Natural Gas Fuel System Overview
The Fuel System provides a means of transporting clean fuel from the fuel tank to the fuel injectors under a controlled pressure. Natural Gas Fuel System
The fuel system consists of a fuel tank, fuel shut-off valve assemblies, fuel supply lines, fuel filter, schrader/service valve, manual fuel shut-off valve, fuel rail, and fuel pressure regulator. Operation of the system is as follows (Figure 73),(Figure 74) and (Figure 75):
The fuel delivery system uses the crankshaft position (CKP) sensor to signal the PCM that the engine is either cranking or running.
The fuel shut-off valve logic is defined in the Fuel System control strategy and is executed in the PCM. The PCM will ground the fuel pump relay for one second during key on and engine off. During crank the fuel pump relay is grounded as long as the PCM receives a signal from the CKP.
The fuel pump relay has a primary and a secondary circuit. The primary side is controlled by the PCM and the secondary side provides B+ to the fuel shut-off valve circuit when the relay is energized.
The inertia fuel shut-off (IFS) switch is used to de-energize the fuel delivery circuit in the event of a collision. The IFS switch is a safety device that should only be reset after a thorough inspection of the vehicle (following a collision).
The fuel injector is used to meter natural gas to each combustion cylinder. Although the NG fuel injector appears very similar to some gasoline fuel injectors, it is unique. Flow capacity of this fuel injector is 6 to 12 times as large as various gasoline fuel injectors.
The fuel tank shut-off solenoid valve is located in the fuel tank. The solenoid valves are on the same circuit as the fuel pump and utilize the same inertia fuel shut-off (IFS) switch as gasoline.
The high pressure fuel filter is used to protect the engine fuel system components. A natural gas coalescing and particulate filter is positioned on the high pressure side of the fuel system just prior to the fuel pressure regulator.
The fuel pressure regulator used on the NG vehicle is a single-staged pressure reducing regulator which expands natural gas from storage pressures of 1,379 to 20,685 kPa (200 to 3,000 psig) to engine fuel pressures of 724 to 862 kPa (105 to 125 psig).
The fuel rail shut-off valve is a normally closed solenoid actuated valve that opens when grounded by the PCM. The valve isolates the fuel injectors from fuel line pressure when the engine is not operating. The fuel rail shut-off valve is wired in parallel with the fuel tank shut-off solenoid valves.
Natural Gas Fuel System
Natural Gas Fuel System Electrical Schematic-Typical
Natural Gas Fuel System Hardware Fuel Rail
The fuel rail (Figure 76) distributes low pressure fuel from the chassis supply line to each fuel injector. Fuel pressure at the top of each fuel injector is maintained within 1% of the other fuel injectors at all times; this is done with nearly symmetric flow paths. The fuel rail is also designed to have minimal flow restriction by increasing the cross-sectional flow area and reducing the flow path length. The fuel rail contains several other parts in assembly (PIA) components that perform crucial functions. These include:
Injection pressure sensor which measures the pressure of the fuel near the fuel injectors. This signal is used by the PCM to adjust the fuel injector pulsewidth and meter fuel to each engine combustion cylinder.
Engine fuel temperature sensor which measures the pressure of the fuel near the fuel injectors. This signal is used by the PCM to adjust the fuel injector pulsewidth and meter fuel to each engine combustion cylinder.
Low pressure solenoid shut-off valve which isolates the fuel rail from the upstream fuel system when the engine is OFF. This minimizes the amount of fuel available to flow through the fuel injectors when the engine is off or leak from a damaged fuel rail during and after a crash. The valve is controlled by the PCM fuel shut-off valve circuit and contains an inertia switch. The valve is only on for one second after a key-on or whenever CKP signals are being received by the PCM.
schrader/service valve which provides a service port to the low pressure fuel system. This valve is needed to relieve the pressure in the system before and during service. This valve could also be used to monitor the pressure near the injectors during diagnostic procedures.
Fuel Rail Components Fuel Injector(s)
The fuel injector (Figure 77) is a solenoid-operated valve that meters fuel flow to the engine. The fuel injector is opened and closed every other crankshaft revolution. The amount of fuel is controlled by the length of time the fuel injector is held open.
The fuel injector is normally closed and is operated by 12 volt VPWR from the power relay. The ground signal is controlled by the PCM.
The fuel injectors are used to meter natural gas to each combustion cylinder. Although the natural gas fuel injectors appear very similar to some gasoline fuel injectors, they are unique. Flow capacity of these fuel injectors is 6 to 12 times as large as various gasoline fuel injectors. Electrical resistance is much lower than typical gasoline fuel injectors (4.6 ohms as opposed to 14.5 ohms). To accommodate this lower resistance, a fuel injector driver module is used to convert the PCM fuel injector driver signal to the signal required by the fuel injector.
Fuel Injector Fuel Pressure Regulator
The fuel pressure regulator (Figure 78) used in the Natural Gas fuel system is a single-stage pressure reducing regulator which expands natural gas from storage pressures of 1,379 to 20,685 kPa (200 to 3,000 psig) to engine fuel injector pressures of 724 to 862 kPa (105 to 125 psig).
The regulator contains a pressure relief device, a 1,896 kPa (275 psig) check valve, which protects the low pressure fuel system. The low pressure fuel system no longer must fulfill the design requirements of the high pressure fuel system, therefore reducing cost, weight and complexity.
