* High Compression: Gasoline engines, even though designed for spark ignition, still achieve significant compression. This compresses the fuel-air mixture, raising its temperature. If the mixture is rich (too much fuel) or the engine is very hot (from hard running), this compressed mixture can reach its autoignition temperature.
* Hot Engine Components: Several components within the combustion chamber retain significant heat after the engine is shut off. This includes the cylinder walls, piston crown, valves, and especially the exhaust manifold and catalytic converter. This residual heat can ignite the fuel-air mixture.
* Carbon Deposits: Buildup of carbon deposits within the combustion chamber can act as glowing embers, further contributing to autoignition. These deposits have a much higher ignition temperature than the fuel-air mixture alone.
* Lean Fuel Mixture: While a rich mixture is more prone to dieseling, an excessively lean mixture can also cause problems. A lean mixture burns slower, leading to sustained high temperatures in the combustion chamber.
* High Ambient Temperatures: Extremely high ambient air temperatures can exacerbate the issue by raising the initial temperature of the incoming air-fuel mixture.
Essentially, dieseling is a runaway combustion event. The initial ignition from residual heat causes combustion, which generates more heat, further igniting more fuel, and so on, until the engine components cool down sufficiently. This is why dieseling usually stops after a short period. However, prolonged dieseling can be damaging to the engine.
