Question: why cant you put diesel into a petrol car and the other way around? I am in first class

Nathan Quinlan answered on 10 Feb 2015:

Hi

That’s a great question. The engine of a car works by burning petrol or diesel in a lot of little fires. Petrol and diesel burn in different ways, and the engine has to be designed to suit the way that petrol or diesel burns. These fires are small and they don’t last long – there are about 100 of them every second. Each fire happens in a cylinder, which is about the size of a small cup, and made of metal. A few drops of petrol or diesel is squirted into the cylinder just before every fire starts.

In a petrol engine, the fire is started by a spark from a spark plug, and then it spreads quickly through all the petrol that’s in the cylinder until all the petrol is burnt. Diesel is harder to burn. If you put diesel in a petrol engine, the fire might not light, or the diesel would burn badly, making a dirty smoke that would damage the engine and cause pollution.

In a diesel engine, the fire starts in a different way. There’s no spark plug. The engine designed to make all of the diesel in the cylinder quite hot before it starts to burn at all. When it gets hot enough, it starts to burn. If you petrol into the diesel engine, it would get hot enough to explode inside the cylinder, instead of burning smoothly. (Remember, diesel is harder to burn than petrol.) These explosions are small, but they damage the engine. Over a long time they can make holes in the engine, even though it’s made of very strong metal.

That’s how I understand it. If you have any more questions I’ll do my best to answer.