Lecture Notes - Internal Combustion Engine

INTERNALLECTURE NOTES:
COMBUSTION

ENGINE

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ENGINE

LECTURE NOTES - INTERNAL COMBUTION ENGINE

TERBITAN EDISI 2020

BUKU “LECTURE NOTES – INTERNAL COMBUTION ENGINE” ADALAH SEBAGAI
RUJUKAN DAN BACAAN UMUM TERUTAMA KEPADA PENSYARAH DAN
PELAJAR POLITEKNIK DAN KOLEJ KOMUNITI MALAYSIA BAGI
MENGAPLIKASIKAN AMALAN TERBAIK DALAM PERLAKSANAAN KAEDAH
PENGAJARAN DAN PEMBELAJARAN BERKONSEPKAN TEKNOLOGI
AUGMENTED REALITY.

EDITOR
MOHD ROZAIMIN ABDUL HAMID

PENULIS

SYED MUHAMMAD FUAD SAYED ABD LRAIHYMAANNA AHMAD BAZULI
MOHD AMIRUDDIN AB AZIZ

DITERBITKAN OLEH

UNIT PEMBELAJARAN DIGITAL
BAHAGIAN INSTRUKSIONAL DAN PEMBELAJARAN DIGITAL
JABATAN PENDIDIKAN POLITEKNIK DAN KOLEJ KOMUNITI ARAS 6, GALERIA PJH,
JALAN P4W, PERSIARAN PERDANA, PRESINT 4,
62100 PUTRAJAYA

Website : www.celt.edu.my
E- mail : [email protected]



CONTENT

1.0 TYPES OF ENGINE
1.1 INTRODUCTION
1.2 TYPES OF IGNITIONS FOR THE INTERNAL COMBUSTION ENGINE
1.3 FOUR STROKE CYCLE
1.4 ROTARY ENGINE
1.5 GAS TURBINE ENGINE
1.6 STERLING ENGINE
1.7 NATURAL ASPIRATE ENGINE
1.8 NUMBER OF CYLINDERS

2.0 PISTON ENGINE PROCESS ANALYSIS
2.1 INTRODUCTION
2.2 THE OTTO CYCLE
2.3 THE DIESEL CYCLE
2.4 THE DUAL COMBUSTION CYCLE

3.0 COMBUSTION AND FUEL CHARACTERISTIC
3.1 NORMAL COMBUSTION
3.2 ABNORMAL COMBUSTION
3.3 SPARK KNOCK
3.4 SURFACE IGNITION
3.5 IGNITION
3.5.1 PRE – IGNITION
3.5.2 IGNITION DELAY
3.6 COMBUSTION PROCESS BASED ON THE PREASSURE VS CRANKSHAFT
POSITION
3.7 THE EFFECT OF ENGINE SPEED ON IGNITION TIMING
3.8 KNOCKING OR DETONATION PROCESS IN SPARK IGNITION ENGINE
3.8.1 TERM OF KNOCKING OR DETONATION
3.8.2 FACTORS THAT CONTRIBUTE KNOCKING
3.8.3 EFFECTS OF KNOCKING DURING ENGINE PROCESS
3.8.4 REDUCE KNOCKING PROBLEM DURING ENGINE PROCESS
3.8.5 EFFECT OF ENGINE OPERATING VARIABLES ON THE ENGINE
KNOCKING (DETONATION)
3.9 MOTOR OCTANE NUMBER (MON)
3.10 CATANE NUMBER
3.11 FUEL ADDICTIVES
3.11.1 EXAMPLE OF FUEL ADDICTIVE
3.12 STOICIOMETRIC RATIO
3.13 RICH MIXTURE
3.14 LEAN MIXTURE

INTERNAL COMBUSTION ENGINE

4.0 ENGINE’S CRITERIONS AND COMPARISON
4.1 THE INDICATOR POWER (I.P)
4.2 THE BRAKE POWER (B.P)
4.3 FRICTION POWER
4.4 THERMAL EFFICIENCY
4.5 SPECIFIC FUEL CONSUMPTION
4.6 INDICATOR MIN EFFECTIVE PRESSURE

5.0 INTRODUCTION
5.1 SWIRL
5.2 TURBULENCE
5.3 SQUISH AND TUMBLE
5.4 EXHAUST BLOWDOWN
5.5 EXHAUST SYSTEM
5.5.1 EXHAUST SYSTEM MANIFOLD
5.5.2 CATALYTIC CONVERTERS
5.5.3 TAILPIPE AND MUFFLER

6.0 PISTON AND PISTON RINGS
6.1 PISTON
6.2 MATERIAL SELECTION FOR PISTON
6.3 PISTON EXPANSION
6.4 PISTON HEAD SHAPES
6.5 PISTON SKIRT
6.6 PISTON PINS
6.7 RING
6.8 COMPRESSION RING
6.9 OIL RING

7.0 VALVE AND COOLING VALVE
7.1 INTRODUCTION
7.2 TYPE OF VALVE LOCATION
7.3 ARRANGEMENT OF VALVE
7.4 VALVE PARTS
7.5 VALVE MATERIALS
7.6 VALVE MATERIALS
7.7 VALVE PROTECTION
7.8 VALVE SEAT
7.9 VALVE ROTATORS
7.10 VALVE TEMPERATURE AND COOLING
7.11 HOW SODIUM COOLED VALVES CAN MAINTAIN WORKING TEMPERATURE
7.12 OVERHEAD CAMSHAFT

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1.0 TYPES OF ENGINE

INTRODUCTION

The internal combustion engine is an engine in which the combustion of a fuel
(normally a fossil fuel) occurs with an oxidizer (usually air) in a combustion chamber.
An external combustion engine (EC engine) is a heat engine where an (internal)
working fluid is heated by combustion in an external source, through the engine wall
or a heat exchanger.
The fluid then, by expanding and acting on the mechanism of the engine, produces
motion and usable work. The fluid is then cooled, compressed and reused (closed
cycle), or (less commonly) dumped, and cool fluid pulled in (open cycle air engine).

