S3 good 159 now
around 260hp V good
Join Date: Jan 2008
Location: United Kingdom
Petrol engines rely on the spark plug to ignite the compressed fuel and air mixture in the cylinder which is drawn in together through the inlet manifold. Petrol is highly flammable and combusts easily.
A diesel engine draws the air only into the cylinder and compresses the air above the piston which creates heat. The fuel is then injected directly into the cylinder or pre combustion chamber and the heat ignites the fuel.
There are several different cylinder head types but basically low compression diesels need heaters to help start when cold. Higher compression diesels should create enough compression heat to start.
Both will have trouble starting if compression has been lost or reduced with mileage, also if one or more heater plugs are not working.
Because the engine is cold, combustion is not complete which causes the knock (or taxi effect on start up). Diesel is not so flammable so relies on heat and atomisation (injector spray mist) to combust. If the injectors are not atomising correctly, the combustion will not be complete causing knock and smoke.
Diesel also combusts over the piston in a different way which creates a different diesel engine sound.
Hope that’s not too technical.
Last edited by Filbyman; 31-07-10 at 22:48.