustion chamber, on the other hand, allows

two completely different methods of operation:

Stratified charge operation: Swirling movements form different layers in the fuel/air mixture, so that mixture with different air/fuel ratios is stratified in the combustion chamber. In stratified operation the mixture only needs to be ignitable in the area around the spark plug. The rest of the combustion chamber contains only fresh gas and residual gas with no unburnt fuel. At idle and in the partial-load range the mixture as a whole is very lean (fuel/air ratio about 1:40). The result is a reduction in the fuel consumption. However, as the system is not operating at = 1, a new catalytic converter technology is necessary.

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Homogeneous operation: In homogeneous mode the mixture is homogeneous throughout the combustion chamber, as with external mixture formation. All the fresh air in the combustion chamber is involved in combustion. For this reason this operating mode is used in the wide open throttle range.

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Exercise 1

a)

b)

c)

Exercise 2

Exercise 3

Exercise 4

a)

b)

c)

3.4 Exercises on mixture formation

What is meant by mixture formation in internal combustion engines?

The mixing of regular gasoline and Super gasoline

The formation of the fuel/air mixture

The formation of the air/fuel ratio and octane number

What do gasoline and diesel consist of?

What are alternative fuels? Name 3!

What is meant by the mixture ratio of the fuel/air mixture? The ratio of ...

Fuel to air.

Diesel fuel to gasoline and air.

Basic gasoline to additives and air.

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Exercise 5

Exercise 6

Exercise 7

a)

b)

c)

d)

Exercise 8

a)

b)

What does combustibility mean? How is combustibility designated in the case of spark ignition

and diesel engines?

What is the meaning of = 1?

Which of the following statements are true?

Spark ignition fuels should have low combustibility.

Spark ignition fuels should have high combustibility.

Diesel fuels should have low combustibility.

Diesel fuels should have high combustibility.

Which statement is true?

A rich mixture has > 1.

A rich mixture has < 1.

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Exercise 9

Exercise 10

a)

b)

c)

Exercise 11

a)

b)

Exercise 12

When is the throttle valve of the engine wide open? Is this related to rpm?

Which engine operating states require a rich fuel/air mixture?

Cold start, warm-up, acceleration, full power

Only at full power

Only at cold start and full power

The mixture ratio of fuel to air depends on...

Engine rpm, temperature and load.

Engine rpm, load and piston weight.

How many injection valves are there in a 4-cylinder engine with central intake manifold

injection?

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Exercise 13

Exercise 14

Exercise 15

Is the fuel quantity adjusted to suit the air volume, or vice versa?

Describe a mixture formation system of your choice!

What are the differences between a Motronic and a K-Jetronic?

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Exercise 16

Which variable tells the controller in the hot wire mass air flow sensor how much air has been

taken in?

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4 Ignition in spark ignition engines 4.1 Ignition in spark ignition engines

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4 Ignition in spark ignition engines

4.1 Ignition in spark ignition engines

Ignition system in spark ignition engines

The purpose of the ignition system is to ignite the compressed air/fuel mixture at the right

instant and thereby initiate combustion. The right instant is called the firing point. In a spark

ignition engine this is accomplished by an electrical spark between the electrodes of the spark

plug. An ignition system that functions reliably under all circumstances is a prerequisite for

proper operation of the engine and catalytic converter. Misfiring results in poor engine

performance, higher consumption and degradation of the catalytic converter due to

overheating. Afterburning of unburnt fuel in the catalytic converter causes it to overheat. High-

voltage systems are generally used to produce the ignition spark. These consist of the spark

plug(s) and the actual ignition system for generating the high voltage required.

Firing point

The instant at which the mixture should be ignited depends primarily on:

Engine speed

The time it takes for a uniform fuel/air mixture to burn completely is constant. About 2 milliseconds pass from the instant of ignition until combustion is complete. As the engine speed rises, the time for this cycle decreases. For this reason ignition must occur earlier at higher engine speeds.

Load

At low loads the mixture leans out, the residual gas quantity increases and the volumetric efficiency decreases. This effect produces a longer ignition delay and a lower combustion rate in the mixture, so ignition must take place earlier.

For these reasons the ignition system includes an adjustment facility to regulate the firing point

according to load and engine speed.

It is usual to reference the firing point to the position of the crankshaft relative to top dead

center (TDC). It is stated as an angle in degrees before TDC. Two adjustment variants exist:

Ignition retard: Adjustment of the ignition angle towards TDC.

Ignition advance: Adjustment away from TDC.

The choice of firing point influences a number of combustion processes and results:

Maximum engine power

Fuel consumption

Driveability

Emissions

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Tendency to knock: Knocking is uncontrolled combustion in the engine. It occurs due to the sudden burning of mixture particles that have not yet been reached by the flame. If this happens, the firing point is too far advanced. Knocking leads to excessive temperatures and steep pressure rises in the combustion chamber. Pressure oscillations occur which superimpose themselves on the normal pressure curve. The cylinder head, cylinder head gasket, bearings and pistons may be damaged. A distinction is drawn between knocking during acceleration (at low rpm and high loads) and high-speed knocking (high rpm and high loads).

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1. Firing point at the correct instant

2. Firing point too early

3. Firing point too late

Spark plug

The spark plug is screwed into each combustion chamber and has the task of bringing the

ignition energy into the combustion chamber. An electrical spark between the electrodes

initiates combustion of the cylinder charge. A ceramic insulator is installed in the plug housing

with gas-tight seals. Usual electrode gaps are 0.6 to 0.9 mm. A longer electrode gap activates a

greater mixture volume, requires a higher ignition voltage and therefore increases the demands

on the ignition system and insulation. With a smaller electrode gap there is the danger of

combustion misfiring as the volume activated is too small. Together with other components of

the engine, the spark plug plays a crucial role in ensuring that the engine operates correctly.

It must enable a reliable cold start and must always guarantee that the engine runs without

misfiring.

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1. Insulator

2. Electrode gap

The demands on a spark plug are enormous:

Electrical: Insulation of high voltages in excess of 30 kV.

Mechanical: Pressure spikes of over 100 bar while remaining gas-tight.

Chemical load: At high temperatures the plug is exposed to the chemical reactions of combustion. Aggressive residues cause deposits that can change the properties.

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Thermal load: Extreme temperature cycles: Combustion up to 2800°C, gas exchange at 60-100°C. For a spark plug to function reliably, 2 limit temperatures must not be violated.

Lower limit temperature 500°C: The insulator nose must be hot enough to burn off carbon residues and thus avoid electric shunts.

Upper limit temperature 900°C: The temperature should not be higher if glow ignition is to be avoided. Otherwise the mixture ignites not due to the controlled initiation of an ignition spark, but by the high temperature in excess of 900°C.

The operating temperature of a spark plug is a temperature equilibrium between heat

absorption and heat release. Heat is absorbed through the combustion temperatures. Heat is

released to the fresh gas and via the center electrode, the insulator nose and the plug housing

through the cylinder head to the coolant. The input of heat depends on the engine design.

