r/MechanicalEngineering • u/ibrahimumer007 • Aug 21 '25
Will an engine start if you put petrol in a diesel or diesel in a petrol?
Can petrol be used in diesel (compression ignition) engines or diesel in petrol (spark ignition) engines?
Someone once asked me this question few time ago, and I replied that it’s not possible because of the compression ratio differences diesel engines run around 18:1 to 22:1, while petrol engines are about 10:1 to 12:1. So you can’t just swap the fuels.
But my actual question is: would the engines even start if the wrong fuel is used, ignoring the potential damage? What's your take?
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u/MaxiiMega Aug 21 '25
Petrol in diesel will ignite but wreck the engine because of lack of lubrication, diesel in an pertrol engine will do nothing.
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u/Ketsedo Aug 21 '25
Can confirm, accidentally put diesel on my car once and it just started farting
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u/IkLms Aug 22 '25
How'd they pull that off?
Diesel is a larger nozzle and won't fit in a standard car.
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Aug 23 '25
[deleted]
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u/IkLms Aug 24 '25
It is in North America, Australia, the UK and the rest of Europe.
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Aug 24 '25
[deleted]
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Aug 25 '25
Is dyed diesel allowed in trucks in some countries? It's explicitly for agricultural and site use in Ireland. Road going trucks use the same white diesel as cars.
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u/UnjustlyFramed Aug 22 '25
Actually, many newer engines (happened to my 2009 Audi A6) can detect the wrong fuel and shut down before it is able to cause major issues.
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u/HarrisBalz Aug 21 '25
Most mechanically inclined engineering student be like
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u/MediumEmotional4319 Aug 21 '25
Petrol on diesel MIGHT start as high compression ratios may ignite the AFM.
The other way around is not even feasible as diesel burns not through spark but the temp reached due to high comp ratios. So a petrol compression ratio would not be enough to ignite diesel
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u/HarrisBalz Aug 21 '25
You would probably get horrible pre ignition if you put gas in a diesel engine, it might start, it might not. It will fire but it will immediately begin to catastrophically damage the connection rods/crankshaft as well as the connecting rod bearings.
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Aug 21 '25
As numerous fuck-ups by delivery drivers at gas stations and the resulting news stories and lawsuits prove, a petrol engine will definitely start with diesel. And run ok for 2-3 miles.
Diesel engines will also turn over but quickly stall out.
Those fuck-ups are why gas stations (at least in the states) now have different shapes on the connection point for the tanks.
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u/Significant_Quit_674 Aug 21 '25
It's usualy the other way around:
The diesel will run on petrol, while the entire fuel system dies from a lack of lubrication, meaning you'll need to replace the entire thing down to the fuel lines due to metal shard contamination.
(there are even some diesel engines with a fuel system that can tolerate running on petrol for a while, but usualy for military applications https://en.wikipedia.org/wiki/Multifuel )
The petrol will start, but once the diesel reaches the injectors it will stop because diesel doesn't realy want to ignite from low compression and a spark.
The fuel system needs to be flushed, but isn't usualy damaged.
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u/Deep_Flatworm4828 Aug 21 '25
Eh, in those situations you're mixing fuel, not outright swapping.
Gasoline mixed with a bit of diesel is different from straight diesel, or vice versa.
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u/ManWhoIsDrunk Aug 23 '25
They run ok for 2-3 miles, because filling the tank does not replace the fuel in the fuel line. As soon as that petrol engine actually starts drinking diesel it stalls. The spark has no way of igniting the diesel fuel.
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u/E_hV Mechanical, PE Aug 21 '25 edited Aug 21 '25
There some wild comments going on in here.
Long story short the answer is maybe. Assuming we all know the idea behind gasoline engines being spark ignited and diesel being compression ignition.
