COMBUSTION IN S.I & C.I ENGINES

COMBUSTION IN SI ENGINES

Combustion may be defined as a relatively rapid chemical combination of hydrogen and carbon in fuel with oxygen in air resulting in liberation of energy in the form of heat.Following conditions are necessary for combustion to take place1. The presence of combustible mixture2. Some means to initiate mixture3. Stabilization and propagation of flame in combustion chamber In SI Engines, carburetor supplies a combustible mixture of petrol and air and spark plug initiates combustion.

Theoretical P-Ѳ diagram

THREE STAGE OF COMBUSTION1. Ignition lag stage(ab)2. Flame propagation stage(bc)3. After burning stage(cd) ab is called as ignition lag or preparation

face in which the growth & development of flame takes place. This is chemical process.

bc is concerned with spreading of flame through out the combustion chamber.

cd is after burning where certain local pockets of air fuel particles undergone combustion.

FACTORS AFFCTING THE FLAME PROPAGATION

Rate of flame propagation affects the combustion process in SI engines. Higher combustion efficiency and fuel economy can be achieved by higher flame propagation velocities.

The factors which affect the flame propagations are 1. Air fuel ratio 2. Compression ratio 3. Load on engine 4. Turbulence and engine speed

PHENOMENON OF KNOCKING IN SI ENGINE

AUTO IGINITION A mixture of fuel and air can react spontaneously

and produce heat by chemical reaction in the

absence of flame to initiate the combustion or

self-ignition. This type of self-ignition in the

absence of flame is known as Auto-Ignition. The

temperature at which the self-ignition takes place

is known as self-igniting temperature.

PRE -IGINITION Pre-ignition is the ignition of the homogeneous

mixture of charge as it comes in contact with hot surfaces, in the absence of spark .

Auto ignition may overheat the spark plug and exhaust valve and it remains so hot that its temperature is sufficient to ignite the charge in next cycle during the compression stroke before spark occurs and this causes the pre-ignition of the charge.

Pre-ignition is initiated by some overheated projecting part such as the sparking plug electrodes, exhaust valve head, metal corners in the combustion chamber, and carbon deposits.

EFFECT OF DETONATION Noise and Roughness. Vibration Mechanical Damage. Decrease in power output and efficiency. Pre-ignition.

Factors affecting Detonation or Knocking in SI engine 1.Compression ratio: The pressure and temperature at the

end of compression increases with increase in compression ratio. This in turn increase the maximum pressure during the combustion and creates a tendency to knock

2.Supercharging: It is the process of allowing the charge at a

pressure higher than atmospheric pressure. Because of supercharging the density and temperature increases there by knocking tendency increases.

Super charging is generally done on CI engines

3.Increasing inlet temperature: It results in increase in knocking.

4.Increase in load: increase in load result in increase in

temperature of the cylinder, there by rising the temperature and hence knocking tendency increasing at high loads.

5.Advancing the spark: when the spark is advance the burning gas is

compressed by rising piston and therefore the increase in temperature will be much more and hence the knocking tendency increases.

To avoid detonation spark must be retarded.

6.Flame travel distance: Flame travel distance should be small to

avoid detonation.7.Spark plug location: spark plug which is located centrally has

minimum tendency for knocking because the flame travel distance decreases.

8.Engine size: For larger engine size the flame travel

distance will be longer and hence knocking chances are high, because of this reason SI engine sizes are generally limited.

9.Location of exhaust valve: There are two hot spot zone in SI engines. Ie

spark plug & exhaust valve. If the exhaust valve is very far from the spark plug the temperature around exhaust valve region will be high. If the spark plug is very far it will take much time and during this time the charge near exhaust valve would autoignitied due to high temperature and hence exhaust valve must be located as close to spark plug as possible. So the flame travel distance is minimized.

10. Turbulence: With increase in turbulence the flame speed

increases and hence the chances of detonation would decreases.

11.Engine speed: Increase in engine speed increases

turbulence & hence knocking tendency decreases.

12. Octane rating of fuel: For better antiknock properties

octane number must be high.

COMBUSTION IN CI ENGINE

STAGES OF COMBUSTION IN CI ENGINE• Ignition Delay period /Pre-flame

combustioni. Physical delayii. Chemical delay

• Uncontrolled combustion• Controlled combustion• After burning

1.Ignition Delay period The fuel does not ignite

immediately upon injection into the combustion chamber. There is a definite period of inactivity between the time of injection and the actual burning this period is known as the ignition delay period.

The ignition delay period can be divided into two parts, the physical delay and the chemical delay.

