Section 2.29 Overview of High Pressure Fuel System for 1998 and Later Series 50G Automotive Engines

Section 2.29
Overview of High Pressure Fuel System for 1998 and Later Series 50G Automotive Engines

For 1998 a new fuel system was introduced for Series 50G automotive model engines. For a schematic diagram of a typical CNG fuel system, see Figure "Schematic Diagram of Typical CNG Fuel System for 1998 and Later Series 50G Engines" .‪

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Figure 1. Schematic Diagram of Typical CNG Fuel System for 1998 and Later Series 50G Engines

The fuel system includes the following components:‪

DDC supplied:‪

  • Venturi Type Gas Mixer
  • PSV (Pulse width modulated Stepper motor Valve)
  • IMPCO Low Pressure Regulator
  • ITT Conoflow High Pressure Regulator
  • Low Pressure DDEC Controlled Gas Shutoff Valve

OEM supplied:‪

  • Fuel Tanks
  • Fuel Lines
  • Manual Shutoff Valves
  • Coalescing Fuel Filters
  • High Pressure DDEC Controlled Gas Shutoff Valve
  • Pressure Gages
  • Fuel Heat Exchanger

See Figure "Series 50G Automotive Engine Component Location" for a component overview of the High Pressure fuel system.‪

The fuel system is designed to accurately deliver clean fuel at the correct air fuel mixture to the combustion chamber at any engine operating condition.‪

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 1. SNEF Module‪

 7. Fuel Pressure Sensor, Fuel Inlet‪

 2. Air Compressor‪

 8. Manifold Air Pressure Sensor‪

 3. Fuel Temperature Sensor‪

 9. Barometric Air Pressure Sensor‪

 4. PSV‪

10. Ignitor Module‪

 5. Throttle Actuator‪

11. Electronic Control Module‪

 6. IMPCO Regulator‪

12. Knock Sensor and Stud‪

Figure 2. Series 50G Automotive Engine Component Location



To avoid injury from an explosion of natural gas, the following precautions must be taken:‪

  • Do not smoke when installing or servicing the engine or fuel system.
  • Installation or servicing of natural gas equipment must only be conducted in well ventilated, natural gas-compatible areas. Do not install or service equipment in an enclosed area where ignition sources are present without first ensuring that an undetected gas leak may be safely vented without being ignited.
  • Natural gas fuel systems are pressurized. Relieve pressure from any fuel system component prior to installation or service of that component. Bleed natural gas lines before installing or servicing any component connected to the fuel lines.


To avoid injury from the explosion of natural gas, the engine must be kept in a well ventilated area away from open flames, sparks, and electrical resistance (heating) coils.‪



To avoid injury from fire, check for fuel or oil leaks before welding or carrying an open flame near the engine.‪

Fuel is stored in the fuel tanks at a pressure as high as 3600 PSI. The fuel passes through high pressure fuel shutoff valves, typically controlled by both DDEC and OEM systems, through primary fuel filters to the high pressure regulator. The pressure is reduced by the high pressure regulator to approximately 110 PSI. Flow continues through the secondary fuel filter, through the DDEC controlled low pressure fuel shutoff valve on to the low pressure regulator. The low pressure regulator is balanced to the air system at the inlet to the fuel mixer assembly. It maintains outlet fuel pressure slightly higher than turbo boost pressure at the mixer inlet. Fuel from the regulator passes through the PSV, enters the venturi mixer and is mixed with incoming air. Air flow through the venturi creates a pressure drop at the throat of the venturi that increases and decreases in proportion to air flow. This pressure differential is used to draw fuel through the venturi insert, the PSV and low pressure regulator. The system is sometimes referred to as a demand system. Increases in pressure drop across the venturi increases the demand for fuel flow required to maintain the fuel outlet pressure at the low pressure regulator. Fine control of fuel flow is achieved by positioning of the gas valve. Changes in gas valve position change the flow restriction between the venturi and low pressure regulator. For a given set of engine speed and load conditions, reducing the valve opening increases restriction resulting in reduced fuel flow and a leaner air fuel mixture. Likewise, increasing the valve opening reduces restriction resulting in increased fuel flow and a richer air fuel mixture.‪

In part, fuel flow is controlled by calibration and matching of the key fuel system components, the venturi mixer, the PSV (gas valve position) and the low pressure regulator. Electronics are used to further refine control. Electric control is provided by DDEC. The DDEC output which allows control of fuel delivery is gas valve position. Input to DDEC for fuel delivery control is provided by engine speed, inlet manifold pressure, inlet manifold temperature, fuel temperature, coolant temperature, throttle position and exhaust gas oxygen sensors. In addition exhaust gas temperature, fuel pressure and knock sensors provide engine protection and diagnostics capabilities unique to the S50 natural gas engine. The system is a closed loop system. This refers to the fact that input or feedback from the exhaust gas oxygen sensor is used to make adjustments in fuel delivery. The system is constantly making use of this feedback to adjust air fuel ratio to the desired value. Electronic control of the fuel system provides the capability to adjust and correct for changes in operating condition, environmental condition, changes in fuel quality, differences in components and component changes that occur over time. For fuel quality specifications refer to "5.1 Fuel" . Refer to "2.11 DDEC III" for details on the DDEC system for Series 50 Natural Gas engines.‪

Series 50 Service Manual - 6SE50
Generated on 10-13-2008

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