The Chemistry of Fuel: Petrol v Diesel #1

We hop in our car, press the accelerator and the car moves.

The energy required for this process comes from the chemical energy stored in the fuel that you put in your tank, whether it be petrol, diesel, or LPG.

The fuel reacts with the oxygen in the air to produce water, carbon dioxide, and energy.

If we consider octane (nominally petrol) the reaction is given by

2C₈H₁₈ + 25O₂ = 16CO₂ + 18H₂O + energy

So we can see that there are two components required for fuel to burn – the fuel and the oxygen.  So the combustion is actually an oxidation process.  Technically, we could say that the combustion process requires the oxidant and the thing that gets oxidised.

Now, of course, in an internal combustion engine we rely upon oxygen from the air – so, if you like, half the components of the reaction that provide the energy for how the car moves are free.  If you have a rocket, however, you cannot rely upon getting oxygen from the air, as there simply isn’t enough of it – so rocket fuels are a two component mixture, including the fuel and the oxidant, but that’s a story for another day.

Back to petrol.  Because we are relying upon oxygen from the air, sometimes there isn’t enough of it, so instead of the reaction above we get

2C₈H₁₈ + 21O₂ = 8CO₂ + 8CO + 18H₂O + energy

so instead of getting all carbon dioxide we now also get carbon monoxide (CO).

Bt we have all seen the black stuff that accumulates in our exhaust pipe – carbon.  This tells us that sometimes there is even less oxygen than this and we get this reaction

2C₈H₁₈ + 16O₂ = 6CO₂ + 6CO + 4C + 18H₂O + energy

Or, expressing it in words:

Octane plus oxygen gives carbon dioxide plus carbon monoxide plus carbon plus water plus energy

By the way, this is why you sometimes see water trickling out of exhaust pipes first thing in the morning, and why exhaust pipes go rusty.  When the motor is still cold, the hot water vapour that has just been formed in the engine condenses in the cold metal of the exhaust pipe and so dribbles out the end – rather like an old hillbilly still.

But when the engine warms up and the exhaust pipe warms up it stays in the vapour phase and you don’t see the dribbles any more.

Petrol and diesel are made up of different chemicals.  They are both mixtures and both come from the same source – crude oil – but have different components.  Petrol consists of short chain hydrocarbons, some aromatics, and is highly flammable with a flash point of -44° C.

Diesel on the other hand, has long chain hydrocarbons, is not flammable (it has a flash point of 75 to 80° C)

And this is the first significant difference between the two – petrol contains aromatics and is therefore carcinogenic, whereas diesel does not contain aromatics and is therefore not carcinogenic.

Also petrol is much more volatile (evaporates more easily) and it therefore is not as easy to store or transport.

So right away we can see that diesel is both safer and an easier fuel to handle.  Tomorrow will look at how they burn and what the implications are for the performance of an engine.

 

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