Fuel System Operation

This 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:
^ The CNG tank is empty.
^ Every 100th start and also when the coolant temperature is below 10°F (-12°C) the vehicle will start and run briefly on gasoline. A switch-over to CNG will occur only during a decel condition.
^ A system fault is detected by the AF ECU. The engine cranks for 8 seconds and fails to start on CNG.

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 the gasoline level (fill if needed).
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 fuel gauge selector switch button. The fuel indicator lamp indicates which fuel system is in operation. The AF ECU 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 Gauge Operation
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 ECU monitors the status of the switch. When the switch is depressed the AF ECU 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 gasoline tank or the fuel level of the AF ECU to be monitored by the PCM. The CNG fuel level is determined by the AF ECU. The AF ECU monitors the fuel tank pressure and fuel tank temperature sensors that are located at the CNG tank HPL. The AF ECU performs a calculation of the CNG pressure and CNG temperature. The AF ECU generates a pulse width modulated signal that can be monitored by the PCM. The PCM then sends the fuel level information to the instrument panel using Class 2 serial data.

Passenger vehicles have a filtered fuel level display, depress the fuel gauge select switch 30 seconds after the vehicle speed sensor indicates the vehicle is at rest in order to avoid inaccurate gauge displays.

Fuel Flow
The alternative fuel engine control unit (AF ECU) controls the fuel system while the vehicle is operating on CNG The AF ECU 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 ECU monitors that the engine is cranking or running. This fuel delivery system utilizes three different pressure stages in order to lower the fuel pressure from tank pressure down to a delivery pressure which is slightly above atmospheric pressure

High Pressure (tank) Stage
When the AF ECU 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 outlet of the HPR is the Intermediate pressure stage. 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.

Intermediate Pressure Stage
The Intermediate Pressure Stage begins at the outlet of the high pressure regulator (HPR). With a pressure between 115-220 psi (793-1517 kPa), the fuel exiting the HPR flows through the intermediate fuel line to the low pressure lock-off (LPL) solenoid. The LPL is controlled by the AF ECU. Fuel flows out of the LPL through another portion of Intermediate fuel line and into the low pressure regulator (LPR). The inlet of the LPR is the Low Pressure Stage.

Low Pressure Stage
The low pressure regulator (LPR) is a two stage regulator. The fuel enters the LPR at the intermediate stage pressure of 115-220 psi (793-1517 kpa). The first stage of LPR operation reduces the fuel pressure down to between 4 and 6 psi (28-41 kPa). The fuel pressure is then again reduced in the second stage of the LPR down to between 2-5 inches (5.1-12.7 cm) of water column. The fuel exits the LPR and flows through the low pressure hose and into the gas mass sensor / mixture control valve (GMS/MCV). The AF ECU monitors the gas mass sensor information and adjusts the amount of fuel entering the engine by varying fuel flow through the mixture control valve. The fuel exits the GMS/MCV and flows through another portion of low pressure hose and into the gas distribution adaptor (GDA). The fuel mixes with inlet air and is drawn through the intake manifold to be burned by the engine.