Diesel Engine Fuel Injection


Diagram of the basic fuel system 
(1) Electronic Unit Injector (2) Solenoid for the fuel injection pump (3) Wastegate Valve (if equipped) (4) Secondary Speed/Timing Sensor
(5) Fuel Injection Pump (6) Primary Speed/Timing Sensor (7) Intake Manifold Pressure Sensor (8) Fuel Manifold Pressure Sensor
(9) Engine Oil Pressure Sensor (10) Intake Manifold Temperature Sensor (11) Coolant Temperature Sensor (12) Diagnostic Connector
(13) Engine Electronic Control Module (ECM)

Low Pressure Fuel System

Low pressure fuel system 
(1) Primary fuel filter (2) Water separator (3) Fuel transfer pump (4) Fuel cooler (optional) (5) ECM 
(6) Secondary fuel filter (7) Fuel Injection Pump
(A) Outlet for High pressure fuel to the high pressure fuel manifold
(B) Return from the Pressure Relief Valve on the High pressure fuel manifold
(C) Return to Fuel tank
(D) Return from the Electronic Unit Injectors
(E) Fuel in from the Fuel Tank

Fuel is drawn from the fuel tank (E) through a 20 micron Primary fuel filter (1) and the Water separator (2) to the Transfer pump (3). The transfer pump increases the fuel pressure to 25 kPa (3.6260 psi). The fuel is pumped through the fuel cooler (optional) (4) to the ECM (5). The fuel cools the Electronic Control Module. The fuel passes from the engine ECM to a 2 micron fuel filter (6). The fuel filter removes particulates from 20 microns to 2 microns in size in order to prevent contamination of the high pressure components in the fuel system. Fuel passes from the fuel filter to the fuel injection pump (7). The fuel is pumped at an increased pressure to the high pressure fuel manifold. Excess fuel from the high pressure fuel pump returns to the tank through a non-return valve. There is a small orifice in the fuel filter base in order to bleed any air back to the tank. The leak off fuel from the electronic unit injectors returns from a connection in the cylinder head to the pressure side of the transfer pump.

High Pressure Fuel System

High pressure fuel system 
(1) Electronic Unit Injector (2) High Pressure Fuel Manifold (3) Fuel Pressure Sensor (4) Fuel Pressure Relief Valve (5) Fuel Transfer Pump
(6) Solenoid for the Fuel Injection Pump (7) Fuel Injection Pump (8) Fuel Pump Gear

The fuel injection pump (7) feeds fuel to the high pressure fuel manifold (2). The fuel is at a pressure of 70 MPa (10152.7 psi) to 130 MPa (18855 psi). A pressure sensor (3) in the high pressure fuel manifold (2) monitors the fuel pressure in the high pressure fuel manifold (2). The engine ECM controls a solenoid (6) in the fuel injection pump (7) in order to maintain the actual pressure in the high pressure fuel manifold (2) at the desired level. The high pressure fuel is continuously available at each injector. The engine ECM determines the correct time for activation of the correct electronic unit injector (1) which allows fuel to be injected into the cylinder. The leakoff fuel from each injector passes into a drilling which runs along the inside of the cylinder head. A pipe is connected to the rear of the cylinder head in order to return the leak off fuel to the pressure side of the fuel transfer pump.

Components of the Fuel Injection System

The fuel injection system has the following mechanical components:
• Primary filter/water separator
• Fuel priming pump
• Secondary fuel filter
• Fuel injection pump
• Fuel injectors
• Fuel manifold
• Pressure relief valve
• Fuel pressure sensor
The following list contains examples of both service and repairs when you must prime the system:
• A fuel filter is changed.
• A fuel line is replaced.
• The fuel injection pump is replaced.

Primary Filter/water Separator
The primary filter/water separator is located between the fuel tank and the priming pump.

Fuel Priming Pump

Hand Fuel Priming Pump

The pump has a plunger (1) which is manually operated in order to prime the fuel system. Air is removed from the fuel system to the fuel return line to the tank. The fuel transfer pump is located in the fuel injection pump.

