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I (like most people) just use regular unleaded gas in my car and always have. But once I accidentally put in the premium kind, mixing it with the unleaded that was already there, and I thought for sure my car was going to explode or something, but it turned out alright.

So I've just gotten to wondering what the harm is in either putting the wrong kind of gas in your car altogether (on its own), or mixing them (the three types....unleaded, premium, and premium+ or w/e it's called).

Lastly, what about diesel? Presumably using that in a normal car or mixing it with normal gas is a death sentence, but I don't understand exactly what would happen.

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Perhaps what you're looking for is in one of the following: mechanics.stackexchange.com/q/498/57, mechanics.stackexchange.com/q/862/57 or mechanics.stackexchange.com/q/39/57 –  Bob Cross Apr 14 at 1:06
    
Definitely informative –  Aerovistae Apr 14 at 12:01

2 Answers 2

up vote 3 down vote accepted

In a newer car, all that would happen is that your electronic fuel management system will notice that your engine is running too lean (not enough fuel) or too rich (too much fuel) and adjust your ignition timing and fuel injection accordingly. Warning: In an older car with a carburetor (no automatic adjustment of ignition timing), you may destroy the engine for one of the reasons described below.

Different fuels burn at different rates, so the spark plug needs to ignite at just the right time during the ignition stroke, otherwise you don't burn all the fuel before the valves open, which could cause a backfire (if you're lucky) or could cause your engine to try to turn in the wrong direction (if you're not) because the explosion happens before the crank shaft is in the right position (called pinging), forcing it back instead of over the crest of the revolution. Or it burns too hot and melts a piston. In either case, this will eventually lead to the engine blowing a nice paper currency-sized hole in your wallet.

Diesel on the other hand should theoretically not ignite in a petrol/gasoline engine because diesel doesn't vaporise very well (which is why diesel is ignited by the compression of hot air under extreme pressure. Diesel engines don't even have spark plugs), leading to a flooding of the cylinders and a stall. But in the event that the diesel actually ignites, the most likely scenario is that the engine will run rather rough and sputter a lot before finally dying and flooding, possibly bending or snapping one or more connection rods.

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Higher octane fuel does not burn as easily as a lower octane fuel. Higher octane fuels are specified where higher compression ratios are present in an engine, or where forced induction (such as turbo charging) is used. By using higher octane fuel where lower is specified, you will create no problems with your engine. It does not (by popular belief) add any power to your engine, but will not harm it at all. Worst case scenario is you've just wasted some money by buying the more expensive fuel.

If you put the lower octane fuel in an engine which specifies high octane fuel, will not cause you any major issues on an electronic fuel injected engine because it has a device known as a "knock sensor" which will pull timing. When a spark is fired at the spark plug, this event actually happens before the piston reaches top dead center (TDC - upper most position in the cylinder). This allows for the air/fuel explosion time to reach it's maximum force by the time the piston is ready to move back down in the cylinder. This may seem weird in the sense that the explosion takes place so quickly, but if you think about how fast the piston is moving (at 3000 rpm, the ignition fires 25 times per second). Pinging occurs when the ignition of the air/fuel occurs before it is supposed to. This can be caused by a hot spot in the combustion chamber (carbon buildup for instance), by the spark occurring too soon, or when the compression ratio is too high for the fuel. As fuel is harder to burn as the octane goes up, it becomes more stable and less likely to burn before it's supposed to. If lower octane fuel is introduced into an engine, the air/fuel will most likely try to burn before it's supposed to and causing the preignition otherwise known as "ping" or "knock". This will be read by the knock sensor and the computer will pull timing out of the cylinder (or cylinders if multiple occurrences) which is having the issue. When I say "pull timing out", I mean to say, the spark will be made not as advanced. For instance, if there is 36 degrees advance - spark occurring 36 degrees Before TDC - the computer may make it only 34 or 32 degrees BTDC. The main effect this has is to reduce the power output of the engine -- the engine will not be as efficient.

When you mix different octanes of fuel, you are either increasing or decreasing the octane of the fuel at hand. It won't cause any real problems for the engine or fuel system at hand (this assumes you are using fuels of the same mixture of ethanol -- mixing E85 fuel into standard fuel to increase octane and introducing it into a fuel system which cannot handle it - read this -- may cause issues with seals and corrosion of parts which are not built to take the higher concentration of ethanol. E10 fuel poses no issues for modern or older vehicles).

As for diesel, it is unlikely that major engine damage would occur due to "hydraulicing" (term used when a large amount of fluid is introduced into a cylinder) as Juann suggests, only because the engine would not run if pure diesel were introduced. A small amount of diesel would be in the cylinder, but not enough to cause damage. It is possible for a mix of diesel and gas to fire, depending on the mix. It would have to be a lot more gas than diesel, but I don't know what the maximum ratio of diesel would be for it to run. Mind you, it would not run as well as straight gas in a vehicle, but theoretically it could run. You'd see a lot of smoke out of your tail pipe (either blue or black) and it would eventually clog your catalytic convertor. So, not a death sentence, but definitely not good for your car.

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