When the fuel is injected into the combustion chamber the fuel mix with air. The time spend in this process is known as physical delay.

Certain pre flame reactions starts and after some time fuel burns automatically and the time consuming during this process is known as chemical delay.

2.Period of Rapid Combustion During total delay period more fuel droplet

come from the injector into the combustion chamber. This group of fuel droplets burn together and this produces uncontrolled combustion is known as knocking and this occurs at the end of delay period or the beginning of combustion.

The rate of heat-release is maximum during this period. This is also known as uncontrolled combustion phase, because it is difficult to control the amount of burning/ injection during the process of burning.

3.Period of Controlled Combustion The rapid combustion period is

followed by the third stage, the controlled combustion. The temperature and pressure in the second stage are so high that fuel droplets injected burn almost as they enter and any further pressure rise can be controlled by injection rate. The period of controlled combustion is assumed to end at maximum cycle temperature.

4.Period of After-Burning Combustion does not stop

with the completion of the injection process. The unburnt and partially burnt fuel particles left in the combustion chamber start burning as soon as they come into contact with the oxygen. This process continues for a certain duration called the after-burning period. This burning may continue in expansion stroke up to 70 to 80% of crank travel from TDC.

Parameters controlling abnormal combustion in CI enginesAbnormal combustion means knocking.1.Compression ratio: Greater the compression the

temperature in the cylinder are higher and hence due to higher molecular activity the ignition delay will be less and the chances of abnormal combustion are less at higher compression ratios.

2.cylinder wall temperature: If the cylinder wall temperatures are high

the delay period will be less and hence knocking tendency will be less.

3.Delay period: The delay period must be small. If the delay

period is large more fuel will be accumulated in the cylinder and may burn suddenly.

4.Inlet temperature: For smaller delay period the inlet

temperature must be high.

5.Self ignition temperature: Self ignition temperature must be low, so

that less time is required to reach that temperature. Therefore only small amount of fuel burning in a controlled manner.

6.Ignition advance angle: If the ignition angle is advanced the fuel

injection starts at low pressure and low temperature and due to this low temperature the delay period will be large and hence more fuel is accumulated in the cylinder and may burn in an uncontrolled manner.

7. engine speed: The fuel injector is geared to

the engine and hence the injection remains constant interms of crank angle. If the speed is increases the duration of injection time decreases and hence at higher speed more fuel is accumulated in less time and hence in CI engines the chances of abnormal combustion are high at high speeds.

wt = Ѳ = constant ↑w X t↓ = constant

8.Using a better fuel: Higher CN fuel has lower delay

period and reduces knocking tendency.9.Knock reducing fuel injector : This type of injector avoid the sudden

increase in pressure inside the combustion chamber because of accumulated fuel. This can be done by arranging the injector so that only small amount of fuel is injected first. This can be achieved by using two or more injectors arranging in out of phase.

COMPARISON OF KNOCKING IN SI AND CI ENGINES

SI NO FACTORS AFFECTING KNOCKING

SI ENGINE CI ENGINE

1 SIT HIGH LOW

2 DELAY PERIOD LONG SHORT

3 CR LOW HIGH

4 INLET TEMPERATURE LOW HIGH

5 INLET PRESSURE LOW HIGH

6 SPEED HIGH LOW

7 COMBUSTION CHAMBER WALL TEMP.

LOW HIGH

1.In spark ignition engines, auto ignition of end gas away from the spark plug, most likely near the end of combustion causes knocking. But in compression engines the auto ignition of charge causing knocking is at the start of combustion.

2.In order to avoid knocking in SI engine, it is necessary to prevent auto ignition of the end gas to take place at all. In CI engine, the earliest auto –ignition is necessary to avoid knocking

3.The knocking in SI engine takes place in homogeneous mixture, therefore , the rate of pressure rise and maximum pressure is considerably high. In case of CI engine, the mixture is not homogenous and hence the rate of pressure is lower than in SI engine.

4.In CI engine only air is compressed, therefore there is no question of Pre-ignition in CI engines as in SI engines.

5.SI fuels should have long delay period to avoid knocking. CI fuels should have short delay period to avoid knocking.

CETANE NUMBER: The cetane number of a diesel fuel

is a measure of its ignition quality.OCTANE NUMBER: The octane number of a petrol fuel

is a measure of its ignition quality.

Define & explain octane number & cetane number. (10 Marks)

Explain pollutants from SI & CI engines. (10

Marks) Explain different types of combustion chambers in

SI engines with neat sketches. (10 Marks) Explain different types of combustion chambers in

CI engines with neat sketches. (10

Marks)