Figure 1.1 : Example of Internal Combustion Engine (Locomotive)

Figure 1.2 : Example of Internal Combustion Engine

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TYPES OF IGNITIONS FOR THE INTERNAL COMBUSTION ENGINE

a) Spark Ignition (SI).

An SI engine starts the combustion process in each cycle using a spark plug. The spark plug
gives a high-voltage electrical discharge between two electrodes which ignites the air-fuel
mixture in the combustion chamber surrounding the plug, many forms of torch holes are used
to initiate combustion from external flame.

b) Compression Ignition (CI).

The combustion process in a CI engine starts when the air-fuel mixture self-ignites due to
high temperature in the combustion chamber caused by high compression.

Figure 1.3 SI Engine Figure 1.4 CI Engine

Figure 1.5

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FOUR STROKE CYCLE

1.1.1 Induction stroke
The air-plus-fuel charge is induced into the cylinder as the piston moves from TDC to BDC.
Due to the movement of the piston the pressure in the cylinder is lower than a value between
the atmospheric pressure. Air flows through the induction system because of the pressure
difference, and the intake valve is opened. Before air is channelled into the cylinder, the air
passes through a carburettor in which the metered amount of petrol is added to the air.
1.1.2 Compression stroke.
With both valves closed, the charge is compressed by the piston. At the TDC position the
charge occupies the volume above the piston, which is called the clearance volume, and it
fills mainly the volume of the combustion chamber. The spark is timed to occur at a point
before TDC. The combustion process occurs at almost constant volume, and there is a large
increase in pressure and temperature of the charge during this process.
1.1.3 Power stroke
When the gas pressure expands, the temperature also rises. This pushes the piston down the
cylinder. This expansion continues and ends near BDC, but in order to assist in exhausting
the gaseous products the exhaust valve opens before BDC.
1.1.4 Exhaust stroke.
The piston travels from BDC to TDC in the exhaust stroke, and it pushes most of remaining
exhaust gases out of the cylinder, and the pressure during this stroke is slightly higher
than atmospheric pressure. In a normal aspirated engine, the clearance volume cannot be
exhausted, and at the commencement of the next cycle this volume is full of exhaust gas at
about atmospheric pressure. The valve exhaust closes after TDC.

Figure 1.6 : Four Stroke Cycle Engine

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Figure 1.7 : Four Stroke Engine

Stroke Spark Ignition Engine

A two-stroke engine is an internal combustion engine that completes the process cycle in
one revolution of the crankshaft (an up stroke and a own stroke of the piston, compared to
twice that number for a four-stroke engine).
This is accomplished by using the end of the combustion stroke and the beginning of the
compression stroke to perform simultaneously the intake and exhaust (or scavenging)
functions.
In this way, two-stroke engines often provide high specific power, at least in a narrow range
of rotational speeds.
The functions of some or all of the valves required by a four-stroke engine are usually served
in a two-stroke engine by ports that are opened and closed by the motion of the piston(s),
greatly reducing the number of moving parts.

Figure 1.8 : Two Stroke Engine

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ROTARY ENGINE

This engine has rotor that provides three equal working spaces. An exhaust release will
occur each time an apex seal overruns the leading edge of an exhaust port, that is, three
times per revolution of the rotor, and this exhaust will continue until the following seal reaches
the trailing edge of the port. Induction will commence in the same space about 60° of rotor
movement earlier. Thus, there are three complete four-stroke cycle per revolution of the rotor
in different working spaces, but all fired by the same spark plug as maximum compression is
reached.

Figure 1.9 : Rotary Engine

GAS TURBINE ENGINE

A gas turbine, also called a combustion turbine, is a type of internal combustion engine.
It has an upstream rotating compressor coupled to a downstream turbine, and a combustion
chamber in-between.
Energy is added to the gas stream in the combustor, where fuel is mixed with air and ignited.
In the high pressure environment of the combustor, combustion of the fuel increases the
temperature.
The products of the combustion are forced into the turbine section. There, the high velocity
and volume of the gas flow is directed through a nozzle over the turbine's blades, spinning
the turbine which powers the compressor and, for some turbines, drives their mechanical
output.
The energy given up to the turbine comes from the reduction in the temperature and pressure
of the exhaust gas.
Gas turbine engines are, theoretically, extremely simple.

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They have three parts:

1) Compressor - Compresses the incoming air to high pressure
2) Combustion area - Burns the fuel and produces high-pressure, high-velocity gas
3) Turbine - Extracts the energy from the high-pressure, high-velocity gas flowing from
the combustion chamber

Compressor Combustion Area Turbine

Figure 1.10 : Schematic Diagram for Jet Turbine

STERLING ENGINE

A Stirling engine is a heat engine that operates by cyclic compression and expansion of air
or other gas (the working fluid) at different temperatures, such that there is a net conversion
of heat energy to mechanical work

Figure 1.11 : Stirling Engine

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Alpha Configuration Operation

An alpha Stirling contains two power pistons in separate cylinders, one hot and one cold.
The hot cylinder is situated inside the high temperature heat exchanger and the cold cylinder
is situated inside the low temperature heat exchanger.
This type of engine has a high power-to-volume ratio but has technical problems because of
the usually high temperature of the hot piston and the durability of its seals. In practice, this
piston usually carries a large insulating head to move the seals away from the hot zone at the
expense of some additional dead space.
The crank angle has a major effect on efficiency and the best angle frequently must be found
experimentally. An angle of 90° frequently locks.
The following diagrams do not show internal heat exchangers in the compression and
expansion spaces, which are needed to produce power. A regenerator would be placed in
the pipe connecting the two cylinders.

