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TM 9-6115-729-24
FUEL SYSTEM - Continued
When the fuel level is low, the fuel level switch will either supply power to the external auxiliary fuel pump for
external fuel supply or shut down the TQG to prevent it from completely running out of fuel. If a diesel engine
runs out of fuel, it is necessary to fill the fuel filters, purge all air from the lines and pumps, and bleed all
injectors. Only hydraulic pressure will open the injectors; air pressure will not.
Auxiliary Fuel System
The auxiliary fuel system consists of an external fuel supply, a fuel hose, a 24 Vdc auxiliary fuel pump with
strainer and solenoid valve, fuel lines, and an auxiliary fuel pump relay. When the EMCP AUX FUEL switch is
ON, the auxiliary fuel pump can transfer fuel from an external fuel supply to the generator fuel tank. The fuel level
float in the fuel tank shuts off power to the auxiliary fuel pump via the auxiliary fuel pump relay when the TQG fuel
tank is full and reactivates the pump as the level drops. The fuel level indicator on the EMCP indicates the fuel
level of the fuel tank from empty (E) to full (F).
The operation of the HEUI fuel system (Figure 3) is different from mechanically actuated fuel injection systems.
The HEUI fuel system requires no mechanical adjustment. Installing different software in the ECM makes
changes in performance.
The engine uses an HEUI in each cylinder. A solenoid on each injector controls the amount of fuel delivered by
the injector. An engine speed/timing sensor detects top dead center (TDC). All fuel injection is timed off the
occurrence of TDC. The ECM determines how much fuel is to be delivered based on the difference between
desired and actual speed. The ECM then sets the timing from TDC for each cylinder based on data received from
the coolant temperature sensor, inlet air manifold temperature, atmospheric pressure sensor, and turbocharger
outlet (boost) pressure sensor. The ECM adjusts timing and fuel delivery for best engine performance, fuel
economy, and white smoke control.
The HEUI fuel system uses a hydraulically actuated electronically controlled unit injector. All fuel systems for
diesel engines use a plunger and barrel to pump fuel under high pressure into the combustion chamber. Fuel is
pumped into the combustion chamber in precise amounts to control engine performance. The HEUI uses engine
oil under high pressure to power the plunger rather than a fuel injection pump camshaft lobe to power the plunger.
The HEUI uses engine lubrication oil that is pressurized from 870 psi (6 Mpa) to 3500 psi (24 Mpa) to pump fuel
from the injector. The HEUI operates in the same way as a hydraulic cylinder to multiply the force of the high-
pressure oil. By multiplying the force of the high-pressure oil, the HEUI produces injection pressures that are very
high. This multiplication of pressure is achieved by applying the force of high-pressure oil to a piston. The piston
is larger than the plunger by approximately six times. The piston pushes on the plunger. The engine lubrication
oil under high pressure is called the actuation pressure of the oil. The actuation pressure of the oil generates the
injection pressure delivered by the unit injector, increasing the pressure by a factor of six. Low actuation
pressures result in low injection pressure of the fuel. During conditions of low speed such as idle and start, low
injection pressure is used. High actuation pressure during high idle and acceleration results in high injection
pressure of the fuel. The HEUI fuel system provides infinite control of injection pressure.


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