# Engine Unit Injector Operation in EMDEC 710 Series
Engine unit injectors (EUIs) operate both electronically and mechanically. EUIs perform metering and timing electronically, and pressurizing and atomizing mechanically.

Engine Control Modules (ECMs) electronically control the metering and timing of the EUIs based on inputs to the software system. Inputs include:
- Speed requests from the interface module.
- Timing and speed data from the timing pick-ups.
- Engine conditions from other sensors.

## Typical Injection Sequence
The fuel injection sequence has five stages:

1. [Piston Rises in the Cylinder](#1-Piston-Rises-in-the-Cylinder)
2. [Fueling Decisions](#2-Fueling-Decisions)
3. [Injection](#3-Injection)
4. [Bleeder Passages](#4-Bleeder-Passages)
5. [End of Injection](#5-End-of-Injection)

Refer to Figure 1 below to follow fuel flow through a typical injection sequence.

Figure 1: Fuel Flow Through EUI
*<small>EMDEC Operating and Troubleshooting Guide for Power Products Applications, Second Edition* (Illinois: Electro-Motive Diesel Inc. Group, 2008), 24.</small>

### 1. Piston Rises in the Cylinder
The piston rises in the cylinder, compressing the fresh air charge to bring it to ignition tempera-ture. As the piston rises, the camshaft and rocker mechanism drive the injector plunger downward. At this point, the fill passage and poppet valve are open, so the injector plunger doesn’t create pressure. Instead, the fuel travels back through the poppet valve into the lower fuel chamber.

### 2. Fueling Decisions
The ECM software measures timing, speed, and performance inputs and makes fueling decisions. It decides:
- When to energize the injector solenoid.
- How long to keep the injector solenoid energized.

The ECM then sends a control signal to energize the injector solenoid at the precise time the igni-tor needs to fire.

### 3. Injection
The energized injector solenoid allows the armature and poppet valve to move upwards closing the passage around the poppet. The injector plunger is still traveling downwards, but now the fuel has nowhere to go. Therefore, fuel pressure rises in the pump chamber.

The high pressure in the pump chamber moves downwards through the column of fuel and the check valve to the spray tip. Fuel also pushes back against the tapered surface of the needle valve. When the pressure in the tip area reaches between 2,000 psi and 2,400 psi, the needle valve lifts off its seat against the needle valve spring. Fuel now flows through the needle valve, through the orifice holes in the spray tip, and into the cylinder.

As the injector plunger continues to move downward, the pressure in the injector rises to the final working pressure between 16,000 psi and 18,000 psi. The injector continues injecting fuel as long as the solenoid is energized (controlled electronically) and the injector plunger is moving down-wards (controlled mechanically).

### 4. Bleeder Passages
Bleeder passages prevent high pressure fuel from escaping to the outside of the injector. During the high pressure process, fuel bleeds upwards to lubricate the injector plunger. This fuel travels through the bleeder passages into the low pressure fuel return system.

Because of the high pressure, some leakage occurs between the components above the injector tip. This fuel goes into a second bleeder passage that also leads into the low pressure fuel return system.

### 5. End of Injection
The ECM software determines when to stop fuel injection based on the fueling decisions made in step two. The ECM sends a control signal to de-energize the solenoid. The return spring then drops the armature and poppet valve against the bench in the lower fuel chamber.