Advanced Ic engines unit 1

ME2041 Advanced Internal Combustion Engines
Department of Mechanical Engineering, St. Joseph’s College of EngineeringUnit I
Syllabus:
• Air-fuel ratio requirements ,
• Design of carburettor –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 I SPARK IGNITION ENGINES

Unit II
Syllabus:
• 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 II COMPRESSION IGNITION ENGINES
Department of Mechanical Engineering, St. Joseph’s College of Engineering

Unit III
Syllabus:
• 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 III ENGINE EXHAUST EMISSION CONTROL

Unit IV
Syllabus:
• 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 IV ALTERNATE FUELS

Unit V
Syllabus:
• 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
Unit V RECENT TRENDS

Unit I
• Carburetion
The process of formation of combustible air-fuel mixture,
by mixing correct amount of fuel and air in a device called
carburetor, before it enters the engine cylinder.
• Factors Affecting Carburetion
1. Carburetor Design
has influence on distribution of air-fuel mixture to cylinders.
2. Ambient Air condition
Ambient pressure and temperature influence the efficiency of
carburetion. Higher ambient temperature increases the
vaporization rate of fuel forming a homogeneous mixture.
3. Fuel Characteristics
Evaporation characteristics (indicated by distillation curve) is critical
for carburetion; presence of volatile HC also is important for quick
evaporation

Unit I
4. Engine Speed and Load
• At higher engine speed, the carburetion time is less causing strain on
carburetor to deliver uniform mixture in a short time; thus provision of
venturi has to be such that the carburetion is done efficiently at higher
pressure drops
• Higher loads will demand richer mixture and lower load leaner mixtures.
• Types of Air-Fuel Mixtures
1. Chemically Correct Mixture
Stoichiometric or balanced chemical mixture in which air is
provided to completely burn the fuel; the excess air factor is unity
2. Rich Mixture
Fuel is in excess of what is required to burn the fuel completely.
The excess air factor is less than unity.
3. Lean Mixture
Air is in excess of what is required to burn the fuel completely. The
excess air factor is greater than unity.

Unit I
• Range of Air-Fuel Ratio in SI Engines
9:1 (rich) to 19:1(lean) ; The stoichiometric value for
gasoline is 14:1, The SI engine will not run for too rich or
too lean mixtures.
• Mixture Requirements at Different Engine
Conditions
The air fuel ratio affects the power output and brake
specific fuel consumption of the engine as shown in the
Figure1.
Power
Output
(kW)
BSFC
(kg/kWh)
Power
BSFC
A/F ratio

Unit I
• Mixture Requirements at Different Engine Conditions
(Contd.)
• The mixture corresponding to maximum output on the
curve is called best power A/F mixture, which is richer
than the stoichiometric mixture.
• The mixture corresponding to maximum BSFC on the
curve is called best economy mixture, which is leaner
than the stoichiometric mixture.
• The actual A/F ratio requirement for an automative
carburetor falls in 3 ranges:
 Idling (rich)
 Cruising (lean)
 High Power (rich)

Unit I
(Contd.)
Idling
A/F
Ratio
Throttle
Opening
1
2
3
4
0 50%
100%
Cruisin
g
Power
Figure 2. A/F Ratio Vs Throttle opening

Unit I
(Contd.)
Idling Range (1-2)
• During idling, engine operates at no load and closed throttle.
• The engine requires rich mixture for starting at idling.
• Rich mixture is required to compensate for the charge dilution due to
exhaust gases from the combustion chamber.
• Also, the amount of fresh charge admitted is less due to smaller throttle
opening.
• Exhaust gas dilution prevents efficient combustion by reducing the contact
between the fuel and air particles.
• Rich mixture improves the contact of fuel and air by providing efficient
combustion at idling conditions.
• As the throttle is opened further, the exhaust gas dilution reduces and the
mixture requirement shifts to the leaner side.

Unit I
(Contd.)
Cruising Range (2-3)
• Focus is on fuel economy.
• No exhaust gas dilution.
• Carburetor has to give best economy mixture i.e.. Lean mixture.
High Power Range (3-4)
• As high power is required, additional fuel has to be supplied to achieve
rich mixture in this range.
• Rich mixture also prevents overheating by reducing the flame temperature
and cylinder temperature.

