Fuel System Description Alternative Fuel

The AF ECM controls the bi-fuel CNG fuel system and determines which fuel system will operate the engine. The AF ECM monitors various engine and vehicle functions to provide the correct amount of CNG fuel under all operating conditions. This provides excellent driveability and fuel economy while maintaining reduced emission levels. When operating on gasoline, the AF ECM will turn the fuel indicator lamp ON. The fuel indicator lamp is located within the fuel gauge select switch. The fuel indicator lamp is also turned ON for a two-second bulb check whenever the ignition is turned ON.

The AF ECM is connected to the vehicles original gasoline control module (VCM, PCM or ECM) wiring harness. This allows the AF ECM to monitor the same engine controls sensors that the gasoline control module monitors. These include but are not limited to the ECT, IAT and TP sensors. Both the AF ECM and the gasoline control module monitor these sensors and circuits for faults. The gasoline control module does not control CNG fuel delivery, however gasoline engine operation and the ignition system are still monitored and controlled by the gasoline control module.

This vehicles primary fuel source is compressed natural gas (CNG). The AF ECM will only allow gasoline operation if one of the following conditions is present:

The instrument panel fuel gauge 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 gauge system utilizes a fuel gauge selector switch and a fuel gauge relay. This allows the two different fuel tank level readings (CNG and gasoline) to be displayed by the one instrument panel fuel gauge.

The fuel gauge selector switch is a momentary-closed type switch located within reach of the vehicle operator. The fuel gauge switch allows the vehicle operator to observe the fuel level of each fuel system regardless of which fuel system is currently in operation. The AF ECM monitors the status of the switch. When the switch is depressed the AF ECM commands the fuel gauge relay either ON or OFF depending on which fuel system is in operation. The fuel gauge relay allows either the fuel level from the PCM or the fuel level of the AF ECM to be monitored by the instrument panel.

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 front tank high pressure lock-off. The AF ECM performs a calculation of the CNG pressure and CNG temperature. The AF ECM generates a pulse width modulated signal that can be monitored by the instrument panel fuel gauge.

The gasoline fuel level is determined by the PCM in a conventional manner using an in-tank variable resistor sensor.

The primary components of the bi-fuel CNG fuel SUPPLY system are as follows:

The CNG Fuel Tanks are constructed of high-strength steel covered with a two-stage epoxy coating finish. A high pressure lock-off solenoid is threaded into the end of each fuel tank and is used to prevent fuel flow during non-cranking or running engine conditions. Total storage capacity of the tanks is about 9.3 gasoline gallon equivalent at 3,600 psi @ 21°C (70°F). The fuel tanks also have a protective cover. The cover is designed to protect the tank from road debris.

The fuel tanks are mounted in tandem, transverse behind the rear axle, under the vehicle.

Regulations require that the fuel tank and brackets be inspected every three years or 36,000 miles (60,000 km), which-ever comes first. Fuel tank inspection results should be recorded in the CNG Fuel Tank Inspection Record area of the owners manual supplement (if applicable). 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.

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 filter element designed to trap contaminants. This filter must be periodically inspected and cleaned. The fill valve is mounted to the rear bumper. Refer to the owners manual supplement for fuel tank filling procedures.

The fill line is constructed of 3/8 in. flexible, steel-braided hose protected by a plastic cover. The fill line connects the fill valve to a line union check valve and then to the fuel tank and is sealed at all connections with an O-ring.

In the event of fill valve leakage, the line union check valve is designed to minimize the amount of CNG fuel leakage. The check valve must be installed with the flow arrow pointing towards the HPL.

The HPL is a normally closed, solenoid valve. Both HPL solenoids, and a low pressure lock-off (LPL) solenoid in the fuel line, prevent fuel flow. The AF ECM commands ONLY the HPLs 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 line. The AF ECM commands BOTH the HPLs and the LPL open when engine RPM indicates the engine is cranking or running on CNG.

The FTP sensor is a pressure transducer. The AF ECM supplies about 5 volts on the FTP reference circuit. The AF ECM also supplies a signal and ground circuit to the FTP. Located at the front fuel tank, threaded into the HPL, the FTP supplies a voltage signal to the AF ECM. With the fuel tanks full (high pressure) a high voltage signal will be monitored by the AF ECM.

The volume of CNG will vary with temperature and pressure. This will vary the amount of fuel vs. the signal voltage the FTP supplies to the AF ECM. In order to compensate for the different volume factors, an HPL mounted (internal to the tank) fuel tank temperature (FTT) sensor is also monitored by the AF ECM. The AF ECM performs a calculation on the FTP voltage vs. the in-tank temperature. The AF ECM will then control the fuel gauge to display the correct fuel level.

The CNG FTP sensor should not be confused with the GASOLINE fuel tank pressure (FTP) sensor that is utilized for Evaporate (EVAP) emissions monitoring.

The FTT 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 signal circuit. When FTT resistance is high (cold sensor) the FTT signal voltage is high. As the FTT warms and resistance drops, less signal voltage is monitored by the AF ECM.

The volume of CNG will vary with temperature and pressure. This will vary the amount of fuel vs. the signal voltage the FTP supplies to the AF ECM. In order to compensate for the different volume factors, an HPL mounted (internal to the tank) fuel tank temperature (FTT) sensor is also monitored by the AF ECM. The AF ECM performs a calculation on the FTP voltage vs. the in-tank temperature. The AF ECM will then control the fuel gauge to display the correct fuel level.

