what is detonation in spark agnition engine? what are its effects and how it can be detected and eliminated?

Answer 1

The fuel in the cylinder is actually not suppose to detonate, but *deflagrate*. Detonation (also called "knocking" due to the characteristic sound it makes) is when when combustion occurs as a reactive shock wave called a detonation wave that travels faster than sound. Deflagration is when the burn wave propagates more slowly (and less destructively) on the time scale of piston cycling. The transition from one to the other is abrupt (it either does one or the other). Octane is a measure of how long it takes the fuel to burn (higher is longer). Increasing octane is needed to prevent detonation for engines with high compression ratio.

Answer 2

Of course the spark plug detonates the air-fuel charge every cycle.

Perhaps you were referring to pre-ignition detonation, which is usually caused by the heat of the compression igniting the air-fuel charge before the spark plug fires. This is typically caused by using low octane fuel, but can also result from carbon deposits on the valves still glowing from the previous cycle