Eli5: Why do Diesel engines have a return line back to the diesel storage tank?

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Eli5: Why do Diesel engines have a return line back to the diesel storage tank?

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Some gasoline cars do as well. This is typically with direct injection engines which use a common rail injection system, where there is a very high pressure line that feeds off to individual injectors in each cylinder. Because the pressure is so high, you couldn’t run without a return line or else you would end up burning out the fuel pump pretty quickly.

Many gasoline vehicles also have return lines, just really depends on how the system is designed.

In most cases, there is a transfer pump in the tank which supplies fuel to the injector rail (or injection pump) at a set flow rate.

Since the engine obviously doesn’t need all the fuel, all the time, some systems have a return line to send the excess fuel back to the tank. This is generally controlled by some sort of pressure regulation device.

This prevents the pump from “dead heading”, and just pushing needlessly. Since many in tank pumps are actually cooled by the fuel they are pumping, the constant flow offered by the return line helps prevent overheating.

Return less systems may have a different style of pump, or a controller which varies the speed of the in tank pump depending upon load.

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.

For older diesels with purely mechanical injectors the moving parts in the injector are lubricated by diesel that then flows back to the tank via the return line. You are unlikely to find such a design in a remotely modern car though.