Reason for explosive combustion of petrol and air mixture in automobile engine cylinder.?

DETONATION

Unburned end gas, under increasing pressure and heat (from the normal progressive burning process and hot combustion chamber metals) spontaneously combusts, ignited solely by the intense heat and pressure. The remaining fuel in the end gas simply lacks sufficient octane rating to withstand this combination of heat and pressure.

Detonation causes a very high, very sharp pressure spike in the combustion chamber but it is of a very short duration. If you look at a pressure trace of the combustion chamber process, you would see the normal burn as a normal pressure rise, then all of a sudden you would see a very sharp spike when the detonation occurred. That spike always occurs after the spark plug fires. The sharp spike in pressure creates a force in the combustion chamber. It causes the structure of the engine to ring, or resonate, much as if it were hit by a hammer. Resonance, which is characteristic of combustion detonation, occurs at about 6400 Hertz. So the pinging you hear is actually the structure of the engine reacting to the pressure spikes. This noise of detonation is commonly called spark knock. This noise changes only slightly between iron and aluminum. This noise or vibration is what a knock sensor picks up. The knock sensors are tuned to 6400 hertz and they will pick up that spark knock. Incidentally, the knocking or pinging sound is not the result of "two flame fronts meeting" as is often stated. Although this clash does generate a spike the noise you sense comes from the vibration of the engine structure reacting to the pressure spike.

Aristotle said there are two meanings for "reason" in this sense: the immediate cause and the purpose behind it. Harry and Terry have the immediate cause covered, so here is the purpose.

It produces heat, and heat produces pressure. The pressure is used by the engine to rotate the output shaft so it can transfer power to whatever the load is.

This is what is known as a "low order" explosion, an explosion that produces a push rather than an impact. The detonation Damon describes is when it goes wrong and the ignition propagates by the pressure front rather than the flame front.

the reason is a spark was introduced into the cylinder under compression.. the real reason is someone decided it would be easier to turn a key than a hand crank.

The spark from the spark plug. Also the mixture can also detoate prematurely, if the cylinder is too hot or, if the mixture is compressed too much.

the spark plugs are there to ignite that air fuel mixture at the right time.