Engines with higher specific outputs generally have higher combustion chamber temperatures.

The temperature is maintained within the limits by varying the insulator nose accordingly.

The thermal load capacity of a spark plug is identified by its heat rating:

Low heat-range code numbers: Short insulator nose and thus a small heat-absorbing surface area, rapid heat dissipation, low resistance to carbon fouling, high resistance to glow ignitions (“short plug”).

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High heat-range code numbers: Long insulator nose and thus a large heat-absorbing surface area, slow heat dissipation, high resistance to carbon fouling, low resistance to glow ignitions (“hot plug”).

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Spark plug designs are distinguished not only by the heat rating (long or short insulator nose),

but also by the material, the shape and the number of the electrodes used. According to the

shape and number of electrodes a distinction is made between:

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Overhead electrode

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Side electrode

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Surface gap spark plug without ground electrode

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Ignition system

The energy required for ignition is provided by the ignition system and allocated to the spark

plug of the relevant cylinder at the firing point. The high voltage necessary for ignition is not

generated until the firing point. The electrical energy for this comes from an energy buffer. The

type of ignition system depends on the nature of this energy storage:

Coil ignition

Capacitor-discharge ignition

Coil ignition

The breaker-controlled coil ignition described here is the simplest version of ignition system in

which all the functions are implemented.

Its elements are:

Ignition coil as inductive energy storage

Distributor with mechanical contact breaker (triggers high-voltage generation at the firing point), timing gear (adjusts the firing point by means of centrifugal and vacuum timing adjusters), distributor finger (distributes the high-voltage pulse to the ignition cables of the individual cylinders) and ignition capacitor.

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Circuit diagram and function of the elements

The ignition energy source for the coil ignition system is the ignition coil. It stores the energy in

the magnetic field and releases it at the moment of ignition as a high-voltage pulse (ignition

pulse) via the ignition lines to the relevant spark plug. The storage method is based on an

induction process (magnetic field with coil). The ignition coil is therefore referred to as an

“inductive energy storage device”.

The ignition coil consists of 2 mutually insulated wire coils, one on top of the other: The primary

coil has few windings of thick copper wire, and the secondary coil has many windings. One end

of the primary coil is connected via the ignition switch to the positive terminal of the battery.

The other end is grounded via the contact breaker. The ignition capacitor is connected in

parallel with the contact breaker. It is operated mechanically so that the contact breaker opens

whenever ignition is supposed to take place. The secondary coil also has one end connected to

ground. The other end is connected via the distributor and ignition lines to the center electrode

of the spark plug.

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When the ignition switch is closed, the primary coil is connected to the positive terminal of the

battery. If the contact breaker is closed, a current flows – the primary current. It rises to a limit

value, the so-called “no-load current”. In conventional coil ignition systems with mechanical

contact breaker, this no-load current is limited to 3 to 4 amps.

At the moment of ignition the contact breaker opens the primary circuit and stops it. In this

instant the magnetic field collapses and induces a voltage in the primary and secondary

windings made of thin copper wire. The primary and secondary windings surround an iron core.

The purpose of this iron core is to intensify the magnetic field and therefore boost the stored

energy.

An important factor in the production of the ignition spark is the magnitude of the secondary

voltage. It is higher,

the faster the magnetic field collapses,

the greater the winding ratio between the primary and secondary coils and

the stronger the primary current.

Because the winding ratio used is 1:100, a high voltage is generated on the secondary side.

Before ignition commences, the gap between the electrodes of the spark plug is completely

non-conductive. The ignition pulse is carried by the ignition cables to the center electrode of

the spark plug, where it causes a steep voltage rise. When a certain voltage (ignition voltage

> 15000 V) is reached, the gap becomes conductive and the spark can cross. Immediately

afterwards the voltage drops steeply down to the burn voltage. This burn voltage is sufficient to

maintain the spark. This ensures that the mixture can still be ignited if this did not already

occur due to the high ignition voltage.

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1. Brief spark head 2. 3. 4. 5.

2. Spark tail as post-discharge with slightly fluctuating burn voltage

3. Spark line

4. Decay phase

5. Primary current flows

As soon as the energy supplied by the storage device drops below a given minimum, the

ignition spark breaks. The spark gap becomes non-conductive again. When the contact breaker

opens, a voltage of 300-400 V is also induced in the primary winding of the ignition coil. If

precautions were not taken, this voltage would produce a strong spark at the contact breaker.

This would lead to energy consumption, severe contact erosion and high transition resistances

at the contacts. The ignition capacitor prevents such negative phenomena. It is connected in

parallel with the contact breaker. At the instant the primary current is interrupted, it takes on

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the electrical charge and thus forms a secondary path to the opening contact breaker. It is

charged to the induced peak voltage. This takes a certain amount of time. The contacts are now

too far apart for the spark to cross. At spark rates below 3000/min the lift speed of the

breaker lever is so small that weak sparking still occurs in spite of the capacitor. This means

that with frequent urban driving the wear on the contact breakers is high and they must be

replaced more frequently.

Further developments of coil ignition

Further developments of the conventional coil ignition are:

Breaker-controlled transistorized coil ignition (TSZ-k),

Breakerless transistorized coil ignition (TSZ).

These developments were necessary because the conventional coil ignition system had the

following shortcomings:

Wear of the contact breakers

Insufficient energy capacity of the ignition coil in some cases.

In the breaker-controlled transistorized coil ignition system (TSZ-k) the function of the contact

breaker as the switch for the primary current is replaced by a transistor. The control current is

interrupted on the basis of the transistor by the mechanical contact breaker in the distributor.

In other words, the transistor here acts as a switch with “on/off” positions. When the contact

breaker is closed, the control current flows and the transistor is conductive (“on” position). This

allows the primary current to flow to charge the ignition coil. When the contact breaker opens,

the control current is interrupted and the interrupted is conductive (“off” position). The primary

current in the ignition circuit is interrupted, initiating the generation of high voltage in the

secondary circuit. The transistor can switch up to 9 amps (higher energy capacity of the ignition

coil)!

In the breakerless transistorized ignition system (TSZ) the mechanical contact breaker for the

control current of the transistor is replaced by electrical switches. Both elements are housed in

the distributor. Because there is no mechanical contact breaker, the firing point remains

constant across the operating duration.

Otherwise the TSZ-k and TSZ systems are identical to conventional coil ignition systems.

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Timing gear in coil ignition systems

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The ignition angle is an important factor in controlling combustion in spark ignition engines.

As the ignition delay and combustion duration change sharply according to the mixture

composition and engine speed, the ignition timing must be adjusted accordingly. Several

different systems are available for timing adjustment based on rpm and load.

Centrifugal timing adjuster: The purpose of the centrifugal timing adjuster installed in the distributor is to advance the firing point as the engine speed increases.

