CHAPTER 5 internal combustion engine

CHAPTER 5

INTERNAL

COMBUSTION ENGINES

1

CO1: Ability to analyze and evaluate mixtures of gases and

vapours, combustion processes and internal combustion

engines.

Contents

1) Introduction

2) ICE-Terminology

3) Four-Stroke Cycle

4) Two-Stroke Cycle

5) Valve Timing

6) Performance Criteria of ICE

7) Factors Influencing Performance

2

Introduction

۩ Theoretical power cycles have been considered in

Chapter 3.

۩ In the theoretical cycles there is no chemical change in

the working fluid (air) and the heat exchanges in the

cycle are made externally to the working fluid.

۩ In the practical cycle the heat supply is obtained from

the combustion of a fuel in air and thus the air charge is

consumed during combustion and the combustion

products must be exhausted from the cylinder before a

fresh charge of air can be induced for the next cycle.

۩ The practical cycle consists of the exhaust and

induction processes together with the compression and

expansion processes as in the theoretical cycle.

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Introduction

۩ Combustion engines may be divided into two types:

External combustion engines.

Internal combustion engines.

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۩ ECE – Combustion of fuel

occurs outside the cylinder.

e.g. steam engines and

turbines where the working

fluids is steam.

۩ ICE – Combustion of fuel

occurs inside the cylinder.

e.g. petrol, oil and gas

engines where the working

fluids is air.

Introduction

۩ ICE work on an open cycle where the working fluid is

renewed at the end of each cycle.

۩ The following four requirements are to be fulfilled by

any ICE:

i. The fuel and air in the correct ratio must be supplied

to the engine.

ii. They must be compressed before or after mixing.

iii. The compressed mixture needs to be burnt and

combustion products expand, at the same time

actuating the engine mechanism.

iv. Combustion products are disposed off to receive

new supply of charge.5

ICE - Terminology

6

۩ Top dead centre (TDC) – theposition of piston when it formsthe smallest volume in thecylinder.

۩ Bottom dead centre (BDC) – theposition of piston when it formsthe largest volume in the cylinder.

۩ Stroke, L – the largest distancethat the piston can travel in onedirection.

۩ Bore, d – the diameter of thepiston.

ICE - Terminology

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۩ Intake valve – the valve where air or air-fuel mixture is

drawn into the cylinder.

۩ Exhaust valve – the valve where combustion products are

expelled from cylinder.

۩ Clearance volume, Vc – minimum

volume formed in the cylinder

when the piston at TDC.

۩ Displacement or swept volume, Vs

– volume displaced by the piston

as it moves between TDC and BDC.

ICE - Terminology

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۩ Compression ratio, r – the ratio of maximum volume to

the minimum (clearance) volume.

۩ Mean effective pressure (mep) – the work done per unit

displacement volume. Can be used as a parameter to

compare performance of reciprocating engines of equal

size. The engine with larger value mep will deliver more

net work per cycle and thus will perform better.

Wnett = MEP X Piston area X Stroke = MEP X Displacement

volume

Four-Stroke Cycle

۩ In 4-stroke engine, the cycle is completed in four

strokes of a piston or two revolutions of the crankshaft.

۩ In petrol and gas engines:

The mixing of fuel and air takes place outside the

cylinder.

This mixture is injected into the cylinder.

Then, it is compressed and fired by spark plugs.

۩ In oil and diesel engines:

Only air is sucked in and then compressed.

Subsequently, fuel is injected into the cylinder

causing it to ignite.

For this reason, these engines do not need spark

plugs or ignition system.9

Four-Stroke Cycle

۩ The working principles of a typical 4-stroke cycle are

as follows:

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Four-Stroke Cycle

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INTAKE / INDUCTION STROKE

This stroke starts with piston at TDC and ends atBDC. During the intake stroke, the piston movesdown. The intake valve is open. The charges flowthrough the intake valve and into the cylinder. Thesecharges comprise of air-fuel mixture in petrol engineor air only in diesel engine. Next, as the piston passesthrough BDC (bottom dead centre), the intake valvecloses.