Figure 1.12 : Alpha Stirling

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Beta configuration operation

A beta Stirling has a single power piston arranged within the same cylinder on the same
shaft as a displacer piston. The displacer piston is a loose fit and does not extract any power
from the expanding gas but only serves to shuttle the working gas between the hot and cold
heat exchangers.
When the working gas is pushed to the hot end of the cylinder it expands and pushes the
power piston. When it is pushed to the cold end of the cylinder it contracts and the momentum
of the machine, usually enhanced by a flywheel, pushes the power piston the other way to
compress the gas.
Unlike the alpha type, the beta type avoids the technical problems of hot moving seals, as the
power piston is not in contact with the hot gas.
Again, the following diagrams do not show any internal heat exchangers or a regenerator,
which would be placed in the gas path around the displacer. If a regenerator is used in a beta
engine, it is usually in the position of the displacer and moving, often as a volume of wire
mesh.

Figure 1.13 : Beta Stirling

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INTERNAL COMBUSTION ENGINE

Gamma configuration operation

A gamma Stirling is simply a beta Stirling with the power piston mounted in a separate
cylinder alongside the displacer piston cylinder, but still connected to the same flywheel.
The gas in the two cylinders can flow freely between them and remains a single body. This
configuration produces a lower compression ratio because of the volume of the connection
between the two but is mechanically simpler and often used in multi-cylinder Stirling engines.

Figure 1.14 : Gamma Stirling

NATURAL ASPIRATED ENGINE

A naturally aspirated engine is an internal combustion engine in which air intake depends
solely on atmospheric pressure and which does not rely air forced through a turbocharger or
a supercharger
While turbocharged engines can provide more power than a naturally aspirated alternative of
the same size, naturally aspirated engines offer other advantages.
Naturally aspirated engines generally respond much more quickly to the accelerator - giving
them what enthusiastic drivers would called greater response - whereas there can be a lag
when suddenly asking for increased speed from turbocharged engines.
This turbo lag is the result of the extra complexity that ultimately allows turbocharged engines
to make additional power.
Similarly, if you let engine speed drop too low, some turbocharged engines can feel very
tardy as the turbocharger requires longer to recover and get back up to speed. This is not a
problem for most naturally aspirated engines.
Furthermore, naturally aspirated cars should be cheaper to buy, more reliable and easier to
maintain because they are less complicated.
Turbocharged models are also not always that economical in real-world driving, especially
when working the engine harder - this can lead to large discrepancies between claimed and
real-world mpg, particularly with turbocharged petrol cars.

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INTERNAL COMBUSTION ENGINE

NUMBER OF CYLINDER

Every vehicle has cylinders, and the engine size is generally denoted by how many cylinders
a vehicle has. A 4 cylinder has 4, a V6 has 6 and so on. Inside each cylinder is a piston,
which moves up and down. Gasoline and air combine inside the cylinder and a spark creates
combustion. The combustion then pushes the cylinder down, which creates motion that is
transferred to the driveshaft, propelling the vehicle. This is why vehicle motors are referred to
as internal combustion.
Most cars are powered by a 4 or 6-cylinder engine, while most trucks have a 6 or 8 cylinder.
The more cylinders in an engine, the more combustion that occurs, creating more movement
to turn the crankshaft and power to move the car. However, more cylinders also require
more gasoline to make the combustion necessary to drive the car and thus are not as
efficient. This means that when you buy a 4-cylinder car, you are sacrificing power in order
to increase efficiency. The 4-cylinder engine has to work harder to move the vehicle, hence
why performance suffers. Manufacturers such as GM have recognized this and are working
to make 4 cylinder engines more enjoyable to drive.

Figure 1.15

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INTERNAL COMBUSTION ENGINE

2.0 PISTON ENGINE PROCESS ANALYSIS

INTRODUCTION

In this unit we are to discuss the meaning of standard cycle, heat supplied at constant
volume and heat supplied at constant pressure. An internal combustion engine can
be classified into three different cycles and they are Otto cycle, diesel cycle and dual-
combustion cycle.

Air Standard Cycles

The air standard cycle is a cycle followed by a heat engine which uses air as the
working medium. Since the air standard analysis is the simplest and most idealistic,
such cycles are also called ideal cycles and the engine running on such cycles are
called ideal engines.

Assumptions:
1. The working medium is a perfect gas with constant specific heats and molecular
weight corresponding to values at room temperature.
2. No chemical reactions occur during the cycle. The heat addition and heat rejection
processes are merely heat transfer processes.
3. The processes are reversible.
4. Losses by heat transfer from the apparatus to the atmosphere are assumed to be
zero
in this analysis.
5. The working medium at the end of the process (cycle) is unchanged and is at the
same condition as at the beginning of the process (cycle).

THE OTTO CYCLE

The Otto cycle is the ideal air standard cycle for the petrol engine, the
gas engine, and the high-speed oil engine. The cycle is shown on p-v diagram in

Otto cycle process:
1 to 2: isentropic compression
2 to 3: reversible constant volume heating, the heat supplied Q1
3 to 4: isentropic expansion
4 to 1: reversible constant volume cooling, the heat rejected Q2

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INTERNAL COMBUSTION ENGINE
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THE DIESEL CYCLE

In diesel cycle, heat is supplied at a constant pressure and rejected at a constant volume.
The process of cycle is:
1 to 2: Isentropic compression
2 to 3: Reversible constant pressure heating, the heat supplied Q1
3 to 4: Isentropic expansion
4 to 1: Reversible constant volume cooling, the heat rejected Q2

