I see much good information above, mixed with a small amount of misinformation and misunderstanding. I understand that this thread has run its course, but for the sake of its historical value to students searching for answers, I’ll put together a good summary based on more than three decades of experience as a mechanical engineer working in and around maintenance shops, both for gasoline and diesel engines.
The simple answer to the original question is, “Yes, under some circumstances you can run a gasoline engine on diesel fuel and visa versa.” But the engines will not run as designed and you may cause harm to them. Under other circumstances, the engines won’t run at all. This is more true of modern engines than those of older design. I’ve done it. I’ve seen it done, and I’ve talked with people who have done it.
Back in the 1950’s and 60’s, when the large over the road trucks were converting from gasoline engines to diesel, it became a sort of urban legend among truck drivers that you can get more power out of a diesel engine if you mix gasoline into the fuel. Some reported that they sometimes ran at nearly 100 percent gasoline, but that only worked if they got the engine hot on diesel first. From what I understand of the science, that was probably true for engines of that day; but probably not of today’s diesel engines. This is backed up by reports from the old timers who said that then engines would run hot when run like this, or maybe they ran hot because the whole purpose of doing it in the first place was to drive faster, or to climb a mountain with a heavy load.
In modern times, I’ve seen many a truck towed into the shop after someone accidently filled the tank with gasoline. We would drain it, clean the fuel system, and refill with the correct fuel. In many cases, the drivers reported that they did not get very far down the road when the engine simply refused to run well at all, or else it quit running all together. When they told us that, we would unscrew the fuel cap and smell the fuel for evidence of gasoline. From what I understand of the science, it is reasonable that modern engines would run or not run differently on this mixture, compared to earlier designs.
Getting rid of the mixed fuel is expensive because it is a controlled waste, so we would add that to the customer’s bill and tell the mechanics to go dispose of the mixture per proper procedure, for which they were all trained. But we all knew that they interpreted that to mean go top off all their personal cars and pickup trucks out in the parking lot. The mixture runs just fine in gasoline engines, especially when further diluted with the gasoline already in their tanks.
All this talks about running the engines on mixtures. What about running these engines on the wrong fuel without mixing them?
The two big differences between these fuels, around which these engines are designed are that gasoline vaporizes and mixes rapidly with air when injected into the engine, but diesel does not. Diesel remains a cloud of very small droplets after being injected. These droplets will burn on the surface. One of the reasons that modern diesel engines are so efficient is because we have learned to reduce the size of these droplets, so they burn more completely. You must have the right fuel/air ratio for a gasoline engine to work, but the diesel cares nothing about the mixture. Most diesel engines pump much unburned air through the engine most of the time. They have no throttle to control the air intake. The second difference is that while they will both combustion ignite, the gasoline will do so at a lower pressure ratio. So if you run gasoline in a diesel engine, you can expect some pre-ignition. If you run diesel in a gasoline engine, the pressures are insufficient for compression ignition, so you must have a mixture that the spark can ignite. I’ve never been able to start a gasoline engine with diesel fuel when the engine is cold, but I have run lawn mowers on diesel fuel when I ran out of gasoline and had to finish the lawn with kerosene or diesel fuel. This works just fine. I’m guessing that the higher temperatures of an engine that is already hot from running on gasoline will vaporize enough of the fuel to ignite from the spark, but not if the engine is cold.
So if you and your friend are stuck a hundred miles off shore with an approaching hurricane, but neither of you have enough fuel to get home; and if your engine is diesel and his gasoline; then go ahead and mix the fuels and abandon one boat. You have a high probability of getting home before the hurricane. This probability is higher if you take the diesel boat home. But under any normal non-emergency conditions, mixing or changing fuels is a bad thing to do.
Also mentioned above is that diesel fuel is also a lubricant. Technically, it is a very poor lubricant, not much better than gasoline. But the fuel does have additives that help lubricate engine parts. This is why when the military runs their diesel equipment on jet fuel, which is the same as diesel fuel without any additives, they will also mix in a small amount of motor oil. If you must run a diesel engine on mixed fuel, it would not be a bad idea to also mix in some motor oil. I’ve been told that the optimum ratio is 100:1 on the oil mix.
Diesel engines will run on any liquid petroleum product. If you add a carburetor, they will run on high mixtures of natural gas and diesel. Caterpillar sells a “heavy fuel” option for their larger diesel engines. In sales demos, I’ve seen them run them on roofing tar while putting out normal power and meeting all the clean air requirements. All you need is a fuel system capable of pre-heating the tar, and pumping it through an appropriate filter before sending it to the engine. I know a commercial fisherman who runs on waste motor oil from automobile maintenance shops. You can buy options with a new diesel engine that will route used oil to the fuel tank and also top off the engine sump with fresh oil. All engine manufacturers offer this option on new trucks, except in Florida where the state is concerned that they can’t collect fuel taxes on the portion of oil used as fuel. So Florida truckers have the “fuel blending” option added at shops in Georgia.
Another factor to consider is that most diesel engines dump some heat from the engine into the fuel tank. They re-circulate fuel to cool the injectors. In the early days, they dumped much more heat into the fuel than do modern engines. The old 2-stroke diesels were famous for getting their fuel tanks very warm. That might help explain some of the anecdotal stories from the 50’s and 60’s from the old truckers, who all swear that diesel engines used to run fine on gasoline. But we could discus many more differences between old and new engines that could also offer an explanation.
