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OBJECTIVES:
To update the knowledge in engine exhaust emission control and alternate fuels
To enable the students to understand the recent developments in IC Engines
UNIT I SPARK IGNITION ENGINES 9
Air-fuel ratio requirements, Design of carburetor –fuel jet size and venture size, Stages
of combustion-normal and abnormal combustion, Factors affecting knock, Combustion
chambers, Introduction to thermodynamic analysis of SI Engine combustion process.
UNIT II COMPRESSION IGNITION ENGINES 9
Stages of combustion-normal and abnormal combustion – Factors affecting knock,
Direct and Indirect injection systems, Combustion chambers, Turbo charging,
Introduction to Thermodynamic Analysis of CI Engine Combustion process.
UNIT III ENGINE EXHAUST EMISSION CONTROL 9
Formation of NOX , HC/CO mechanism , Smoke and Particulate emissions, Green
House Effect , Methods of controlling emissions , Three way catalytic converter and
Particulate Trap, Emission (HC,CO, NO and NOX , ) measuring equipments, Smoke and
Particulate measurement, Indian Driving Cycles and emission norms
UNIT IV ALTERNATE FUELS 9
Alcohols , Vegetable oils and bio-diesel, Bio-gas, Natural Gas , Liquefied Petroleum
Gas ,Hydrogen , Properties , Suitability, Engine Modifications, Performance ,
Combustion and Emission Characteristics of SI and CI Engines using these alternate
fuels.
UNIT V RECENT TRENDS 9
Homogeneous Charge Compression Ignition Engine, Lean Burn Engine, Stratified
Charge Engine, Surface Ignition Engine, Four Valve and Overhead cam Engines,
Electronic Engine Management, Common Rail Direct Injection Diesel Engine, Gasoline
Direct Injection Engine, Data Acquisition System –pressure pick up, charge amplifier PC
for Combustion and Heat release analysis in Engines.
TOTAL: 45 PERIODS
TEXT BOOK:
1. Heinz Heisler, ‘Advanced Engine Technology,” SAE International Publications,
USA,1998
2. Ganesan V..” Internal Combustion Engines” , Third Edition, Tata Mcgraw-Hill ,2007
REFERENCES:
1. John B Heywood,” Internal Combustion Engine Fundamentals”, Tata McGraw-Hill
1988
2. Patterson D.J. and Henein N.A,“Emissions from combustion engines and their
control,” Ann Arbor Science publishers Inc, USA, 1978
3. Gupta H.N, “Fundamentals of Internal Combustion Engines” ,Prentice Hall of India,
2006
4. Ultrich Adler ,” Automotive Electric / Electronic Systems, Published by Robert Bosh
GmbH,1995
SPARK IGNITION ENGINES
Air-fuel Requirement in SI Engines
The spark-ignition automobile engines run on a mixture of gasoline and air. The amount of mixture the
engine can take in depends upon following major factors:
(i) Engine displacement.
(ii) Maximum revolution per minute (rpm) of engine.
(iii) Volumetric efficiency of engine.
There is a direct relationship between an engine’s air flow and it’s fuel requirement. This relationship is
called the air-fuel ratio.
Air-fuel Ratios
The air-fuel ratio is the proportions by weight of air and gasoline mixed by the carburetor as required for
combustion by the engine. This ratio is extremely important for an engine because there are limits to how
rich (with more fuel) or how lean (with less fuel) it can be, and still remain fully combustible for efficient
firing. The mixtures with which the engine can operate range from 8:1 to 18.5:1 i.e. from 8 kg of air/kg of
fuel to 18.5 kg of air/kg of fuel. Richer or leaner air-fuel ratio limit causes the engine to misfire, or simply
refuse to run at all.
Stoichiometric Air-Fuel Ratio
The ideal mixture or ratio at which all the fuels blend with all of the oxygen in the air and be completely
burned is called the stoichiometric ratio, a chemically perfect combination. In theory, an air fuel ratio of
about 14.7:1 i.e. 14.7 kg of air/kg of gasoline produce this ratio, but the exact ratio at which perfect
mixture and complete combustion take place depends on the molecular structure of gasoline, which can
vary somewhat.
Engine Air-fuel Ratios
An automobile SI engine, as indicated above, works with the air-fuel mixture ranging from 8:1 to 18.5:1.