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INTERNAL COMBUSTION ENGINE

THE DUAL COMBUSTION CYCLE

Modern oil engines known also as diesel engine, use solid injection of the fuel. The ideal
cycle which is used as a basis for comparison is called the dual combustion cycle or the
mixed cycle, and is shown on a p-v diagram in Figure. 3.1 In this cycle, heat is supplied in
two parts; the first part at constant volume and the second in constant pressure. Hence the
name ‘dual combustion’. Dual cycle process:
1 to 2: Isentropic compression
2 to 3: Reversible constant volume heating, the heat supplied Q1
3 to 4: Reversible constant pressure heating, the heat supplied
4 to 5: Isentropic expansion
5 to 1: Reversible constant volume cooling, the heat rejected Q2

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3.0 COMBUSTION AND FUEL
CHARACTERISTIC

COMBUSTION PROCESS TERMS:

Normal Combustion
A combustion process which is initiated solely by a timed spark and in which the
flame front moves completely across the combustion chamber in a uniform manner
at a normal velocity.
Abnormal Combustion
A combustion process in which a flame front may be started by hot combustion-
chamber surfaces either prior to or after spark ignition, or a process in which some
part or all of the charge may be consumed at extremely high rates.

Figure 3.1 : Diagram for Normal and Abnormal Combustion

Spark Knock
A knock which is recurrent and repeatable in terms of audibility. It is controllable by
the spark advance; advancing the spark increases the knock intensity and retarding
the spark reduces the intensity.
Knock is the name given to the noise which is transmitted through the engine
structure when essentially spontaneous ignition of a portion of the end gas—the fuel,
air, residual gas, mixture ahead of the propagating flame—occurs.

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INTERNAL COMBUSTION ENGINE

There is an extremely rapid release of most of the chemical energy in the end-gas, causing
very high local pressures and the propagation of pressure waves of substantial amplitude
across the combustion chamber.

Figure 3.2 : Spark knock phenomenon
Surface Ignition
Surface ignition is ignition of the fuel-air charge by any hot surface other than the spark
discharge prior to the arrival of the normal flame front. It may occur before the spark ignites
the charge (pre-ignition) or after normal ignition (post-ignition).
Surface Ignition is ignition of the fuel-air mixture by a hot spot on the combustion chamber
walls such as an overheated valve or spark plug, or glowing combustion chamber deposit:
i.e., by any means other than the normal spark discharge. Following surface ignition, a flame
develops at each surface-ignition location and starts to propagate across the chamber in an
analogous manner to what occurs with normal spark-ignition.

Figure 3.3 : Normal and Premature Combustion
Figure 3.3: (a) Normally, fuel is ignited by the spark plug, and combustion spreads
uniformly outward. (b) Gasoline with an octane rating that is too low for the engine
can ignite prematurely, resulting in uneven burning that causes knocking and
pinging.

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INTERNAL COMBUSTION ENGINE

The octane scale was established in 1927 using a standard test engine and two pure
compounds: n-heptane and isooctane (2,2,4-trimethylpentane). n-Heptane, which causes a
great deal of knocking on combustion, was assigned an octane rating of 0, whereas isooctane,
a very smooth-burning fuel, was assigned an octane rating of 100.

Figure 3.4 : The Octane Ratings of some Hydrocarbons and Common Additives
Chemists assign octane ratings to different blends of gasoline by burning a sample of each
in a test engine and comparing the observed knocking with the amount of knocking caused
by specific mixtures of n-heptane and isooctane. For example, the octane rating of a blend
of 89% isooctane and 11% n-heptane is simply the average of the octane ratings of the
components weighted by the relative amounts of each in the blend. Converting percentages
to decimals, we obtain the octane rating of the mixture: 0.89(100) +0.11(0) =89
A gasoline that performs at the same level as a blend of 89% isooctane and 11% n-heptane
is assigned an octane rating of 89; this represents an intermediate grade of gasoline. Regular
gasoline typically has an octane rating of 87; premium has a rating of 93 or higher.

Figure 3.5 : The Distillation of Petroleum

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INTERNAL COMBUSTION ENGINE

Ignition
The ignition system is designed to ignite the air and fuel that have been mixed in the fuel
system. It is important to improve this system. Each year, ignition is becoming
more and more computerized. Today’s ignition systems are almost totally computer
controlled for improved combustion.
Pre- Ignition
This Is one process where the spark is heated up before the ignition begins. It causes
rough running and in extreme cases, can do damage to the engine.
Causes of pre-ignition include the following:
1. Carbon deposits form a heat barrier and can be a contributing factor to pre-ignition.
2. Glowing carbon deposits on a hot exhaust
3. A sharp edge in the combustion chamber or on top of a piston
4. Sharp edges on valves that were reground improperly
5. An engine that is running hotter than normal due to a cooling system problem
6. Auto-ignition of engine oil droplets.
Ignition Delay
Ignition delay is defined as the time (or crank angle interval) from when the fuel injection
starts to the onset of combustion.

Figure 3.6 : TPressure - Crank Angle diagram for a four-stroke cycle
Delay period is the commencement of injection and it is indicated by the dot on the compression
line 15° before dead centre. The period 1 is the delay period during which ignition is being
initiated, but without any measurable departure of the pressure from the air compression
curve which is continued as a broken line in the diagram as it would be recorded if there were
no injection and combustion.

Figure 3.7: Pressure vs crankshaft position diagram

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INTERNAL COMBUSTION ENGINE

Combustion Process Based On the Pressure Vs Crankshaft Position

0 ° – 180 ° = Intake valve open (IVO), intake process occurred and piston moving from Top
Dead Center (TDC) to Bottom Dead Center (BDC). The mixture of fuel and O2 is entering
the cylinder. Exhaust valve close (EVC) during this stroke (intake stroke) and the pressure
remain constant.

180 ° – 360 ° = Intake valve close (IVC), piston climb up from Bottom Dead Center (BDC)
to Top Dead Center (TDC). The mixture of fuel and O2 is compressed and the pressure is
raising up. Spark plug will ignite before TDC (Ignition Delay).

