What's the difference between petrol and diesel?

13 July 2008

Question

What is the difference between cars that run on petrol and cars that run on diesel?

Answer

The answer to this is it's a totally different fuel source. The fuels totally differ in the way in which they behave inside the engine. Petrol engines have spark plugs and diesel engines don't. That's the simplest difference. In a petrol engine what happens is you have the piston going down in the cylinder. It pulls in some air and at the same time some fuel is added - sprayed in if you have an injection engine, or just drawn in with the air if you have a normally aspirated engine. The next thing that happens is that the piston goes up again and it compresses the mixture of petrol and air. This makes it a bit warmer because when you compress things they do heat up but it doesn't make it hot enough to ignite the petrol. Just before the piston gets to the top of the cylinder the spark plug kicks in, ignites a spark which ignites the fuel-air mixture. This burns very fast and this turns a liquid into a gas which takes up many, many times more space. This increase in volume inside the cylinder drives the piston back down inside the cylinder creating power. That's the power stroke. On the way up again the exhaust valves open and you blow the exhaust out. That's how a petrol engine works.

With a diesel engine the difference there is that you're entirely depending on the compression of the engine to make the explosion happen. What happens is the piston goes down. If you've got a normal, old fashioned diesel engine like they used to have on tractors and things this just drew in a cylinder full of air above the piston: a bit like you pulling on a syringe and filling a syringe with air. The next thing that happens is that the piston would go up and as the piston goes up it compresses the air that it's drawn in. If you've got a turbocharger on your engine actually what happens is it forces a bit more air into the cylinder under pressure. You have more air than you would normally have in the cylinder. As the cylinder comes up it compresses all of the air and when you compress air (just like putting your thumb over the end of a bicycle pump) it gets very, very hot. The heat is hundreds of degrees Celsius and just at the top of the piston compressing the air, right at the top of the cylinder, the fuel pump turns on and it sprays a fine mist of diesel fuel into this superheated air right at the top of the cylinder. This mist of diesel immediately starts to burn and just like the petrol engine it produces enormous amounts of gas. This expands very rapidly and that's what produces the power stroke. No spark plugs in a diesel engine. That's basically, in a nut shell, the difference.

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