@beamerguide

PV = nRT, where P = pressure, V = volume, n = mole, R = gas constant, T = temperature
From the above formula we come to know that pressure and volume is directly proportional to temperature-let we have taken 1 mole of gas and constant can be ignored. When we reduce the volume of gas or compress it -or apply pressure to gas- then temperature of gas would increase.

If temperature of the gas on reaching the combustion chamber is high enough for the gas to burn, then the gas inside the combustion chamber would burn before the ignition of the spark plug. This phenomena is called Knocking. Knocking can seriously damage an engine!