What happens when gas is burned in a diesel engine? Bernie


Gasoline has an octane rating. This is a measurement of the resistance of the gasoline to ignite.

Diesel fuel has a cetane rating. This is measurement of how easily the diesel fuel will ignite.

When gasoline is burned in a diesel engine, it might not ignite completely. So any unburned gasoline can end up in the exhaust system where it can ignite. If enough gasoline ends up in the exhaust system and ignites, it can blow up the exhaust system.

Also, diesel engines use injector pumps. Running gasoline through an injector pump and the injectors of a diesel engine can destroy these components. Especially if the gasoline contains any ethanol.

The best thing to do when gasoline is accidently introduced into a diesel engine fuel tank is drain the tank of gasoline and put in diesel fuel. Otherwise, it could cost hundreds if not thousands of dollars to repair the damage caused from running gasoline through a diesel engine.



“When gasoline is burned in a diesel engine, it might not ignite completely. So any unburned gasoline can end up in the exhaust system where it can ignite.”

Once again, this is not true. Gasoline will ignite and combust at a much lower pressure (compression) than diesel. Typical gasoline will pre-ignite at a compression ratio of about 10:1, while a compression ratio of about 20:1 is required for diesel. Also, the gasoline combustion is much faster. If you inject gasoline into a diesel engine (at about 20-25 BTDC) you will have serious/distructive pre-ignition and the exhaust system will be the least of your problems. I do agree that running gasoline in a diesel engine is a very bad idea.


The only time I’ve seen this happen it did significant damage to the diesel engine.




“What happens when you use gasoline in a diesel engine? Either something expensive or something very expensive. Since gasoline is designed to be resistant to self-ignition, gasoline in a diesel engine either won’t ignite or will ignite at the wrong time. Some diesel engines run leaner than gasoline engines (meaning that the air-fuel mix has a higher proportion of air than a gasoline engine). That increases the chances that the gasoline won’t ignite and that unburnt fuel will be sent into the hot exhaust system–where, ironically, it could ignite, leading to possible exhaust damage.”

I’m not sure who wrote that, but they have a “limited” understanding of how diesel engines work. “Some diesel engines run leaner than gasoline engines,” is a ridiculous statement. The term “lean” is meaningless in a diesel, all (not some) diesel engines run with significant excess oxygen. Saying " gasoline in a diesel engine either won’t ignite or will ignite at the wrong time," is partially true; it will almost certainly ignite at the wrong time, much too early. The only reason gasoline would not ignite is if the amount was too small (too lean) to support combustion, then the engine wouldn’t be running anyway. Maybe he’s trying to say that the exhaust could be damaged if unburned fuel entered the exhaust after the engine failed to start right away, but that is not clear. Has this guy ever seen what happens if you put low octane gasoline in a engine with a 10:1 compression ratio? What does he think would happen at 20:1? Be careful what you believe from the internet.


Guys, the OP asked “What happens when gas is burned in a diesel engine.” The website Tester referenced above answers the question “Can I use diesel fuel instead of regular gas?”(in a gas engine)

These are two completely different questions.


Very true, but if you read the the entire linked article, the author also gives his (questionable) opinion of the OP’s question. I do think we all agree that “bad stuff” can happen if you run gasoline in a diesel engine.


From the owner’s manual from my pre TDI (prior to 1992) VW diesel: Your Diesel engine has been specifically designed to operate on Diesel fuel only. Therefore, do not use home heating oil or regular gasoline. The properties of those fuels may cause serious damage to the fuel injection system and to the engine.

Skipping a few paragraphs it goes on to say about cold (below 20F) weather oeration: Only if neither No. 1 Diesel Fuel, nor winterized Diesel Fuel No. 2, nor kerosene are available, use up to 30% leaded or unleaded gasoline.

The 30% is not a typo! I have no knowledge of what is permitted with TDI diesels.


The very old benz diesel owners manuals (before about 1980) allowed the use of some gasoline to thin the diesel for very cold temperatures. After about 1980 the allowance for using gasoline was removed. The manuals for mine (82 and 83) only allow no. 1 diesel (basically kerosine) to be mixed with no. 2 diesel (which you sometimes get at the pump in cold climates).


it wouldnt be a problem about gas igniting in your exhaust, it will just burn off in your cats, i agree with craig though


I must agree with Craig. You MIGHT get away with 30% gasoline WHEN THE TEMPERATURE IS BELOW 20 DEGREES.

Virtually ALL todays gasoline is blended with 10% alcohol which may or may not mix with diesel oil. The whole mess could separate.

Rudolf Diesel destroyed MANY engines as he worked out the first successful Diesel engine. He was badly injured when one of his prototypes exploded…

When gasoline is used to fuel a compression ignition engine, the burn rate is so fast it can only be described as destructive detonation.

When diesel oil and pure gasoline are mixed, the resulting fuel will have completely different combustion characteristics that either of the two unmixed fuels…These characteristics will depend on the proportions of the mixture…Some engines may indeed tolerate such a mixture, depending on compression ratio and intake air temperature.

I have burned a 20% mixture of diesel in a gasoline engine (just to dispose of the diesel) without any major problems…


Usually, the engine siezes or you damage the pump or injectors. Engine runs way too hot on gasoline. Jet engine fuel works great however.