Two pivoting centrifugal weights are mounted on the carrier plate which is connected to the drive shaft. As the engine speed increases, centrifugal force swings them increasingly further away from their rest position against the force of the extension springs. The drivers adjust the pivoting breaker cams in the drive shaft relative to the drive shaft

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Vacuum timing adjuster: The vacuum adjuster changes the firing point in relation to the intake manifold vacuum. A law of physics dictates that a vacuum is produced in the intake manifold due to the high flow rates of the air. In the partial-load range, the spark ignition engine is operated with a less ignitable, slower burning mixture which must therefore be ignited earlier. The vacuum in the intake manifold is used as a measure of the engine load. The vacuum-dependent firing point advance function is integrated in the distributor and operates additively to the rpm-dependent centrifugal adjustment. The vacuum immediately after the throttle valve acts on the membrane. In the partial-load range this is pressed against the effect of the spring and rotates the breaker plate in the opposite direction to the breaker cam via the pull rod. The spring is preloaded so that no adjustment takes place at wide open throttle. The adjustment range is limited by end stops on the pull rod.

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Electronic ignition systems – Capacitor-discharge ignition (CDI)

A further stage of development was the integration of electronics in ignition systems and the

transition to capacitor-discharge systems. These are characterized by the fact that the ignition

energy is stored in a capacitor. They may be breaker-controlled or breakerless. The CDI

consists of a switching unit and an ignition transformer. The switching unit contains a charging

device, the capacitor and the necessary electronic circuitry. The charging device is a voltage

converter, which transforms the battery voltage into the considerably higher charging voltage

for the capacitor. The capacitor is charged to 300-400 volts. The charging currents (up to

100 amps) are switched by a thyristor (electronic switch). The ignition transformer transforms

the discharge current coming from the storage capacitor into the high voltage required to

generate the ignition spark.

The advantages of CDI over coil ignition are:

Higher ignition voltages

Almost complete insensitivity to fouled spark plugs

Timing adjustment in electronic ignition systems

Ignition systems with centrifugal and vacuum-controlled firing angle adjustment can only follow

simple adjustment characteristics in relation to engine speed and load. However, these

systems take inadequate account of different operating states and mixture compositions. By

using electronic ignition systems it is possible to obtain the optimum ignition of the fuel/air

mixture in all engine operating states. The mechanical centrifugal or vacuum-controlled timing

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adjuster is replaced by an ignition map stored in the control unit. With the aid of input factors

such as the engine speed/crankshaft position, load (intake manifold vacuum) and other

parameters (e.g. engine temperature, data from the knock sensor etc.), the control unit

determines the optimum firing point to each operating point.

Maintenance note

on ignition systems

The maintenance operations commonly carried out on modern electronic ignition systems

without mechanical distributor are limited to the replacement of spark plugs according to

the specified maintenance intervals (since the distributorless ignition and factory-set firing

points mean that there are no mechanisms susceptible to failure). On conventional

systems work is also performed to adjust the ignition timing, renew the contact breakers

and replace the capacitors. Before replacing the spark plugs, the areas around the plugs

must be cleaned and then the ignition cables disconnected. Care must be taken to ensure

that the ignition cables are reconnected in their original positions. This can be done using

an adhesive label if there is not already a cylinder marking on the cable. The spark plugs

can then be unscrewed using a spark plug wrench. The exact heat rating of the spark plug

for the vehicle must be noted. The new spark plug is then screwed in and the ignition

cables are connected. Observe the tightening torques (see Workshop Information System

WIS).

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Exercise 17

Exercise 18

Exercise 19

Exercise 20

4.2 Student exercise on ignition in spark ignition engines

What is meant by mixture formation in internal combustion engines?

a The mixing of regular gasoline and Super gasoline

b The formation of the fuel/air mixture

c The formation of the air/fuel ratio and octane number

What do gasoline and diesel consist of?

What are alternative fuels? Name 3!

What is meant by the mixture ratio of the fuel/air mixture? The ratio of ...

a Fuel to air.

b Diesel fuel to gasoline and air.

c Basic gasoline to additives and air.

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Exercise 21

Exercise 22

Exercise 23

Exercise 24

Exercise 25

What does combustibility mean? How is combustibility designated in the case of spark ignition

and diesel engines?

What is the meaning of = 1?

Which of the following statements are true?

a Spark ignition fuels should have low combustibility.

b Spark ignition fuels should have high combustibility.

c Diesel fuels should have low combustibility.

d Diesel fuels should have high combustibility.

Which statement is true?

a A rich mixture has � > 1.

b A rich mixture has � < 1.

When is the throttle valve of the engine wide open? Is this related to rpm?

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Exercise 26

Exercise 27

Exercise 28

Exercise 29

Exercise 30

Which engine operating states require a rich fuel/air mixture?

a Cold start, warm-up, acceleration, full power

b Only at full power

c Only at cold start and full power

The mixture ratio of fuel to air depends on...

a Engine rpm, temperature and load.

b Engine rpm, load and piston weight.

How many injection valves are there in a 4-cylinder engine with central intake manifold

injection?

Is the fuel quantity adjusted to suit the air volume, or vice versa?

Describe a mixture formation system of your choice!

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Exercise 31

Exercise 32

What are the differences between a Motronic and a K-Jetronic?

Which variable tells the controller in the hot wire mass air flow sensor how much air has been

taken in?

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5 Exhaust treatment in spark ignition engines

5.1 Exhaust treatment in spark ignition engines

The formation of pollutants

As a reminder: The fuel is composed of hydrocarbon compounds. If the carbon (C) and

hydrogen (H) components in the fuel could be completely burned with the oxygen (O2) from the

combustion air in the combustion chamber, the exhaust gas would principally consist of water

(H2O) and carbon dioxide (CO2). Unfortunately, this so-called ideal combustion is not possible

for technical reasons.

Rather, we are dealing with real-life combustion in which a number of by-products are

produced.

The by-products (exhaust gases) produced by spark ignition combustion are classified as:

Non-pollutants: Nitrogen (N2), carbon dioxide (CO2), oxygen (O2), water vapor (H2O). However, the non-pollutant carbon dioxide (CO2) has negative effects in the context of global warming.

Pollutants: Carbon monoxide (CO), hydrocarbons (HC) and nitrogen oxides (NOX).

Carbon monoxide (CO) is particularly hazardous as it acts as a respiratory poison and can be

fatal in higher concentrations. Hydrocarbons (HC) occur increasingly when the combustion

conditions are not good, for example with a rich mixture and in the “hidden” spaces in the

combustion chamber (top land).

Nitrogen oxides (NOX) occur at high combustion temperatures when the nitrogen (N2) in the

combustion air combines with the oxygen (O2). The action of sunlight breaks nitrogen dioxide

(NO2) down into nitrogen oxide (NO) and in one atom of oxygen (O). This free oxygen atom

bonds with the oxygen (O2) in the air to form ground-level ozone (O3). In conjunction with the

hydrocarbons, ground-level ozone is responsible for the formation of smog.

