Figure 5-21. - Spring-loaded ball-check valve.

maintained by a spring-loaded ball-check return fitting (fig. 5-21) in the governor cover of the pump.

Nozzle

The injector nozzle, used with the DB2 fuel injection pump, is opened outward by high fuel pressure and closed by spring tension (fig. 5-22). It has a unique feature in that it is screwed directly into the cylinder head An outward opening valve creates a narrow spray that is evenly distributed into the precombustion chamber. Both engine compression and combustion pressure forces assist the nozzle spring in closing an outward opening valve. These factors allow the opening pressure settings of the nozzle to be lower than those of conventional injectors.

During injection, a degree of swirl is imparted to the fuel before it actually emerges around the head of the nozzle. This forms a closely controlled annular orifice with the nozzle valve seat, which produces a high- velocity atomized fuel spray, forming a narrow cone suitable for efficient burning of the fuel in the precombustion chamber.

Figure 5-22. - Injector nozzle.

The nozzle has been designed as basically a throwaway item. After a period of service, the functional performance may not meet test specifications. Nozzle testing is comprised of the following checks:

Nozzle opening pressure

Leakage

Chatter

Spray pattern

Each test is done independently of the others (for example, when checking the opening pressure, do not check for leakage). If all the tests are satisfied, the nozzle can be reused. If any one of the tests is not satisfied, replace the nozzle. For testing procedures, consult the manufacturer's service manual.

CAUTION When testing nozzles, do not place your hand or arms near the top of the nozzle. The high-pressure atomized fuel spray from the nozzle has sufficient penetrating power to puncture flesh and destroy tissue and may result in blood poisoning. The nozzle tip should always be enclosed in a receptacle, preferably transparent, to contain the spray.

DETROIT DIESEL UNIT INJECTION SYSTEMS

The fuel system used by Detroit diesel is known as a low-pressure fuel system, owing to the fact that fuel delivered to the unit injectors averages 45 to 70 psi. This is much lower than the average 2,500 to 300 psi that passes through the fuel line from the injection pump and nozzles used in other systems.

The four main functions of the fuel system used with a Detroit diesel engine are as follows:

1. To supply clean, cool fuel to the system by passing it through at least a primary and secondary filter before the pump and injectors.

2. To cool and lubricate the injectors, as the fuel flows through them, and return to the tank (recirculatory system).

3. To maintain sufficient pressure at all times through the action of the positive displacement