Fuel System Description (LC8FHZ)
Fuel System Description (LC8\FHZ)
Fuel System Overview
The compressed natural gas (CNG) fuel system operates similar to the gasoline version. The main differences being the storage, delivery, and safety devices within the high pressure CNG system. The engine control module (ECM) energizes the fuel pump relay that powers the CNG control module. The CNG control module supplies power and ground to the 4 high pressure lock-out solenoid valve and fuel pressure regulator. This allows CNG to flow through the high pressure lines through the fuel pressure regulator up to the fuel rail. The ECM controls the injector pulse width and timing as needed for proper engine performance.
Compressed Natural Gas (CNG) Fuel Tanks
The CNG fuel tanks are constructed of steel and conform to NGV2-1 (Type 1) specifications. A high pressure lock-off solenoid valve is threaded into the end of each fuel tank and is used to prevent fuel flow during non-operational running conditions. Metal shields are used to protect the fuel tanks from road debris or other contact conditions that may occur.
The system utilizes 4 CNG fuel tanks 2 that are mounted in tandem behind the rear axle, 1 mid-ship under the vehicle, and 1 in the cargo compartment.
Note:
Federal Government Regulations require that the fuel tanks and brackets be inspected every three years or 60,000 km (36,000 m), whichever occurs first. Inspection results should be recorded in the inspection record section of the CNG owners manual supplement. The service life of a type 1 CNG fuel tank is 15 years from the date of manufacture. All CNG fuel tanks regardless of inspection results must be removed from service after this 15 year period
CNG High Pressure Lock-Off (HPL) Solenoid Valves
The HPL solenoid valve is a normally closed solenoid valve. The HPL solenoids, along with the high pressure regulator (HPR) solenoid prevents fuel flow when in the closed position. The ECM commands fuel pump relay "ON" at every ignition cycle for 4.8 seconds. This supplies power to the CNG control module to prime the fuel system and allow the fuel tank pressure (FTP) sensor to monitor the pressure for fuel gauge display. The CNG control module commands all solenoids "ON" when the fuel pump relay is energized. The ECM energizes the fuel pump relay when engine RPM indicates a crank or run condition is present.
CNG Tank Pressure Relief Device (PRD)
Note:
All external PRD devices are connected directly to fuel storage pressure and cannot be isolated from the high pressure system. Do not attempt to service these devices or connect tubing/hoses unless you are absolutely certain that the system is completely empty of CNG fuel.
Each HPL solenoid valve contains an integral thermally-activated Pressure Relief Device (PRD). The PRD will activate when exposed to temperatures of approximately 108°C (220°F). In addition, there are three externally mounted PRD devices, two are equipped with a thermally and pressure-activated PRD and one is a thermally-activated PRD only. The pressure activated PRD function provides additional protection and activates when CNG tank pressure becomes too high for safe operation (approximately 37,231 kPa (5400 psig).
The combination Temperature and Pressure devices are located as follows:
* One between the aft axle CNG tanks
* One at the mid-ship (between axles) CNG tank
The external thermal only device PRD is located at the opposite-end of the HPL solenoid valve of the mid-ship tank.
CNG Fill Receptacle and Lines
Note:
Before refueling, the O-ring must be inspected and replaced if missing or damaged. The vehicle is shipped with three replacement O-rings placed in the glove compartment. Replacement O-rings are available through the GM parts network.
The CNG fill receptacle is a NGV1 profile and mates to any NGV1 fill dispenser valve. The fill dispenser seals to the receptacle with an O-ring. The fill receptacle is mounted in the vehicle fill pocket behind the fuel access door. Refer to the CNG owners manual supplement for refuel procedures.
The fuel fill line is a combination of flex hose and tubing. All connections are sealed by O-rings. The fill line runs from the receptacle to the check valve. The check valve is intended to minimize the amount of fuel leakage in the event the receptacle develops a leak.
CNG Fuel Lines
The CNG fuel system utilizes different fuel line types depending upon the working pressure and vehicle interface requirement the fuel line is required to handle.
Note:
* O-rings must be replaced with the correct replacement part when inspection reveals damage, etc.
* Hose, lines, and fittings must be replaced with approved GM service part
High Pressure Line and Hose
The high pressure line is a combination of stainless steel tubing and stainless steel jacketed PTFE hose that are certified to NGV 3.1. All connections between lines and components are of the O-ring Face Seal (ORFS) design and are sealed by specific O-rings manufactured specifically for CNG operation.
Low Pressure Line and Hose
The low pressure line is a combination of stainless steel tubing and stainless steel jacketed PTFE hose that are certified to NGV 3.1. All connections between lines and components are of the O-ring Face Seal (ORFS) design and are sealed by specific O-rings manufactured specifically for CNG operation.
CNG 1/4 Turn Isolation Valve
The high pressure fuel system is equipped with a manually-operated isolation valve located forward of the rear wheels on the left side of the vehicle at the top inboard side of the LH frame rail. A label applied to the lower body panel indicates the approximate location of this valve. The purpose of the valve is to isolate the high pressure side of the fuel system for some service procedures. If this valve is inadvertently left in the "OFF" position, the vehicle will not be operable. Refer to the CNG owners manual supplement for operating instructions.
