COMBUSTION IN SI ENGINES
The combustion process of SI engines can be divided into three broad regions: (1)ignition and flame development, (2) flame propagation, and (3) flame termination. Flame development is generally considered the consumption of the first 5% of the air-fuel mixture (some sources use the first 10%). During the flame development period, ignition occurs and the combustion process starts, but very little pressure rise is noticeable and little or no useful work is produced (Fig. 7-1). Just about all useful work produced in an engine cycle is the result of the flame propagation period of the combustion process. This is the period when the bulk of the fuel and air mass is burned (i.e., 80-90%, depending on how defined). During this
time, pressure in theexpansion stroke. The final 5% (some sources use 10%) of the air-fuel mass which burns is classified as flame termination. During this time, pressure quickly decreases and combustion stops. Inan SI engine, combustion ideally consists of an exothermic subsonic flame progressing through a premixed homogeneous air-fuel mixture. The spread of the flame front is greatly increased by induced turbulence, swirl, and squish within the cylinder. The right combination of fuel and operating characteristics is such that knock is avoided or almost avoided.
Types of combustion:
1. Normal Combustion.
2. Abnormal Combustion.
Normal combustion rarely occurs in a real engine without some trace of auto ignition appearing. After ignition, the flame front travels across the combustion chamber. The gas a heat of the flame front called the "end gas ". The end gas receives heat due to compression by expanding gases and by radiation from the advancing flame front, therefore, its temperature and density increases. If the temperature exceeds the self – ignition temperature and the un-burnt gas remains at or above this temperature for a period of time equal to/or greater the delay period, spontaneous ignition(or auto ignition) will occurs at various locations. Shortly after words an audible sound called knock appears.
If the end gas does not reach its self-ignition temperature, the combustion will be normal.