C15 and C18 Tier 4 Final Engines Fuel System Caterpillar

Fuel System
1.1. Fuel System Electronic Control Circuit
2.2. Electronic Controls
3.2. Fuel Injection Timing and Delivery
4.1. Electronic Unit Injector Mechanism
5.1. Electronic Unit Injector
6.1. Fuel Heaters

Illustration 1g01915500
(1) Fuel filter base
(2) Temperature sensor
(3) Fuel pressure sensor (secondary filter inlet)
(4) Fuel pressure sensor (secondary filter outlet)
(5) Cylinder head
(6) Fuel injector
(7) Low-pressure regulator
(8) Fuel tank
(9) Electric priming pump
(10) Primary fuel filter
(11) Fuel transfer pump
(12) Electronic control module (if equipped)

The fuel supply circuit is a conventional design for electronic unit injector diesel engines. Fuel transfer pump (11) of the circuit is a gear-type pump with positive displacement. The fuel transfer pump is located in the front of the engine at the lower left corner. The fuel transfer pump is mounted to the front timing gear cover. The fuel transfer pump is driven by the front gear train.

The fuel transfer pump draws fuel from fuel tank (8). The fuel passes through primary filter/water separator (9). The fuel passes into the fuel transfer pump and the fuel is then pressurized. The fuel transfer pump incorporates a check valve in order to permit a fuel flow around the gears for hand priming. The fuel transfer pump also incorporates a relief valve in order to protect the system from extreme pressure. The excess fuel flow that is provided by the fuel transfer pump is used in order to cool the electronic unit injectors. The excess fuel flow also purges the air from the fuel system.

The fuel from the fuel transfer pump flows through the fuel filter at fuel filter base (1). Fuel temperature sensor (2) is mounted in the fuel filter base. The Engine Control Module (ECM) (12) uses the sensor to monitor the temperature of the fuel that is entering the engine. The information is used by the ECM to calculate a fuel correction factor during engine operation. Fuel priming pump (8) is positioned on the fuel filter base in order to prime the system when air has been introduced into the fuel system. For more information on priming the fuel system, refer to Testing and Adjusting, "Fuel System - Prime".

The fuel leaves the fuel filter base and flows through the fuel supply line to the cylinder head. The fuel enters the cylinder head at the front of the engine. Fuel is delivered to the electronic unit injectors through fuel manifold that is drilled into the cylinder head during the manufacturing process.

Excess fuel exits the cylinder head at the rear. The fuel returns to the fuel filter base through the fuel return line to the pressure regulating valve that is incorporated into the fuel filter base. The pressure regulating valve maintains a sufficient amount of back pressure in the system in order to fill the electronic unit injectors. After the fuel passes through the pressure regulating valve, the fuel returns to the fuel tank. The fuel flows continuously from the fuel supply through the electronic unit injectors in the head and back to the tank.

Fuel System Electronic Control Circuit

The injection pump, the fuel lines, and the nozzles that are used in the traditional AVSpare diesel engines have been replaced with a mechanically actuated electronically controlled unit injector in each cylinder. The engine has two solenoids on each injector that controls the amount of fuel that is delivered by the injector. The ECM sends a signal to each injector solenoid in order to provide complete control of the fuel to the engine.

Electronic Controls

The electronic control system provides complete electronic control of all engine functions. The electronic control system consists of the following three types of components: input, control and output. Sensors monitor engine operating conditions. This information is sent to the ECM. The ECM has three main functions. The ECM provides power for the engine electronics and monitors input signals from the engine sensors. The ECM also acts as a governor to control engine rpm. The ECM stores active faults, logged faults, and logged events. The Personality Module is the software in the ECM which contains the specific maps that define power, torque, and rpm of the engine. The ECM sends electrical current to the output components in order to control engine operation.

Fuel Injection Timing and Delivery

This engine uses an injection system control that allows variable control of the spray pattern of the injector during the duration of fuel injection into the cylinder. A pilot injection takes place in order to start fuel combustion in the cylinder. When the pilot injection has been completed, the fuel injection delivery is then controlled by the ECM in order to optimize the combustion efficiency for the current operating conditions. The pilot injection requires a new camshaft with a modified injector lobe.

The ECM controls the amount of the fuel that is injected by varying the signals to the electronic unit injectors. The electronic unit injectors will inject fuel only if the electronic unit injector solenoid is energized. The ECM sends 120 VDC to the solenoid in order to energize the solenoid. By controlling the timing of the 120 V signal, the ECM can control injection timing. By controlling the duration of the 120 V signal, the ECM can control the injected fuel amount.

