Engineering Question #1923

Damanjit Singh, a 21 year old male from New Delhi, India asks on February 20, 2004,

What will happen if we use petrol in a diesel engine or vice versa?

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The answer

John Jones answered on February 20, 2004

Petrol in a diesel engine

[I have inadvertently investigated this question myself; while on a cross-country trip in 1979, I filled a VW Diesel Rabbit with petrol at a pump in North Dakota. The engine wouldn't run at all, though I don't think it was damaged.]

Diesel fuels are designed to ignite when compressed, whereas gasoline is designed not to -- if the fuel in a petrol engine ignites under compression, this causes the noise we call `knock'. The octane number is an indication of how reluctant the fuel is to ignite under pressure -- the higher the octane, the less likely it is to knock.

It is possible to design a diesel engine that will run on petrol, and this is sometimes done for diesels designed for military use. The chief change made is to increase the compression ratio; a typical diesel engine has a compression ratio of 17 or 18, whereas a multi-fuel military diesel will have a compression ratio of 21 or higher. The higher the octane of the petrol, the higher the compression ratio would need to be. Once you've got the compression ratio right, there doesn't seem to be a problem with the injector. And timing isn't really an issue in the diesel, since a spark plug is not used.

Diesel fuel in a petrol engine

This is much less likely to work; the problem is that the diesel is much less volatile than gasoline, so you are unlikely to be able to get it to burn at all. So a conventional carburetor is unsuitable, since it relies on the fuel vaporising. A fuel-injector would be a better bet, but there is a high likelihood of it plugging up with deposits. The hotter the engine, the more likely it is that you'll be able to get the diesel to burn. You might, for example, start the engine on petrol, let it warm up, then gradually dilute the petrol with diesel. There is likely to be a lot of black smoke in the exhaust, due to unburned and partially burned fuel, and I would expect the cylinder to build up carbon deposits rather quickly.

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