Unit I
• Principle of Operation of Simple Carburettor

Unit I Department of Mechanical Engineering, St. Joseph’s College of Engineering
• The carburettor works on Bernoulli's principle: the faster air moves, the
lower its static pressure, and the higher its dynamic pressure.
• The throttle (accelerator) linkage does not directly control the flow of liquid
fuel. Instead, it actuates carburettor mechanisms which meter the flow of
air being pulled into the engine. The speed of this flow, and therefore its
pressure, determines the amount of fuel drawn into the airstream.
• A simple carburetor consists of a float chamber, fuel discharge nozzle, a
metering orifice, a venturi a throttle valve and choke.
• The float and needle valve maintain the fuel level
• Fuel strainer is used to trap debris from the fuel and prevent choking of
the fuel nozzle. It is removed periodically for cleaning.
• During suction stroke air is drawn through the venturi.
• Venturi accelerates the air causing a pressure drop.

• This pressure drop provides vacuum necessary to meter the air-fuel
mixture to the engine manifold.
• Fuel is fed to the fuel discharge jet, the tip of which is located at the throat
of the venturi
• Pressure drop is proportional to the throttle opening or load on the engine.
• Throttle valve achieves governing of SI engine by varying the A/F ratio. It
is a butterfly valve located after the venturi tube. When the load is less, the
throttle is in near closed position and if the load is high throttle is fully
opened.
• The choke valve is located between the entrance and venturi throat. It is
also of butterfly type. When choke is partly closed, a large pressure drop
occurs at the venturi throat, which provides a rich mixture by induction of
large amount of fuel as required during idling or high load conditions.
Choke valves are spring loaded to prevent excessive choking and are
sometimes automatically controlled by thermostat.
• For providing rich mixture during idling, an idling adjustment is provided. It
has an idling passage and idling port.

• The system operates at starting and shuts off when 20% throttle opening
is reached.
• Normal venturi depression is not sufficient to provide rich mixture due to
lower throttle opening. But this low pressure causes fuel rice in idling
passage and it is discharged through idling port downstream of the throttle
valve.
• The idling air bleed sucks some air for mixing with the idling fuel and
vaporizes the mixture. The additional fuel-air supply makes the mixture
rich for idling.
• Simple carburettor has the drawback of providing rich mixture with
increasing throttle opening.

• Compensating systems in Carburettors
• For part load conditions, the carburettor must supply economic air-fuel
ratio mixture. The main metering system will not satisfy this requirement.
The following compensating systems are used to achieve this:
• Air Bleed Jet
• Compensating Jet
• Emulsion Tube
• Back Suction Control Mechanism
• Auxiliary Air Valve
• Auxiliary Air Port
• Altitude Compensating Device

Air Bleed Jet
• It contains an air bleed to the main
nozzle.
• Air flow through the bleed passage is
restricted by orifice.
• When engine is not operating the bleed
passage is filled with fuel.
• When the engine starts the fuel from the
bleed passage is displaced by air flow
from the orifice.
• The air and fuel form an emulsion at the
tip of the bleed passage.
• This causes faster delivery of fuel due to
low viscosity and fuel discharged rises.
• Thus uniform mixture ratio is supplied.

Compensating Jet
• The purpose of this is to make the
mixture leaner as the throttle opens
progressively.
• An additional jet called compensating jet
is provided with the main jet.
• This jet is also connected to the fuel well
and the fuel is metered through
compensating orifice.
• As the throttle opening increases the
main jet makes the mixture richer by
adding more fuel.
• The compensating jet makes the
mixture leaner proportionately. The total
mixture will make A/F ratio constant.
• When the main jet is lean,
compensating jet is rich.

Emulsion Tube
• It is also known as submerged jet
device.
• Here, the main metering jet is kept at a
level 25 mm below the fuel level in float
chamber.
• The jet is called submerged jet. The jet
is placed in a well that has holes
exposed to atmosphere.
• When the throttle opening increases, the
holes in the well are uncovered causing
additional fuel and air to enter the air-
fuel stream, causing the faster A/F
mixture delivery during part load
operation.

Back Suction Control Mechanism
• In this device, the top of the fuel
chamber is connected to air entry by
means of a large vent line fitted with a
control valve.
• The second line connects the fuel float
chamber to venturi throat via a metering
orifice.
• When the control valve is opened, the
pressure in float chamber is p1 and the
throat pressure is p2 which is lower than
p1. This causes the fuel to flow. When
the valve is closed, there is no
difference in pressure and hence no fuel
flow.
• Thus the control valve achieves the
desired air fuel ratio during part load
operation.

Auxiliary Air Valve
• When the engine is not in operation, the
pressure p1 acting on the valve is
ambient. The pressure p2 acting at the
venturi is negative (vacuum). This
pressure differential lifts the auxiliary
valve against the spring tensile force.
• Additional air is thus infused in the air-
fuel mixture preventing rich mixture
during part load operation.

Auxiliary Air Port
• If the butterfly valve is opened,
additional air passes through this port,
reducing air flow through venturi. Thus
pressure differential is comparatively
smaller. Thus fuel drawn is reduced to
compensate for loss in density of air at
high altitudes.

