With heaps of air, the engine reacts by trying to throw more fuel into it to try create the perfect mix of air and fuel for combustion. What is this perfect mix? Going back to chemistry, it is ideal for every 14.7 parts of air to have 1 part of fuel. If there is more air added, then it’s known as a leaner mix, less air = richer mix.
From the tank, fuel is pumped along lines and through filters down into the engine (most of the time the pump is in the tank). In the engine bay, the fuel lines become pressurized as it reaches the dead end known as the fuel rail (Below with fuel injectors)
Tis me unbolting the fuel rail btw :D
Anyway, along the fuel rail are small injectors which are electronically controlled and closed when the engine is switched off. As long as they’re closed the fuel system will remain at a constant pressure of around 36 psi + boost pressure from a turbo (if any). When the computer decides it’s the right time to inject fuel in, it sends a voltage to the injector which forces it open for a specific duration (Pulse duration). With this, the air fuel ratio can be controlled by lengthening pulse duration. I could talk about fuel injectors all day but that’ll be for another time.
There are two types of fuel systems: the return and non-return variety. Return fuel systems (beloooowwww) let the excess fuel run back to the tank once the system has reached a certain pressure (monitored by the fuel pressure regulator). This means the pump is constantly working at a constant load.
The non-return type allows the pump to vary it’s pressure so only enough fuel will be pumped in to keep the system pressurized. Whichever you have, they’re both awesome systems and there’d be no need to switch from one to the other.
Usually if you work on your car and have to relieve the pressure of the fuel system (WHICH YOU MUST DO IF YOU PLAN ON REMOVING ANYTHING FUEL RELATED), just open the fuel filler cap. the you can remove the rails as you please
Also note, when you put everything back together, you need to “prime” the injectors before you can run them: switch the car on but don’t start it. Leave it for a few seconds and switch it off. Repeat a few times then start ‘er up.