But the ideal ratio would be one that provides both the maximum power and the best economy, while
producing the least emissions. But such a ratio does not exist because the fuel requirements of an engine
vary widely depending upon temperature, load, and speed conditions. The best fuel economy is obtained
with a 15:1 to 16:1 ratio, while maximum power output is achieved with a 12.5:1 to 13.5:1 ratio. A rich
mixture in the order of 11:1 is required for idle heavy load, and high-speed conditions. A lean mixture is
required for normal cruising and light load conditions. Figure 9.36 represents the characteristic curves
showing the effect of mixture ratio on efficiency and fuel consumption.
Practically for complete combustion, through mixing of the fuel in excess air (to a limited extent above
that of the ideal condition) is needed. Lean mixtures are used to obtain best economy through minimum
fuel consumption whereas rich mixtures used to suppress combustion knock and to obtain maximum
power from the engine. However, improper distribution of mixture to each cylinder and
imperfect/incomplete vaporization of fuel in air necessitates the use of rich mixture to obtain maximum
power output. A rich mixture is also required to overcome the effect of dilution of incoming mixture due to
entrapped exhaust gases in the cylinder and of air leakage because of the high vacuum in the manifold,
under idling or no-load condition. Maximum power is desired at full load while best economy is expected
at part throttle conditions. Thus required air fuel ratios result from maximum economy to maximum
power. The carburetor must be able to vary the air-fuel ratio quickly to provide the best possible mixture
for the engine’s requirements at a given moment.
The best air-fuel ratio for one engine may not be the best ratio for another, even when the two engines are
of the same size and design. To accurately determine the best mixture, the engine should be run on a
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dynamometer to measure speed, load and power requirements for all types of driving conditions.
With a slightly rich mixture, the combustion flame travels faster and conversely with a slightly weak
mixture, the flame travel becomes slower. If a very rich mixture is used then some “neat” petrol enters
cylinder, washes away lubricant from cylinder walls and gets past piston to contaminate engine oil. A very
sooty deposit occurs in the combustion chamber. On the other hand, if an engine runs on an excessively
weak mixture, then overheating particularly of such parts as valves, pistons and spark plugs occurs. This
causes detonation and pre-ignition together or separately.
The approximate proportions of air to petrol (by weight) suitable for the different operating conditions are
indicated below:
Starting 9 : 1
Idling 12 : 1
Acceleration 12 : 1
Economy 16: 1
Full power 12 : 1
It makes no difference if an engine is carburetted or fuel injected, the engine still needs the same air-fuel
mixture ratios.
Carburetion
Introduction
Spark-ignition engines normally use volatile liquid fuels. Preparation of fuel-air mixture is done
outside the engine cylinder and formation of a homogeneous mixture is normally not completed in the inlet
manifold. Fuel droplets, which remain in suspension, continue to evaporate and mix with air even during
suction and compression processes. The process of mixture preparation is extremely important for sparkignition
engines. The purpose of carburetion is to provide a combustible mixture of fuel and air in the
required quantity and quality for efficient operation of the engine under all conditions.
Definition of Carburetion
The process of formation of a combustible fuel-air mixture by mixing the proper amount of fuel
with air before admission to engine cylinder is called carburetion and the device which does this job is
called a carburetor.
Requirements of an automotive carburetor
The spark ignition engines fitted to automotive vehicles have to operate under variable speed and
load conditions. These engines present the most difficult and stringent requirements to the carburetors.
