Pre-ignition happens when the air and fuel mixture in the cylinder of an engine is ignited before the spark plug fires. This is typically caused from a hot spot in the compression chamber or by a spark plug that is too hot. Often called dieseling, the engine can run on after the key is turned off and if allowed to continue, can cause catastrophic engine failure.
Many high-compression racing engines suffer from pre-ignition. The fuel enters the engine and is ignited early by the heat generated from the ultra-high compression pressures, much like a diesel engine. This often causes the engine to break pistons or bend connecting rods as the other cylinders attempt to fire in the correct timing sequence. The best-case scenario will result in engine knocking or pinging.
In some early engines, this run-on was due to excess fuel being drawn into the engine by poorly functioning carburetors after the ignition was turned off. The engine would continue to stutter and pop as the raw fuel entered the hot combustion chamber and pre-ignition occurred. The advent of fuel injection has all but stopped this type of engine behavior.
Pre-ignition as well as engine knock both drastically increase the temperature within the combustion chamber. This virtually ensures that the occurrence of one of these conditions will bring on the other. Pre-ignition results in poor engine performance and is often accompanied by a rough running engine.
The pre-ignition problem is often easy to correct and can be eliminated with minimal effort. Changing to a cooler operating spark plug can often cure pre-ignition in an engine. Proper adjustment of the carburetor and a corrected air-fuel mixture can also cure most pre-ignition conditions. Another cure for the problem can be the addition of a cleaning agent to the vehicle's fuel supply. Also, cleaning the combustion chambers of carbon deposits will often solve the problem.
The timing sequence of combustion in a piston engine is an exact science. Any combustion occurring either too early or too late can cause sever problems. The fuel-air mixture entering the engine must be allowed to travel to its intended destination before igniting. Any variance of this timed event can result in engine damage or destruction.
Engine builders often spend many hours removing sharp edges from engine components. This is done not only to reduce stress risers and prevent parts breakage, but also to prevent hot spots that can cause pre-ignition. By removing the sharp edges with sand paper or other methods, the builder can continue assembling the engine with less fear of problems later on.