How HEUI Injectors Work
Hydraulic Electric Unit Injectors
HEUI Injector History
HEUI injectors are found on 7.3L & 6.0L Power Stroke diesels. The acronym HEUI stands for Hydraulic Electric Unit Injector, or more descriptively, Hydraullically actuated Electronic controlled Unit injector. An HEUI injector uses high pressure oil to pressurize fuel inside the injector. A high pressure oil pump (HPOP) is used to pressurize engine oil between approximately 500 - 3,000 (3,770 for 7.3L) psi. The high pressure oil pump is seperate from the engine's oil pump, which provides oil pressure for lubrication. Rather, the HPOP is dedicated to providing pressure to the HEUI injectors only. The HEUI injector was invented by Caterpillar and International adopted the technology for Power Stroke diesels between 1994 and 2007 before switching to a high pressure common rail system (6.4L). While the HEUI injector has become technologically outdated, it was considered cutting edge when it originally entered the market.
How HEUI Injectors Work
HEUI injectors are relatively simply in design. The injector can be broken down into 2 basic sections, a fuel chamber and an oil chamber. A low pressure fuel pump supplies fuel to the injector, while a high pressure oil pump supplies pressurized oil to the injector. During the injection cycle, an actuator allows high pressure oil to enter the oil chamber of the injector body, applying pressure to an intensifier piston. On the other side of this piston is the fuel chamber of the injector. The intensifier piston pressurizes fuel at a rate of 7 times the oil pressure. Fuel becomes pressurized before an electric actuator releases it through the injector nozzle. Maximum fuel pressure for the 7.3L is 21,000 psi (7 x 3,000) and 26,000 psi for the 6.0L (7 x 3,770).
HEUI Injector Advantages & Disadvantages
HEUI injectors do not have many advantages over today's high pressure common rail systems, thus the reason International designed the 6.4L without an HEUI system. When HEUI systems were founded, they were competing against mechanical injectors, and were found to have many benefits, such as higher fuel pressures at low engine speeds (injection pressure in a HEUI system is independent of crankshaft speed), improved throttle response, and often better fuel economy.