Basic Requirements of a Good Combustion Chamber
The main requirements of spark ignition combustion chamber design are high power output with minimum octane requirement of the fuel, high rate of thermal efficiency, and smooth engine operation at all gears.
For getting a high power output from the same size of the engine, the compression ratio of the engine has to be increased. The compression ratio is limited by the detonation and knocking and depends on the fuel quality and chamber design. The second factor is that no excess air should be used and complete utilization of the air should be made. Needless to say, no dead pockets should remain inside the combustion chamber. The third factor is high volumetric efficiency that is achieved by having large diameter valves along with good timing and straight passageways to allow flow of the gases with the least pressure drop.
For getting a high thermal efficiency, a high compression ratio is. The second factor for thermal efficiency is to have minimal heat loss during combustion, which can be achieved by giving a small surface to volume ratio to the combustion chamber. Turbulence also speeds up the combustion and reduces the combustion time loss, hence allowing the speed of the engine to be increased. The third factor for getting a high thermal efficiency is to have good scavenging of the exhaust gases, because any remaining exhaust gas inside the combustion chamber will lead to the dilution of the charge and loss of efficiency.
For getting smooth engine operation we must have an absence of detonation and a moderate rise of pressure during combustion. These in turn depend basically upon having a compact combustion chamber design which allows for a short distance of flame travel. The second factor is the correct location of the spark plug and the exhaust valve to allow the proper prorogation of the flame front and the exhausting. The third factor is that the cooling of the spark plugs should be done so that the hot spark plug points do not trigger any pre-ignition.