Detonation in SI Engines:

Detonation is a condition in which the unburned air-fuel mixture in the engine cylinder explodes instead of burning smoothly. This sudden and rapid combustion of the air-fuel mixture produces a knocking sound, which is why detonation is also known as engine knock. Detonation can cause significant damage to the engine and reduce its performance.

Causes of Detonation in SI Engines:

There are several reasons why detonation can occur in SI engines, including:

1. High Compression Ratio: SI engines with a high compression ratio are more prone to detonation. This is because the high pressure and temperature in the combustion chamber can cause the air-fuel mixture to ignite before the spark plug fires.

2. Lean Air-fuel Mixture: A lean air-fuel mixture has a higher chance of detonating because there is less fuel to cool the combustion chamber and absorb the heat generated by the combustion process.

3. Hot Spots: Hot spots in the combustion chamber, such as carbon deposits or sharp edges, can cause the air-fuel mixture to ignite prematurely.

4. Low Octane Fuel: Low octane fuel has a lower resistance to detonation, which can lead to engine knock.

Auto-Ignition and Detonation:

Auto-ignition is a process in which the air-fuel mixture ignites spontaneously, without the need for a spark plug. Auto-ignition can occur when the temperature and pressure in the combustion chamber exceed the auto-ignition temperature of the fuel. When auto-ignition occurs after the spark plug fires, it is known as detonation.

Answer:

Option C is the correct answer, which states that detonation in SI engines occurs due to auto-ignition of the charge after the spark is struck. This means that the air-fuel mixture ignites on its own, without the need for a spark plug, after the spark plug has fired. Detonation can occur due to a combination of factors, including high compression ratio, lean air-fuel mixture, hot spots, and low octane fuel.