A great deal of effort is therefore invested in minimizing pollutant emissions. The concepts

extend from engines designed for optimum fuel consumption through to technical concepts for

exhaust treatment. These technical concepts can be broken down into:

Engine-related measures

Exhaust aftertreatment measures using catalytic converters

Engine-related measures

These are always compromises. What is good for reducing one pollutant may encourage the

formation of another:

The problem is to reduce the quantity of hydrocarbons (HC) as much as possible. This is done by means of small combustion spaces where there is little opportunity for the mixture to creep into “hidden” locations, from which it would be exhausted again unburnt.

A high compression ratio improves thermal efficiency. This reduces fuel consumption and the quantity of pollutants. However, greater efficiency leads to higher combustion temperatures and thus to higher nitrogen oxide (NOX) levels.

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The location of the spark plug in the combustion chamber, the spark temperature and duration: The ignition of the mixture and the heat release depend on these factors. And these are accompanied by the formation of pollutants. A central location for the spark plug is therefore preferred. The reason is that this results in very short flame propagation distances and allows almost complete combustion of the mixture.

The firing point and the valve timing also have a very strong influence on the pollutant level and fuel consumption.

Exhaust gas recirculation: This system feeds a small percentage of the exhaust gas back to the intake system and mixes it with the inducted fuel/air mixture. The recirculated gas lowers the combustion temperature, which causes a reduction in the NOX levels in the exhaust gas. A maximum of 8-10% of the fresh gas volume is recirculated gas.

Only the map-controlled exhaust gas recirculation of the Motronic operates perfectly. The control unit contains a characteristics map for exhaust gas recirculation. This lists the air mass required for each operating point. From the mass air flow sensor the control unit receives information about the air mass currently being inducted. It then compares this with the specified values in the map and allocates the exhaust gas recirculation quantity accordingly. This is done by controlling the EGR valve.

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Exhaust aftertreatment measures using catalytic converters

The combination of oxygen sensor and 3-way catalytic converter is the most efficient method of

keeping the pollutant levels of spark ignition engines low.

3-way catalytic converter

The 3-way catalytic converter derives its name from the breakdown (Greek “katalysis”) of the

3 pollutants carbon monoxide, hydrocarbons and nitrogen oxides. But this breakdown is more

of a conversion.

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Catalytic converters exist in ceramic and metal catalyst versions. The ceramic catalyst is

discussed here. The only difference is the material used for the substrate.

The ceramic catalytic converter is composed of the following elements:

Substrate: The substrate consists of a ceramic honeycomb structure, the channels and cells of which provide a high surface area for the chemical conversion. One liter of ceramic honeycomb provides a total surface area of about three liters.

Washcoat: To further increase the surface ares, the cell walls of the honeycomb structure are coated with an intermediate layer (washcoat) of aluminum oxide and so-called promoters. The promoters intensify the catalytic effect of the precious metal coating (catalytically active layer, see below). They increase the surface area by about 7000 times.

Catalytically active layer: This is applied to the washcoat and consists of the precious metals platinum, rhodium and/or palladium.

Housing with insulation: The ceramic substrate is extremely brittle and also has a different thermal expansion rate from the housing. For this reason it is embedded in an insulation layer, a wire mesh or ceramic fiber matting. Metal substrates do not require an insulation layer.

The task of the catalytic converter is to bring about the parallel oxidation and reduction

processes. Oxidation means that oxygen (O) is attached to the exhaust gases. Reduction means

that oxygen is taken away from the pollutant. The carbon monoxide (CO) and hydrocarbon (HC)

pollutants are converted into harmless carbon dioxide (CO2) and in water vapor (H2O) through

oxidation. The nitrogen oxide (NOX) pollutants are reduced by reduction into nitrogen (N2) and

oxygen (O2). The conversion rates on new catalytic converters is about 90-98% at a

temperature between 400 and 800° C.

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Lambda ( ) control

The conversion rates of the pollutants in the catalytic converter are most effective at around

= 1. It must be attempted to keep the fuel/air mixture in this range. This range is also

designated as the so-called lambda window.

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This is what the Lambda sensor is for. It is located after the exhaust manifold and before the

catalytic converter and determines oxygen content of the exhaust. This allows conclusions to

be made about the lambda ratio of the fuel-air mixture. It must determine whether the fuel/air

mixture is richer or leaner than = 1. A lean mixture contains more air, a rich mixture, in

contrast, less air. Each deviation from Lambda =1 must be transmitted to the control unit so

that it can make a corresponding change of the injection volume within the lambda window.

Mode of operation and design of the lambda control

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The oxygen sensor is a gas-permeable ceramic body made of zirconium dioxide. This becomes

permeable to oxygen at temperatures of 350°C and above. Above this temperature the

difference in the electrical charge between the outside air and exhaust sides allows a voltage

comparison to be made which provides an indication of the oxygen content of the exhaust gas.

As the measurement procedure is highly complex, it will not be covered any further here.

One more thing: Since the sensor can only measure, and thus regulate, at a temperature of

350°C and above, these sensors are usually heated so that the catalytic converter is able to

operate approx. 20-30 seconds after a cold start even at low temperatures.

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Problems in the use of catalytic converters

There are also problems in the use of catalytic converter technology:

Degradation of efficiency due to poisoning (filling with leaded fuel)

Thermal aging due to overheating

Aging of the oxygen sensor

Increased exhaust back pressure

Use of rare materials

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Exercise 33

Exercise 34

Exercise 35

5.2 Student on exhaust treatment in spark ignition engines

What measures can be taken to reduce pollutants?

What is the function of the oxygen sensor?

What does an EGR valve do?

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Exercise 36

Exercise 37

Exercise 38

Is complete combustion of the air/fuel mixture possible? Explain your answer!

In which air/fuel ratio range does the catalytic converter operate most effectively? What is it

called?

Complete the following sentence: “If the combustion temperature drops, the following is

reduced”:

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6 Mixture formation in diesel engines

6.1 Mixture formation in diesel engines

The diesel engine has an internal mixture formation system, i.e. the fuel/air mixture is only

produced and rendered ignitable in the combustion chamber. Ignition occurs due to

autoignition of the mixture, in other words no ignition device is necessary.

In the 2nd stroke (compression) at about 15-20° crank angle before TDC the fuel delivered by

an injection pump is injected into the combustion space via the injection nozzle. But the fuel,

injected into air at 500-900°C, does not ignite immediately. It first has to warm up, then

evaporate and mix with the combustion air. Only then is a combustible mixture formed, which

ignites at several points simultaneously. Because of the ignition the temperature in the

combustion chamber exceeds 2000°C, so the fuel injected afterwards ignites immediately

without a delay. The injection period is up to about 20° crankshaft angle.

Ignition delay

The time period from the start of injection to the start of ignition is the ignition delay, and is

usually approx. 1 ms.

Under the following conditions, however, the ignition delay can last for up to 2 ms:

Low engine temperature

Injection nozzles not atomizing

Injection pump start of delivery set too early

Bad fuel (e.g. gasoline)

This has certain consequences: A hammering noise known as combustion knock occurs. The

reason for this is that, due to the extended ignition delay, more fuel has accumulated in the

combustion chamber, which then ignites all the more violently. This sudden ignition sounds like

hammering or knocking.

