What is the difference between diesel and petrol, and why can’t cars take both?
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The difference between the two is how easy they are to ignite.
To ignite a hydrocarbon, first you need it to evaporate into a gas and mix with the surrounding air to create a suitable fuel to oxygen mixture that will burn.
Petrol evaporates very easily – and does this at a fairly normal temperatures and atmospheric pressures. Leave petrol in an open container and it will start to evaporate naturally and form a slowly expanding explosive atmosphere around that container. Allow a source of ignition to touch this cloud of vapour and it will ignite – it doesn’t have to touch the liquid petrol at all.
This is why dousing a bonfire in petrol to light it is so dangerous – leave that petrol time to evaporate and your pile of wood will be sitting inside an explosive cloud that will all light very suddenly and explosively.
Diesel on the other hand doesn’t evaporate until a higher temperature. So to get diesel to ignite you first need to heat it up until it starts to evaporate, and only then will ignition occur.
To can continue each way up the grades of hydrocarbon – something like propane or butane evaporates into a gas at very low temperatures so we will rarely see a liquid form (and is suitably explosive), while heavier oils won’t evaporate until much, much higher temperatures.
What this all means is that we need to account for how the fuel ignites when we design an engine.
A petrol engine works because the fuel will vaporise on its own, and all you need is a spark to ignite it – so we squirt fuel into the engine cylinders and light it with the electrical spark created by a spark plug.
A diesel engine first has to turn the fuel into vapour, so does this by compressing the fuel – as it is compressed it heats up rapidly, causing the fuel to first vaporise, and then ignite itself from the temperature.
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