In our discussion of the UFI, we will use the needle valve injector as our main example. Differences between the needle valve and the crown valve are within the tip assembly, and will be pointed out during our discussion. Refer to figures 9-22 through 9-25 as we discuss the operation of the unit injector fuel system. Fuel, under pressure, enters the injector at the inlet side through a filter cap and filter. (Refer to fig. 9-23.) From the filter, the fuel passes through a drilled passage into the supply chamber, the area between the plunger bushing and the spill deflector, in addition to that area under the injector plunger within the bushing. The plunger operates up and down in the bushing, the bore of which is open to the fuel supply in the annular ring-shaped chamber by two funnel-shaped ports in the plunger bushing.

The motion caused by the injector rocker arm is transmitted to the plunger by the follower, which bears against the follower spring. (See fig. 9-23.) In addition to the reciprocating up-and-down motion, the plunger can also rotate around its own axis by the gear, which meshes with the control rack. For the metering of the fuel, an upper helix and a lower helix are machined in the lower part of the plunger. The relation of the helices to the two fuel ports will change as the rotation of the plunger changes.

As the plunger moves downward (due to the force of the injector rocker arm), a portion of the fuel trapped under the plunger is displaced into the supply chamber through the lower port. This action will occur until the port is closed off by the lower end of the plunger. A portion of the fuel trapped below the plunger is then forced up through a central passage in the plunger into the fuel metering recess and into the supply chamber through the upper port until that port is closed off by the upper helix of the plunger. (See fig. 9-23.) With the upper and lower ports both closed off, the remaining fuel under the plunger is subjected to increased pressure by the continuing downward movement of the plunger. When sufficient pressure builds up, the flat, nonreturn check valve opens. The fuel in the check valve cage, spring cage, tip passages, and tip fuel cavity is compressed until the force of the pressure acting upward on the needle valve is sufficient to open the valve against the downward force of the valve spring. As soon as the needle valve lifts off its seat, as illustrated in figure 9-23 (or as soon as the injector valve is lifted off its seat as illustrated in fig. 9-22), the fuel is forced through the small orifices in the spray tip and atomized into the combustion chamber. When the lower land of the plunger uncovers the lower port in the bushing, the fuel pressure below the plunger is relieved. The valve spring then closes the needle valve (or injector valve), and injection stops. A pressure relief passage is provided in the spring-cage. This passage permits any bleed-off of fuel that may leak past the needle pilot in the tip assembly. The check valve, located directly below the bushing (the needle valve in fig. 9-23) or mounted in the spray tip (the crown valve in fig. 9-22), prevents leakage from the combustion chamber into the fuel injector in case the valve is accidently held open by a small particle of dirt. The injector plunger is then returned to its original position by the injector follower spring.

Figure 9-25 shows the various phases of injector operation indicated by vertical travel of the injector plunger. On the return upward move-ment of the plunger, the high-pressure cylinder within the bushing is again filled with fuel through the ports. The constant circulation of fresh, cooler fuel through the injector renews the fuel supply in the chamber, helps carry heat from the injector, and also effectively removes all traces of air that might otherwise accumulate in the system and interfere with accurate metering of the fuel. The fuel injector outlet opening, through which the excess fuel returns to the fuel return manifold and then back to the fuel tank, is directly adjacent to the inlet opening.

A change in the position of the helices, by rotation of the plunger, retards or advances the closing of the ports and the beginning and ending of the injection period. At the same time, a change in the position of the helices increases or decreases the amount of fuel injected into the cylinder.

Look again at the various plunger positions in figure 9-25. With the control rack pulled out all the way (no injection as shown in view A), the upper port is not closed by the helix until after the lower port is uncovered. Consequently with the rack in this position, all of the fuel is forced back into the supply chamber and no injection of fuel takes place. With the control rack pushed all the way in (full injection), the upper port is closed shortly after the lower port has been covered, thus a maximum effective stroke and maximum injection is produced. From this no injection position in view A to the full injection position (full rack movement) in view D, the contour of the upper helix advances the closing of the ports and the beginning of injection.

Figure 9-25.-Injection and metering principles of a unit injector.

Purity of the fuel cannot be overstated in any injection system. The unit injectors have spray tip holes as small as 0.005 inch. Pressures of approximately 20,000 psi are developed by a combination of spray hole area restriction and plunger/bushing design. Typically, the plunger and bushing are matched to a diametrical clearance of 60 millionths of an inch to prevent leakage during the injection cycle. As you can see, impurities of any kind can damage the unit.