HEUI Fuel Injection System
FIGURE 54-30
A hydraulically actuated electronically controlled unit injector (HEUI) fuel injection system.

Another popular fuel injection system is the hydraulically actuated electronically controlled unit injector (HEUI) system Figure 54-30. This diesel fuel injection system 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. Then, fuel flows 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 PCM. The PCM controls the duration of fuel injection pulses based on a range of input variables discussed earlier in the chapter. Once the injector solenoid has been activated, oil from a high-pressure oil pump hydraulically actuates the injectors. In other words, the injectors are operated by hydraulic pressure instead of a cam and mechanical linkage.

By varying the oil pressure, injection can be controlled independently of the engine speed, or crankshaft and camshaft position. 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 18,000 to 24,000 psi (124,000 to 165,000 kPa). The area of the head of the intensifier piston is approximately seven times the area of its plunger. As a result, a 7:1 pressure increase on the fuel beneath the plunger can be achieved.

When working on modern diesel engines with high-pressure fuel systems, be sure you have the correct manufacturer’s service information for the engine. On some models, residual pressure may exist in the fuel lines even though the engine is not running.