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With increased demand to lower emissions from diesel engines, the flexibility and improved performance offered by electronic control was an important driver for many engine manufacturers to introduce electronically controlled fuel injection systems in the late 1980s and early 1990s. An important tool for lowering emissions from diesel engines produced during this period was fuel injection timing that could be varied over the speed and load range of the engine. While injection timing could be varied with a purely mechanical approach, electronic control offered a much more flexible and a potentially simpler way to achieve this while also providing the option of introducing a number of other desirable features. Some of the first electronically controlled fuel injection systems in heavy-duty engines appeared in the Detroit Diesel Series 92 in 1985 and the Series 60 in 1987 . Caterpillar applied it to the 3176 in 1988 .
The unit injectors used in these engines lent themselves well to early adoption of solenoid actuated electronic fuel injectors. Solenoid actuator designs of that period were still relatively large and bulky and a unit injector for a heavy-duty engine provided ample room for it. It took several years for manufacturers to refine the actuator design to make it compact enough to use in common rail systems for light-duty applications  and to produce a heavy-duty unit injector, Delphi’s E1 in 2000, which replaced the bulky side mounted actuator with a more compact design that could be integrated into the injector body.
Manufacturer’s quickly learned that electronic control offered not only the ability to control injection timing according to speed and load but also according to the type of driving the vehicle was experiencing. In the 1990s, it was common to program engine controllers to adjust injection timing to optimize fuel consumption in heavy-duty diesel engines when the operating conditions indicated highway cruise conditions. In some cases, this injection timing conflicted with that required to meet regulated emission limits.
As emission regulations continued to tighten, the demands placed on fuel systems increased further and it was not sufficient to simply provide flexibility in injection timing control. Additional drivers that pushed the evolution of diesel fuel injection systems included:
A number of major engine manufacturers developed their own, often unique fuel injection systems. The following are examples of internally designed injection systems:
In other cases, major heavy-duty engine manufacturers were able to acquire patented technologies and further develop the concepts for their own engine line. An example is the Bendix Diesel Engine Controls unit injector system that was licensed by Cummins and used in the CELECT unit injector.
While this paper outlines the evolution of electronic fuel injection systems for two specific engine manufacturers—Cummins and Caterpillar—it should be acknowledged this by no means covers the entire range of injection systems available in heavy-duty diesel engines. Fuel systems from suppliers such as Bosch, Delphi, Siemens/Continental, Denso and others are also very common.