1 INTERNAL COMBUSTION ENGINES - Nathi | Loves Jesus · PDF file11/2/2013 · MECHANICAL ENGINEERING DEPARTMENT. ... INTERNAL COMBUSTION ENGINES 1. IC Engine Fundamentals. 2

ADDIS ABABA UNIVERSITY

INSTITUTE OF TECHNOLOGY

MECHANICAL ENGINEERING DEPARTMENT

DIVISON OF THERMAL AND ENERGY CONVERSION

INTERNAL COMBUSTION ENGINES1

IC Engine Fundamentals2

Engine Systems An engine must at least include the following systems to

operate

Air supply and Exhaust system

Fuel supply system

Combustion system

Ignition system (SI)

Cooling system

Lubricating system

Starting system

Electrical power supply (battery or generator)


areaforcepressure =

force = pressure x area


areaforcepressure =

force = pressure x area


IC Engine Fundamentals

Ignition system

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Crank mechanism

Ignition system


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Spark Ignnition System9

Distributer contains:- A rotating cam driven by the engine drive

- A set of breaker points,

- A condensor(capacitor)

- A roter and distributer cap

o The ignition coil consit of two transformer windings sharing acommon megnetic core- the primary (200 to 300 turns) andsecondary windings (21,000 turns).

o An alternating current in the PW induces magnetic field in thecoil’s core so that it serves as a step up transformer.

Spark Ignnition System10

For an ignition coil, one end of windings of both the primaryand secondary are connected together.

This common point is connected to the battry ( ussually throughthe current –limiting ballast rsistor)

The other end of the primary is connected to the point in thedistributer

The other end of the SW is connected via the the distributercap and rotor to the spark plugs

Intake system

Ignition system

Crank mechanism


Exhaust system

Ignition system

Crank mechanism

Intake system


Cooling system ThermostatIntake system Exhaust system

Ignition system

Crank mechanism


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Lubrication system

Crankcase vent

Ignition system

Exhaust systemIntake systemCooling system Thermostat


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Otto cycles

Intake

Compression,

Combustion

Power

Exhaust

fuel

air air + fuel

volume

pres

sure

TDC BDC

1. INTAKE STROKE


Stoichiometric mixture

volume

pres

sure

TDC BDC


volume

pres

sure

TDC BDC


2. COMPRESSION STROKE

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volume

pres

sure

TDC BDC


volume

pres

sure

TDC BDC


3. Power stroke

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volume

pres

sure

TDC BDC


volume

pres

sure

TDC BDC


4. Exaust stroke

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volume

pres

sure

TDC BDC


Exhaust gas residual

volume

pres

sure

TDC BDC

Negative work

positive work

Work = (pressure x volume)



CxHxCONOx

Mixture preparation

Intake tuning

Combustion, auto ignition

Wear

Inertia

lubrication

Exhaust tuning Emissions

IC Engine Fundamentals-Some Problems

1. Performance2. Fuel economy3. Emission

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The Diesel Engine

volume

pres

sure

TDC BDC

Negligible negative work

IC Engine Fundamentals-Diesel Engine29

10000 - 20000 psi

IC Engine Fundamentals-Diesel Engine

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• Intake Stroke

• The piston travels from TDC to BDC with theintake valve open and exhaust valve closed.

• Piston draws air and fuel mixture intocylinder

IC Engine fundamental-SI Engine32


• Compression Stroke• When the piston reaches BDC, the intake

valve closes and the piston travels backto TDC with all valves closed.

• Rising piston compresses mixture raisingboth the pressure and temperature in thecylinder.

• Near the end of the compression stroke,the spark plug is fired and combustion isinitiated. (constant-volume combustion).


• Expansion Stroke

• With all valves closed, the high pressurecreated by the combustion process pushesthe piston away from TDC.

• As the piston travels from TDC to BDC,cylinder volume is increased, causingpressure and temperature to drop.

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• Exhaust Stroke

• With the exhaust valve remainingopen, the piston now travels from BDCto TDC in the exhaust stroke.

• This pushes most of the remainingexhaust gases out of the cylinder intothe exhaust system

IC Engine fundamental-SI Engine

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Intake Stroke

Intake stroke in Diesel engine is the same as the intakestroke in an SI engine.

The major difference being no fuel is added to theincoming air.

