Eli5: Why do Diesel engines have a return line back to the diesel storage tank?
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Diesel engines need high pressure fuel, higher then the compression generated by the pistons. So they have a high pressure fuel pump. A lot of gasoline engines have a similar pump as well but at lower pressures. But fuel can not be compressed. This makes pump designs a bit harder. If you put just a single droplet of fuel into the fuel system without letting it escape it would have to find a way to get out. Older diesel fuel systems have the diesel pump synchronized with the timing of the engine so that the fuel from the pump gets injected into the cylinders at the exact same time. However it can cause issues with timing if you want it controlled by computers. So instead modern engines have the diesel pump make as much high pressure fuel as possible and then if the pressure is too high it gets released back to the fuel tank. So there is always high pressure fuel available for the injectors, not just when needed.
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