Only the fuel tank temperature sensor of the front tank HPL is monitored by the AF ECM.

The fuel system uses three different fuel line types (not including the fill line). The type used is dependent upon the pressure the fuel line type is designed to handle.

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 that are specific for gaseous fuel operation. These O-rings must be replaced with the correct replacement part whenever a line is opened.

When installing NPT (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 solid stainless steel line with a flexible line for connection at the low pressure regulator (LPR) that is designed to isolate alternative fuels components from vibration.

Low Pressure Hose

Low pressure gas is supplied from the LPR to the fuel mixer with Gann® VH-100 vapor hose. This hose is a patented non-permeable hose.

All hose, lines and fittings must be replaced ONLY with the correct replacement GMSPO part number.

The electric low pressure lock-off solenoid provides fuel shut-off prior to the low pressure regulator. The normally closed solenoid has a high temperature coil and is both UL and CGA approved. The LPL has a 1/4 in. diameter fuel orifice. The LPL is only commanded ON when engine cranking or running is detected.

The primary components of the bi-fuel CNG fuel metering system are as follows:

The CNG fuel system is controlled by the alternative fuels engine control module (AF ECM). The AF ECM receives input from various engine sensors in order to determine the amount of fuel necessary to maintain a precise air/fuel mixture across the entire engine operational range. The AF ECM then sends a command to the fuel control solenoid to meter the appropriate quantity of fuel. The fuel control solenoid meters the fuel flow by changing the pressure on the LPR diaphragm from atmosphere to vacuum. This system will also automatically sense and compensate for changes in altitude.

The HPR is supplied with fuel from the CNG fuel tank (at up to 3,600 psi) through high pressure stainless steel lines. The HPR reduces fuel pressure to between 115-220 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 400 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.

The low pressure regulator (LPR) is a negative pressure two-stage configuration that is closed when the engine is not running. When the engine is cranking or running a partial vacuum is created in the fuel line to the regulator. This opens the regulator, permitting fuel to flow to the fuel mixer.

The regulator receives CNG fuel at about 170 psi. The pressure is reduced in two stages to slightly less than atmospheric. In the first stage the pressure is reduced to approximately 5 psi. In the second stage, the CNG fuel is drawn into the secondary chamber where the pressure is reduced to approximately -0.5 in. of water column.

The fuel mixer is an air-fuel metering device and is completely self contained. The fuel mixer does not require linkage or an idle vacuum line connected to the intake manifold. The mixer is an air-valve design, utilizing a relatively constant pressure drop to draw fuel into the mixer from cranking to full load.

During normal engine operation, the fuel is drawn from the secondary chamber of the low pressure regulator by the vacuum created as the air passes through the fuel mixer. This pressure drop is most commonly referred to as air valve vacuum. As the air valve vacuum increases or decreases, the amount of fuel drawn from the secondary chamber will increase or decrease. In the fuel mixer, the CNG vapor is combined with air to form a combustible air/fuel mixture.

The vacuum applied to the air valve, working against a spring located on top of the valve, correctly positions the air valve to meter the proper amount of fuel for any given engine speed and load. The pressure drop that is controlled by the air valve spring provides the force to draw fuel into the air stream. The upward movement of the diaphragm controls the fuel flow in the venturi air valve.

A pressure drop under the air valve of approximately 0.2 psi (6 in H2O) of pressure is required to lift the air valve of its seat. Approximately 0.5 psi (13.8 in H2O) lifts the valve to the top of its travel in the full open position. Lowered pressure communicated to the top of the diaphragm varies with engine speed and the position of the throttle valve opening. The air valve assembly measures the air flow into the engine by moving precisely in response to the demands of the engine and throttle valve position.

The controlled pressure drop of 0.2-0.5 psi (6-13.8 in H2O) set up by the metering spring provides the signal or force necessary to draw fuel into the air stream within the mixer. The gas metering valve is attached to the air valve assembly and is shaped to admit the correct amount of fuel from the gas jet to mix with incoming air at any opening of the air valve.

Fuel Control Solenoid (FCS)

The FCS is an electronically controlled solenoid used to trim fuel mixtures by metering air valve vacuum to control the pressure on top of the LPR secondary diaphragm. This changes the LPR output pressure, and the amount of fuel flowing out of the LPR. The Alternative Fuels ECM controls the FCS with an electrical duty cycle which is converted into the regulated air valve vacuum signal communicated to the top of the LPR. As the FCS duty cycle increases, more vacuum on top of the LPR secondary diaphragm reduces the fuel flow out of the LPR which leans the air fuel mixture. As the FCS duty cycle decreases, less vacuum on top of the LPR secondary diaphragm increases the fuel flow out of the LPR enriching the air fuel mixture. The fuel control authority of the FCS increases with engine air flow.

Idle Control Solenoid (ICS)

The ICS is an electronically controlled solenoid used to meter supplemental fuel from the LPR to the mixer to increase fuel control authority during idle and low speed operation. The Alternative Fuels ECM controls the ICS with an electrical duty cycle which is converted into the regulated fuel flow from the LPR to the mixer. As the ICS duty cycle increases, more supplemental fuel flows out of the LPR into the mixer enriching the air fuel mixture. As the ICS duty cycle decreases, less supplemental fuel flows out of the LPR into the mixer which leans the air fuel mixture. The fuel control authority of the ICS decreases with engine air flow.