IC Engine fundamental-SI Engine

IC Engine fundamental-Diesel Engine37

Compression Stroke The same as in SI engine except that only air is compressed and

compression is to higher pressures and temperature.

Late in the compression stroke fuel is injected directly into thecombustion chamber where it mixes with the very hot air. This causesthe fuel to evaporate and self-ignite causing combustion to start.

Combustion is fully developed by TDC and continues at aboutconstant pressure until fuel injection is complete (i.e. after Cut - off)and the piston has started towards BDC.

IC Engine fundamental-Diesel Engine38

Power Stroke

o The power stroke continues as combustion ends and the piston travels towards BDC.

Exhaust Stroke

o The same as SI engine

IC Engine Fundamentals- 2 Stroke Engine39

The two stroke engine employs the crankcaseas well as the cylinder to achieve all theelements of the Otto cycle in only two strokesof the piston.

One power stroke in one revolution of thecrankshaft

The main difference between two stroke andfour-stroke engines is in the method of fillingthe fresh charge and removing the bunt gasesfrom the cylinder.

IC Engine Fundamentals- 2 Stroke Engine

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In a two-stroke engine, the filling process is accomplished by thecharge compressed in crankcase or by a blower.

The induction of the compressed charge moves out the product ofcombustion through exhaust ports.

Therefore, no piston strokes are required for these two operations.

Two strokes are sufficient to complete the cycle, one for compressingthe fresh charge and the other for expansion or power stroke.

IC Engine Fundamentals-2 Stroke Engine

A compression stroke (intake +compression) starts by closing theintake and exhaust ports, and thencompresses the cylinder contents anddraws fresh charge into thecrankcase.

The fuel/air mixture is drawn into thecrankcase by the vacuum that iscreated during the upward stroke ofthe piston.

As the piston approaches TDC,combustion is initiated.

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IC Engine Fundamentals- 2 Stroke Engine

A power or expansion stroke (combustion+ exhaust)

At the end of compression stroke thespark plug ignites the fuel mixture. Theburning fuel expands, driving the pistondownward, to complete the cycle.

At the same time, another crankcasecompression stroke is happening beneaththe piston.

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IC Engine Fundamentals-2 Stroke Engine

During downward movement of the pistonfirst the exhaust ports and then the intakeports are uncovered.

When the inlet ports are uncovered, thefresh charge which has been compressed inthe crankcase flows into the cylinder.(Scavenging)

The piston and the ports are generallyshaped to deflect the incoming charge fromflowing directly into the exhaust ports and toachieve effective scavenging of the residualgases

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IC Engine Fundamentals-2 Stroke Engine44

Upward stroke of the piston During the downward stroke

Intake Valve open Crank Case compression

Transfer port OpeningScavenging

IC Engine Fundamentals -2 Stroke Engine45

Upward stroke of the piston

Cylinder compression-another intake stroke is happening beneath the piston

Power Stroke-Crankcase compression

Down ward stroke of the piston

Exhaust Stroke-Scavenging


Comparison of SI & CI Engines

Description SI Engine CI Engine

Basic Cycle Otto Cycle Heat addition @ Constant volume

Diesel cycle, Heat addition @ constant pressure

Fuel Gasoline, Highly volatile fuel, self ignition temp is high

Diesel oil, non volatile fuel, self-ignition temp is comparatively low

Introduction of Fuel

Fuel-air mixture introduced during suction stroke (Carburetor is necessary)

Fuel directly injected to the combustion chamber at high pressure (fuel pump and injector is necessary)

Load control Throttle controls the quantity of mixture introduce

The quantity of fuel is regulated in the pump. Air quantity is not control

Ignition Required an ignition system with spark plug

Ignition system & sparkplug are not necessary

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Comparison of SI & CI Engines

Description SI Engine CI Engine

Compression ratio

6-10, upper limit is fixed by anti knock quantity of fuel

16-20 upper limit is limited by weight increase of the engine

SpeedThey are high speed engines (light weight & homogeneous combustion)

They are low speed engines ( heavy weight & heterogeneous combustion)

Thermal Efficiency

Lower thermal Efficiency (lower compression ratio)

Higher thermal efficiency (higher compression ratio)

weightLighter due to lower peak pressure

Heavier due to higher peak pressure

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4-Stroke Engines 2-Stroke Engine

