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In unit injector (UI) and unit pump (UP) systems, each engine cylinder is served by a separate injection pumping element or injection pump in close proximity to the cylinder. Unit pump (UP) systems enable short high pressure fuel lines by locating the pump close to the injector. Combining the pumping element and the injector into one assembly as in unit injector (UI) systems, allows these lines to be eliminated altogether. The elimination—or length reduction—of high pressure fuel lines in the UI/UP injection systems results in two benefits:
With regards to fuel pressure, it should be noted that common rail fuel injection system pressures have risen as well and in some systems have reached or exceeded the pressures available from UI/UP systems. While there is no technical reason keeping UI/UP pressures from rising even further, engine manufacturers are increasingly using common rail systems in applications traditionally dominated by UI/UP systems. For this reason, UI/UP systems will likely see little evolution beyond their current peak pressures of about 250 MPa.
Both the UI and UP systems are driven from the engine camshaft. In one common mechanical system design, fuel control was typically achieved by rotation of the pumping element (plunger) in the same way as is done in P-L-N systems. With the introduction of electronics to diesel engines, electronic unit injector (EUI) and electronic unit pump (EUP) systems were developed. These employ an electromagnetically operated spill valve for fuel control.
Due to the presence of fuel lines, the unit pump system can be classified as a variant of the P-L-N injection system. However, the design of unit pump and unit injector systems is often similar, making it convenient to discuss these systems together. In fact, some manufacturers offer their injection systems in both UI and UP versions (compare Figure 4 and Figure 11).
The commercial application of unit injectors started in the 1930s on Winton (a GM subsidiary) and GM diesel engines. Winton continued to supply engines to the Electro-Motive Corporation (EMC), while GM transferred diesel engine production to its Detroit Diesel Division. The Detroit Diesel Corporation’s two-stroke engine line is one of the better known applications of unit injector technology. From the 1930s to the mid-1980s, Detroit Diesel used a mechanical unit injector design. In 1985, Detroit Diesel’s Series 92 two-stroke engine became the first heavy-duty diesel engine to adopt electronically controlled unit injection . Since this introduction of electronic control, unit injectors continued to evolve to higher levels of sophistication. The evolution for light-duty and heavy-duty applications followed different paths.
Possibly the most advanced design of unit injector for light-duty applications is the PPD injector produced briefly by Volkswagen Mechatronic (a joint-venture between Volkswagen and Siemens VDO) starting in 2004 for model year 2006 Euro 4 applications. This injector used a piezoelectric actuator and was capable of up to 2 pilot injections and 2 secondary injections in addition to the main injection event. However, it came at a time when common rail systems had already taken hold in light-duty applications and were quickly gaining ground. The PPD injector could not compete with common rail systems and was phased-out soon after its launch. Starting in 2007, it was replaced with common rail for Euro 5 applications. Common rail systems have since become the preferred choice for light-duty applications and unit injectors are quickly disappearing from new engine designs.
For heavy-duty applications, electronic unit injectors continued to evolve. The evolution of some of these designs is described in the paper on injection systems in HD engines. The pinnacle of heavy-duty unit injector design is represented by the two-valve designs of Delphi’s E3 and Caterpillar’s MEUI-C injectors for engines meeting US EPA 2007 on-road emission standards. While these advanced unit injector designs have capabilities such as rate shaping and multiple injections, common rail systems for heavy-duty applications have evolved to the point were they are replacing unit injectors in many new engine designs for markets with the most demanding emission standards. To facilitate this switch, fuel injection equipment manufacturers have designed common rail systems that can easily be fitted to engine platforms that were originally designed for unit injector or unit pump systems and thus avoiding the need for a completely new engine design.