The Chemistry of Fuel: Petrol vs Diesel #2

Petrol and diesel both come from the same place – crude oil.  In other words, they are natural.

That’s right – they’re natural.

Crude oil that is taken out of the ground is comprised of hundreds and possibly thousands of chemicals.  They are separated into their various components by boiling point by a process called fractional distillation.

There are several implications of this.  Firstly, as we have seen, petrol is carcinogenic and diesel isn’t.

Secondly, petrol is flammable and diesel isn’t.  If you toss a match into a bucket of diesel nothing would happen but if you tossed a match into a bucket of petrol it’d be a different story.  The flammability of petrol has resulted in several fires at petrol stations, mostly caused by static electricity that can jump between the driver and the body of the car as they are fuelling it.

The flammability of petrol of course means that it ignites with a spark plug inside your engine, but diesel won’t.

If you accidentally put diesel into your petrol car by mistake then you would find that initially a lot of smoke would come out and then it would just stop.

Diesel must be heated to a high temperature before it will ignite and this is how diesel motors work – the air is heated to above the auto ignition temperature by the compression of the air in the engine and the fuel is then vapourised into the combustion chamber by the fuel injectors.  The tiny droplets of diesel are then ignited by the hot air.

With petrol, the combustion process only starts at one point – at the spark plug.  The flame must then travel across the cylinder until all the fuel is ignited.  This is obviously a less efficient process than diesel where all the droplets ignite at the same time giving a more uniform combustion.

This may be the reason why diesel motors last longer than petrol motors – the uniform combustion places less mechanical stresses on the engine than a petrol engine which has uneven combustion.

Because petrol is a more volatile chemical, by the time the petrol gets inside the combustion chambers it is in the vapour phase, regardless of whether it has come through an old-fashioned carburettor or whether it is through a fuel injection system.

With diesels, however, the less volatile nature of the fuel has meant that in the past the diesel that is injected is not fully vaporised, but present as lots of small droplets, each of which must individually burn.  The implications of this are twofold.

Firstly, the time taken for the drops to burn detracts from the efficiencies of the engine, and secondly, and probably more importantly, often all the fuel simply does not burn.  This is why old diesel cars and trucks have that residual oily diesel smell about them – it’s all the unburnt fuel.

But modern common rail turbodiesel motors are far more efficient, as the result of extremely high injection pressures.  The much higher pressures result in much more efficient vaporisation, and much more efficient burning of the fuel. That’s why modern diesel motors do not give your car an oily diesel smell.  They also do not blow great clouds of black smoke (which is unburnt fuel) which was an issue on older type diesels.

Tomorrow, we’ll look at the implications of these combustion processes on the mechanical efficiency, performance, and economies of diesel and petrol motors.