Combustion in SI Engine1

7/28/2019 Combustion in SI Engine1

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1 09/08/2010


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FLAME Gas rendered luminous start from point

of ignition to end of combustion chamber.

Self Ignition Temperature Temperature at whichfuel will ignite itself without flame.

Combustion rapid and high temperature oxidationof fuel with liberation of heat energy.

Spatial velocity velocity of flame depend onshape and size of combustion chamber.

Concepts and definition


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It is essential to burn the fuel completely for the economical

working of the engine

Therefore the mixture supplied to the engine should possessrequired A:F ratio otherwise, combustion cannot be initiatedor if initiated it cannot be sustained.

Combustion in S. I. Engines


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There is a ignition limit for any fuel to start the combustion andsustain it till the complete fuel burns by the flame generated with

spark plug.

In addition to this, the temperature of the mixture to initiate theignition is equally important.

It is also known that the flame will propagate if the temperature ofthe burnt gases exceeds 1500 K for S.I. engine fuels.

The upper and lower limit of A:F ratio for ignition depends upon the

temperature of a particular fuel.

Combustionin S. I. Engines


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Ignition Lag

Flame propagation

After burning

Stages of Combustion in S. I. Engines


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Motoring curve - Engine is not under firing.

ABCD - Actual combustion curve.

A- point of spark supply.

Spark Advance - Crank angle before TDC atthe instant of spark.

Ignition Lag- Time between instant of sparkand tiny flame reappear (AB).

Ignition Lag


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High Temperature & Pressure molecules nearer

reduces ignition lag.

High compression ratio reduces ignition lag.

High self ignition temp high ignition lag.

High speed ignition advance.

Residual gas increase ignition lag.

Effect of engine variable


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High compression ratio Increase flame speed.

Small size combustion chamber reduces time forpropagation.

Turbulence Intensifies rate of heat transfer &increase flame speed.

A/F Ratio 10% more than stoichiometric mixture.

Effect of engine variable


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Burning continues due to left over fuel.

Flame velocity decreases

After burning


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Time of flame travel

(A B) Reaction rate flame get into unburned gas.

Transposition rate physical movement of flame due

to pressure differential. Low transposition rate & low turbulence.

Quiescent zone (B-C) More turbulence.

Burnt more mass mixture.

(B-C) Low turbulence.


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residual gas - diluent it lowers the laminar burning velocity.

Because of lower burning velocity overall burn angle increasesso need to increase spark advance.

In modern engines the ECU sets the spark advance based onengine data such as: throttle position, intake manifold pressureand engine speed.

Spark Timing


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Abnormal combustion

A combustion process in which a flame front may

be started by hot combustion-chamber surfaceseither prior to or after spark ignition, or a process inwhich some part or all of the charge may beconsumed at extremely high rates.

Abnormal combustion


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Knock is the engine sound that results from

spontaneous ignition of the unburned fuel-airmixture ahead of the flame (the end gas).

Surface ignition is the ignition of the fuel-air

mixture by any hot surface, other than the sparkdischarge, prior to arrival of the flame

Knock and Surface Ignition


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A knock which is recurrent and repeatable in terms

of audibility. It is controllable by the spark advance;

advancing the spark increases the knock intensityand retarding the spark reduces the intensity.


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Abnormal Combustion in SI Engine

nock is the term used to describe a pinging noise emitted from a SI enginendergoing abnormal combustion.

he noise is generated by shock waves produced in the cylinder whennburned gas autoignites.


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Engine parameters that effect occurrence of knock are:

Compression ratioAt high compression ratios - high pressure

and temperature which promotes auto ignition.

Engine speedAt low engine speeds the flame velocity is slow

and thus the burn time is long, this results in more time for auto

ignition.

SuperchargingIncreases knock.

Air Intake TemperatureIncreases knock.

Sparktiming

Advance the spark. Temperature of walls and cooling watercooler walls less tend

to knock.

Electrode gap of spark plugreduces with increases in

compression ratio.

Knock


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High compression ratio high efficiency but also

tend to knock.

Maximum compression ratio limited by knock

High useful compression ratio


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Fuel Additives

The most effective antiknock agents are lead alkyls;

(i) Tetraethyl lead (TEL), (C2H5)4Pb(ii) Tetramethyl lead (TML), (CH3)4PbA manganese antiknock compound known as MMT was introduced to

supplement TEL

Low-lead and unleaded gasoline were introduced over toxicologicalconcerns with lead alkyls (TEL contains 64% by weight lead).

Alcohols such as ethanol and methanol have high knock resistance.

Another alcohol methyl tertiary butyl ether (MTBE) has been added togasoline to increase octane number.


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Factors which reduce Knock in S.I. engine

S.No. Factors affecting Knock S.I. engines

1. Self ignition temperature High

2. Delay period of fuel Long

3. Compression Ratio Low

4. Inlet temperature Low

5. Inlet Pressure Low

6. Speed High

7. Cylinder size Small

8. Combustion chamber wall Temp Low


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Noise: human ear is remarkably sensitive

Mechanical vibration: accelerometer mounted on

the head or block

Cylinder pressure fluctuation: pressure transducer

in cylinder head

Light emission fluctuation: optical probe incylinder head

Knock Detection


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Effect of knock

Mechanical failure erosion of piston crown, pitting of cylinderhead and valves.

Local melting of piston and ring high rate of heat transfer dueto high temperature of gases.

Overbeating of spark plug violent gas vibration due to hotgases flow in and out of spark plug cavity change in electrical

characteristic and render it useless.

Noise and roughness

Power and efficiency

Pre-ignition hot spot in combustion chamber and spark plug.


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Engine Damage From Severe Knock

mage to the engine is caused by a combination of high temperature andh pressure.

Piston Piston crown

Cylinder head gasket Aluminum cylinder head

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Combustion in SI Engine1