The Alternative Fuels supply system contains the following components:

Fuel Tank

The CNG Fuel Tank is constructed of steel and is wrapped in carbon fiber set in epoxy. The high pressure lock-off solenoid is contained within the fuel tank and is used to prevent fuel flow during non-cranking or running engine conditions. Fuel tanks have a protective cover. The cover is designed to protect the tank from damage due to shifting cargo and also for ultra-violet (UV) radiation protection. The cover is not designed to support a load. Regulations require that the fuel tank and brackets be inspected every three years or 60,000 km (36,000 miles) , which-ever comes first. Fuel tank inspection results should be recorded in the CNG Fuel Tank Inspection Record area of the owners manual supplement. The service life of the CNG fuel tank is 15 years from the date of manufacture. All CNG fuel tanks regardless of inspection results must be replaced after  15 years. Refer to Fuel Tank Inspection procedures.

Fill Valve and Fill Line

The fill valve receives fuel from the fill nozzle at the CNG dispensing station. The fill valve seals to the dispensing nozzle with an O-ring. Before re-fueling this O-ring must be inspected and replaced if missing or damaged. The fill valve contains an internal 380 micron filter element designed to trap contaminants. This filter must be periodically inspected and cleaned. Refer to Fill Valve Filter Replacement for filter inspection and cleaning procedures. The fill valve mounting location varies with vehicle model. Refer to the owners manual supplement for fill valve location and fuel tank filling procedures. The fill line is constructed of high pressure line. The fill line connects the fill valve to the fuel tank and is sealed to the fuel tank with an O-ring.

Fill Line Union Check Valve

The fill line is equipped with a check valve. In the event of fill valve leakage, the check valve is designed to minimize the amount of CNG fuel leakage. The check valve must be installed with the flow direction arrow pointed towards the high pressure lock-off solenoid.

High Pressure Lock-Off (HPL) Solenoid

The HPL is a normally closed, solenoid valve. The HPL solenoid and a low pressure lock-off (LPL) solenoid in the fuel line prevent fuel flow. The alternative fuels engine control module (AF ECM) commands ONLY the HPL open for 1 second at EVERY ignition ON for a CNG prime pulse. The CNG prime pulse is performed in order to charge the fuel line and also allows the fuel tank pressure (FTP) sensor to monitor the amount of fuel pressure in the tank. The AF ECM commands BOTH the HPL and the LPL open when engine RPM indicates the engine is cranking or running on CNG.

Fuel Tank Pressure (FTP) Sensor

The FTP sensor is a pressure transducer. The AF ECM supplies about 5 volts on the FTP sensor reference circuit. The AF ECM also supplies a signal and ground circuit to the FTP sensor. Located at the end of the fuel tank, threaded into the HPL, the FTP sensor supplies a voltage signal to the AF ECM. With a full fuel tank a high voltage signal will be monitored by the AF ECM. The volume of CNG varies with temperature and pressure. This will vary the amount of fuel vs. the signal voltage the FTP sensor supplies to the AF ECM. In order to compensate for the different voltage factors, a tank mounted fuel tank temperature (FTT) sensor is also monitored by the AF ECM. The AF ECM performs a calculation on the FTP sensor voltage vs. the in-tank temperature. The AF ECM will then signal the correct fuel level to the PCM.

Fuel Tank Temperature (FTT) Sensor

The FTT sensor is a thermistor mounted inside the HPL and is not serviceable separately from the HPL. The FTT thermistor has high resistance when cold and low resistance when hot. The AF ECM supplies about 5 volts to the FTT sensor signal circuit. When the FTT sensor is cold its resistance is high and the signal voltage is high. As the FTT sensor warms and resistance drops, less signal voltage is monitored by the AF ECM. The volume of CNG varies with temperature and pressure. This will vary the amount of fuel vs. the signal voltage the FTP sensor supplies to the AF ECM. In order to compensate for the different voltage factors the tank mounted FTT sensor is also monitored by the AF ECM. The AF ECM performs a calculation on the FTP sensor voltage vs. the in-tank temperature. The AF ECM will then signal the correct fuel level to the PCM.