When gas expands, the fuel temperature drops significantly causing extreme cold temperatures (-177°C or -160°F) that may damage synthetic fuel system components as well as cause water vapor within the fuel to condense, freeze and plug the lines, valve and injectors. To prevent this, engine coolant is routed through the fuel pressure regulator to warm the fuel before it expands.
The regulator has an internal thermostat to control the flow of engine coolant. This prevents overheating and subsequent thinning of the fuel which may cause lean combustion. Outlet coolant flow is restricted by the thermostat when it rises above approximately 82°C (100°F).
Fuel Pressure Regulator High Pressure Fuel Filter
The high pressure fuel filter (Figure 79) is used to protect the engine fuel system components. A natural gas coalescing and particulate filter is positioned on the high pressure side of the fuel system just prior to the pressure regulator. The filter is part of the regulator assembly. The filter can be disassembled to service the element. The drain plug on the bottom of the housing can be removed to drain any water that accumulates.
Fuel Filter Fuel Lines and Fittings
A fuel line assembly (Figure 80) consists of flexible hose and/or stainless steel seamless tubing, end fittings and tube nuts. The hose is a conductive polytetrafluoroethylene (PTFE) liner reinforced with a stainless steel wire braided covering. The fittings are inserted into the hose ends and crimped into place. The stainless steel tubing contains end fittings which are brazed to the tube. There are high pressure fuel lines that are identified by either 1/4-inch or 3/8-inch outer diameter and a low pressure fuel line identified by a 1/2-inch outer diameter. The low pressure fuel line has a quick-connect at one end for connection to the fuel rail. The other fittings used on the natural gas vehicle to connect fuel components are SAE O-ring face seal tube fittings. There are two end types: an O-ring face seal end and a straight thread end. On tee and elbow fittings, a washer and a positionable nut are provided to aid in orientation of the fitting.
Fuel Line Assembly Fueling Connector Flange Assembly-Fuel Tank Fill The flange assembly (Figure 81) is designed for 20,685 kPa (3,000 psi) service pressure and is the refueling connection to fill the vehicle. The assembly is mounted behind the fuel filler door and attached to the fuel filler housing, similar to a gasoline vehicle. This assembly consists of an NGVP1 type receptacle with a 150 micron filter (which can be serviced), a spring loaded check valve to allow filling of the vehicle and a manually opened bypass to provide safe venting of the fuel system. The vehicle is refueled by attaching the fuel station fill nozzle to the receptacle and locking into place.
Flange Assembly Fuel Tank Shutoff Valve
The fuel tank shutoff solenoid valve (Figure 82) is located in the fuel tank. The solenoid valves are on the same circuit as the gasoline fuel pump and utilize the same Inertia fuel shutoff (IFS) switch as gasoline. When the key is in the off position, the shutoff valves are closed and fuel in the tanks is isolated. During refueling, the shutoff valve acts as a check valve and allows flow due to pressure differential between the fuel being added from the fill station and the fuel in the tank.
The internal solenoid valves also have the capability of being "manually locked down." If, while servicing the vehicle, it becomes necessary to remove the fuel tank, the lock down feature provides an added measure of safety. In addition, the valve has an internal Canadian Gas Association (CGA) type 9 fusible link pressure relief device (PRD) that senses the internal fuel tank gas temperature. The contents in the tank are vented when the internal fuel tank gas temperature reaches 103°C (217°F) and melts the fusible link. The escaping gas is vented through a vent line to the atmosphere.
Fuel Tank Shutoff Valve Inertia Fuel Shutoff (IFS) Switch
The inertia fuel shutoff (IFS) switch (Figure 83) is used in conjunction with electric fuel close valves. The purpose of the IFS switch is to close the fuel shut-off valves if a crash occurs. It consists of a steel ball held in place by a magnet. When a sharp impact occurs, the ball breaks loose from the magnet, rolls up a conical ramp and strikes a target plate which opens the electrical contacts of the switch and closes the electric fuel shut-off valve.Once the switch is open, it must be manually reset before restarting the vehicle. On some vehicles a fuel reset light illuminates. Refer to the Owner Guide for the location of the IFS.
Check for natural gas leaks in the engine compartment.
Note: In the closed position, the button can be depressed an additional 1.57 cm (1/16 inch) against a spring. If no natural gas leak is apparent, reset the IFS by pushing the reset button on the top of the switch (refer to Owner Guide).
Turn key to on or start position for a few seconds, then off again.
WARNING: IF YOU SMELL NATURAL GAS AT ANY TIME OTHER THAN DURING FUELING, DO NOT RESET THE IFS SWITCH. Again, check for leaking natural gas.
Fuel Rail Shut-Off Valve
The fuel rail shut-off valve (Figure 84) is a normally closed solenoid actuated valve that opens when (along with all of the tank valves) Pin 80 is grounded by the PCM. The valve isolates the fuel injectors from fuel line pressure when the engine is off. Nominal resistance of the coil is 11 ohms. The fuel rail shut-off valve is wired in parallel with the four tank valves. Fuel Rail Valve Circuit Operation When the key is turned to the ON position, the power relay is turned on. The power relay provides power to the PCM and the control side of the fuel shut off valve relay. The relay provides voltage to the fuel rail valve. If the ignition switch is not turned to the START position, the PCM will shut off the fuel rail valve after one second. The PCM will open the valve (along with the four tank valves) to provide fuel while cranking. The valve will remain open when the engine is running unless the inertia fuel shut-off switch is "tripped."
