Figure 5.33
Classification of diesel engine fuel systems [Sher].
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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.
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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.