High Pressure Line

The high pressure line is stainless steel tubing with matching high pressure O-ring face seal (ORFS) fittings. The individual tubes are pre-formed with fittings installed. All O-ring face seal fittings use nitrile rubber O-rings and might be identified with a yellow or white band. These O-rings must be replaced with the correct replacement part whenever a line is opened.

When installing pipe thread fittings DO NOT use Teflon® tape. Tape fragments could lodge in a regulator and allow fuel pressure to exceed specified levels. The use of pipe sealant with Teflon® is required.

Intermediate Pressure Fuel Hose

The intermediate pressure fuel hose assembly is a unique two-piece stainless steel braided hose and is designed to isolate AF components from vibration.

Coalescing CNG Fuel Filter

The coalescing CNG fuel filter is located between the high pressure lock-off solenoid and the high pressure regulator. The coalescing filter housing contains a 6 micron filter element as well as a drain fitting. The coalescing filter is designed to trap contaminates and liquids that may damage the gaseous fuel injectors. This filter must be periodically inspected, drained and replaced.

The Alternative Fuels metering system contains the following components:

High Pressure Regulator (HPR)

The HPR is supplied with fuel from the CNG fuel tank at up to 24821 kPa (3600 psi) through high pressure stainless steel lines. The HPR reduces fuel pressure to between 758-1448 kPa (110-210 psi). The outlet of the HPR is the Intermediate pressure stage. Fuel flows out of the HPR and into the low pressure lock-off (LPL) solenoid. The pressure drop within the regulator causes fuel temperature to drop. In order to prevent HPR freeze-up, the HPR is connected to the vehicle engine cooling system. The HPR contains an over-pressure relief device (PRD) which will not allow pressure above 1896 kPa (275 psi) on the output stage of the HPR. The PRD is removable in order to allow the installation of a pressure gauge for diagnostic purposes. The HPR has an internal, serviceable 40 micron filter. This filter must be serviced periodically. Refer to High Pressure Regulator Filter Replacement for replacement procedures.

Low Pressure Lock-Off (LPL) solenoid

The LPL is a normally closed, solenoid valve. The LPL is mounted in the Intermediate pressure line. The AF ECM controls LPL solenoid operation. The AF ECM energizes the LPL solenoid only when engine cranking or running is detected. Anytime the engine is stopped the AF ECM de-energizes the LPL solenoid which stops fuel flow. The AF ECM operates the LPL and the high pressure lock-off (HPL) solenoid at the same time except at ignition ON. During ignition ON only the HPL is opened while the LPL remains closed in order to charge the fuel system with CNG. The LPL is supplied fuel from the HPR with fuel pressure between 758-1448 kPa (110-210 psi). The outlet of the LPL supplies the intermediate pressure regulator (IPR) with fuel.

Intermediate Pressure Regulator (IPR)

Fuel enters the IPR at the intermediate stage pressure of 758-1448 kPa (110 to 210 psi). The IPR reduces the fuel pressure down to about 276 kPa (40 psi). The fuel exits the IPR and flows to the CNG fuel rails. Refer to Fuel System Diagnosis for pressure checking procedures.

Gaseous Fuel Injectors

The gaseous fuel injectors are pulse-width modulated and sequentially fired. The injector voltage is supplied by an injector relay. The AF ECM supplies the ground path.

Fuel Rail Pressure Sensor

The fuel rail pressure (FRP) sensor is a pressure transducer. The AF ECM supplies about 5 volts on the FRP sensor reference circuit. The AF ECM also supplies a signal and ground circuit to the FRP sensor. When fuel pressure is high the signal voltage is high. The FRP signal is used by the AF ECM for fuel trim.

Fuel Rail Temperature Sensor

The fuel rail temperature (FRT) sensor is a thermistor. The FRT thermistor has high resistance when cold and low resistance when hot. The AF ECM supplies about 5 volts to the FRT sensor signal circuit. When the FRT sensor is cold its resistance is high and the signal voltage is high. As the FRT sensor warms and resistance drops, less signal voltage is monitored by the AF ECM. The FRT signal is used by the AF ECM for fuel trim.

The bi-fuel vehicles primary fuel source is compressed natural gas (CNG). The vehicle will only switch-over to gasoline operation if one of the following conditions is present:

60 Day Cycle

It is recommended that every 60 days the vehicle be cycled between CNG and gasoline. This ensures the gasoline fuel system stays in good working condition and also accommodates for seasonal changes in gasoline formulation. Perform the following steps every 60 days.

