Naked Science Forum
Non Life Sciences => Chemistry => Topic started by: chris on 16/12/2010 03:30:46
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How cold must it become before diesel and petrol begin to freeze?
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Without knowing the composition of the particular fuels, it's not possible to give a very precise answer. However, petrol (gasoline) has a much lower freezing point than diesel fuel. The range for petrol seems to be somewhere between -40°C and -60°C, so it's unlikely you will ever have a problem with your fuel freezing if you run your car on petrol.
There are also a lot of different types of diesel fuel, but the "pour point" might fall in a range between -6°C and -18°C (*), although these values can be lowered with additives. The pour point is the temperature at which diesel turns into a sort of jell which can no longer be pumped, so it's essentially useless as an automotive fuel at, or below, those temperatures.
(* Marks Standard Handbook for Mechanical Engineers)
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As I recall, the 'waxing' temperature of winter diesel in the UK is -15oC. But as Geezer pointed out, addatives can be used to lower the waxing temperature to around -22oC.
Diesel is usually refined to be suitable for the region and season it is to be used in.
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Why do the two different fuel types, petrol and diesel, have different freezing points in the first place?
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Because they are substantially different molecules. Diesel consists mostly of straight chains of 12 - 20 carbon atoms, like paraffin wax and poyethylene, whereas petrol is a mixture of shorter chains around C8 and C6 rings.