Basically you have to distinguish between the low pressure and the high pressure system. The low pressure system feeds the fuel electrically to the high-pressure pump. Both systems are adapted constantly to the demands of the engine control system. The power input of the pump and consequently the pressure are adjustable. There is no fuel return from the high-pressure pump. Just, vapor bubbles are led back from the fuel filter to the tank. The electric fuel pump feeds just enough fuel. This way the fuel remains cold. No energy is wasted and the service life of the fuel pump is increased. In certain operating conditions, e.g., the opening of the driver's door or shutting-off of the engine, there is a specific preliminary or after-run of the fuel feed pump perhaps with higher pressure.
The pressure in the high-pressure system is also variable. However, because this pump is driven mechanically, an excess-pressure valve is used. Thus, 30 bar are possible, e.g., withdeceleration fuel shut off or 120 bar at full performance. A pressure sensor transmits this information to the control device. Because the high pressure is substantially higher than the supply pressure, its return pipe does not go back to the tank, but into the feed pipe just before the high-pressure pump. Apparently, due to the high pressure there are no problems with vapour locks or ray formations.
The injectors must be sealed carefully, comparable to the spark plugs, against the combustion chamber. Plastic gaskets are used. They are not comparable to the injectors of the Diesel-Common-Rail. The pressure is substantially lower and there is no sharp, bundled up ray, yet a delicate atomized ray with an exactly predefined angle and twist, possibly, e.g., by twist plates. Also the positioning of the injectors varies a lot and influences the air flow together with the ray formation. 07/07