What is the primary cause of engine knocking (detonation) in a Spark Ignition (SI) petrol engine?

1. Delayed ignition of the fuel-air charge resulting in low cylinder pressure.
2. Pre-ignition or auto-ignition of the unburned end gas before the normal flame front arrives.
3. Incomplete combustion due to insufficient air in the cylinder.
4. Low compression ratio which causes the fuel to ignite too early.