ummm... i think its somehting you have to do with the activation enery doesn't it? We can explain it by the petrol and diesel engine activity in vehicles. The difference between the two engines are totally based on the type of the fuel used.
Gasoline does not self-ignite when compressed, because it does not generate enough heat. It needs activation energy to explode, and this energy is generated by a spark plug at the top of the cylinder. Diesel on the other hand, does not require a spark plug to ignite it because it generates enough heat when it is completely compressed to ignite itself and cause explosion. The compression pressure is directly related to the combustion as well. Diesel engines have a much higher compression pressure than gasoline engines. The higher compression pressure in diesels explains the difference in the methods of ignition used in gasoline and diesel engines. The reason for this higher pressure is that in a diesel engine, only air is compressed. The fuel is then directly injected into the cylinder, and this allows for a much higher compression ratio.