Back in the 1990's, powerstroke engines that you would find in diesel engines needed to be more fuel efficient while still offering powerful engine torque output. In addition, these powerstroke engines had to operate in a more cleaner method to changing air emission standards. So new technology was invented that consisted of the Hydraulically actuated Electronic Unit Injection (HEUI) to address these issues.
What Is A HEUI? A HEUI was created by Caterpillar and International for all powerstroke engines manufactured in the early 1990's and into the early 2000's until the injector was replaced by a high pressure common rail system in 2007. The HEUI injector consists of two parts: a high pressure oil chamber and a low pressure fuel chamber. The basic function of the HEUI was to pressurize fuel through the plungers using high pressure oil.
As the fuel pump pushes fuel into the injector, the oil pump places high pressure oil into the injector. The actuator lets the high pressure oil into the injector's oil chamber to pressurize the intensifier piston. The intensifier piston pressurizes the fuel at a rate (7.1) times higher than the pressurized oil. The powertrain control module (PCM) controls the operation of the HEUI.
Advantages of the HEUI The HEUI had many benefits compared to the mechanical injectors. When at low engine speeds, the engine could produce higher fuel pressure for better fuel economy. Injection timing could be controlled along with the fuel rate through electronic control means. Basically, the HEUI lowered emissions that caused air pollution while increasing performance of powerstroke diesel engines no matter at what speed the vehicle was traveling.
For diesel engines today, many HEUI injectors are being replaced with the high pressure common rail systems. One engine, the International 6.4L, was developed without an HEUI.