internal combustion engines are discussed including combustion behaviour

Engine Performance

Prof. (Dr.) MP PooniaDirector, NITTTR

Chandigarh (India)

Purpose of an engine

Converts the heat of burning fuel into useful energy

Let's take a look at how the engine was invented.

When expressed as a percentage, the thermal efficiency must be between 0% and 100%. Due to inefficiencies such as friction, heat loss, and other factors, thermal engines' efficiencies are typically much less than 100%. For example, a typical gasoline automobile engine operates at around 25% efficiency. The largest diesel engine in the world peaks at 51.7%.

Otto’s practical internal combustion engine is used to power automobiles, motorcycles and motorboats. Also, the Diesel engine is a form of internal combustion engine, which employs a four-stroke cycle that is similar to Otto’s. Nikolaus August Otto died on January 26, 1891.

Basic Terminology

•Bore & Stroke

•Engine Displacement

•Compression Ratio

•Power

•Torque

Stroke

•Distance

TDC-BDC

•Distance piston travels

up or down

Figure 3 : Engine Terminology

Cylinder Displacement

0.7854 x D2 x stroke

or

π x r2 x Stroke

15

Compression Ratio

Area of cylinder at BDC

compared to

Area of cylinder at TDC

600 cm3 to 8 cm3

=

7:5 compression ratio

16

Kinds of Horsepower

• Brake Horsepower

• Indicated Horsepower

• Frictional Horsepower

• Rated Horsepower

• Corrected Horsepower

17

Corrected Horsepower

• corrected for elevation (sea level)

• corrected for temperature

• barometric pressure

• quality of fuel

• humidity

Efficiency

• In general, energy conversion efficiency is the ratio between the useful output of a device and the input. For thermal efficiency, the input, to the device is heat, or the heat-content of a fuel that is consumed. The desired output is mechanical work, or heat, or possibly both. Because the input heat normally has a real financial cost, a memorable, generic definition of thermal efficiency is;

Mechanical Efficiency

Some of the power generated in the cylinder is used to overcome engine friction and to pump gas into and out of the engine.

The term friction power, , is used to describe collectively these power losses, such that:

gi

f

gi

fgi

gi

bm W

W

W

WW

W

W

,,

,

,

1

−=

−==η

.

The mechanical efficiency is defined as:

bgif WWW −= ,

• The final parameter to be defined is the volumetric efficiency of the engine; the ratio of actual air flow to that of a perfect engine is

• In general, it is quite easy to provide an engine with extra fuel; therefore, the power output of an engine will be limited by the amount of air that is admitted to an engine.

How about gasoline?

• Gasoline is a product obtained by refining crude oil (petroleum) obtained from wells drilled into the earth.

• The crude oil is treated in various ways to produce gasoline

• Since gasoline is a mixture of carbon and hydrogen atoms, it is termed a hydrocarbon

Fuel Properties

Fuel Heating value, QR (J/kg)

f at stoichiometric

Gasoline 43 x 106 0.0642

Methane 50 x 106 0.0550

Methanol 20 x 106 0.104

Ethanol 27 x 106 0.0915

Coal 34 x 106 0.0802

Paper 17 x 106 0.122

Fruit Loops 16 x 106 Probably about the same as paper

Hydrogen 120 x 106 0.0283

U235 fission 82,000,000 x 106 1

Fuel Requirements

• Gasoline is a mixture of hydrocarbons (with 4 to approximately 12 carbon atoms) ,SIT 450 oC

• Diesel fuel is a mixture of higher molar mass hydrocarbons (typically 12 to 22 carbon atoms), SIT 200 oC.

• Fuels for spark ignition engines should vaporize readily and be resistant to self-ignition, as indicated by a high octane rating.

• Fuels for compression ignition engines should self-ignite readily, as indicated by a high cetane number.

Octane number Standard measure of the anti-knock

properties (i.e. the performance) of a motor or aviation fuel. The higher the octane number, the more compression the fuel can withstand before detonating.

• The octane or cetane rating of a fuel is established by The octane or cetane rating of a fuel is established by comparing its ignition quality withcomparing its ignition quality with respect to respect to reference fuels in CFR (Co-operative Fuel Research) reference fuels in CFR (Co-operative Fuel Research) enginesengines

• RON is determined by running the fuel in a test RON is determined by running the fuel in a test engine with a variable compression ratio under engine with a variable compression ratio under controlled conditions, and comparing the results with controlled conditions, and comparing the results with those for mixtures of iso-octane and n-heptane.those for mixtures of iso-octane and n-heptane.

Simple Combustion Equilibrium

• A stoichiometric mixture contains the exact amount of fuel and oxidizer such that after combustion is completed, all the fuel and oxidizer are consumed to form products.

• Equivalence Ratio:

• Lambda is the ratio of the actual air-fuel ratio to the stoichiometric air-fuel ratio defined as

Methods of Quantifying Fuel and Air Content

of Combustible Mixtures

• If less air than the stoichiometric amount is used, the mixture is described as fuel rich.

• If excess air is used, the mixture is described as fuel lean.

• Fuel-Air Ratio (FAR): The fuel-air ratio, f, is given by

Octane rating: The octane rating indicates how well the gasoline will resist detonation (burning too rapidly) in the cylinders.

The lower the octane rating the faster the fuel burn

Slower burning fuel provides more even combustion throughout the power stroke of the piston.

Unleaded gasoline

All gasoline sold today is unleaded.

Unleaded gasoline contains no tetraethyl lead.

Tetraethyl lead quickly destroys catalytic converters.

Preparing the fuel

As you know, gasoline burns readily. However to get the most power from this fuel, and in fact, to get it to power an engine, special treatment is required.

