Diesel Engine Fuel Rail Systems

• This document covers the fundamentals of fuel rail systems, specifically for diesel engines.
• Module 8 focuses on the structure and function of common rail diesel fuel systems, while module 9 looks at fuel pumps and regulators. Module 10 concerns the types of fuel control systems. Finally, module 11 details instruments for frequency measurement and noise control.
• After completing this module , one will know the fundamentals of fuel rail systems for diesel engines.

Common Rail Diesel Fuel Systems

• The fuel system is divided into these basic circuits:
  • Low pressure supply circuit
  • High pressure delivery circuit
  • Fuel leak back and return

High-Pressure Pump

• The high-pressure pump serves as the interface between the low and high-pressure sides of the fuel system.
• Ensures delivery of enough fuel at sufficient pressure across the engine's entire operating range.
• Provides sufficient fuel for rapid engine start and increases pressure in the rail.
• The transfer pump moves fuel from the fuel tank to the high-pressure pump's pumping chambers.
• A fuel metering valve regulates fuel intake volume into the pumping chambers.
• The pump includes the fuel metering control valve (solenoid), fuel supply inlet, fuel return, high pressure fuel outlet, gear-type transfer pump, pressure valve, suction valve, pump shaft with eccentric cam and polygon ring.
• 3 pumping pistons are operated by a polygon ring on an eccentric cam on the pump shaft.
• As the polygon ring rotates, it moves in a circular motion to operate the pump pistons.
• An electric pre-supply pump in fuel tank can substitute a transfer pump.
• Some systems use a combination of electric and transfer pumps.
• Located at the back of high pressure pump.
• The valve controls amount of fuel to the pump.
• Receives voltage from the engine. _ Energized by PWM at ~180Hz.
• When solenoid is de-energized, the valve is open, resulting in low fuel volume intake.
• When solenoid is energized, the valve is closed, resulting in high fuel volume intake.

Fuel Volume Intake Control Advantages

• Only supply the volume of fuel required to the common rail from the high pressure pump.
• Reduced fuel flow results in lower fuel return flow temps.
• Reduced parasitic engine load from the high pressure pump contributes to lower exhaust emissions.

Fuel Metering Control Valve Failure

• Solenoid circuit is monitored by the engine ECM.
• An open or short circuit will cause:
  • Engine stops or will not start
• DTC stored and MIL illuminated
• Mechanical failure doesn't necessarily keep the engine from starting.
• Mechanical faults will trigger DTCs that relate to positive or negative rail pressure deviations.

High Pressure Regulator Valve

• It is a system variant fitted at the back of HP pump.
• It controls high pressure fuel delivery to the common rail. _ Excess fuel returns to the tank.

High Pressure Accumulator (Common Rail)

• Stores fuel and distributes it to the individual injectors.
• Dampens pressure vibrations from pump and injection.
• Volume typically is 16-20cm3.
• Fuel rail pressure:
  • 300-400 Bar (4410–5880 psi) when idling or running at temperature
  • Typically a maximum value of 1600-2000 Bar (23520–28400 psi) is possible.

Fuel Rail Pressure Sensor

• It is located on the fuel rail and monitors fuel pressure.
• It's a piezo resistive type sensor with three wires.
  • 5V supply from engine ECM
  • Sensor ground via engine ECM
• Linear signal voltage output to ECM
• ECM applies a stabilized 5V supply to the wire, the resistive value of the sensor creates a change in voltage on signal wire that is relative to fuel rail pressure.

Typical Signal Voltages from Rail Pressure Sensor

• Engine stationary: 0.5 volts
• Engine idling: 1.32 volts
• Snap acceleration: 3.77+ volts

Rail Pressure Limiter Valve

• A mechanical pressure limiter valve is fitted to some systems at the fuel rail's end.
• Function: Relieves rail pressure when system pressure is abnormally high.
• Excessive fuel pressure opens a return port.
• Excess fuel is relieved back to the fuel tank.
• Valve opens at ~230 MPa (2300 Bar) and closes at ~50 MPa (500 Bar) on Denso HP3 systems.

Fuel Rail Pressure Control Valve Solenoid

• A rail pressure control solenoid is fitted to the common rail on some systems.
• The valve controls fuel pressure, when opening/closing a return port in the rail. _ Excess fuel returns to the fuel tank.
• ECM supplies a battery voltage. _ ECM energized via a negatively triggered PWM.
• Operating frequency is approximately 1000Hz.
• Provides accurate and control, especially when pressure during overrun is reduced.

Rail Pressure Control Valve

• More fuel is returned via return port when de-energized.
• Rail pressure decreases
• Most likely the engine will stop or not start for failure symptoms and diagnosis. _ Open or short circuit for circuit that can be monitored by the Engine ECM.
• May cause DTC and MIL to illuminate (engine may not start).
• A minimum amount of fuel rail pressure is needed to enable the engine to start, typically approximately 200-300 Bar.

Testing Rail Pressure Control Valve

• Connect a multimere to test internal resistance.
• Run diagnostic scan tool.
• Use an Oscilloscope to test for supply levels. _ Test for waveform stability.

Fuel Injectors

• Fuel Injectors are controlled by a solenoid or piezo actuator, sequentially energized by the engine ECM.
• The ECM instantaneously switches a live voltage supply and an earth for each injector.
• Multiple injection functions per cylinder combustion are possible.

Fuel Injector Operation

• Fuel pressure is supplied to the injector needle's seat area; fuel is relieved on top of the injector's piston to return fuel tank.
• When solenoid is energized, the injector valve opens and pressure drops in the injector piston, which returns fuel to return ports.
• This creates a pressure difference above and below the injector piston.
• Fuel pressure below the injector needle then lifts the needle to inject fuel.
• Maximum stroke of solenoid valve: approximately 50 micrometers (0.05 mm).

Piezo Injector Primary Advantage

• Quicker response time; up to four times faster than solenoid injectors.

Piezo Injector Features

• The stack has several hundred thin slices of Piezo crystal material.
• Valve opens when stack expands and the voltage is applied. _ Mechanical principle is like the solenoid injector.
• Has a code that must be programmed into the engine ECM, since code relates to calibrated flow rate of the injector.
• Enables the ECM to correct injection quantity, to tolerances.

Engine Management

• Engine management can utilize closed loop control systems.
• Rail pressure calculations are measured against pre-set values.
• Actual fuel rail pressure value can affect duty cycle.

Fuel System Diagnosis

• Tests for fuel system diagnosis include considering the basics first, determining the system's fuel pressure generating capability, and a leak back test.
• Basics steps: check for fuel, contamination, leaks/damage/ charge and adequacy/crank.
• The system should generate enough fuel pressure - between 200 and 300 Bar.
• During engine cranking.
• Practice figures during cranking usually are higher.
• If engine cranks, check for injector leak.

Types of Fuel Control Systems for Petrol Engines

• Gravity feed: tank is at the highest position, feeding fuel into a carburetor float chamber by gravity, simple and cheaper to produce/maintain, needs for placement over the carburetor.
• Pressure feed: uses a hermetically sealed fuel tank, air pulp to create preesure, creates pressure and forces fuel to flow to the system.
• Vacuum system: engine suction pressure to suck fuel from the main rank to to auzillary tank.
• Pump feed: Use a steel tank to pump fuel through the filter, most present day cars use this system.
• Fuel injection: Uses a perol injection system for each cylinder.

Diesel Engine vs Petrol Engine Fuel Feed Systems

• Fuel take
• Filters
• Fuel pump
• Fuel Lines
• Carbueretor or Injectors
• Engine management system
• Fuel Gaguee