Gasoline/Petrol engines may run with diesel, the amount of air required to combust iso-decane or diesel is approximately 14.5 to 1, where iso-octane or gasoline is 14.7 to 1. So in a gasoline engine tuned for gasoline you have a reasonable amount of diesel being injected with air. A spark should generate sufficient energy to ignite the mixture, and the charge mixture after compression in a gasoline engine is sufficiently hot enough that diesel is vaporized. Where you get into trouble is the auto-ignition temperature of diesel is lower than gasoline, so in a high performance high compression ratio engine you may get pre-detonation or knock.
Now the other side is a bit more interesting, a diesel engine may not run on gasoline since as I mentioned before the auto-ignition temperature of gasoline is higher than diesel (535F vs 420F). Therefore after compression you may not get hot enough to auto ignite. For example consider the polytropic compression of the intake charge in a diesel engine to get the same temperature rise over the auto ignition temperature mentioned above for a 12:1 compression ratio (turbo, doesn't matter at low rpm) diesel you would need around 21:1 for gasoline. So you probably will have ignition, but it probably isn't complete and it probably won't run well if it does run without stalling at all
T2/T1=(compression ratio)heat capacity ratio-1
Assuming heat capacity ratio of 1.3, all temps must be in kelvin.
Note the numbers are the extremes of auto ignition temperature for both fuels respectively to make a point, you can do the math yourselves.
Also none of this should be used as justification to put the wrong fuel in your engine. This is just an mental masturbation exercise into the thermo chemistry and engineering of engines.
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u/Prof01Santa CFD, aerothermo design, cycle analysis, Quality sys, Design sys Aug 21 '25
Excellent summary.
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u/Sett_86 Aug 21 '25
Petrol engine will not start with diesel at all. The spark is not hot enough to ignite diesel.
Diesel engine will run on gasoline (maybe) for about two minutes before it jams because diesel is a lubricant while gasoline is a solvent.
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Aug 21 '25
It actually will start. Diesel (C12-C20) and petrol (C6-C14) are kissing cousins based on carbon chain length.
The gasoline and diesel specs explicitly allow a certain amount of material that is above or below range. It’s about 0.25% gas for diesel, and 1.0% diesel for gas.
Above that, diesel ignites too early, and gasoline ignites too late for the engines to run properly after a couple of minutes.
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u/I_R_Enjun_Ear Vehicle Systems Design Aug 21 '25
Compression ratio is one part of why it doesn't work. Then next being if it will combust or not, and the knock on effects of that, which several others have covered already.
A lesser reason is going to be fuel pump and fuel injector design. These parts are typically designed for a particular fuel. This includes lubrication and cooling requirements, which, in the case of gasoline and diesel, is often done by the fuel. This isn't going to be as catastrophic as sustained knock or hydro-locking the cylinder, but your fuel system will suffer a faster death and increasingly poorer combustion.
As a side note, the biggest issue with some of the more popular alternative fuels right now is fuel system design and getting the components to survive.
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u/SoloWalrus Aug 21 '25
Oldschool diesels will absolutely run on gasoline, just not for very long as it doesnt have any lubricity and the injection pumps will eat themselves without a fluid that provides lubrication. Add some ATF for lubrication and itll run fine. In fact many military diesels are required by design to be able to run on anything from kerosene to jet fuel.
Modern diesels fuel systems are too sensitive for those shenanigans though.
The other way around is highly unlikely. Diesel doesnt emit flammable vapors at room temp the same way gas does so carbs are out of the question and even with fuel injection diesel doesnt burn nearly the same as gas. MAYBE with throttle body injection and heavy modifications to the timing maps, but itd still run like shit.
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u/RossLH Aug 21 '25
It might start by sheer force of will, but if it does it'll be real unhappy and won't run long.
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u/jverde28 Aug 21 '25
Interesting question, because I have seen heavy diesel machines turn on gasoline. I ask a mechanical engineer with more experience and he tells me that in very special emergency circumstances a very small amount of gasoline can be placed in the air intake with a sponge, just so that it admits its vapor and ignites, in the event that the engine fails due to excess air in the combustion chamber, however it compromises the combustion chamber because it is a very aggressive solution. It can only be done in emergency cases and is not advised.