Altitude Compensation Device
• This was used in high altitude car driving and for aircrafts.
• At high altitudes, air density decreases and hence engine power
output is affected.
• A/F ratio is affected at high altitudes as carburettors are designed to
operate on sea level.
• To compensate for the change in air density, fuel flow has to be
reduced from the calibrated value at sea level.
• A mixture control system comprising a needle valve, which restricts
fuel flow in proportion to altitude change acts as an altitude
compensating device.

• Calculation of A/F ratio for a Simple Carburettor
• Let be the difference in height between the tip of the nozzle and fuel
level in the float chamber
• 21,CC
21, pp

- Pressures at inlet and exit
- Air density
- Air velocities at inlet and exit
Z
Applying Bernoulli‟s Equation across the venturi,

2
2
21
2
1
22
pCpC

As ,21 CC 

2
2
21
2
pCp


• Calculation of A/F ratio for a Simple Carburettor

p
C


2
2
Mass flow rate of air through the venturi,
;ACCm da  ;
2


p
ACm da

 pACm da  2
Similarly Mass flow rate of fuel,
;fffdf CACm f
 )(2 ZgpACm fffdf f
 
Due to the
difference in level
between tip of jet
and fuel level in
chamber
A/F ratio is,
)()(2
2
Zgp
p
A
A
C
C
Zgp
p
A
A
C
C
m
m
fffd
d
fffd
d
f
a
ff










Where , A- area of venturi, Af – Area of fuel jet, f – density of fuel

• Combustion in SI Engines
• Combustion is the process of oxidation of fuel resulting into the
release of energy equivalent to calorific value of fuel. Energy released
in combustion is in the form of heat.
• Combustion process in spark ignition engine has requirement of the
• mixture of fuel and air in right proportion
• mechanism for initiation of combustion process and
• stabilization and propagation of flame for complete burning
• For complete combustion of every fuel there is chemically correct fuel-
air ratio also called stoichiometric fuel-air ratio.
• This fuel air ratio may be rich or lean depending upon the proportion of
fuel and air present in mixture. In SI engine this fuel air ratio generally
varies between 1 : 7 to 1 : 30 with lean mixture at 1 : 30 and rich mixture
at 1 : 7.
• Stoichiometric fuel-air ratio is around 1 : 14 to 1 : 15 for hydrocarbon
fuel. The extreme values of fuel-air ratio permissible in SI engine on
both rich and lean ends put limits as „lower ignition limit’ and ‘upper
ignition limit’.

• Combustion in SI Engines
• Varying fuel-air ratio is required in SI engine due to varying loads on
engine between no load to full load on engine. The ratio of actual fuel-
air ratio to stoichiometic fuel-air ratio is given by „equivalence ratio‟ or
„relative fuel-air ratio‟.
• Appropriate fuel-air ratio is maintained in SI engines through
„carburettor‟ (the fuel metering system).

• Stages of Combustion in SI Engines

Combustion in SI engine may be described to be occurring in following
significant phase:
(i) preparation phase
• After compression of fuel-air mixture in cylinder the high temperature
spark is delivered by spark plug in the compressed fuel-air mixture.
Temperature at the tip of spark plug electrode may go even more than
10,000ºC at the time of release of spark.
• Sparkles released have sufficiently high temperature to initiate the
combustion of fuel. For complete combustion of fuel mere initiation of
combustion does not serve the purpose instead a sustainable combustion
process is required.
• After setting up of combustion, a sustainable flame front or flame nuclei is
needed so that it proceeds across the combustion space to ensure
complete combustion. Thus, this phase in which spark is first released
followed by setting up of sustainable flame front is called “preparation
phase” and may consume around 10º of crank angle rotation.

• Crank angle rotation consumed in “preparation phase” depends upon the
speed of engine, constructional feature of cylinder, piston, location of spark
plug, strength of spark, characteristics of fuel, fuel-air ratio etc.
• Preparation phase is shown to occur from „a’ to ‘b’ with small or negligible
pressure rise as initially rate of burning is very small.
(ii) Flame Propagation Phase
• After sustainable combustion flame is set up, then the flame nuclei get
scattered due to excessive turbulence in combustion space causing
pressure to rise from „b’ to ‘c’.
• This phase of combustion depends upon the turbulence inside cylinder,
strength of combustion nuclei, fuel-air ratio, strength of spark, cylinder
geometry, fuel properties etc.
• This phase of combustion is called as “flame propagation phase” and is
accompanied by the excessive pressure rise. Flame propagation phase
should also be as small as possible.