They are as follows:-
1. Ease of starting the engine, particularly under low ambient conditions.
2. Ability to give full power quickly after starting the engine.
3. Equally good and smooth engine operation at various loads.
4. Good and quick acceleration of the engine.
5. Developing sufficient power at high engine speeds.
6. Simple and compact in construction.
7. Good fuel economy.
8. Absence of racing of the engine under idling conditions.
9. Ensuring full torque at low speeds.
Factors Affecting Carburetion
Of the various factors, the process of carburetion is influenced by
i. The engine speed
ii. The vaporization characteristics of the fuel
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iii. The temperature of the incoming air and
iv. The design of the carburetor
Principle of Carburetion
Both air and gasoline are drawn through the carburetor and into the engine cylinders by the suction
created by the downward movement of the piston. This suction is due to an increase in the volume of the
cylinder and a consequent decrease in the gas pressure in this chamber. It is the difference in pressure
between the atmosphere and cylinder that causes the air to flow into the chamber. In the carburetor, air
passing into the combustion chamber picks up discharged from a tube. This tube has a fine orifice called
carburetor jet that is exposed to the air path. The rate at which fuel is discharged into the air depends on the
pressure difference or pressure head between the float chamber and the throat of the venturi and on the
area of the outlet of the tube. In order that the fuel drawn from the nozzle may be thoroughly atomized, the
suction effect must be strong and the nozzle outlet comparatively small. In order to produce a strong
suction, the pipe in the carburetor carrying air to the engine is made to have a restriction. At this restriction
called throat due to increase in velocity of flow, a suction effect is created. The restriction is made in the
form of a venturi to minimize throttling losses. The end of the fuel jet is located at the venturi or throat of
the carburetor. The geometry of venturi tube is as shown in Fig.16.6. It has a narrower path at the center so
that the flow area through which the air must pass is considerably reduced. As the same amount of air must
pass through every point in the tube, its velocity will be greatest at the narrowest point. The smaller the
area, the greater will be the velocity of the air, and thereby the suction is proportionately increased
As mentioned earlier, the opening of the fuel discharge jet is usually loped where the suction is maximum.
Normally, this is just below the narrowest section of the venturi tube. The spray of gasoline from the
nozzle and the air entering through the venturi tube are mixed together in this region and a combustible
mixture is formed which passes through the intake manifold into the cylinders. Most of the fuel gets
atomized and simultaneously a small part will be vaporized. Increased air velocity at the throat of the
venturi helps he rate of evaporation of fuel. The difficulty of obtaining a mixture of sufficiently high fuel
vapour-air ratio for efficient starting of the engine and for uniform fuel-air ratio indifferent cylinders (in
case of multi cylinder engine) cannot be fully met by the increased air velocity alone at the venturi throat.
The Simple Carburetor
Carburetors are highly complex. Let us first understand the working principle bf a simple or
elementary carburetor that provides an air fuel mixture for cruising or normal range at a single speed.
Later, other mechanisms to provide for the various special requirements like starting, idling, variable load
and speed operation and acceleration will be included. Figure 3. shows the details of a simple carburetor.
The simple carburetor mainly consists of a float chamber, fuel discharge nozzle and a metering orifice, a
venturi, a throttle valve and a choke. The float and a needle valve system maintain a constant level of
gasoline in the float chamber. If the amount of fuel in the float chamber falls below the designed level, the
float goes down, thereby opening the fuel supply valve and admitting fuel. When the designed level has
been reached, the float closes the fuel supply valve thus stopping additional fuel flow from the supply
system. Float chamber is vented either to the atmosphere or to the” upstream side of the venturi.During
suction stroke air is drawn through the venturi.
As already described, venturi is a tube of decreasing cross-section with a minimum area at the
throat, Venturi tube is also known as the choke tube and is so shaped that it offers minimum resistance to
the air flow. As the air passes through the venturi the velocity increases reaching a maximum at the venturi
throat. Correspondingly, the pressure decreases reaching a minimum. From the float chamber, the fuel is
fed to a discharge jet, the tip of which is located in the throat of the venturi. Because of the differential
pressure between the float chamber and the throat of the venturi, known as carburetor depression, fuel is
discharged into the air stream. The fuel discharge is affected by the size of the discharge jet and it is
chosen to give the required air-fuel ratio. The pressure at the throat at the fully open throttle condition lies
between 4 to 5 cm of Hg, below atmospheric and seldom exceeds8 cm Hg below atmospheric. To avoid
overflow of fuel through the jet, the level of the liquid in the float chamber is maintained at a level slightly
below the tip of the discharge jet. This is called the tip of the nozzle. The difference in the height between
the top of the nozzle and the float chamber level is marked h in Fig.3.