If you were to place a small amount of gasoline in a jar and drop a match into it, it would burn.

Such burning is fine to produce heat but it would not give us the explosive force needed to operate an engine.

OXYGEN,GOTTA HAVE IT

In order to burn, gasoline must combine with oxygen in the air.

For purposes of illustration imagine that a gasoline particle is square. It will burn on all sides. However it will still not burn quickly enough to for use in an engine

To make the gasoline burn more rapidly, we can break it up into smaller particles. Notice that as you divide it into smaller particles, you expose more surface area to the air.

Internal Combustion Engines – Carnot cycle

The Ideal Air Standard Otto Cycle

Ignition and Combustion in Spark Ignitionand Diesel Engines

Spark ignition (SI) engines usually have pre-mixed combustionCompression ignition (CI) engines the combustion is controlled primarily by diffusion.

December 20, 2014 I.C. Engines Laboratory Slide 40

Premixed vs. Non-premixed Charge Engines

Flame front Fuel spray flame

Premixed charge (gasoline)

Non-premixed charge (Diesel)

Spark plug Fuel injector

Fuel + air mixture Air only

I.C. Engines Laboratory



Combustion in SI engine

I.C. Engines Laboratory

Flames detected between

Type of cycle

- 450 and TDC early burn cycle

TDC and 45° Fast burning cycles

450 and 900 slow burn cycles

90° and BDC Late burn cycles

BDC and TDC delayed burn cycles

Flames not detected misfires and partial burn cycles

Good combustion is almost entirely made up of fast burn cycles. poor combustion consists of a high proportion of late and delayed burn cycles.

COMBUSTION QUALITY

EFFECT OF ENGINE VARIABLES ON FLAME PROPAGATION

Fuel-air ratio :



Compression Ratio


P-t diagram of a normal cycle


p-t diagram of a knocking cycle

Pressure-Volume Graph 4-stroke SI engine

One power stroke for every two crank shaft revolutions

1 atm

Spark

TDCCylinder volume

BDC

P

Exhaust valveopens

Intake valvecloses

EVC

IVO

• Otto cycle efficiency predicts an efficiency of 60% and the fuel-air cycle predicts an efficiency of 47% for stoichiometric operation. In reality, such an engine might have a full throttle brake efficiency of 30%, and this means 17 percentage points must be accounted

Diesel engines have a higher maximum efficiency than spark ignition engines for three reasons:

• 1. The compression ratio is higher.• 2. During the initial part of compression,

only air is present.• 3. The air-fuel mixture is always weak of

stoichiometric.

The Ideal Air Standard Diesel Cycle

What is Diesel Fuel?

Various Petroleum Components:

• Paraffins

• Isoparaffins

• Napthenes

• Olefins

• Aromatic Hydrocarbons

Cetane Number

CN is a measurement of the combustion quality of diesel fuel during compression ignition.

Cetane Number

• Measures readiness of fuel to auto-ignite.• High cetane means the fuel ignite quickly• Most fuels have cetane numbers between 40

and 60.• ASTM requires a minimum cetane number of

40• Premium Diesel fuel typically has a cetane of

47

Cetane

Ignition Delay: The period that occurs between the start of fuel injection and the start of combustion; the higher the cetane number, the shorter the ignition delay and the better the quality of combustion.

Cetane

Low Cetane Impact

Poor Ignition QualityLong ignition delayAbnormal CombustionPossible High Combustion PressureIncreased Engine stressExcessive Engine KnockSmoke on Cold start

Direct Injection vs. Indirect Injection

Swirl :

• The orderly motion of the air particularly parallel to the axis of the engine.

• Very much required for diesel engines.

Squish/Squash :

• The radial inward motion of the air-fuel mixture towards (squish) and away from the axis of the engine (squash).

• Very much required for the diesel engines.

Turbulence :

• Random mixing of the burned and unburned gases


Effect of Fuel-Air Ratio on Power Output of CI Engine


Stages of Combustion in CI Engines


Combustion Rate (CI)


Effect of varying the amount of fuel injected on P-θ diagram


Effect of speed on ignition delay in a diesel engine





CI Engine Combustion Chambers

• Open Chamber or DI engine

• Divided Chamber or IDI engine

Diesel Ignition System

• Glow plug

• Glow plug relay

• Fusible Link

• Glow Plug Temp Sensor

• Heat Sink

Current Modern fuel injector design- The fuel injection systems on the

John Deere Power Tech Plus engines

operate at 2,000 barPhotos compliments of the National Alternative Fuel Training Consortium

In-Line Injection Pumps

• An injection pump with a separate plunger for each engine cylinder.

• Plunger is rotated by a rack to determine metering helical cuts on the pump plungers.

• The plungers are driven off a camshaft, which usually incorporates a centrifugal controlled timing advance mechanism.

A diesel fuel injection system employing a common pressure accumulator

The rail is fed by a high pressure fuel delivery pump.

The injectors, are activated by solenoid valves.

The solenoid valves and the fuel pump are electronically controlled.

Also known as CRD, Common Rail Diesel Technology

Common Rail Injection

• ic engines fdp\IC Engines Videos\In-line Injector Pump Part (1 _ 2).mp4

• Video not found

Common Rail Injection Vehicles

Advantages of DI Engines

Fuels of poorer ignition quality can be used.

Single-hole injection nozzles and moderate injection pressures can be used and can tolerate greater degrees of nozzle fouling.

Higher fuel-air ratios can be used without smoke.

Disadvantages of IDI Engines

More expensive cylinder construction.

More difficult cold starting because of greater heat loss through the throat.

Poorer fuel economy due to greater heat losses and pressure losses through the throat, which result in lower thermal efficiency and higher pumping loss.

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Engineering

internal combustion engines are discussed including combustion behaviour