1. With the vehicle operating on CNG check and fill the gasoline level.
2. Operate the vehicle until the CNG fuel tank is empty and the vehicle switches over to gasoline operation.
3. Operate the vehicle until the gasoline tank is near empty.
4. Refill both fuel tanks.
5. The vehicle will switch-over to CNG operation.
6. Repeat procedure in 60 days.

Fuel Indicator Lamp (FIL)

The fuel indicator lamp (FIL) is contained within the instrument panel. The fuel indicator lamp indicates which fuel system is in operation. The PCM controls the operation of the fuel indicator lamp. The fuel indicator lamp illuminates ONLY when the engine is operating on gasoline. The fuel indicator lamp will also illuminate for several seconds during start-up as a bulb check.

Fuel Gage Operation

The instrument panel fuel gage displays the level of the fuel system that is operating the engine until the operator of the vehicle requests the non-operating fuel system level to be displayed. Once requested, the non-operating fuel system level will be displayed for about 10 seconds. The Bi-fuel CNG fuel gage system utilizes a fuel gage selector switch. This allows the two different fuel tank level readings ,CNG and gasoline, to be displayed by the one instrument panel fuel gage. The fuel gage select switch is a momentary-closed type switch located within reach of the vehicle operator. The fuel gage switch allows the vehicle operator to observe the fuel level of each fuel system regardless of which fuel system is currently in operation. The PCM monitors the status of the switch. When the switch is depressed the PCM sends an appropriate fuel level serial data message to the instrument panel (IP) depending on which fuel system is in operation. The PCM allows either the fuel level from the gasoline tank or the fuel level of the AF ECM to be monitored by the IP. The CNG fuel level is determined by the AF ECM. The AF ECM monitors the fuel tank pressure and fuel tank temperature sensors that are located at the CNG tank HPL. The AF ECM performs a calculation of the CNG pressure and CNG temperature. The AF ECM generates a pulse width modulated signal that is monitored by the PCM. The PCM then sends the fuel level information to the instrument panel using Class 2 serial data.

The dedicated CNG fuel system operation is functionally the same as the bi-fuel CNG fuel system. The vehicle does not contain any gasoline fuel system components. The vehicle does utilize a PCM that operates in conjunction with the AF ECM. The calibration programmed into the PCM has disabled the gasoline specific DTC's. The PCM still performs diagnostics on the vehicle sensors such as ECT, TP, IAT, etc.

The AF ECM controls the fuel system while the vehicle is operating on CNG. The AF ECM receives signals from various engine sensors and determines the correct amount of fuel required by the engine. The fuel is stored in a cylindrical fuel tank. A full tank can contain fuel at 3600 psi (24,820 kPa). A high pressure lock-off (HPL) solenoid in the tank and a low pressure lock-off (LPL) solenoid in the fuel line prevents fuel flow until the AF ECM monitors that the engine is cranking or running. The fuel delivery system utilizes three different pressure stages in order to lower the fuel pressure from tank pressure down to a delivery pressure.

Tank Pressure Stage

When the AF ECM commands the HPL open fuel begins to flow at up to 3600 psi through the high pressure steel lines up to the high pressure regulator (HPR). The HPR reduces fuel pressure to between 115-220 psi (793-1517 kPa). The pressure drop within the regulator also causes fuel temperature to drop. In order to prevent HPR freeze-up, the HPR is connected to the vehicle engine cooling system.

High Pressure Regulator Stage

The high pressure regulator stage begins at the outlet of the high pressure regulator (HPR). With a pressure between 110-210 psi (758-1448 kPa), the fuel exiting the HPR flows through the fuel line to the low pressure lock-off (LPL) solenoid. The LPL is controlled by the AF ECM. Fuel flows out of the LPL into the intermediate pressure regulator (IPR).

Intermediate Pressure Regulator Stage

Fuel enters the IPR at the pressure of 110-210 psi (758-1448 kPa). The fuel pressure is reduced in the IPR down to about 40 psi (277 kPa). The fuel exits the IPR and flows through pressure hose and into the CNG fuel rails. The AF ECM monitors various sensor information and adjusts the amount of fuel entering the engine by varying the pulse width of the gaseous fuel injectors.