360 ° – 540 ° = Intake valve close (IVC) and exhaust valve close (EVC) during power stroke.
Piston moving downward from Top Dead Center (TDC) to Bottom Dead Center (BDC). At the
end of this process the exhaust valve open (EVO). The pressure is decreasing.

540 ° – 720 ° = Piston moving upward from Bottom Dead Center (BDC) to Top Dead Center
(TDC). The product of combustion is expelled out from the cylinder. At the of this process,
intake valve open (IVO) and continue for next cycle.

The Effect of Engine Speed on Ignition Timing

1) Ignition timing, in a spark ignition internal combustion engine (ICE), is the process
of setting the angle relative to piston position and crankshaft angular velocity that a
spark will occur in the combustion chamber near the end of the compression stroke.

2) "Timing advance" refers to the number of degrees before top dead center (BTDC)
that the spark will ignite the air-fuel mixture in the combustion chamber during the
compression stroke. Igniting the mixture before the piston reaches TDC will allow the
mixture to fully burn soon after the piston reaches TDC.

3) “Retarded timing” can be defined as changing the timing so that fuel ignition happens
later than the manufacturer's specified time. For example, if the timing specified by
the manufacturer was set at 12 degrees BTDC initially and adjusted to 11 degrees
BTDC, it would be referred to as retarded.

4) If the air-fuel mixture is ignited at the correct time, maximum pressure in the cylinder
will occur sometime after the piston reaches TDC allowing the ignited mixture to
push the piston down the cylinder with the greatest force. Ideally, the time at which
the mixture should be fully burnt is about 20 degrees ATDC.

5) If the ignition spark occurs at a position that is too advanced relative to piston
position, the rapidly expanding air-fuel mixture can actually push against the piston
still moving up, causing knocking (pinging) and possible engine damage.

6) If the spark occurs too retarded relative to the piston position, maximum cylinder
pressure will occur after the piston is already traveling too far down the cylinder. This
results in lost power, overheating tendencies, high emissions, and unburned fuel.

7) The ignition timing will need to become increasingly advanced (relative to TDC) as
the engine speed increases so that the air-fuel mixture has the correct amount of
time to fully burn.

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INTERNAL COMBUSTION ENGINE

8) As the engine speed (RPM) increases, the time available to burn the
mixture decreases but the burning itself proceeds at the same speed, it
needs to be started increasingly earlier to complete in time.

9) Poor volumetric efficiency at lower engine speeds also requires
increased advancement of ignition timing.

Figure 3.8

The Effect of Engine Speed on Ignition Timing

1) The correct timing advance for a given engine speed will allow for maximum cylinder
pressure to be achieved at the correct crankshaft angular position. When setting the
timing for an automobile engine, the factory timing setting can usually be found on a
sticker in the engine bay.

2) The ignition timing is also dependent on the load of the engine with more load (larger
throttle opening and therefore air: fuel ratio) requiring less advance (the mixture
burns faster). Also it is dependent on the temperature of the engine with lower
temperature allowing for more advance. The speed with which the mixture burns
depends also on the octane rating of the fuel and on the air-fuel ratio.

Figure 3.9 : Example of Timing Map Diagram

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INTERNAL COMBUSTION ENGINE

Knocking or Detonation Process in Spark Ignition Engine

Term of Knocking or Detonation

This is one process that happens within the combustion chamber. It sounds like a small
ticking or rattling noise within the engine. In long term, the piston and ring can be damaged
as well as the spark plug and valve.

Factors That Contribute Knocking

Detonation occurs when several conditions / factors inside the combustion chamber
exist at the same time: -
1) Increased compression
2) High temperatures
3) Lean fuel/air mixture
4) Advanced ignition timing
5) Lower octane fuels are all factors that PROMOTE detonation conditions

Effects of knocking during engine process

The effects of knocking during engine process are: -
1) A drop in engine performance.
2) Pollution of gases from the combustion is incomplete.
3) High consumption of fuel.

Reduce knocking problem during engine process

In this case we have three options to reduce knocking during engine process: -
1) Increase the ignition combustion engine.
2) Reduce the heat the final combustion; enriching the air-fuel ratio which alters the
chemical reactions during combustion, reduces the combustion temperature and
increases the margin above detonation.
3) Use high quality fuel, the use of a fuel with high octane rating, which increases the
4) Combustion temperature of the fuel and reduces the proclivity to detonate.

According to Ricardo, there are three stages of combustion in SI Engine as shown: -

1) Ignition lag stage

2) Flame propagation stage

3) After burning stage

Figure 3.10 : Ricardo Diagram with Important Points

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Ignition lag stage:
There is a certain time interval between instant of spark and instant where there is a noticeable
rise in pressure due to combustion. This time lag is called IGNITION LAG. Ignition lag is the
time interval in the process of chemical reaction during which molecules get heated up to self-
ignition temperature, get ignited and produce a self-propagating nucleus of flame.
The ignition lag is generally expressed in terms of crank angle (q1). The period of ignition lag
is shown by path ab. Ignition lag is very small and lies between 0.00015 to 0.0002 seconds.
An ignition lag of 0.002 seconds corresponds to 35 deg crank rotation when the engine is
running at 3000 RPM. Angle of advance increase with the speed. This is a chemical process
depending upon the nature of fuel, temperature and pressure, proportions of exhaust gas and
rate of oxidation or burning.
Flame propagation stage:
Once the flame is formed at “b”, it should be self-sustained and must be able to propagate
through the mixture. This is possible when the rate of heat generation by burning is greater
than heat lost by flame to surrounding.
After the point “b”, the flame propagation is abnormally low at the beginning as heat lost is
more than heat generated. Therefore, pressure rise is also slow as mass of mixture burned
is small. Therefore, it is necessary to provide angle of advance 30 to 35 deg, if the peak
pressure to be attained 5-10 deg after TDC. The time required for crank to rotate through an
angle q2 is known as combustion period during which propagation of flame takes place.
After burning:
Combustion will not stop at point “c” but continue after attaining peak pressure and this
combustion is known as after burning. This generally happens when the rich mixture is
supplied to engine.

