The basic fuel flow is as follows:

The fuel pump draws fuel from the tank past a one-way no-return check valve into the primary filter. Here the fuel passes through a 30-micron- filtering-capacity, cotton-wound, sock-type element. From the primary filter it passes up to the suction side of the fuel pump. Here the fuel is forced out at 65 to 75 psi to the secondary filter that is a pleated paper element of lo-micron filtering capacity.

Fuel then passes up to the inlet fuel manifold of the cylinder head where it is distributed through the fuel jumper lines into each injector.

All surplus fuel (not injected) returns from the injectors through fuel jumper lines to the return fuel manifold, through the restricted fitting, which maintains adequate fuel pressure in the cylinder head at all times, then back to the tank.

Injectors

The fuel injector, or what is often referred to as a unit injector (fig. 5-25), is used by Detroit diesel in all series of engine that they build. Certainly, there are some variations in basic design and in the actual testing procedures used; however, the function and operation is the same for all.

Unit injectors were designed with simplicity in mind both from a control and adjustment outlook. They are used on direct-injection, open-type, two-cycle combustion chamber engines manufactured by General Motors. No high-pressure fuel lines or air-fuel mixing or vaporizing devices are required with these injectors. The fuel from the fuel pump is delivered to the inlet fuel manifold (cast internally within the cylinder head) at a pressure of 65 to 75 psi. The fuel then flows to the injectors through fuel pipes called jumper lines. Once the fuel from the pump reaches the injector, it performs the following functions:

1. Times injection: liming of the injector is accomplished by movement of the injector control rack, which causes rotation of the plunger within the injector bushing. Since the plunger is manufactured with a helical chamber area, this rotation will either advance or retard closing of the ports in the injector bushing, and therefore the start and end of the actual injection period. Pushrod adjustment establishes the height of the injector follower above the body. In turn, this factor establishes the point or time that the descending plunger closes the bushing ports, allowing injection to begin.

Figure 5-25. - Unit injector.

2. Meter the fuel: The rotation of the plunger by movement of the injector control rack will advance or retard the start and end of injection. If the length of time that the fuel can be injected is varied, the amount of fuel will be varied.

3. Pressurizes the fuel: Fuel that is trapped underneath the plunger on its downward stroke will develop enough pressure to force its way past the check valve or needle valve, therefore entering the combustion chamber.

4. Atomizes the fuel: Fuel under pressure that forces its way past the check or needle valve must than pass through small holes or orifices in the injector spray tip. This passage breaks the fuel down into a finely atomized spray, as it enters the combustion chamber.

The two-stroke Detroit diesel engine unit fuel injector is located in the cylinder head. The injector sits in a copper tube in the head that is surrounded by water for cooling purposes. The injector is placed in the cylinder head by a dowel pin on the underside of its body. The injector is held in place by a single bolt and