Skip to main content


The purpose of the fuel system is to store the fuel, keep it clean and free from air, water, or other impurities and to deliver it to the combustion chamber at the correct pressure, temperature, air and fuel mixture. The Series 50G engine requires an adequate fuel supply for proper performance and to develop full rated power. This section describes the function requirements for the Detroit Diesel Series 50G high pressure natural gas fuel system. ‪

The following are DDC supplied parts: ‪

  • Gas Mixer
  • Compuvalve
  • GFI pressure regulator
  • Hose from compuvalve to Gas Mixer

The following are OEM supplied parts and are not serviced by Detroit Diesel: ‪

  • Tescom regulator
  • Fuel tank
  • Line from tank to regulator
  • GFI pressure regulator bracket
  • Fuel filters and brackets
  • Hose from regulator to coalescing fuel filter
  • Manual gas shutoff valve
  • DDEC controlled shutoff valve
  • Pressure gages
  • Vehicle vent system
  • Hose from coalescing fuel filter to compuvalve. See Figure 25032 and see Figure 25033 .

The Series 50G spark ignited natural gas engine utilizes both Detroit Diesel Electronic Controls (DDEC) and the Gaseous Fuel Ignition (GFI) system to provide engine control. DDEC provides ignition timing, throttle position, and engine protection. The GFI system provides control and metering of the natural gas, based on input from sensors mounted directly on the engine. ‪

The fuel system uses computer calculations for both fuel flow and air flow, and maintains a lean air - fuel mixture for low emissions and good fuel economy. Calculations are based on knowing the temperature and pressure of the air and fuel. The manifold absolute pressure (MAP), barometric absolute pressure (BAP), fuel absolute pressure (FAP), and fuel regulated temperature (FRT) are sensors internally located in the metering valve. The intake air temperature (IAT) sensor is mounted in the intake tube between the throttle and the air intake manifold. The additional oil temperature sensor (OTS) (displayed as MST on GFI fuel monitor) that is used by the fuel system is located in the left rear of block a few inches above the current DDEC oil temperature sensor. ‪

Compressed Natural Gas Fuel System

The Series 50G engine has been designed for operation with two types of high pressure fuel systems, either compressed natural gas (CNG) or liquefied natural gas (LNG). ‪

In CNG installations, fuel is stored in the fuel tanks under pressure as high as 3615 PSIA (3600 PSIG). In some installations, a high pressure fuel line supplies natural gas from the fuel tank to a Tescom regulator where it is reduced to an approximate pressure of 515 PSIA (500 PSIG) prior to entering the GFI pressure regulator. In newer CNG installations the TESCOM regulator has been eliminated and fuel is supplied to a different GFI regulator at pressures up to 3615 PSIA (3600 PSIG). Regardless of which installation type is used, the fuel pressure out of the GFI regulator should remain above 125 PSIA (110 PSIG). From the regulator, the fuel passes through a 1 micron coalescing gas filter to the computer and metering valve (compuvalve). The compuvalve then regulates fuel flow by means of a series of electronically controlled solenoids and injectors. Fuel is then passed to the gas mixing unit located in the air intake system upstream of the engine throttle. See Figure 25032 . ‪

Compressed Natural Gas Fuel System Description

Liquefied Natural Gas Fuel System

In LNG installations fuel is stored in cryogenic storage tanks under pressure at extremely cold temperatures. The LNG fuel flows from the tank as a liquid or mixture of liquid and vapor through the manual shutoff valve to the vaporizer. The fuel is heated in the vaporizer and any liquid natural gas is vaporized or changed to a gaseous state. Regulators and control valves in the vehicle fuel system control the rate that fuel is supplied to the vaporizer and control the pressure of the gaseous fuel supply exiting the vaporizer. The vaporized fuel passes through an ignition controlled fuel shutoff valve to a one micron fuel filter and then on to the GFI regulator and compuvalve. The minimum inlet pressure to the GFI regulator 135 PSIA (120 PSIG). The pressure may be as high as 265 PSIA (250 PSIG). The GFI regulator and compuvalve for the LNG fuel system are different than those for the CNG fuel system. Fuel is metered from the compuvalve and passed to the gas mixing unit located in the air intake system upstream from the throttle. See Figure 25033 . ‪

Liquefied Natural Gas Fuel System