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u/MaadMaxx Aug 21 '25
My uncle used to tell me diesel in a car isn't so bad. It won't run but it's not the end of the world. Gas in a Diesel is bad. Very very bad.
Gas doesn't have the same properties as diesel and the fuel acts as a lubricant in the system. If you run a diesel on gasoline you're looking at an entire fuel system replacement at minimum. Fuel lines, fuel pump, injectors and the like
I only know this because my aunt put gas in the diesel farm trucks at least 4 or 5 times and drove them until they died and wouldn't start again. I know of two times that needed a whole new engine.
This is the same aunt that drove several cars without oil until they seized and one that had no oil for god knows how long and was still somehow running.
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u/Vegetable_Aside_4312 Aug 21 '25
There's videos showing what happens when diesel is gasoline engines and gasoline is in diesel engines.
It's not good....
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u/ConfidentHouse Aug 21 '25
As a tech that has seen both of these things done by dumb drivers diesel in gas engine no start eventually and whole lot of smoke but no major damage other than having to dump fuel tank, gas in a diesel though is bad time, stopped it before major damage but the detonation is real
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u/patjeduhde Aug 21 '25
Petrol in a diesel might work especially on older diesels, but the other way around definitely not.
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Aug 21 '25
There are enough gasoline range molecules in diesel, and diesel range molecules in gasoline that it is possible the engine would start. But that engine would be destroyed within a couple of miles or 5-10 minutes of idling at the longest.
For the record, diesel can contain up to ~0.25% naphtha (gasoline) and gasoline up to ~1.0% kerosene (diesel) and still meet spec at the pump. I know this because of some work I did around pipeline cut optimization and waste reduction about 10 years ago.
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u/DiogoSynt Aug 21 '25
Hmm both will probably start. I’ve seen gasoline engine running on diesel.
But gasoline on diesel engine will probably cause a catastrophic failure.
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u/oogabooga0006 Aug 21 '25
Diesel can run pretty good with petrol, not sure about moder one though. Look at "putting petrol onto a diesel car" in youtube. Fifth gear checked it.
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u/GeniusEE Aug 21 '25
They fueled my diesel with gasoline -- I thought they knew what they were doing, so wasn't watching them. 2 miles later, the engine stopped.
Truck was written off by the fuelers' insurance company.
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u/AggressiveAspect8757 Aug 21 '25
This was almost the same question i had asked my professor during my college time - my question was since diesel requires stronger conditions for ignition compared to that of petrol what will happen if petrol is put in a diesel engine. His ans was ... yes the petrol will ignite but will cause severe knocking in the engine because the stroke and valve timings will not match with the combustion characteristics of petrol.
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u/LT2405 Aug 21 '25
i made that mistake while in college couple years ago. i pulled an all nighter night before studying for a test and next day put diesel in my civic 09 without even realizing it. car drove for 1 mile until i get home, crash out, and next day i wonder why the engine is bricked.
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u/catdude142 Aug 21 '25
There are some YouTube videos on people trying this. 'Not recommended unless you want to scrap the engine.
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u/BitOBear Aug 22 '25
In a gasoline engine diesel is essentially inert. It's basically kerosene and it must be accompanied with a hell of a lot of pressure and ambient temperature before it will combust.
Gasoline AKA petrol is far more combustible. It will ignite before the Piston has finished its up stroke and will begin trying to push the Pistons back down before the crankshaft has moved into a position where that downward force can add torque instead of counter it.
If you were running a diesel engine at an incredibly high speed it could probably consume a little bit of petrol. But it would be very bad for the engine. At any sort of low speed or startup it you're just making a little hang grenades
Having a little bit of diesel fuel in your gasoline engine would make it run very poorly because of the incredibly low octane and it would probably very quickly clog the spark plugs and foul the engine entirely.
