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The Diesel engine noise is largely a result of the pressure needed to inject fuel into the cylinder, this pressure also resulting in atomisation of the fuel. The pressures can be 200bar or more. The engine is better than a petrol engine because the fuel is injected progressively into the cylinder over a period starting just before top center of crankshaft (top of piston stroke), finishing maybe ten or fifteen degrees crankshaft movement after top. This means that combustion takes place in a more linear fashion than in the petrol engine, where it is triggered by the ignition spark. Many discussions describe the combustion process in an engine (petrol or Diesel) as an "explosion". This is not true, as fuel in both engines has a well defined "flame from propogation front", in fact a period of time is involved as opposed to a sudden bang.