Figure 5.33 Classification of diesel engine fuel systems [Sher].
70 Rotary fuel systems This class of fuel system is typified by compact fuel pumps of four-, five-, six-, and eight-cylinder configuration - the main applications being turbocharged diesel engines used in passenger cars and light commercial vehicles. The rotary or distributor pump was popular for many years in cost-sensitive applications that did not require the highest levels of injection system performance, for example, in light marine, agricultural, and power generation applications. The rotary pump is the fuel system of choice for the generation of passenger-car, high-speed, direct-injection (HSDI) engines developed from the mid-1980s to the present day for the European market. In order to meet the many needs of the passenger-car application, these mechanical pumps have become very complex as they attempt to perform complex functions with simple hydromechanical control elements. Typical mechanical control features include idle and high-speed governors and engine/ambient temperature compensation, including cold start excess fuel, transient air-fuel ratio (boost) control, and injection timing control. Electronic unit injectors Electronic unit injectors were first introduced in heavy-duty truck engines in the United States in the late 1980s. Although a number of implementations exist, having somewhat different characteristics, the underlying principle of operation and inherent advantages relative to pump-line-nozzle systems are the same.
71 Figure 5.34 Example of a direct-metered EUI [Sher]. The electronic unit injector (EUI) is a modular, integrated fuel injection system mounted directly in the cylinder head of the engine. The pumping element and delivery nozzle are integrated into a single unit, the pumping plunger being driven by the engine camshaft, usually via a rocker lever. In the direct-metered unit injector, the space between the plunger and the nozzle is opened and closed by a fast-acting solenoid valve as shown in Figure 5.34. The valve is closed to initiate injection and opened to terminate injection, thereby providing control of injected quantity and injection timing. Typically, injection pressure is related to injection timing since the cam profile does not generally allow constant injection rate over a wide range of timings. The key to the success of the unit injector is the very small dead volume between the pumping plunger and the injector nozzle. This allows very high-pressure operation with a relatively high efficiency. EUIs with pressure capability as high as 1600 bar are in service and pressures as high as 2400 bar have been demonstrated. The EUI is the preeminent fuel system for on-highway heavy-duty trucks at this time, combining very high-pressure capability with low power consumption and high reliability. The disadvantages of the EUI for smaller HSDIs are the difficulties of packaging, and the impact of the EUI on the total engine design and manufacturing process.