Combustion knock can have the same consequences as in a spark ignition engine (see relevant

chapter).

The ignition delay can be influenced by changing the start of injection. When injection occurs

later, the temperatures and pressure in the combustion chamber are higher and the ignition

delay is shorter.

Mixture formation processes

The shape of the combustion chamber together with the positions of the injection jets

characterize the mixture formation process. The mixture formation processes are therefore

classified according to these features,

A distinction is made between:

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Chamber process (indirect injection): In the chamber combustion process the combustion chamber is divided into at least 2 chambers which are connected by relatively narrow transfer ports. The gas flow between the chambers speeds up the mixture formation process considerably. Such processes include the prechamber process and the swirl chamber process. •

Direct injection: In direct injection systems the combustion chamber is not divided, or any division is not significant. The chamber is formed from flatter or deeper cavities in the cylinder head or in the pistons. The task of mixture formation in these systems is often transferred largely to the injection nozzle. Mixture formation may be assisted by rotating air movements generated during the induction process. These systems include central injection, the M-process and the D-process.

The important variants of these combustion systems are outlined below:

Prechamber process

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The picture illustrates a typical layout with eccentrically arranged prechamber. One or more

transfer ports connect the prechamber with the main combustion chamber. A single-hole nozzle

(pintle nozzle) injects fuel directly into the prechamber with the spray aligned in the longitudinal

axis of the prechamber. As the piston rises, a violent air flow is created in the prechamber,

which activates mixture formation in the chamber. Because the walls of the prechamber are

hot, ignition occurs in the prechamber itself after a relatively short ignition delay. When

combustion has started, the majority of the rich (inert) mixture formed in the prechamber is

ejected into the main combustion chamber where it mixes with the air.

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Swirl chamber process

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The swirl prechamber is a spherical or cylindrical chamber in which turbulence is produced by

the tangential entry of the air flow from the transfer port. Fuel is injected by the nozzle in the

same direction as the air flow. Assisted by the centrifugal effect, the mixture is stratified in the

swirl chamber, with rich, heavy inert mixture around the periphery. After ignition in the

chamber, the gas containing the unburnt or only partially burnt rich mixture is ejected into the

main combustion chamber.

Direct injection

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The illustration shows a combustion system with one centrally located multi-hole nozzle.

The fuel jets are aimed radially outwards towards the cylinder wall so that as much of the

combustion cavity volume as possible is covered. The nozzle is primarily responsible for mixture

formation. An extremely high injection pressure ensures good atomization.

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M-process

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The M-process arose from the efforts to improve smooth running characteristics with low

consumption. The fuel jet from the eccentrically arranged nozzle is directed against the wall of

a spherical combustion cavity located deep in the piston. The jet sprays in the direction of the

air turbulence so that atomization is reduced. This produces a film of fuel on the wall of the

combustion chamber. Only a small proportion of the fuel is used for ignition, which means that

combustion is gentle. Accelerated evaporation of the film from the chamber wall, final mixing

and combustion proper are achieved by means of the hot combustion gases only after

combustion has commenced.

D-process

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In the D-process, too, the nozzle is eccentrically arranged in the cylinder head. Two jets are

aimed approximately parallel to the cylindrical wall of the oblique combustion cavity in the

piston. A vortex about the combustion chamber axis, which is produced as normal during

induction, creates an intense transverse flow at the injection jets and therefore guarantees

intensive atomization of the injected fuel. Again, a layer of rich mixture is formed, not on the

chamber wall, but in the vortex. At the end of the ignition delay, this inert layer is initially

immune from rapid combustion, which ensures that the engine runs smoothly.

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Comparison of chamber processes with direct injection

The chamber (indirect injection) processes are characterized by remarkably low combustion

noise levels. The reasons for this are the short ignition delay (hot chamber walls) and the rich

mixture in the chamber at the start of combustion.

A particular disadvantage of the indirect injection processes is the longer combustion durations

compared to the direct injection processes. Other negative aspects are the losses due to

transfer between the parts of the combustion chamber, meaning that fuel consumption is

5-10% higher than in engines with direct injection. In addition, the high flow rates increase

thermal losses. For cold starts a starting aid (starter glow plug) is necessary to heat the

contents of the prechamber.

Injection systems

Since good combustion requires that the fuel be distributed as finely as possible, the pressures

upstream of the injection valves should be as high as possible. A variety of injection system

variants are available for this, which will be described in more detail below.

The conventional diesel injection systems are:

In-line injection pumps (PE)

Individual injection pumps (PF)

Distributor injection pumps (VE)

More recent variants are:

Common rail systems

Pump-Nozzle Unit (PDE)

Pump-Line-Nozzle (PLD).

In-line injection pump

An in-line injection pump consists of two elements: a delivery pump and a high-pressure pump.

The delivery pump delivers the fuel from the fuel tank to the high-pressure pump at a pressure

of approx. 2 bar.

The high-pressure pump consists of piston pump elements. Piston pumps in turn consist of a

pump cylinder and a pump piston. There is one pump element for each cylinder.

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The pump piston is moved in the delivery direction by an engine-driven camshaft and back

again by the piston spring. Via the plunger the cam actuates the pump piston against the force

of the spring. The fuel is pumped via the pressure valve in the injection line to the nozzle.

Fuel delivery begins as soon as the top edge of the pump piston has closed the feed bore from

the delivery pump. It continues until the diagonal bottom edge of the pump piston opens the

feed bore again. The fuel displaced by the further movement of the piston can now flow back

through this bore. The end of delivery, and thus the injection quantity, can be adjusted by

rotating the pump piston with a toothed rack (control rod) and gear wheel to change the

position of the diagonal edge. The pump chamber is charged via the feed bore by filling the

cavity that is formed during the upward movement of the piston with the pressure valve closed

during the dwell period of the bore. To shut the engine off, the piston is rotated until its

longitudinal groove is constantly adjacent to the bore (zero delivery). These piston pumps are

connected in series in an injection pump block and are driven by a common camshaft and

rotated by a common control rod.

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Injection timing devices for in-line injection pumps

The higher the engine speed, the less time there is available for the combustion cycle. To

ensure that the mixture has sufficient time to combust, the fuel must be injected earlier. This is

why injection timing devices are used. Centrifugally controlled injection timing devices are

installed in the drive train between the engine and the injection pump. As the engine speed

increases, the centrifugal weights (1) turn the camshaft (2) of the injection pump against the

drive shaft in the “earlier delivery” direction.

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Individual injection pumps

To avoid the need for excessively long injection lines, individual pumps are used on large

engines. The pumps are flange-mounted on the crankcase itself as close as possible to the

associated cylinder. The pump pistons are operated by additional cams on the engine timing

camshaft. The modern Pump-Nozzle variants were developed to do away with injection lines

entirely.

Distributor injection pumps

A distinction is drawn between distributor fuel injection pumps with mechanical control and

those with electronic diesel control; there are versions with rotary solenoid adjustment

mechanisms and versions with solenoid valve control.

