eli5: Difference between petrol and diesel?
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There are two methods of ignition in internal combustion engines; spark-ignition (SI), and compression ignition (CI). For SI engines, a spark plug ignites the fuel-air mixture, for CI the fuel-air mixture is compressed until it gets to the proper pressure and temperature where it ignites on it’s own.
Petrol/gasoline and diesel are the fuels optimized for these two cycles in automotive internal combustion engines. Petrol is designed to be resistant to autoignition, with ignition (and thus combustion control) being given over to the ECU via the timing of spark ignition. Diesel is designed to deliberately autoignite under compression, and control is achieved by the precise timing the fuel is injected (as the ignition delay period between injection and ignition is something we have extensive correlations for).
The result of this is that petrol/gasoline is more volatile and tends to vaporize very easily on it’s own, whereas diesel is heavier and needs a little help vaporizing (which is also why diesel engines need glow plugs for cold start conditions). Diesel also has a lower combustion velocity, thus they tend to top out at a lower RPM than SI engines. There is a mild difference in energy density, but contrary to popular belief the energy density differences have absolutely no real bearing on fuel economy or power between the two cycle types. Instead, fuel economy is driven by entirely by the compression ratio and aspiration of the engine (i.e. turbocharging), and power is driven by the amount of ***air*** that the engine can ingest, as fuel delivery and injector design is basically a solved problem; the injectors are always oversized, and you can always get more fuel than you actually need at any given moment.
Beyond these fuels, there are others that are suitable as stand-ins. Ethanol alcohol is actually an excellent spark-ignited fuel, and has excellent resistance to autoignition, hence why it’s used as a high-performance fuel. On the other end, Kerosene and Jet Fuel are both extremely similar to diesel, to the degree that basically any diesel engine can run on kerosene or jet fuel with no modifications in the short-term (in the longer-term there can be lubricity issues, but that’s largely a function of engine design and can be countered in well-designed/well-calibrated engines).
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