HEUI stands for Hydraulic Electronic Unit Injector, and it refers to a type of fuel injection system used in some diesel engines. The HEUI system is primarily associated with engines manufactured by Caterpillar and is commonly found in various applications, including trucks (Powerstroke engines build by International), buses, and some industrial equipment.

Here's a basic overview of how an HEUI fuel system works:

Hydraulic System: Unlike traditional fuel injection systems, HEUI uses engine oil to

pressurize and actuate the fuel injectors. A high-pressure oil pump delivers oil to the

injectors at high pressure.

Electronic Control: The injection process is electronically controlled. The engine

control module (ECM) precisely manages the timing and duration of the fuel injection

based on various engine parameters such as speed, load, and temperature.

Unit Injectors: Each cylinder has its own unit injector. The injector contains a plunger

and a solenoid valve. When the solenoid is energized, it opens the injector, allowing

high-pressure oil to act on the plunger, which in turn pressurizes and injects the fuel into

the combustion chamber.

Oil-Fueled Injection: The high-pressure oil not only actuates the injectors but also

serves as the medium to pressurize and inject the fuel into the combustion chamber.

This is a departure from traditional systems where fuel is pressurized separately from

the engine oil.

The HEUI injector’s job is to inject fuel at high pressure when the PCM/ECM commands

it. The PCM does so by sending signals to the IDM (Injector Driving Module) or FICM

(Fuel injection control module). The IDM controls the solenoid by controlling the voltage.

Depending on the voltage, the solenoid releases the force on the poppet valve. Since

the poppet valve is counter-pushed by springs, as the solenoid releases it, it is pushed

up by the springs. This opens a pathway for the pressurized oil to fill in the cavity. This

pressure is decided on engine load rather than engine speed.

The pressurized oil then pushed the intensifier piston. The piston in return pushes the

fuel in the cavity located at the injector’s nozzle. Again the injector nozzle valve is kept

closed using the spring action.

Only when the fuel reaches a preset pressure does it overcome the spring force and

exit the nozzle. Since the nozzle exit is incredibly small, the high-pressure fuel going out

of an even smaller area gets atomized. The atomized fuel enters the combustion

chamber and combustion happens.   

The advantages of the HEUI system include precise control over the fuel injection

process, which can contribute to better fuel efficiency, reduced emissions, and improved

engine performance. However, these systems can be complex and may require special

diagnostic tools for troubleshooting.

It's worth noting that HEUI systems are just one type of fuel injection system used in

diesel engines, and there are other systems like Common Rail Injection (found in many

modern diesel engines) and others, each with its own set of advantages and

disadvantages.