Figure 3.11 : Ricardo Diagram
In Figure 3.12 in the Ricardo diagram Line 1 explains the good condition of combustion.
Line 2 explains the overhead and the curve in Line 3 explains late ignition

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Figure 3.13: Normal Combustion Figure 3.14: Combustion with light
with no Knocking Knocking

Effect of engine operating variables on the engine knocking (detonation):

The various engine variable affecting knocking can be classified as:
1) Temperature factors
2) Density factors
3) Time factors
4) Composition factors

Temperature factors

Increasing the temperature of the unburned mixture increase the possibility of knock in
the SI engine. We shall now discuss the effect of following engine parameters on the
temperature of the unburned mixture:

1) RAISING THE COMPRESSION RATIO.
Increasing the compression ratio increases both the temperature and pressure
(density of the unburned mixture). Increase in temperature reduces the delay period
of the end gas which in turn increases the tendency to knock.

2) SUPERCHARGING.
It also increases both temperature and density, which increase the knocking
tendency of engine

3) COOLANT TEMPERATURE
Delay period decreases with increase of coolant temperature, decreased delay
period increase the tendency to knock.

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4) TEMPERATURE OF THE CYLINDER AND COMBUSTION CHAMBER WALLS
The temperature of the end gas depends on the design of combustion chamber.
Sparking plug and exhaust valve are two hottest parts in the combustion chamber
and uneven temperature leads to pre-ignition and hence the knocking.

Density factors

Increasing the density of unburnt mixture will increase the possibility of knock in the engine.
The engine parameters which affect the density are as follows:

1) Increased compression ratio increases the density.
2) Increasing the load opens the throttle valve more and thus the density.
3) Supercharging increase the density of the mixture.
4) Increasing the inlet pressure increases the overall pressure during the cycle.
The high pressure end gas decreases the delay period which increase the tendency
of knocking.
5) Advanced spark timing: quantity of fuel burnt per cycle before and after
TDC position depends on spark timing. The temperature of charge increases
by increasing the spark advance and it increases with rate of burning and does
not allow sufficient time to the end mixture to dissipate the heat and increase the
knocking tendency.

Time factors

Increasing the time of exposure of the unburned mixture to auto-ignition conditions increase
the possibility of knock in SI engines.

1) Flame travel distance: If the distance of flame travel is more, then possibility of
knocking is also more. This problem can be solved by combustion chamber design,
spark plug location and engine size. Compact combustion chamber will have better
anti-knock characteristics, since the flame travel and combustion time will be shorter.
Further, if the combustion chamber is highly turbulent, the combustion rate is high
and consequently combustion time is further reduced; this further reduces the
tendency to knock.
2) Location of sparkplug: A spark plug which is centrally located in the combustion
chamber has minimum tendency to knock as the flame travel is minimum. The flame
3) Travel can be reduced by using two or more spark plugs.
4) Location of exhaust valve: The exhaust valve should be located close to the spark
5) plug so that it is not in the end gas region; otherwise there will be a tendency to
knock.
6) Engine size: Large engines have a greater knocking tendency because flame
requires a longer time to travel across the combustion chamber. In SI engine
therefore, generally limited to 100mm.
7) Turbulence of mixture: decreasing the turbulence of the mixture decreases the
flame speed and hence increases the tendency to knock. Turbulence depends on
the design of combustion chamber and one engine speed.

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Composition

The properties of fuel and A/F ratio are primary means to control knock:

1) Molecular Structure: The knocking tendency is markedly affected by the type of
the fuel used. Petroleum fuels usually consist of many hydro-carbons of different
molecular structure. The structure of the fuel molecule has enormous
effect on knocking tendency. Increasing the carbon-chain increases the knocking
tendency and centralizing the carbon atoms decreases the knocking tendency.
Unsaturated hydrocarbons have less knocking tendency than saturated hydrocarbons.

Paraffins:
i. Increasing the length of carbon chain increases the knocking tendency.
ii. Centralising the carbon atoms decreases the knocking tendency.
iii.Adding methyl group (CH to the side of the carbon chain in the centre position
decreases the knocking tendency.

Olefins:
Introduction of one double bond has little effect on anti-knock quality but two or three double
bond results less knocking tendency except C and C.

Napthenes and Aromatic:
i. Napthenes have greater knocking tendency than corresponding aromatics.
ii. With increasing double-bonds, the knocking tendency is reduced.
iii. Lengthening the side chains increases the knocking tendency whereas branching of
the side chain decreases the knocking tendency.

2) Fuel-air ratio. The most important effect of fuel-aft ratio is on the reaction time
or ignition delay. When the mixture is nearly 10% richer than stoichiomiric (fuel-
air ratio = 0.08) ignition lag of the end gas is minimum and the velocity of flame
propagation is maximum. By making the mixture leaner or richer (than F/A 0.08) the
tendency to knock is decreased. A too rich mixture is especially effective in
decreasing or eliminating the knock due to longer delay and lower temperature of
compression.

3) Humidity of air. Increasing atmospheric humidity decreases the tendency to knock
by decreasing the reaction time of the fuel.

Research Octane Number (RON)

The most common type of octane rating worldwide is the Research Octane Number (RON).
RON is determined by running the fuel in a test engine with a variable compression ratio
under controlled conditions, and comparing the results with those for mixtures of iso-octane
and n-heptane.

