A high-pressure direct injection fuel pump can vary the pressure to the injectors from around 300 to 5,000 psi. So, how does it do it? This is the job of this electric solenoid on the side of the unit that controls the pressure and the volume of fuel.
During the inflow sequence of the cycle, the plunger or piston travels downwards. The control solenoid opens, and fuel enters the high-pressure side of the pump. The next part of the sequence is called the “spill” process or cycle. This happens when the piston or plunger starts to travel upwards, the control solenoid remains open for a specific period that is measured in milliseconds. Some of the fuel will spill back into the low side or supply side of the pump. The longer the control solenoid stays open, less pressure will be generated. A shorter open period will create more pressure.