The Hydraulically actuated, Electronically controlled Unit Injector or HEUI system of diesel fuel injection operates by drawing fuel from the tank using a tandem high and low pressure fuel pump. The pump circulates fuel via the 'low' side of the pump at low pressure through a combination of fuel filter, water separator, and heater bowl, and then back through the 'high' side of the pump at high pressure into the fuel galleries located in the cylinder head, and through to the injector units.
The injectors are controlled by a Powertrain Control Module or PCM. Although the PCM controls the duration of fuel injection pulses based on a range of inputs, oil from a high-pressure oil pump hydraulically actuates the injectors.
By varying the oil pressure, injection can be controlled independently of the position or speed of the engine crankshaft or camshaft. A solenoid-actuated valve controls the high-pressure oil flow which is applied to the top of an intensifier piston in the injector. This can increase injection pressures to 1250 to 1800 Bar or 18,000 to 24,000 PSI.
The area of the head of the intensifier piston is approximately 7 times the area of its plunger. As a result a 7:1 pressure increase on the fuel beneath the plunger can be achieved.