COMPRESSION STROKE

After the piston passes BDC, it starts moving up. Bothvalves are closed. Near or at TDC, the charges areignited either by spark plugs in petrol engines or byfuel injection in compressed air which has reachedfiring temperature in diesel engines. Combustioncauses the temperature and pressure of fluid in thecylinder to increase.

Four-Stroke Cycle

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POWER STROKE

The high temperature causes very high pressure which

pushes down the piston to BDC. The downward

movement of the piston is transmitted through the

connecting rod to the crankshaft which turns to move

the drive wheels.

EXHAUST STROKE

As the piston approaches BDC on the power stroke,

the exhaust valve opens. After passing through BDC,

the piston moves up again. The burnt gases escape

through the open exhaust port. When the piston passes

through TDC and starts moving down again, the

exhaust valve closes. Another intake stroke begins and

the whole cycle repeats.

Two-Stroke Cycle

۩ In the two-stroke cycle, all four processes are carried

out in only two stroke of the piston movement and one

crank revolution. This can be done on two ways:

By compressing air in a compressor outside the

cylinder so that air can be forced into the cylinder.

This compressor usually is part of the engine and is

driven by it, or

By designing the crank casing so that it acts as a

compressor

۩ The working principles of a typical 2-stroke cycle are

as follows:

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Two-Stroke Cycle

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STROKE 1 – COMPRESSION & INTAKE

The piston moves upwards. The stroke starts from BDC and ends atTDC. Before the stroke commences, the suction valve opens,allowing fresh charge to come into the crank casing, where it is beingcompressed. The charge then enters the cylinder through hole T thuspushing out the remaining exhaust gases through hole E. When thecylinder is full of fresh charge both holes are closed and thecompression begins until TDC. Just before TDC, the mixture isignited.

Two-Stroke Cycle

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STROKE 2 – POWER & EXHAUST

The piston moves down. This stroke, which is from the TDC to the

BDC is the power stroke. Energetic combustion gases expand and

approximately 80% of the stroke, the piston no longer closes the

exhaust hole, and the gases are discharged to the atmosphere. The

intake of fresh charge helps the discharge process.

Valve Timing

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۩ The timing of the opening and closing of inlet and

exhaust valves as well as the exact point of ignition are

very important .

۩ This is to make sure a successful running of the engine.

۩ The main factors that affect the timing of valves are the

high velocity of the charge at the entry to the cylinder

and the high velocity of the exhaust gases at the exit

from the cylinder.

۩ Note that, different engines apply different valve

timing.

۩ The angular positions shown is refer to the crank angle

position in relation to the TDC and BDC positions of

the piston.

Valve Timing

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IO Inlet valve opens.

The actual position is between 10deg before

TDC and 15deg after TDC

IC Inlet valve closes.

This occurs 20deg to 40deg after BDC.

S Spark occurs.

This is 20 to 40deg before TDC when the

ignition is fully advanced, and is at TDC when

ignition is fully retarded.

EO Exhaust valve opens.

At about 50deg before BDC.

EC Exhaust valve closes.

This occurs 0deg to 10deg after TDC

Valve timing diagram

for 4-stroke SI engine

Valve Timing

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IO Inlet valve opens.

Up to 30deg before TDC

IC Inlet valve closes.

Up to 50deg after BDC.

Injection Injection of fuel occurs.

About 15deg before TDC.

EO Exhaust valve opens.

About 45deg before BDC.

EC Exhaust valve closes.

About 30deg after TDC.Valve timing diagram

for 4-stroke CI engine

Valve Timing

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Valve timing diagram for 2-stroke engine

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Performance Criteria of ICE

۩ An engine is selected to suit a particular application.

۩ The main consideration being its power/speed

characteristics.

۩ Important additional factors are initial capital cost and

running cost.

۩ Different types of engine can be compared to each other

using a number of performance criteria.

۩ These include indicated power, brake power, friction

power, mechanical efficiency, brake mean effective

pressure, thermal efficiency, fuel consumption, and

volumetric efficiency.