Electric Fuel Priming Pump

The electric fuel priming pump can be installed on some engines.

Secondary Fuel Filter


The secondary fuel filter (1) is located after the priming pump. The filter is always before the fuel injection pump.

Fuel Pump Assembly

The fuel pump assembly consists of a low pressure transfer pump and a high pressure fuel injection pump. The pump assembly is driven from a gear in the front timing case at half engine speed. The fuel injection pump has two pistons that are driven by a camshaft. There is a cam for each piston and each cam has three lobes. The fuel injection pump delivers a volume of fuel six times for each revolution. The stroke of the pistons is fixed. The injector will use only part of the fuel that is delivered by each stroke of the pistons in the pump. The solenoid for the fuel injection pump is controlled by the engine ECM in order to maintain the fuel manifold pressure at the correct level. The solenoid allows excess fuel to be diverted away from the fuel manifold and back to the tank. A feature of the fuel injection pump allows fuel to return to the tank continuously.

Fuel Injection Pump


The fuel injection pump has the following operations:
• Generation of high pressure fuel
The fuel output of the fuel injection pump is controlled by the ECM in response to changes in fuel pressure.

Fuel Transfer Pump


The fuel transfer pump is a serviceable component. The fuel transfer pump provides a relatively low fuel pressure to the fuel injection pump. The fuel transfer pump has a regulating valve in order to control the low pressure. The fuel transfer pump circulates fuel through the primary fuel filter and the secondary fuel filter. The fuel transfer pump has a fuel bypass valve in order to allow the low pressure fuel system to be primed.

Shutoff
The engine shuts off by interrupting the fuel supply. The engine electronic control module (ECM) specifies the amount of fuel. The quantity of the fuel that is required by the ECM is set to zero.

Control

Electronic control for the fuel system

The Engine Electronic Control Module determines the quantity, timing and pressure of the fuel in order to be injected into the fuel injector.
The Engine Electronic Control Module uses input from the sensors on the engine. These sensors include the speed/timing sensors and the pressure sensors. The ECM controls the fuel pressure by increasing or decreasing the flow of fuel from the fuel injection pump. The ECM controls the timing and the flow of fuel by actuating the injector solenoid. The amount of fuel is proportional to the duration of the signal to the injector solenoid.

Fuel Injectors


The fuel injectors are not serviceable. When the engine ECM sends a signal to the injector solenoid, a valve inside the injector opens. The valve allows the high pressure fuel from the fuel manifold to enter the injector. The pressure of the fuel pushes the needle valve and a spring. When the force of the fuel pressure is greater than the force of the spring, the needle valve will lift up. The timing and duration of injection is controlled by a solenoid valve in the injector. The valve has two positions. In the closed position, the valve closes the inlet to the injector. In this position, fuel above the injector needle is allowed to vent through the leak off port. In the open position, the valve opens the inlet to the injector. Simultaneously, the valve closes the leakoff port in order to allow high pressure fuel to flow to the needle. When the solenoid valve is closed, some fuel escapes past the valve in order to vent through the leakoff port. A certain volume of fuel always flows from the leakoff port. If the volume of fuel increases beyond a critical level, the high pressure fuel pump will not be able to maintain pressure in the fuel manifold. The faulty electronic unit injector must be identified and replaced. When the signal to the injector ends, the valve closes. The fuel in the injector changes to a low pressure. When the pressure drops the needle valve will close and the injection cycle stops. When the needle valve opens, fuel under high pressure will flow through nozzle orifices into the cylinder. The fuel is injected into the cylinder through the orifices in the nozzle as a very fine spray. The needle valve has a close fit with the inside of the nozzle. This makes a positive seal for the valve.

Fuel Manifold


The fuel manifold (1) stores high pressure fuel from the fuel injection pump. The high pressure fuel will flow to the injectors. The fuel pressure sensor (2) measures the fuel pressure in the fuel manifold (1). The relief valve (3) will prevent the fuel pressure from getting too high.