What is fuel pressure

Question

(in cars)

In: 0

Answer 1

Fuel pressure is simply the pressure the fuel has in the fuel system. If you were to disconnect the fuel line from a fuel injector, fuel would spray out of it. If there was more pressure, the fuel would spray out faster/farther. If there was no pressure, the fuel would not spray out at all.

Fuel pressure in cars is important for two reasons. First, the fuel tank is usually at the same height or lower than the height of the fuel injectors. Something has to push the fuel up to the engine. Otherwise, it would just stay in the tank. Even worse, if the fuel tank is mostly empty, there is even more of a height difference. The fuel pump — typically in the tank, but can be somewhere else, or even mounted to the engine — provides the pressure to push fuel to the fuel injectors.

The second reason, specifically for fuel injected engines, is that the injectors have to spray a very fine mist into the intake or cylinders. It cannot merely dribble drops of fuel. Think of how a can of spray paint or window cleaner sprays out. This allows it it mix with the air. In order to get a good spray/mist, it has to be pressurized. (The spray paint can is pressurized with chemicals, and the window cleaner is sprayed when you squeeze the trigger, which is a manual pump.)

There is a difference between pressure and flow. When you take your car to the station to fill your tank up, you are not worried about the pressure of the station’s pump. All that matters is that it flows fuel quickly enough to fill your tank before you get too bored standing there. Your car’s fuel system has to flow enough fuel, to keep the air-fuel mixture correct, as well as the pressure required to spray/mist the fuel through the injectors.

This can actually get much more complicated, as in the case of modern vehicles that aim to be as efficient as possible, or race/performance cars that need to work as consistently as possible through high-speed turns and rapid acceleration, and can be much less complicated, in much older cars and small engines.