The gasoline engine is quantity governed, which means that when power output is to be varied at a
particular speed, the amount of charge delivered to the cylinder is varied. This is achieved by means of a
throttle valve usually of the butterfly type that is situated after the venturi tube. As the throttle is closed
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less air flows through the venturi tube and less is the quantity of air-fuel mixture delivered to the cylinder
and hence power output is reduced. As the” throttle is opened, more air flows through the choke tube
resulting in increased quantity of mixture being delivered to the engine. This increases the engine power
output. A simple carburetor of the type described above suffers from a fundamental drawback in that it
provides the required A/F ratio only at one throttle position. At the other throttle positions the mixture is
either leaner or richer depending on whether the throttle is opened less or more. As the throttle opening is
varied, the air flow varies and creates a certain pressure differential between the float chamber and the
venturi throat. The same pressure differential regulates the flow of fuel through the nozzle. Therefore, the
velocity of flow of air II and fuel vary in a similar manner. At the same time, the density I of air decrease
as the pressure at the venturi throat decrease with increasing air flow whereas that of the fuel remains
unchanged. This results in a simple carburetor producing a progressively rich mixture with increasing
throttle opening.
The Choke and the Throttle
When the vehicle is kept stationary for a long period during cool winter seasons, may be
overnight, starting becomes more difficult. As already explained, at low cranking speeds and intake
temperatures a very rich mixture is required to initiate combustion. Some times air-fuel ratio as rich as 9:1
is required. The main reason is that very large fraction of the fuel may remain as liquid suspended in air
even in the cylinder. For initiating combustion, fuel-vapour and air in the form of mixture at a ratio that
can sustain combustion is required. It may be noted that at very low temperature vapour fraction of the fuel
is also very small and this forms combustible mixture to initiate combustion. Hence, a very rich mixture
must be supplied. The most popular method of providing such mixture is by the use of choke valve. This is
simple butterfly valve located between the entrance to the carburetor and the venturi throat as shown in
Fig.3.
When the choke is partly closed, large pressure drop occurs at the venturi throat that would
normally result from the quantity of air passing through the venturi throat. The very large depression at the
throat inducts large amount of fuel from the main nozzle and provides a very rich mixture so that the ratio
of the evaporated fuel to air in the cylinder is within the combustible limits. Sometimes, the choke valves
are spring loaded to ensure that large carburetor depression and excessive choking does not persist after the
engine has started, and reached a desired speed. This choke can be made to operate automatically by
means of a thermostat so that the choke is closed when engine is cold and goes out of operation when
engine warms up after starting. The speed and the output of an engine is controlled by the use of the
throttle valve, which is located on the downstream side of the venturi.
The more the throttle is closed the greater is the obstruction to the flow of the mixture placed in the
passage and the less is the quantity of mixture delivered to .the cylinders. The decreased quantity of
mixture gives a less powerful impulse to the pistons and the output of the engine is reduced accordingly.
As the throttle is opened, the output of the engine increases. Opening the throttle usually increases the
speed of the engine. But this is not always the case as the load on the engine is also a factor. For example,
opening the throttle when the motor vehicle is starting to climb a hill may or may not increase the vehicle
speed, depending upon the steepness of the hill and the extent of throttle opening. In short, the throttle is
simply a means to regulate the output of the engine by varying the quantity of charge going into the
cylinder.
Stages of Combustion in SI Engine
In a spark-ignition engine a sufficiently homogeneous mixture of vaporized fuel, air and residual
gases is ignited by a single intense and high temperature spark between the spark plug electrodes (at the
moment of discharge the temperature of electrodes exceeds 10,000°C), leaving behind a thin thread of
flame. From this thin thread combustion spreads to the envelop of mixture immediately surrounding it at a
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rate which depends primarily upon the temperature of the flame front itself and to a secondary degree,
upon both the temperature and the density of the surrounding envelope. In this manner there grows up,
gradually at first, a small hollow nucleus of flame, much in the manner of a soap bubble. If the contents of
the cylinder were at rest, this flame bubble would expand with steadily increasing speed until extended
throughout the whole mass. In the actual engine cylinder, however, the mixture is not at rest. It is, in fact,
in a highly turbulent condition the turbulence breaks the filament of flame into a ragged front, thus
presenting a far greater surface area from which heat is radiated; hence its advance is speeded up
enormously. The rate at which the flame front travels is dependent primarily on the degree of turbulence,
but its general direction of/movement, that of radiating outward from the ignition point, is not much
affected. According to Ricardo the combustion can be imagined as if developing in two stages, one the
growth and development of a semi propagating nucleus of flame called ignition lag or preparation phase,
and the other, the spread of the flame throughout the combustion chamber