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a) Indicated Power (ip)

It is defined as the rate of work done by the gas on the

piston as evaluated from an indicator diagram (shown in

Fig.) obtained from the engine.

The area of the indicator diagram represents the

magnitude of the net work done by the system in one

engine cycle.

Net work done per cycle (area of power loop – area

of pumping loop)

Therefore, indicated mean effective pressure, Pi, is

defined in the following way:

constant diagram oflength

diagram of areanet ip The constant depends on

the scales of the recorder


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Power loop

Pumping loop

Indicator diagram for an engine


۩ Considering one engine cylinder.

where A is the area of piston and L the length of stroke

Power output = work done per cycle x cycles per

minute

Or ip = piAL x (cycles/unit time)

۩ The number of cycles per unit time depends on the type

of engine; for four-stroke engines the number of cycles

per unit time is N/2, and for two strokes the number of

cycles per unit time is N, where N is engine speed25

LApi cycleper doneWork


۩ The formula for ip then becomes for four-stroke

engines.

۩ For two-stroke engines

Where n is the number of cylinders.

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2ip

ALNnpi

ALNnpiip


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b) Brake Power (bp)

It is the measured output of the engine.

Measurement of the brake power involves the

determination of the torque and the angular speed of the

engine output shaft.

The torque can be measured using a dynamometer that

is connected to the engine. The torque is given by:

Torque, T = net load, W x radius from axis of rotation, R

T = WR

The brake power, bp is then given by:

bp = 2πNT


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c) Friction power (fp) and mechanical efficiency, ηM

The difference between the ip and the bp is the friction power,

(fp), and is that power required to overcome the frictional

resistance of the engine parts.

fp = ip – bp

The mechanical efficiency of the engine is defined as:

ηM usually lies between 80% and 90%

ip

bpM

d) Brake mean effective pressure (Pb)

From mechanical efficiency equation, we get; bp = ηM x ip

For four-stroke engine:

bp =

Since ηM and pi are difficult to obtain, they may be combined

and replaced with pb,

bp =

(where pb = ηM x pi)

pb also can be written as:

(where K is a constant)


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The Pb may be thought of as that mean

effective pressure acting on the pistons

which would give the measured bp if the

engine were frictionless


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e) Thermal efficiency and specific fuel consumption

The overall efficiency of the engine is given by the brake

thermal efficiency, ηBT,

(where mf is the mass of fuel consumed per unit time, and Qnet,v

is the net calorific value of the fuel)

The indicated thermal efficiency, ηIT, is defined the similar way

to ηBT.


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Next, dividing these equations gives:

The specific fuel consumption (sfc), is the mass flow rate of fuel

consumed per unit power output, and is a criterion of economical

power production.


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f) Volumetric efficiency (ηV)

Volumetric efficiency can be defined as the ratio of volume of air

induced, measured at the free air conditions to the swept volume

of the cylinder.

where V = volume of air induced

Vs = swept volume

Examples

Try Example 13.1 and Example 13.2 in the textbook

(Eastop and McConkey)

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Factors Influencing Performance

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a) SI engines

The thermal efficiency of the Otto cycle depends on

compression ratio.

A graph of air standard thermal efficiency against

compression ratio is shown below:

This graph indicates the form engine development

should take, and over the early years increases in

compression ratio were made.


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The ability to use higher ratios has depended on the

provision of better-quality fuels and of improved

designs of combustion chamber.

The main features of the combustion chamber are the

distances to be travelled by the flame after initiation of

combustion, and the gas flow pattern established.

It is evident that if a petrol–air mixture is compressed

sufficiently it will ignite spontaneously.

This suggests one limit to the compression ratio if

controlled combustion is to be obtained from spark

ignition.

However, before this limit is reached for the whole

charge, spontaneous ignition can occur in the unburnt

charge after combustion has commenced normally.


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The unburnt gas, compressed by the advancing flame

front, is raised in temperature and may reach the point

of self ignition.

This produces an uncontrolled combustion and its

occurrence may be heard as a knocking sound.

A critical condition can be reached which is called

detonation, or ‘heavy knock’.