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1 Regulating valve 4 Shutoff valve

2 Solenoid valve for injection timing device 5 Flow controller

3 Pump piston 6 Fuel rack position sensor

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Distributor fuel injection pumps have only one delivery unit for all cylinders, which supplies the

injection nozzles of the individual cylinders in sequence according to the firing order. The fuel is

pumped from the reservoir into the chamber of the injection pump by a vane-type supply pump

mounted on the drive shaft of the injection pump. Any surplus fuel delivered is directed back to

the reservoir by a return line connected to the highest point of the pump chamber for bleeding.

The fuel quantity circulating in this way is used to lubricate and cool the moving components of

the pump. The pump is fitted with a mechanical rpm governor which regulates the fuel quantity

via the movement of the regulating valve. The pump also possesses an injection timing device

operated by the rpm-proportional pressure, which turns the roller ring via an appropriate

adjustment mechanism. This influences the start of delivery and thus the start of injection. An

electromagnetic shutoff valve closes the fuel supply to the pump plunger when the engine is

switched off. The operating principle of the distributor fuel injection pump (for high-pressure

generation) and fuel distribution is described in detail below:

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The pump plunger is connected with the cam plate and the rolling motion of the cam plate on a

roller ring produces a combined linear/rotary movement of the plunger. The fuel is supplied via

grooves in the pistons. The rotary motion interrupts the fuel inflow and the linear travel opens

the bore to an injection line while the pressure builds up. The end of fuel delivery is determined

by the position of the regulating valve. After a quarter turn (in a four-cylinder pump) the process

starts again from the beginning. Compared with in-line injection pump, distributor pumps are

more susceptible to malfunctions. All injection pumps feature a governor for the low idle speed

and the maximum engine speed (governed maximum). Between these two speeds the load is

adjusted via the accelerator pedal, which moves the control rod, for example. To reduce the

exhaust emissions of diesel engines while keeping fuel consumption low, injection systems with

electronic control are increasingly being used. In these the injection process can be flexibly

adapted to suit the operating state of the engine by an electronic engine management system

as with a characteristics map.

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Radial piston distributor pump

A very high injection pressure (up to 1500 bar) can be produced by a so-called radial piston

distributor pump. The main components of this kind of pump are the feed unit, consisting of the

cam ring, the roller shoes, the rollers and the delivery piston, as well as the solenoid valve

needle operated by the pressure solenoids and the injection timing piston.

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Function description

Filling: When the solenoid valve is open, the fuel flows at low pressure to the feed unit of the radial piston pump.

The rotary motion of the drive shaft causes the rollers to roll up the cams of the cam ring and push the pistons inwards. The fuel between the pistons is compressed. When the pressure solenoid is energized, the high-pressure system is closed by the solenoid valve needle, the fuel is subjected to high pressure and conveyed to the injection nozzle. The injection timing device can turn the cam ring and change the position of the delivery process on the delivery cam. In combination with the solenoid valve control it is therefore possible to adjust the delivery rate within certain limits independently of the delivery time and quantity allocation by shifting the position of the delivery process on the delivery cam.

Defining the injection quantity: When the solenoid valve is opened, the high-pressure side of the pump is connected with the low-pressure side, interrupting the delivery process.

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Common rail system

In the common rail injection system the injection pressure is produced by a high-pressure

pump, stored in a pressure reservoir (common rail) and from there distributed by the solenoid

valve-controlled injectors.

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The main feature of the system is that the injection pressure can be generated regardless of

the engine speed and the injection quantity. The rail acts as a high-pressure reservoir for the

fuel. The storage capacity of the rail is such that neither the removal of fuel by the individual

injections nor the work of the high-pressure pump cause significant pressure fluctuations.

A high-pressure piston pump of the radial piston design produces the pressure. The desired rail

pressure is regulated by a pressure regulating valve located either at the pump or at the rail.

The high pressure generated by the high-pressure pump and regulated via a regulation circuit is

present at the injector. The injector is the core of the system and its task is to inject the fuel

correctly into the combustion chamber. A pulse given at the right moment by the control unit to

the solenoid valve in the injector initiates the injection process. The opening duration and fuel

pressure dictate the incoming fuel quantity.

Injector

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The fuel coming from the rail is constantly present at the high-pressure connection (1) of the

injector. It floods the chamber of the injection nozzle (3) and also, via a feed bore, the valve

control cavity (2). The same pressure prevails in the chamber of the injection nozzle and in the

valve control cavity as long as the solenoid valve is closed. To guarantee that the injection

nozzle remains leaktight, a pressure area ratio of approx. 1.5 of the control plunger surface

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area to the nozzle needle is generated. This means that the hydraulic force of the control

plunger predominates and the valve control plunger presses the nozzle needle into its seat.

When the solenoid valve is energized, the pressure in the valve control cavity is abruptly

reduced. The pressure in the chamber lifts the nozzle needle off its seat and the fuel is injected

into the combustion chamber.

Pump-Nozzle Unit

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The Pump-Nozzle Unit consists of a high-pressure injection pump and a multi-hole injection

nozzle. This is the so-called Pump-Nozzle Unit. There is also a gear pump that supplies the

Pump-Nozzle Unit with fuel under an initial pressure of 3.5 bar. The Pump-Nozzle Unit is

installed in the top of the cylinder head and there is one for each cylinder of the engine. It is

driven, i.e. the pump piston is operated, by the engine camshaft with a cam provided especially

for this purpose. The Pump-Nozzle Unit has no control rod and the pump piston has no control

edge. Instead there is a fast-switching solenoid valve actuated electrically by a control unit.

The solenoid valve is installed in the return duct between the high pressure chamber of the

pump element and the fuel tank. The Pump-Nozzle Unit can produce injection pressures of up

to 2000 bar.

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Pump-Line-Nozzle

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In this system each engine cylinder has its own feed unit, consisting of a single-cylinder high-

pressure injection pump, a short injection line and the injection nozzle. The injection pump is

driven by the engine camshaft with an additional injection cam. The pump has no control rod,

no pressure valve and the pump element has a piston with no diagonal control edge. Instead

there is a fast-switching, electrically controlled solenoid valve installed in the return duct to the

fuel tank. The Pump-Line-Nozzle system is installed in Mercedes-Benz commercial vehicle

engines and produces injection pressures of up to 1800 bar.

Injection nozzle and injection valve

The fuel delivered by the injection pump is sprayed into the combustion chamber under high

pressure by an injection nozzle. The most common type of nozzle features a spring-loaded valve

needle which is lifted off its seat by the fuel pressure and opens the nozzle hole.

Two different designs of nozzle may be distinguished:

Multi-hole nozzle

Multi-hole nozzles are mainly used in the direct injection processes.

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Throttling pintle nozzle

These have only one nozzle aperture and are mostly used for engines with divided combustion chamber.

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Injection nozzles are usually replaceably installed in the nozzle holder, which also contains the

spring for loading the valve needle.

Nozzle holder TT_00_00_018972_FA

The injection nozzle is pressed directly against the nozzle holder by a union nut with flattened

surfaces. The compression spring acts on the nozzle needle via a spring plate. The whole unit is

known as the nozzle holder. The connection connects the nozzle holder with the injection

pump.