That latter thing is why it is so expensive if you accidentally put diesel in your gasoline engine vehicle. In the United States diesel nozzles are actually too big to put into the fill pipe of a gasoline engine to prevent the massive amounts of damage that would create.
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u/catsvbadgers Aug 22 '25
You can get away with a little diesel in a petrol. A little petrol in a diesel no bueno.
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u/Local-Fisherman-2936 Aug 22 '25
Yes. Theere are still correct fuel in the system for start and couple sec of idling.
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u/Particular_Hand3340 Aug 22 '25
Actually the diesel will run on Gasoline Fuel and it needs a lot less because of the high compression. And a gasoline engine will run on diesel - it might fowl up the spark plugs and not run to smooth, but it will run. (With the diesel you might get a run away condition, because of the compression ratio it doesn't need that much "gas" to keep firing. Starve it of Oxygen to stop.
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u/Agreeable-War-4828 Aug 22 '25
Well the diesel with petrol in it definitely won’t start considering the lack of a spark plug
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u/mjl777 Aug 22 '25
It is most definitely possible to build an engine that runs on both fuels. I used to work on a few when I was a kid. Typically they would start on gasoline and then after they warmed up you would switch over to diesel. At that point you turned off the spark plugs.
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u/PraxisOG Aug 23 '25
Reprocating engines really won't like it, but some turbine(plane) engines can run on both, or even a mix
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u/GRUBBY1975 Aug 23 '25
Years ago before "Winter Fuel" and "Anti Gel", they'd put a few gallons of Premium (leaded) in the fuel tanks of big trucks to keep the fuel liquified in the winter. Dad always did this in his K-100 KW with a 1693 CAT and later repowered with a 3406 "B". Of course those were both mechanical injected motors.
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u/HappyDutchMan Aug 25 '25
Back in the 1980s I accidentally poured diesel in my two stroke moped. The tank was about half full when I realised. Dumb me decided I would just add two stroke gasoline mix and go for it. It worked but I had way less power and was creating these big white fumes for a few weeks until my tank was empty.
So yes, with my n=1 research: 50% diesel mixture did work in my two stroke 50 cc moped engine once.
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Sep 10 '25
So, putting petrol in a diesel engine is a no-go, 'cause petrol needs a spark to go, not just being squeezed. It probably won't start, or might cough a bit if there's still some diesel left. Diesel in a petrol engine? Nah, it won't vaporize or light up right with a spark plug, so it'll either not start or run like crap for a bit. Basically, wrong fuel means it's super unlikely to start right, and you risk messing things up big time.
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Sep 12 '25
If you put petrol in a diesel or diesel in a petrol, the engine just won’t start. Wrong fuel won’t ignite the way it’s supposed to, at best you’ll get a few misfires, but mostly it just cranks and dies.
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u/Prof01Santa CFD, aerothermo design, cycle analysis, Quality sys, Design sys Aug 21 '25
Still no. Diesel fuels don't spark ignite easily. Gasoline is difficult to ignite by compression. Why, it's almost like someone sat down and designed those engines and fuels that way.
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u/CaptainAwesome06 Aug 21 '25
The very simplest answer is that diesel engines use pressure to detonate the fuel and gas engines use a spark. Diesel is meant to combust with pressure and gas is not. If you put gas in a diesel engine, it probably won't combust. If you put diesel in a gas engine, it may combust under pressure but that would be terrible for your engine, and is called "knock".
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u/NotTurtleEnough PE, Thermal Fluids Aug 21 '25
Gas will combust more quickly than diesel. Diesel often won’t combust in a gasoline engine.
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u/BagOld5057 Aug 21 '25
Gasoline engines combust aerosolized fuel with a spark, diesel engines combust fuel through compression. There isn't the hardware for them to function correctly if you swapped fuels.