The ECM sets certain limits on the amount of fuel that can be injected. The Fuel Ratio Control FRC fuel position is a limit that is based on boost pressure in order to control the air/fuel mixture for the emission control. When the ECM senses an increase in the boost pressure, the ECM increases the FRC fuel position. The rated fuel position is a limit that is based on the horsepower rating of the engine. The rated fuel position is similar to the rack stops and the torque spring on a mechanically governed engine. The rated fuel position provides the horsepower and the torque curves for a specific horsepower rating. The limits are programmed by the factory into the personality module.

Injection timing depends on three factors: the engine speed, the engine load and other engine data. The ECM determines the top center position of cylinder number 1 from the signal that is provided by the crankshaft position sensor. The ECM determines when the fuel injection should occur relative to the top center position. The ECM then provides the signal to the electronic unit injector at the correct time.

Electronic Unit Injector Mechanism

Illustration 2g01377898
Electronic unit injector mechanism
(41) Electronic unit injector
(42) Adjustment mechanism
(43) Rocker arm assembly
(44) Camshaft

The electronic unit injector mechanism provides the downward force that is required to pressurize the fuel in the unit injector. Electronic unit injector (41) injects fuel in the combustion chamber at the correct time. The camshaft gear is driven by two idler gears and a cluster gear that is driven off the crankshaft gear. The timing marks on the crankshaft gear, on the cluster gear, and on the housing of the timing cover are aligned in order to provide the correct relationship between the piston and the valve movement. Camshaft (44) has three lobes for each cylinder. Two lobes operate the inlet valves and the exhaust valves, and one lobe operates the electronic unit injector mechanism. Force is transmitted from the injector lobe on the camshaft to the top of the electronic unit injector through rocker arm assembly (43). Adjustment mechanism (42) allows the injector lash to be adjusted. For the proper setting of the injector lash, refer to the topic on adjustment of the electronic unit injector in Testing and Adjusting, "Electronic Unit Injector - Adjust".

Electronic Unit Injector

Illustration 3g01377900
Electronic unit injector
(45) Spring
(46) Plunger assembly
(47) Connector assembly
(48) Seal
(49) Solenoid valve assembly
(50) Spring
(51) Seal
(52) Spacer
(53) Check valve
(54) Spring (nozzle)
(55) Body
(56) Seal
(57) Tip assembly

Fuel at low pressure from the fuel supply manifold enters the electronic unit injector at the fill port through drilled passages in the cylinder head. Seals (48) and (51) are used in order to seal the fuel injector into the injector sleeve that is in the cylinder head.

As the electronic unit injector mechanism transfers the force to the top of the electronic unit injector, spring (45) is compressed and plunger (46) is driven downward. The electronic unit injectors have independent control of the fuel fill and fuel spill. The fuel inside the injector is pressurized mechanically when the rocker arm depresses the injector using a dedicated lobe on the camshaft. The engine ECM electronically controls the start, stop, and duration of the fuel injection by energizing and de-energizing the one solenoid, which controls two coil assemblies, contained within the injector.

One coil controls the fill/spill valve which allows or prevents fuel to freely flow into and out of the injector. When this valve is closed, the injector is able to pressurize the fuel contained within the injector as the rocker arm depresses the injector.

The second coil controls the Direct Operated Check (DOC) valve. The purpose of the DOC valve is to hold the nozzle check in the closed position, preventing fuel from injecting. The DOC valve will remain closed until sufficient pressure (as determined by the engine ECM) is developed inside the injector. After the correct amount of fuel has been discharged into the cylinder, the ECM discontinues the signal that is sent to the solenoid connection. The solenoid valve assembly is de-energized and the valve in the solenoid valve assembly is allowed to open.

The high-pressure fuel is then dumped through the spill port and into the fuel return manifold. The fuel then returns to the fuel tank. As the pressure in the tip of the injector decreases, the check valve in the injector tip returns to the seated position.


The electronic unit injectors utilizing the DOC valve allow for higher injection pressures, regardless of engine speed and load. Higher injection pressures yield greater atomization of the fuel, which results in a more complete combustion of the fuel in the cylinder.

As the injector lobe of the camshaft rotates past the point of maximum lobe lift, the force on top of the electronic unit injector is removed and the spring is allowed to expand. The plunger returns to the original position. The fuel supply passage is uncovered into the plunger barrel in order to refill the injector pump body. The fuel at low pressure then circulates through the fuel injector body. After circulating through the fuel injector body, the fuel flows out of the spill port. This process continues until the solenoid valve assembly is again re-energized.

Fuel Heaters

Fuel heaters prevent the plugging of the fuel filters in cold weather. This plugging is called waxing. The engine will not dissipate enough heat in order to prevent waxing. Heaters that are not thermostatically controlled can heat the fuel in excess of 65 °C (149 °F). High fuel temperatures will reduce the engine performance, the transfer pump check valve, and the system reliability.

Note: Never use fuel heaters that do not have some type of temperature regulator that is incorporated into the system.