h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Maybe I should have posted this in EngEx, but whatever.

What's the difference between diesel and regular fuel? What is the difference between the corresponding engines? And why could filling your car's tank with the wrong one lead to disaster?

(More specifically -- legend has it that in one direction the mistake is worse than the other: using diesel in a regular engine, or using regular petrol in a diesel engine. Which one is is, and what are the consequences of either mistake?)

That's a huge question, but to try and break it down to some specifics:

Let's start with crude oil. Crude oil is a mixture of hydrocarbons (molecules containing hydrogen and carbon) of a whopping variety of carbon chain lengths. We separate it via fractional distillation, meaning we can draw apart various carbon chain lengths because they have different boiling points. The chain length mixture is the main chemical difference between petrol and diesel. Petrol is a mixture of carbon chains between 5 and 12 atoms long; diesel is a mixture of carbon chains between 8 and about 20.

Practically, there is a helluva lot more chemistry goes into the composition of your fuel, but if you want to know about cracking, isomerism and oxygenates it might be better left for another thread.

The main difference between petrol and diesel engines is in the method of ignition. Diesel engines use the Ideal Gas Law (which, contrary to its name, does work in real-life applications), to ignite the fuel. Air is pressurised in thye cylinders, and as gases are pressurised, the temperature increases to an extent where the fuel will ignite (react with the oxygen in the air) spontaneously. As such, they are very efficient by engine standards, being able to convert somewhere in the region of 40% of the fuel's stored energy.

Petrol engines use spark plugs to ignite a mix of fuel vapour and air. There is a lot of chemical wastage in this process, so petrol engines are only about 20% efficient.

Diesel fuel is incidentally more energy-efficient than petrol for another reason. Because it has longer carbon chains, there is more inter-molecular force between molecules and hence it is more dense. Therefore there are more intra-molecular bonds per litre, and hence more energy density.

Diesel in a petrol engine probably will not ignite, therefore it will flood the carburettor and cylinders. It will require an engine and fuel-line drain, which will cost you a couple of hundred quid and a day off work.

Petrol in a diesel engine could be potentially catastrophic. It has a lower auto-ignition temperature than diesel and is much more volatile. If it bleeds into the air compression system, then say goodbye to your engine. Equally, it will create a higher vapour pressure than a diesel engine is designed to tolerate, and hence a higher force under ignition. Again, bye-bye engine.

B

It depends on the petrol/diesel mix. It used to be (still is?) common practice in colder climates to add a small amount of petrol to a diesel tank to make it easier to start up the engine in sub-zero weather, diesel being prone to become waxy when it's cold.

I once mistakenly put petrol in a diesel tank. The mix was about half and half and I got a couple of miles before it conked out. We towed the van to the mechanic, drained the engine, refilled it with diesel and it fired up. He told me that the only problem I might have had was that the petrol could perish a rubber seal in the fuel pump.

Thanks for these neat and clear answers!

I too thought this was a nice, clearly written question and answer thread!

And now for the requisite tangent: It's been my (unfortunate) experience that the main problem with putting gasoline (petrol as you say on that side of the pond) in a diesel-powered vehicle is the potential destruction of the relatively high pressure diesel fuel pump, which is a rather expensive part to replace. I think diesel motors can handle gasoline for a short period without failure. If the detonation of the gasoline vapors doesn't overheat or otherwise damage the rings on the cylinder, I wouldn't imagine there's much problem with it in the motor itself.

- JD

Yes, of course. I should have considered the fuel pump. Diesel pumps are designed to delive a more viscous fluid, so could easily blow apart under pressure when thin petrol is put through. On the flipside, a petrol fuel pump will just clog on diesel and can be drained.

B

In addition to the pressure issues the diesel pump seals are lubricated by the fuel. Petrol (gas - over in the colonies) causes them to perish and when they disintegrate the pump and fuel injection system are toast.

t.