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INTERNAL COMBUSTION ENGINE

Motor Octane Number (MON)

There is another type of octane rating, called Motor Octane Number (MON), is determined
at 900 rpm engine speed instead of the 600 rpm for RON.MON testing uses a similar test
engine to that used in RON testing, but with a preheated fuel mixture, higher engine speed,
and variable ignition timing to further stress the fuel's knock resistance. Depending on the
composition of the fuel, the MON of a modern pump gasoline will be about 8 to 12 octane
lower than the RON, but there is no direct link between RON and MON. Pump gasoline
specifications typically require both a minimum RON and a minimum MON.

Catane Number

There is a delay between the time that fuel is injected into the cylinder and the time that the
hot gases ignite. This time period or delay is expressed as a catane number. Catane number
ranges from 30 to 60 on diesel fuel. Catane number is an indication of the ignition quality of
the diesel fuel. The higher the catane number, the better the ignition quality of the fuel. High
catane numbers should be used to start an engine in cold weather. A catane number of 85 to
96 is often used for starting diesel engines in cold weather. If a low catane number is used
in diesel engine, some of the fuel may not ignite. The fuel will then accumulate within the
cylinder. When combustion finally does occur, this excess fuel will explode suddenly. This
may result in a knocking sound as in gasoline.

Figure 3.13 : Pump Station

Fuel Additives

Additives - Chemicals are added to gasoline in very small quantities to improve and maintain
gasoline/fuel quality.

Effects of fuel additives:
• To improve combustion and pollutant emissions,
• To ensure reduced wear and limit deposit formation during the engine life cycle of
several hundreds of thousands of miles.

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INTERNAL COMBUSTION ENGINE

Example of Fuel Additives:
1. Oxygenates
Alcohols: Methanol (MeOH)
Ethers: Methyl tert-butyl ether (MTBE), now outlawed in many states of the U.S. for
road use, mostly because of water contamination.
2. Antioxidants, stabilizers
Butylated hydroxytoluene (BHT)
3. Antiknock agents
Toluene

Stoichiometric Ratio

The stoichiometric mixture for a gasoline engine is the ideal ratio of air to fuel that burns all
fuel with no excess air. For gasoline/petrol fuel, the stoichiometric air–fuel mixture is about
14.7:1 i.e. for every one gram of fuel, 14.7 grams of air are required.

The fuel oxidation reaction is:

Figure 3.16: Ignition limits for hydrocarbons

Rich Mixture

Fuel-air mixtures with less than stoichiometric air can also burn. With less than stoichiometric
air you get fuel rich combustion, there is insufficient oxygen to oxidize all the C and H in the
fuel to CO2 and H2O.
Get incomplete combustion where carbon monoxide (CO) and molecular hydrogen (H2) also
appear in the products.

where for fuel rich mixture have insufficient air; γ< 1

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INTERNAL COMBUSTION ENGINE

Lean Mixture

Fuel-air mixtures with more than stoichiometric air, excess air, can burn. With excess air you
get fuel lean combustion, the extra air appears in the products in unchanged form.
where for fuel lean mixture have excess air so γ> 1

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INTERNAL COMBUSTION ENGINE

4.0 ENGINE’S CRITERIONS
AND COMPARISONS

THE INDICATOR POWER (I.P.)

This is defined as the rate of work done by the gas on the piston as evaluated from
an indicator diagram obtained from the engine.
Indicator power represents the maximum power from the engine under ideal or perfect
condition. I.p is calculated on the basis of engine size, displacement, operational
speed and the pressure developed theoretically in the cylinder. Ip will always be
more than b.p.

Figure 4.1 : Formula & Equation

Indicator Power, Ip = PiLANK
For 4 stroke engine, Ip = PiLANK (N=N/2)
For 2 stroke engine, Ip = PiLANK (N=N)
Which is;
A = Area
L = Length of Displacement
Pi = Indicate Mean Effective Pressure
N = Revolution per Minute (RPM)
K = Number of cylinder

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INTERNAL COMBUSTION ENGINE

THE BRAKE POWER (B.P)

Brake power is known as the indicated power output, bp is the power developed inside the
engine cylinder by the combustion of the charge. It is also called brake power because a
brake is used to slow down the shaft inside a dynamometer. Brake horsepower is often
used to compare engines and their characteristics. Automotive manufacturers use brake
horsepower to show the differences between engines.
Brake power, bp = 2πNT
Which is;
N = rpm,
T = torque = W x R,
W = weight in Newton,
R = radius from rotation point

FRICTION POWER

Friction power (fp) is defined as the horsepower being used to overcome internal friction.
Anytime two objects touch each other while moving, friction is produced. Friction has a
tendency to slow down the engine.
f.p = i.p – b.p
Mechanical efficiency, ηm = bp/ip

THERMAL EFFICIENCY, (ꞃbt)

The power output of the engine is obtained from the chemical energy of the fuel supplied.
The overall efficiency of the engine is given by the brake thermal efficiency,

OR

(mf is the mass of fuel consumed per unit time, and Qnet, v is the lower calorific
value of the fuel)

SPECIFIC FUEL CONSUMPTION, (s.f.c)

Specific fuel consumption (s.f.c) is the mass of fuel consumed per kW develop per
hour, and is a criterion of economic power production,

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INTERNAL COMBUSTION ENGINE

INDICATOR MIN EFFECTIVE PRESSURE, i.m.e.p. Pm

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Morse Test
1) The Morse test is only applicable to multi-cylinder engines.
2) One cylinder is cut out, by shortening the plug if S.I engine is under test, or by
disconnecting an injector if a C.I. engine is under test.
3) If the value of i.p. of the cylinders denoted by I1, I2, I3, and I4 (considering a four-
cylinder engine), and the power losses in each cylinder are denoted by L1, L2, L3,
and L4, then the value of b.p., B, at the test speed with all cylinders firing is given by:
B = (I1 – L1) + (I2 - L2) + (I3 – L3) + (I4 – L4)
4) If number 1 cylinder is cut out, then the contribution I1 is lost; and if the losses
due to that cylinder remain the same as when it is fired, then the b.p., B1, now obtain
at the same speed is:
B1 = (0 – L1) + (I2 - L2) + (I3 – L3) + (I4 – L4)
5) Subtracting the second equation from the first gives B – B1 = I1
6) By cutting out each cylinder in turn the values I2, I3, and I4 can be obtained from
equations similar to B – B1 = I1. Then, for the engine, I = I1 + I2 + I3 + I4