The cycle completed in 4 strokes of piston or 2-revolution of crankshaft

The cycle is completed in twoA strokes of the piston or one power strokes obtained in every revo. of crankshaft

Turning moment is not uniform & hence a heavier flywheel is needed

Turning moment is more uniform & hence a lighter flywheel can be used

The power produced for the same size engine is less (2 revo gives 1 power)

Power produced for the same size of engine is more (Theoretically twice, actually 1.3 times) due to 1 Power stroke in 1 revo. Of Cra Sh)

Lesser cooling & lubrication requirementLower rate of wear & tear

Greater cooling & lubrication requirementsHigher rate of wear & tear

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Comparison of 2-Stroke & 4-Stroke Engines


4-S Engines 2-S Engine

Contains valve & Valve mechanisms to open & close valves

No valves but ports (some 2-s engines are fitted with exhaust valves or reed valve)

The initial cost is very high (Coz of heavy wt. & complicated valve mechanism)

Initial cost of the engine is less (Coz odlight wt. & simplicity)

Volumetric Efficiency is more (Coz of more time for induction)

Volumetric Efficiency is low (Coz of lesser time for induction)

Thermal Efficiency is higher, part load efficiency is better

Thermal Efficiency is less, part load efficiency is poor compared 4-s engine

Used where Efficiency is important( in Cars, Buses, aero planes, etc…

Used where low cost, compactness & Light Wt. are important (in Scooters, Motorcycles

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IC Engine Fundamentals-Wankel Engine

Alternative to the reciprocating enginegeometry

The intake, compression, combustion andexhaust process happen in different cavitiescreated between the rotor and the part ofthe housing.

In Wankel engine there are two rotatingparts: the triangular shaped rotor and theoutput shaft with its integral eccentric (lobe).

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The rotor revolves directly on the eccentric

The rotor has an integral timing gear whichmeshes with the fixed timing gear on one sideof the housing to maintain the correct pulserelationship between the rotor and theeccentric shaft rotations.

As the rotor makes one complete rotation,during which the eccentric shaft rotatesthrough three revolutions, each chamberproduces one power stroke.

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IC Engine Fundamentals-Wankel Engine52

A rotary engine has an ignition system and a fuel-delivery system that aresimilar to the ones on piston engines.

The Rotor The rotor has three convex faces, each of which acts like a piston.

Each face of the rotor has a pocket in it, which increases the displacementof the engine, allowing more space for air/fuel mixture.

At the apex of each face is a metal blade that forms a seal to the outside of the combustion chamber


The rotor has a set of internal gearteeth cut into the center of one side.

These teeth mate with a gear that isfixed to the housing.

This gear mating determines the pathand direction the rotor takes throughthe housing.

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The housing is roughly oval in shape.

its shape is designed so that the threetips of the rotor will always stay incontact with the wall of the chamber,forming three sealed volumes of gas.

Each part of the housing is dedicatedto one part of the combustion process.

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The four sections are:◦ Intake◦ Compression◦ Combustion◦ Exhaust

The intake and exhaust ports arelocated in the housing. There are novalves in these ports.

The exhaust port connects directly tothe exhaust, and the intake portconnects directly to the throttle.

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IC Engine Fundamentals-Wankel Engine56

Output Shaft The output shaft has round lobes mounted eccentrically,

meaning that they are offset from the centerline of theshaft.

Each rotor fits over one of these lobes. The lobe acts sort oflike the crankshaft in a piston engine.

As the rotor follows its path around the housing, it pushes onthe lobes.

Since the lobes are mounted eccentric to the output shaft,the force that the rotor applies to the lobes creates torquein the shaft, causing it to spin.


ADVANTAGES

higher output for similardisplacement and physical size

simple and contain far fewermoving parts

the shape of the Wankelcombustion chamber and theturbulence induced by themoving rotor prevent localizedhot spots from forming

DISADVANTAGES

seals exposed to heating andcooling cycles in use, led to a veryhigh incidence of loss of sealing

incomplete combustion of the air-fuel charge, with the remainingunburned hydrocarbons releasedinto the exhaust.

Is difficult to expand the engineto more than two rotors

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1 INTERNAL COMBUSTION ENGINES - Nathi | Loves Jesus · PDF file11/2/2013 · MECHANICAL ENGINEERING DEPARTMENT. ... INTERNAL COMBUSTION ENGINES 1. IC Engine Fundamentals. 2