Some of us drive Diesel cars, some Petrol (gas) of the leaded kind and some of the unleaded variety. We all can intuitively understand that an engine built to use one type will have trouble using another. But why and how much trouble is data we normally don’t posses until it’s too late.

Found myself driving the other day with a friend and he told me that someone borrowed his petrol car and put in the tank a gallon of diesel and drove it for a bit. When my friend found out he consulted with a mechanical authority – his dad – that said that for that amount just fill the tank with petrol and the car should run fine on the mix. So he did and it did.

The thing is that if we don’t understand the problem we can’t really understand how we should act.

Lets start with the easy stuff – Leaded petrol is considered bad for the environment and so it is becoming harder to find, and the use of unleaded petrol in a “leaded” engine is considered safe enough and shouldn’t cause problems of any significant scale for cars made in the 1980’s and onward, they designed it that way. The opposite is a bit more problematic as “unleaded” engines come equipped with a catalytic converter that can and will be damaged by the lead residue if using leaded fuel, one single wrong fill-up shouldn’t damage it too much but repeated miss-fueling will.

Now to the interesting part – diesel and petrol engines have two major differences that come from the differences of the fuels they use, as petrol is much more light and flammable then diesel. A diesel engine has no spark plug and the combustion is due to the high compression ratio (~1:20) that results in high temperature of the fuel/air mixture and ignition. A petrol engine gets to about 1:10 and so needs a spark. And so if you put petrol in a diesel engine the engine is likely to be damages very fast – the gas will ignite before the engine reaches the end of compression stroke and so the timing of the ignition is wrong and the pistons are damaged. Another thing to note is that in a diesel engine the fuel also lubricates some parts and if used with petrol (with worse lubricant characteristics then diesel) those parts will be greatly damaged (the fuel pump is normally the first to go).

A petrol engine with diesel fuel will seize up pretty fast as its compression ratio is not enough to ignite the heavy diesel, even with the added spark plug. A good mechanic should get it started again after some good cleaning of all the pipes and cylinders.

A bad fuel/air mix can cause havoc...

A bad fuel/air mix can cause havoc…

And about putting “just a little” of the wrong fuel in – as long as it mixes with the proper fuel at a ratio that will not really affect anything (5-10%, don’t make a habit out of it Though), the car should run good. If the ratio is worse then avoid starting it until you get it low enough. Trust me, it’s better to spend two extra hours at the gas station fixing what you did then getting stranded on a god forsaken road with a broken car.