Injection nozzles have extremely high mechanical and thermal load capacities.

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Exercise 39

a)

b)

Exercise 40

a)

b)

c)

Exercise 41

a)

b)

c)

d)

Exercise 42

a)

b)

c)

d)

Exercise 43

a)

b)

c)

d)

6.2 Student exercise on mixture formation in diesel engines

Which statement is true?

The diesel engine has internal mixture formation.

The diesel engine has external mixture formation.

What is meant by the ignition delay? The time span from the start of injection until ...

the maximum combustion pressure.

the start of autoignition.

the fuel has burned completely.

Which control system meters the fuel quantity in the in-line injection pump?

Edge control by the pump piston

Pressure valve control in the feed bore

Solenoid valve in the injection line

Edge control by a control valve ring on the pump piston

What fresh gas charge does the diesel engine receive during induction?

Diesel/air mixture

Oxygen/fuel mixture

Clean air

Pure oxygen

Which pollutants result from combustion in a diesel engine?

Carbon dioxide, oxygen, soot particles

Unburnt hydrocarbons, soot particles, nitrogen

Soot particles, nitrogen oxides, unburnt hydrocarbons

Nitrogen oxides, carbon dioxide, lead

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Exercise 44

Exercise 45

Exercise 46

Is the air/fuel ratio in a diesel higher or lower than in a spark ignition engine?

Describe the advantages and disadvantages of the chamber combustion process and the direct

injection engine!

Why are direct injection diesel engines louder than chamber combustion engines?

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Exercise 47

How is the delivery rate regulated in a distributor fuel injection pump?

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7 Exhaust treatment in diesel engines

7.1 Exhaust treatment in diesel engines

In diesel engines, too, great importance is attached to reducing the 3 pollutants CO, HC and

NOX. With diesels engines soot formation is also a concern (soot has been proved to be

carcinogenic).

Pollutants from diesel engines can also be reduced by measures relating to the engine and by

an exhaust aftertreatment process.

Engine-related measures

The injection timing point plays a similar role to the firing point in spark ignition engines, as it can be used to influence the start and progress of combustion. If the injection timing point is retarded, soot emissions are increased and NOx emissions are reduced. There is hardly any effect on HC emission levels.

High injection pressures and electronic control (modern diesel technology) reduce the exhaust emissions.

Exhaust gas recirculation: Another measure to reduce nitrogen oxides (NOX) is exhaust gas recirculation as in gasoline engines. This can significantly reduce the NOx concentrations. Lowering the combustion temperature reduces the NOX component in the exhaust gas without any compromises in terms of efficiency. At the same time there are positive effects on HC emissions in the low partial-load range. Particulate emissions, however, remain unaffected. The design of the system is approximately the same as for spark ignition engines.

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Exhaust aftertreatment

Exhaust aftertreatment to reduce nitrogen emission levels is not yet possible for diesel

automotive engines due to the presence of oxygen in the exhaust gas, which is inherent in the

process. So-called DeNOx catalytic converters, which allow NOx reduction even with surplus

air, are still in the development stage. At present the following methods of exhaust

aftertreatment are available:

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Oxidation catalytic converters: With the aid of these, hydrocarbons (HC) and carbon monoxide (CO) can be oxidized to harmless carbon dioxide (CO2) and water vapor (H2O) at exhaust temperatures above 250°C. It is not possible to oxidize soot particles at these temperatures, but particulate emissions are reduced thanks to the afterburning of the hydrocarbons (HC). Some of the soot particles are attached to the HC. In the upper load range, an increase in particulate emissions can only be achieved by reducing the sulfur content of the diesel fuel.

Particulate filter: The greatest prospects of success in reducing particulate emissions are anticipated in regenerative trap oxidizers (particulate filters). On the basis of certain processes in the filter (diffusion and adsorption processes), the particles are retained by the filter bed and burned off after a certain period of time. Filter come in two types: surface-type filters and deep-bed filters. In surface-type filters, particles form a layer of soot on the surface of the filter material. Particles in deep-bed filters penetrate into the filter material through pores. When a certain charge level is reached, e.g. when there is an increase in the flow resistance, the particles are burned off (filter regeneration). Burn-off can be achieved either by heating electrically, by means of special burners or by increasing the exhaust temperature.

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8 Increasing output by forced induction

8.1 Increasing output by forced induction

The purpose of forced induction is to increase the power output and torque of the internal

combustion engine without increasing the engine speed. With diesel engines another aim can

be to reduce noise emissions.

There are a number of alternatives available to increase the power output of engines:

Increasing the displacement

Increasing the engine rpm

Improving the charge of fuel mixture in the combustion chamber (forced induction)

A larger displacement means more material, more weight and more bulk. Any rpm increase is

accompanied by higher mechanical stresses and shorter service life. Both measures therefore

mean less economic efficiency. In forced induction (charged) engines, air is compressed in a

compressor (the so-called charger) and forced into the cylinder. This increases the density of

the intake air, which increases the charge of the cylinder so that more fuel can be burned in

each working cycle. In spark ignition engines forced induction is limited by the start of

combustion knock. In diesel engines it is limited by the strength of the components.

Forced induction processes

The forced induction (charging) processes differ in terms of the method used to drive the

charger:

Utilizing the exhaust energy to compress the charge

Mechanical charging

Combined process

Utilizing the exhaust energy to compress the charge

In this process the compressor is driven by a turbine wheel with no mechanical connection to

the engine. This is called a turbocharger.

The shaft is installed so that the turbine wheel turns in the exhaust tract and the compressor

impeller in the intake tract. The two wheels are connected by a shaft so that they cannot rotate

separately and are lubricated by the oil circuit of the engine.

Operating principle

When the driver demands more power and torque from the engine via the accelerator pedal,

the throttle valve is instantly opened and more fuel is injected. The increases the combustion

emissions and the rate of combustion. The rotational speed of the turbine wheel rises. Because

of the non-rotating connection between the turbine and compressor sides, the intake air is

forced at higher speed towards the combustion chambers. This also increases the intake

pressure in the cylinders. With each intake process there is now more air in the cylinders than

without turbo compression. The mass/volume air flow meter registers this and allows the

engine control to inject more fuel. The vehicle thus now has more power available at

approximately the same engine speed. It takes a certain length of time for the boost pressure

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to build up. In other words, the power increase is only available a few seconds after the

accelerator pedal is depressed. This effect is also referred to as “turbo lag”. The direct form of

energy transfer from the exhaust to the fresh charge is more efficient than mechanical

supercharging. No additional mechanical energy from the combustion process is consumed in

its operation. On the contrary, it exploits the kinetic energy from the exhaust stroke which

would otherwise escape unused into the atmosphere from the exhaust pipe.

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1. Compressor impeller

2. Turbine wheel

3. Air outlet

4. Exhaust inlet

5. Air inlet

6. Exhaust outlet

7. Rotor shaft

Boost pressure regulation

In order to assure the best possible cooperation between engine and turbocharger in all

operating ranges (wide open throttle – partial load), it is necessary to regulate the boost

pressure.

