1. Combustion in S.I. Engine

2. 8.1 introduction
Combustion may be defined as a relatively rapid chemical combination of hydrogen and carbon in the fuel with the oxygen in the air resulting in liberation of energy in the form of heat. Combustion is very complicated phenomenon and has been subject of intensive research for many years. The conditions necessary for combustion are:
1. The presence of a combustible mixture.
2. Some means of initiating combustion, and
3. Stabilization and propagation of flame in the combustion chamber.
In S.I. engines the combustible mixture is generally supplied by the carburetor and the combustion is initiated by an electric spark plug
8.2 STAGES of combustion in S.I. Engine
The combustion process of S.I. engines can be divided into three board regions:
Ignition and flame development: is generally considering the consumption of the first (5-10) % of the fuel and air mixture. In this period, ignition occurs and the combustion process started, but very little pressure raise and no useful work produced. This region occurs (10-30) before TDC.
Flame propagation: all useful work produced in the engine cycle is the result of flame propagation period of the combustion process. This period burns (80-90) % of air and fuel mixture. In this period, pressure of cylinder is greatly increased, and this provides the force to produce work in expansion stroke.
Flame termination: during this time, pressure quickly decreased and combustion stop, this period burned (5-10) % of air and fuel mixture. This region occurs (15-20) after TDC.

3. 8.3 abnormal combustion
In normal combustion the flame started by the spark travel across the combustion chamber in a fairly even way. Under certain operating conditions “abnormal combustion” may take place which is detrimental to life and performance of the engine. There are a variety of ways in which abnormal combustion can occur. The important abnormal combustions are detonations or knock, pre-ignition, run-on, and rumble. Knock is the most important because it puts a limit on the compression ratio at which an engine can be operated, which in turn, controls the efficiency and to some extent, power output.
8.3.1 Detonation (knock)
In Fig.(8.1) a normal flame front travels across the combustion chamber from A toward D. as the flame front advances it compresses the unburned charge raising its temperature. The temperature is also increased by radiation from the advancing flame front and due to reaction taking place in the unburned mixture itself. If this unburned charge does not reach its critical temperature for auto-ignition, it will not auto-ignite and the flame front will move across the unburned charge to the farthest point of the chamber D. in the up normal combustion called detonation or knocking, the end gas auto–ignites before the flame front reaches it. In order to auto-ignite, the last unburned portion of the charge must reach above the certain critical temperature. To distinguish this violation we must have:
1. High pitch (knocking) sound is heard.
2. Heat transfer to the walls increased causing decrease in the power.
3. The surface of the engine is reduced due to the impingement of the shock wave at the chamber wall.
4. Overheating of the engine.
5. Deformation of the P- diagram.

4. Important factors which effect on knock
1. Compression ratio: if is increased P & T of the cylinder is increased which yields knock is increased.
2. Engine speed: if (rpm0 of the engine is increased required time of the flame termination is decreased which causes knock to decrease.
3. Ignition timing: if ignition timing is increased P & T of the end gas is increased which yields increased in knock, but high advancing would cause decreased in the knock.
4. Throttle setting: if the opening of the throttle setting is increased P & T of end gas is increased which causes knock to increase.
5. Inlet air temperature: if inlet air temperature is increased P & T of end gas is increased which causes knock to increase.
6. Cooling wall temperature: if cooling wall temperature is increased T of the cylinder increased which causes knock to increase.
7. Location of spark plug and exhaust valve: the spark plug should be located to allow for the minimum distance for the normal flame travels. The exhaust valve should be located closed to the spark plug so that not in the end gas region.
8. Octane number: it’s numerical scale generated by comparing the self ignition temperature of the fuel to that of standard fuel in specific engine test at specific operating condition. To avoid knock use low octane number with low compression ration and high octane number with high compression ratio.
To reduce knock:
1. Higher speed rpm.
2. Good state of cooling system to reduced inlet temperature.
3. Use petrol with a suitable octane number.
4. Reduced inlet pressure.
5. Reduced spark advance (ignition timing).

5. 8.3.2 Pre-ignition
Further increase in hot spot temperature would cause the mixture to ignite before the spark ignites causes loss in power because increasing heat transfer to the walls. Pre-ignition follows heavy knocking (due to brake down of the cylinder wall boundary layers.
To reduce pre-ignition:
1. Good state of cooling system 2. Not to over load the engine
8.3.3 Running-on
When the ignition is switch-off and the throttle closed, the engine may continue to fire this may due to a hot surface in the cylinder. To reduce running-on: 1. decrease spark advance Take care of cooling system.
3. Decrease air-fuel ratio at adlerun.
8.3.4 Rumble
This an ignition associated with the combustion deposits. This might occur with sudden power demand from the engine causing the deposits to glow and ignite the fuel and air mixture. To reduce phosphor compounds the deposits become chemically inactive and do not cause ignition.