(iii) After Burning Phase
• After the maximum amount of fuel-air mixture is burnt, the residual gets
burnt after the piston has moved across the TDC.
• This last phase is termed as “after burning phase” and occurs during the
expansion stroke.
• Hence, it can be summarised that the complete combustion in SI engine
occurs in three distinct zones i.e. preparation phase, flame propagation
phase and after burning phase.
• In order to have complete combustion in smallest possible time the flame
propagation phase and preparation phase should be shortened.
• Out of total distance travelled in combustion space in first phase i.e.
Preparation phase about 10% of combustion space length is covered in
about 20–30% of total time for combustion.

• Flame propagation phase is spread in about 80% of combustion space
length and is covered in 60–70% of total time of combustion.
• „After burning‟ occurs in less than 10% of combustion space in less than
10% of total combustion time.
• Abnormal Combustion
• Combustion may also sometimes occur abnormally. “Abnormal combustion”
is said to occur when combustion begins inside the cylinder on its‟ own
before the stipulated time for it.
• This abnormal combustion may be due to pre-ignition (i.e. ignition of fuel
even before spark plug ignites it) or auto-ignition (i.e. Ignition of fuel due to
hot spots in the combustion space like valve seats, spark plug) and results
in uncontrolled pressure rise.
• Abnormal combustion is also termed as detonation or knocking and can be
felt by jerky operation of engine, excessive noise, reduced power output etc

• Factors affecting knock
• Fuel
A „low self ignition temperature‟ fuel promotes knock.
• Induction pressure
Increase of pressure decreases SIT and increases induction time; tendency of knock
increases. Eg. At full throttle knock tends to occur more.
• Engine Speed
Low engine speed will give low turbulence and low flame velocity and hence knock
tendency is more.
• Ignition Timing
Advancing ignition timing increases peak pressure and promotes knock.
• Compression Ratio
High compression ratio increases cylinder pressures and increases the tendency for
knock.
• Combustion Chamber Design
Poor design results in long flame path, low turbulence and insufficient cooling all of
which increase knock tendency.
• Cylinder Cooling
Poor cylinder cooling increases the temperature and hence the chances of knock
temperature‟ fuel promotes knock.

• Combustion Chambers

Unit I
THERMODYNAMIC ANALYSIS OF SI ENGINE COMBUSTION
Because combustion occurs through a flame propagation process, the
changes in
state and the motion of the unburned and burned gas are much more
complex
than the ideal cycle analysis.
The gas pressure, temperature and density changes as a result of
changes in volume due to piston motion.
During combustion, the cylinder pressure increases due to the release of
the fuel's
chemical energy.
As each element of fuel-air mixture burns, its density decreases by about
a factor of four.
This combustion-produced gas expansion compresses the unburned
mixture ahead of the flame and displaces it toward the combustion
chamber walls.
The combustion-produced gas expansion also compresses those parts of
the charge which have already burned, and displaces them back toward
• Burned and Unburned Mixture States

Unit I
During the combustion process, the unburned gas elements move away
from the spark plug; following combustion, individual gas elements move
back toward the spark plug.
Further, elements of the unburned mixture which burn at different times
have different pressures and temperatures just prior to combustion, and
therefore end up at different states after combustion.
The thermodynamic state and composition of the burned gas is,
therefore, non-uniform.
A first law analysis of the spark-ignition engine combustion process
enables us to
quantify these gas states.
Work transfer occurs between the cylinder gases and the piston (to the
gas before TC; to the piston after TC).

Unit I
Heat transfer occurs to the chamber walls, primarily from the burned
gases.
At the temperatures and pressures typical of spark-ignition engines it is a
reasonable approximation to assume that the volume of the reaction zone
where combustion is actually occurring is a negligible fraction of the
chamber volume even though the thickness of-the turbulent flame may
not be negligible compared with the chamber dimensions.
With normal engine operation, at any point in time or crank angle, the
pressure throughout the cylinder is close to uniform.
The conditions in the burned and unburned gas are then determined by
conservation of mass :

Unit I
The conservation of energy:
where V is the cylinder volume, m is the mass of the cylinder contents, v
is the specific volume, xb is the mass fraction burned, Uo is the
internal energy of the cylinder contents at some reference point 0, u is
the specific internal energy, W is the work done on the piston, and Q is
the heat transfer to the walls. The subscripts u and b denote unburned
and burned gas properties, respectively.
The work and heat transfers are:
Where is the instantaneous heat-transfer rate to the chamber walls.

Unit I
Useful results can be obtained by assuming that the burned and
unburned gases are different ideal gases, each with constant specific
heats. i.e.
Combining these eqns.

Unit I
The above equations may be solved to obtain
If we now assume the unburned gas is initially uniform and undergoes
isentropic compression, then

Advanced Ic engines unit 1

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