Question: How do petrol and diesel differ in properties and refinement?

Chris Armstrong answered on 23 Jun 2015:

Petrol and Diesel are both hydrocarbon chains found in Crude oil, and almost a blanket term for a collective rather than a single hydrocarbon type.

Getting both of these out of crude oil requires the same process – fractional distillation. This takes advantage of the fact that increasing lengths of hydro carbons have increasing boiling points.

Petrol has a lower boiling point than diesel so these two can be separated that way.

However there’s far longer chains than diesel present in crude oil so processes like “cracking” can help convert these long chains into diesel or even diesel into petrol. “Unification” works in the other way combining short chains into the longer ones.

In terms of properties they differ quite a lot! Petrol requires spark ignition, whereas diesel is ignited via incredibly compressed air. (Air under intense pressure but fixed volume must increase in temperature by the ideal gas law). Diesel (by volume) contains more energy than petrol and generally provides a higher mpg than petrol engines can. Largely down to the efficiency of the compression ignition, the fuel is injected into the hot compressed air which evenly dissipates the fuel allowing even combustion, in petrol engines they have to encourage this perfect air/fuel mix in other ways, rather than it being aided by the air pressure.