What is the difference between diesel and petrol, and why can’t cars take both?
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Each fuel was designed to operate in a different type of engine. That is, it is more that the fuel was optimized for the type of engine rather than the engine being optimized for the fuel.
Spark ignition engines — which is what petrol engines are — generally have the mixed fuel and air in the cylinder before ignition occurs. It’s not good if this mixture ignites before the spark occurs. Because of this, petrol has additives that decrease how fast it will auto-combust under compression. This is what the octane rating measures. A higher octane rating means more delay before auto-combustion occurs.
Diesel engines inject the fuel at the last instant. Here it is best if the fuel auto-combusts immediately. Diesel has additives that make auto-combustion occur faster. Diesel has a cetane number. A higher cetane number means less delay.
Because of the difference in operation between the two engines (spark ignition vs auto-ignition), the two fuels need somewhat opposite traits — fast vs slow auto-ignition. Thus, they don’t work well in each others engines.
Bonus note: Gas turbine engines (aka jet engines) have a continuous flame burning in them after they are started. Thus how quickly the fuel starts to burn (in tiny fractions of a second) is not important. This type of engine can be fairly easily designed to work on diesel, gasoline, and other flammable liquids.
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