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Volumetric efficiency, ꞃv
1) The power output of an I.C engine depends directly upon the amount of charge
which can be induced into the cylinder.
2) This is referred to as the breathing capacity of the engine and is expressed
quantitatively by the volumetric efficiency, which is defined as the ratio of the
volume of air induced, measured at the free air conditions to the swept volume of the
cylinder,

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INTERNAL COMBUSTION ENGINE

The volumetric efficiency of an engine is affected by many variables such as;
i. compression ratio
ii. valve timing
iii. induction and port design
iv. mixture strength
v. latent heat of evaporation of the fuel
vi. heating of the induced charge
vii. cylinder temperature
viii. atmospheric condition.
The power output of an engine depends on its capacity to breathe, and if a particular engine
had a constant thermal efficiency then its output would be in proportion to the amount of air
induced.
The volumetric efficiency with normal aspiration is seldom above 80%, and to improve on
this figure, supercharging is used. Air is forced into the cylinder by a blower or fan which is
driven by the engine.

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INTERNAL COMBUSTION ENGINE
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5.0 FLUID MOTION IN
COMBUSTION CHAMBER

INTRODUCTION

This chapter discusses air, fuel, exhaust gas motion that occurs within the cylinders
during the compression stroke, combustion, and power stroke of the cycle. It is
important to have this motion to speed evaporation of the fuel, to enhance air-fuel
mixing, and to increase combustion speed and efficiency. In addition to the normal
desired turbulence, a rotational motion called swirl is generated on the air-fuel mixture
during intake.
Near the end of the compression stroke, two additional mass motions are generated:
squish and tumble. Squish is a radial motion towards the centre-line of the cylinder,
while tumble is a rotational motion around a circumferential axis. One additional flow
motion will be discussed: that of crevice flow and blow-by. This is the flow into the very
small crevices of the combustion chamber due to the very high pressures generated
during compression and combustion, (Willard W. Pulkrabek).

SWIRL

Swirl greatly enhances the mixing of air and fuel to give a homogeneous mixture
in the very short time available for this in modern high-speed engines. It is also a
main mechanism for very rapid spreading of the flame front during the combustion
process, (Willard W. Pulkrabek).

Figure 5.1 :
(a) Swirl motion within engine cylinder. Methods to generate

(b) Air entering cylinder from tangential direction

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INTERNAL COMBUSTION ENGINE

Figure 5.2 :
(c) countered intake runner, (d) countered valve
Swirl ratio is a dimensionless parameter used to quantify rotational motion within the cylinder.
Figure 5.3 shows how ratio changes through a cycle of the engine swirl. During intake, it is
high, decreasing after BDC in the compression stroke due to viscous drag with the cylinder
walls.
Combustion expands the gases and increases swirl to another maximum part way into the
power stroke. Expansion of the gases and viscous drag quickly reduce this again before
blow-down occurs. One-fourth to one-third of angular momentum will be lost during the
compression stroke, (Willard W. Pulkrabek ).

Figure 5.3:
Average cylinder swirl ratio as a function of crank angle for a typical SI engine.
Swirl is high during the intake process, with a maximum near TDC. It then is reduced by
viscous drag during the compression stroke. There is a second maximum near ~e end
of compression when the radius of rotation is decreased near TDC and expansion from
combustion occurs. Viscous drag with the cylinder walls during the expansion stroke quickly
reduces this again before blowdown occurs, (Willard W. Pulkrabek

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INTERNAL COMBUSTION ENGINE

TURBULENCE

Due to the high velocities involved, all flows into, out of, and within engine cylinders are
turbulent flows. The exception to this are those flows in the corners and small crevices of
the combustion chamber where the close proximity of the walls dampens out turbulence.
Because of turbulence, thermodynamic transfer rates within an engine are increased by
an order of magnitude. When flow is turbulent, particles experience random fluctuations in
motion superimposed on their main bulk velocity. These fluctuations occur in all directions,
perpendicular to the flow and in the flow direction. This makes it impossible to predict the
exact flow conditions at any given time and position. There are many levels of turbulence
within an engine. Large-scale turbulence occurs with eddies on the order of the size of the
flow passage (e.g., valve opening, diameter of intake runner, height of clearance volume,
etc.), (Willard W. Pulkrabek ).

Turbulence in a cylinder is high during intake, but then decreases as the flow rate slows near
BDC. It increases again during compression as swirl, squish, and tumble increase near TDC.
Swirl makes turbulence more homogeneous throughout the cylinder. The high turbulence
near TDC when ignition occurs is very desirable for combustion. It breaks up and spreads the
flame front many times faster than that of a laminar flame.

The air-fuel is consumed in a very short time, and self-ignition and knock are avoided. Local
flame speed depends on the turbulence immediately in front of the flame. This turbulence is
enhanced by the expansion of the cylinder gases during the combustion process. The shape
of the combustion chamber is extremely important in generating maximum turbulence and
increasing the desired rapid combustion, (Willard W. Pulkrabek ).

Figure 5.4: Turbulence level off gas flows in engines as a function of engine speed

Turbulence intensity is a strong function of engine speed (Fig. 5.4). As speed is increased,
turbulence increases, and this increases the rate of evaporation, mixing, and combustion.
One result of this is that all engine speeds have about the same burn angle (i.e., the crank
angle through which the engine turns as combustion takes place). The one phase of this
process which is not totally changed by the increase in turbulence is ignition delay, (Willard
W. Pulkrabek ).

43