The high air temperature near the end of the compression stroke causes spontaneous combustion of the mixture as the fuel is injected.
Broken seals or a broken turbocharger may cause large amounts of oil mist to enter the inlet manifold, whereas defective injection pumps may cause an unintentionally large amount of fuel to be injected directly into the combustion chamber.
This should be the last option because it can result in catastrophic damage to the whole transmission, mainly the gearbox, but this operation can save the engine.
After the refinery's blowdown stack malfunctioned and started releasing a cloud of raffinate vapor into the air, a pickup truck that had been parked near the stack with its engine idling was engulfed by the vapor cloud released and the engine began to race.
As staff at the refinery attempted to stop the truck's now-overheating engine, it backfired, igniting the vapor cloud and triggering the disaster.