What properties of diesel fuel make it ignite simply from the piston compression?

05 June 2011

Question

What properties of diesel fuel make it ignite simply from the piston compression?

Answer

Chris - When you compress air, it gets very, very hot. You're squeezing the air molecules together, you'll get a dramatic rise in temperature, and therefore, you'll need a fuel that in fact can tolerate being put to quite a high temperature before it begins to burn and that's one of the properties of diesel. It's less volatile than petrol. If you put petrol into a diesel engine then the petrol will burn too quickly and it won't run properly which is why you mustn't do that - it will actually damage the engine. But with diesel, what happens is you compress the air in the cylinder and just before the piston gets to the very top of the cylinder, you then open the injector and squirt a mist of fuel in - that's called atomisation. The fuel then burns in this very compressed, very, very hot gas, the oxygen in the air is fuelling that. That then leads to a chemical reaction producing large volumes of carbon dioxide and water and some other partially burned hydrocarbons. This gas takes up a lot more space than the original liquid fuel did, maybe expanding by a factor of 600 to 1,000 and that expansion drives the piston down in the cylinder again and that's the work stroke in which you extract energy from the cylinder by connecting that piston to the crankshaft. So you need a choice of fuel which can tolerate being injected at those very high temperatures and not explode. Petrol will get so hot when you compress it to push it into the cylinder in the first place, it will try and burn in the fuel line and that's what does the damage to the engine.

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