The advancing flame front is suddenly accelerated by

the occurrence of a high-pressure wave and the flame

front and the shock wave traverse the cylinder together.

The detonation wave suffers successive reflections,

and a high-frequency noise is created.

These combustion phenomena are usually referred to

collectively as ‘knock’.


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One of the results of knock is that local hot spots can

be created which remain at a sufficiently high

temperature to ignite the next charge before the spark

occurs.

This is called pre-ignition, and can help to promote

further knocking.

These result is a noisy, overheated, and inefficient

engine, and perhaps eventual mechanical failure.

The compression ratio which can be utilized depends

on the fuel to be used and a scale has been developed

against which the knock tendency of a fuel can be rated.

The rating is given as an octane number.

The fuel under test is compared with a mixture of iso-

octane (high rating) and normal heptane (low rating), by

volume.


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The octane number of the fuel is the percentage of

octane in the reference mixture which knocks under the

same conditions as the fuel.

The number obtained depends on the conditions of the

test and the two main methods in use (the research and

the motor methods) give different ratings for the same

fuel.

Fuels have been developed which have a higher anti-

knock rating than iso-octane and this has led to an

extension of the octane scale.

Aviation conditions of operation lead to another scale

which gives a better indication of the detonation

characteristic: this is the performance number (PN).

The relationship between octane number (ON), above

100 and performance number is given by


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With higher compression ratio engines, other

phenomena are observed.

From compression ratios of 9.5/1 upwards there are

high rates of pressure rise which have their origin in the

additional flame fronts started from surface deposits in

the cylinder.

At about 9.5/1 compression ratio the low-frequency

engine vibrations produced are called rumble or

pounding.

At compression ratios of 12/1 the pressure rise is about

8.3 bar per degree crank angle with a peak pressure of

83 bar.

3

100PN100100 above ON


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The engine noises produced are known as thud or

pressure rap; surface ignition is not present and fuel

characteristics have little influence.

a) CI engines

The effect of compression ratio in the CI engine is

somewhat simpler than in the Si engine.

The efficiency of the cycle increases with higher

values of compression ratio and the limit is a

mechanical one imposed by the high pressures

developed in the cylinder, a factor which adversely

affects the power-weight ratio.

The normal range of compression ratios is 13/1 to

17/1, but may be anything up to 25/1.

The main factor is the delay period.


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A long delay period means more combustible mixture

has had time to form, and so more charge will be

involved in the initial combustion.

As the speed increases the rate of pressure rise in this

phase also increases.

This is because the delay period is a function of time if

surrounding conditions remain constant, and at the

higher engine speeds more mixture will be formed in

the delay period.

The initial rapid combustion can give rise to rough

running and a characteristic noise called diesel knock.

It has been stated that the delay period depends on the

nature of the fuel, and a fuel with a short delay period,

or high ignitability, is required.


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The ignitability of a fuel oil is indicated by its cetane

number, and the procedure for obtaining it is similar to

that for obtaining the octane number of petrols.

Engines are affected in performance by the atmosphere

in which they operate and some allowance must be

made in performance figures quoted for variations in

pressure, temperature, and relative humidity.

The variations in performance can be represented

graphically, but the normal values quoted apply up to 30oC and 150 m altitude from sea-level, for normally

aspirated engines.

The reduction in output per 300 m of altitude above

150 m is about 3%, and for every 5 K for above 30 oC

the reduction is also about 3%.

THE END

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C: Crankshaft

E: Exhaust camshaft

I: Inlet camshaft

P: Piston

R: Connecting rod

S: Spark plug

V: Valves. Red: exhaust, Blue:

intake

W: Cooling water ducts

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Engine Torque and Power

Torque is measured using a dynamometer.

Load cell

Force FStator

Rotor

b

N

The torque exerted by the engine is: T = F b with units: J

The power P delivered by the engine turning at a speed N and

absorbed by the dynamometer is:

P = T = (2 N) T w/units: (rad/rev)(rev/s)(J) = Watt

Note: is the shaft angular velocity with units: rad/s

CHAPTER 5 internal combustion engine

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