One type of unit fuel injector is commonly known as a HEUI injector, the four-letter acronym standing for hydraulically-actuated, electrically-controlled unit injector.
A HEUI injector has a fuel inlet port communicated to a source of fuel under pressure, such as pressurized fuel in a fuel rail. It also has an oil inlet port communicated to a source of hydraulic fluid under pressure, such as pressurized oil in an oil rail. Fuel is injected out of the injector through orifices in a nozzle having a tip end disposed within the head end of an engine cylinder.
Injection of fuel is controlled by an electric actuator that when actuated opens a valve that allows oil from the oil rail to pass through the oil inlet port and apply hydraulic force to a piston that is disposed at one end of a plunger. The hydraulic force is transmitted through the piston and the plunger to fuel that the fuel pressure in the fuel rail has forced into the fuel injector, increasing pressure on fuel in the injector. The increased pressure is applied both to an inlet check and to a reverse flow check.
When the actuator is not actuated, the inlet check allows fuel to flow from the fuel inlet port through an inlet through-passage to replenish fuel in the injector. When the actuator is actuated to increase pressure on fuel in the injector, the inlet check is forced to close the inlet through-passage, thereby preventing the replenished fuel from back-flowing out of the injector through the fuel inlet port. The increased fuel pressure also forces the reverse flow check to open a high-pressure injection passage to the nozzle so that increased fuel pressure applied along the high-pressure injection passage can unseat a spring-biased needle from a seat in the nozzle and allow the hydraulic force being applied to the piston to displace the piston and the plunger and force fuel through the high-pressure injection passage to the nozzle and out of the nozzle orifices.
When plunger displacement ceases, fuel injection out of the nozzle orifices ceases. A return spring forces the plunger and the piston to retract, and the reverse flow check operates to substantially close the high-pressure injection passage. By substantially closing the high-pressure injection passage, the reverse flow check avoids the creation of a sudden large pressure drop in the high-pressure injection passage that could otherwise occur as the retracting plunger is creating low pressure that opens the inlet check and draws replenishment fuel into the injector.
When the injector has been replenished, the next actuation of the actuator again forces the inlet check closed to prevent backflow of fuel out of the injector through the fuel inlet port, while forcing the reverse flow check to open. The increased fuel pressure along the high-pressure injection passage unseats the needle against the opposing spring bias to open the high-pressure injection passage to the nozzle orifices, allowing fuel to be injected into an engine cylinder as the plunger extends. When the actuator ceases being actuated, the pressure that the plunger is applying to the fuel drops, allowing a bias spring to re-seat the needle and thereby terminate injection.
Fuel that enters a unit fuel injector has typically already been filtered by one or more filtration devices in the fuel system upstream of the unit fuel injector to remove particulate matter that may be present.