For this there are 2 concepts available:

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Wastegate control

This is a valve in the exhaust tract upstream of the turbine wheel. When a defined boost pressure is reached, a part of the exhaust gases is directed into the exhaust pipe bypassing the turbine. The reference variable available to the wastegate valve via a pressure line is the intake pressure (boost pressure). In vehicles with mechanical boost pressure control, the boost pressure acts on a diaphragm in the wastegate and opens the valve when the boost pressures are too high. With electronic boost pressure control the motor electronics control unit controls the valve. This control guarantees that good performance is achievable even at low engine speeds (with only low exhaust pressure). It can also provide the high torque rise (overboost) which gives the engine high pulling power, e.g. when driving uphill. The wastegate valve is opened when the motor electronics control unit determines that the engine is overloaded, e.g. due to an excessively high oil temperature, a tendency to knock etc.

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Variable turbine geometry (VTG)

In this design the exhaust turbine side is additionally equipped with a guide vane system. It changes the cross section and angle of incidence at the turbine wheel. It is therefore possible to have the effect of a small or large turbocharger which varies its boost pressure by changing the geometry of the turbine. This concept has no need of a wastegate.

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Mechanical charging

In this process the compressor is coupled with the engine and is driven by it. Examples of

mechanical chargers are the Roots-type supercharger (“Kompressor”) and the scroll or spiral-

type supercharger (“G-Lader”).

Kompressor

The two two-lobe rotary pistons of the Roots supercharger turn with no contact between each

other or between them and the housing. The size of the pocket produced is dictated by the

design, the materials used and the manufacturing tolerances. The two rotary pistons are

synchronized by a pair of gear wheels outside the working chamber, which are usually driven by

the crankshaft via V-belts.

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G-Lader

The G-Lader is a scroll-type supercharger. The displacer is eccentrically mounted and guided on

the housing side so that it performs an oscillating motion of twice the eccentricity when the

drive shaft is rotated. In this way the working chambers are opened for charging, closed for

delivery, and reopened again for ejection at the hub in phases. This also compresses and heats

the inducted air.

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Dynamic pressure charger (Comprex)

A cell rotor is driven mechanically by the crankshaft (requiring about 1% of the engine power for

operation) and rotates at approx. 2 - 3.5 times the engine speed. The cell rotor turns in a

cylindrical housing. The processes are relatively complex, so they will not be described in more

detail here. Basically, the supercharger generates pressure waves (at the speed of sound)

which cause an energy exchange between the exhaust gas and the fresh air. This exchange

takes place in the cells of the rotor (hence the term cell rotor). Due to the very good boost

pressure curve even at low engine speeds, the torque curve is excellent. Because the pulses

are transferred directly, there is no delay effect during acceleration as there is, for example,

with the turbocharger.

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1. Air inlet

2. Exhaust exit

3. Charge line

4. Housing

5. Cell

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6. V-belt

7. Housing

8. Cell rotor

9. Exhaust line

Intercooling

So that the compressed air can be utilized even more effectively, the intake air is usually cooled

before the intake turbine by a so-called charge air cooler. Cooler air has a lower density, which

additionally improves the charge in the cylinders.

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Exercise 48

Exercise 49

Exercise 50

8.2 Student exercise on increasing output

How can the power output of engines be increased?

Describe wastegate control!

Name the components.

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Exercise 51

Exercise 52

What does this picture show?

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What limits the forced induction of engines?

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9 Power and torque 9.1 Power and torque

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9 Power and torque

9.1 Power and torque

Power is expressed in kilowatts (kW) or in horsepower (hp). Torque is expressed in newton

meters (Nm). The two parameters are usually shown together in an engine graph. The power

and the torque are plotted against the engine speed (see below). The engine speed is

expressed in revolutions per minute (rpm).

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To explain the graph (“What can be read from an engine graph?”): The power curve is the one

that looks like a walking-stick. It is also referred to as an inverted J-curve.

The power in kW is read on the left.

The torque curve is the other one.

The torque in Nm is read on the right.

The engine has a peak output of 103 kW at a speed of 5600 rpm. The engine has a torque of

205 Nm at a speed of 4000 rpm. Supercharged engines can be recognized by the torque curve,

which is shifted up and to the left. This means that supercharged engines have a higher torque

that is available in a low rpm range.

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Exercise 53

9.2 Student exercise on power and torque

This engine graph illustrates 2 engines: a forced induction engine and a naturally aspirated

engine. Label the graph with the designations for power and torque and state which torque

curve belongs to the forced induction engine. Explain your statement!

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10 Exhaust system

10.1 Exhaust system

The exhaust system includes the following subsystems; their functions are given in brackets

(in sequence from the engine exhaust to the rear of the vehicle in a front-engine vehicle):

Exhaust manifold or collector (collects the exhaust gases from all the cylinders in a cylinder bank and conveys them towards the catalytic converter)

Oxygen sensor (subsystem for exhaust treatment; see above)

Exhaust gas recirculation system (subsystem for exhaust treatment; see above)

Catalytic converter(s) (subsystem for exhaust treatment; see above)

Center muffler (soundproofing)

One or more rear mufflers (soundproofing)

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The primary function of the exhaust system is to convey the exhaust gases from the engine to

the exhaust outlet under the vehicle rear. The outlet is at the rear so that exhaust gases do not

enter the interior through the ventilation intakes when driving. The installation of silencers in

the form of the center and rear mufflers provides an acoustic improvement of the combustion

noise level. Pipes connect the mufflers to each other and to the catalytic converter and

manifold. These pipes are connected together by exhaust clamps and by bolted flange

connections with film seals. The entire exhaust system is mounted on rubber mounts at a

distance from the body. For this purpose the exhaust systems and the underside of the body

have special eyelets which are connected by rubber buffers. The rubber mounting and body

clearances are necessary to prevent the transfer of exhaust vibrations and heat to the body,

and to allow for changes in length due to thermal expansion.

Repair information

As the products of combustion consist of aggressive acids, the exhaust can be expected to

corrode during operation. For this reason, some manufacturers equip their vehicles with

exhaust systems made of stainless steel. Conventional exhausts are made of aluminized steel

and other materials. The mufflers are particularly susceptible to corrosion. The reason is their

bathtub shape. When the engine is switched off, condensation and acid accumulate in the floor

of the muffler, which accelerate the oxidation process. The exhaust is especially susceptible

when repeatedly operated “cold”. This occurs with frequent short-distance trips. The exhaust

does not heat up completely, and the gases are unable to evaporate after the engine has been

switched off. To replace a muffler, the clamps or bolted flanges are unfastened and the muffler

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is pulled out. With the clamp design the pipes overlap, which causes contact corrosion. This

means that these parts can only be separated with difficulty. In this case a grinder or similar

must be used to cut a slit in the overlapping pipe. The new muffler is always installed using new

clamps/bolts and new rubber elements.

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