Fuel Tank Pressure (FTP) Sensor
The fuel tank pressure (FTP) sensor is a pressure transducer. The CNG control module supplies a 5.0 V reference signal to the transducer. The transducer output varies from approximately 4.5 V when system is full to 0.5 V when the system is empty. The CNG control module provides a signal to the ECM to that will indicate the CNG system fuel level.
Note:
When refueling in cold ambient conditions, the fuel gauge may not display "FULL" even though the temperature-compensated refueling event produces a temperature-compensated "FULL" condition. This is due to commercial refueling station output regulated by a temperature vs. pressure strategy to prevent the vehicle from becoming over-pressurized if relocated to a warmer ambient location after refueling.
CNG Fuel Filter
The CNG fuel filter is a high pressure coalescing media filter located forward of the CNG 1/4 turn Isolation Valve. This filter requires periodic service intervals which can be found in the CNG owners manual supplement or the service manual.
Compressed Natural Gas (CNG) Control Module
The CNG control module provides two functions when energized.
* Provides a 5 V reference voltage to the FTP sensor and converts the value to a signal that it supplies to the ECM for proper fuel level display.
* Provides power and ground to the high pressure lock-out (HPL) solenoid valve and high pressure regulator (HPR) solenoid when the fuel pump relay is energized.
High Pressure Regulator (HPR)
The high pressure regulator (HPR) is supplied with fuel by the fuel supply system at pressures up to 24821 kPa (3600 psig) at 21°C (70° F). Fuel flow begins when the ignition is cycled and the HPL and HPR solenoids energized. The outlet pressure is regulated to 7 - 8 bar (90 - 110 psig) and delivered through the low pressure lines to the fuel rail and CNG injectors. The HPR is connected to the engine cooling system by an inlet and outlet circuit which intercepts the heater hoses between the engine and heater core to prevent icing. The HPR also has an integral pressure burst disc (PRD) which will rupture if outlet pressure exceeds 1896 kPa (275 psig). The solenoid assembly and ORFS O-rings are the only serviceable components of the HPR.
CNG Fuel Injectors
The CNG Fuel Injectors are specific to Gaseous Fuel use. The operation of these injectors is controlled by the ECM. The injectors are installed in the base vehicle fuel rail and require spacers between the injector and manifold injector boss for proper installation. The injectors are sealed with O-rings in an identical manner to the gasoline version
On-Board Refueling Vapor Recovery System (ORVR)
The On-Board Refueling Vapor Recovery System (ORVR) is an on-board vehicle system designed to recover fuel vapors during the vehicle refueling operation. ON CNG vehicles this system is not functional although the purge solenoid is still required for proper vehicle operation.
Fuel Rail Assembly
The fuel rail assembly attaches to the engine intake manifold. The fuel rail assembly performs the following functions:
* Positions the injectors (3) in the intake manifold
* Distributes fuel evenly to the injectors
Fuel Metering Modes of Operation
The engine control module (ECM) monitors voltages from several sensors in order to determine how much fuel to give the engine. The ECM controls the amount of fuel delivered to the engine by changing the fuel injector pulse width. The fuel is delivered under one of several modes.
Starting Mode
When the ignition is first turned ON, the ECM supplies voltage to the CNG control module for 4.8 seconds. While this voltage is being received, the CNG control module provides power and ground to the HPL and HPR solenoid valves. The ECM calculates the air/fuel ratio based on inputs from the engine coolant temperature (ECT), mass air flow (MAF), manifold absolute pressure (MAP), and throttle position (TP) sensors. The system stays in starting mode until the engine speed reaches a predetermined RPM.
Clear Flood Mode
If the engine floods, clear the engine by pressing the accelerator pedal down to the floor and then crank the engine. When the TP sensor is at wide open throttle (WOT), the ECM reduces the fuel injector pulse width in order to increase the air to fuel ratio. The ECM holds this injector rate as long as the throttle stays wide open and the engine speed is below a predetermined RPM. If the throttle is not held wide open, the ECM returns to the starting mode.
Run Mode
The run mode has 2 conditions called Open Loop and Closed Loop. When the engine is first started and the engine speed is above a predetermined RPM, the system begins Open Loop operation. The ECM ignores the signal from the heated oxygen sensors (HO2S). The ECM calculates the air/fuel ratio based on inputs from the ECT, MAF, MAP, and TP sensors. The system stays in Open Loop until meeting the following conditions:
* Both front HO2S have varying voltage output, showing that both HO2S are hot enough to operate properly.
* The ECT sensor is above a specified temperature.
* A specific amount of time has elapsed after starting the engine.
Specific values for the above conditions exist for each different engine, and are stored in the electrically erasable programmable read-only memory (EEPROM). The system begins Closed Loop operation after reaching these values. In Closed Loop, the ECM calculates the air/fuel ratio, injector ON time, based upon the signal from various sensors, but mainly from the HO2S. This allows the air/fuel ratio to stay very close to 14.7:1.
Acceleration Mode
When the driver pushes on the accelerator pedal, air flow into the cylinders increases rapidly. To prevent possible hesitation, the ECM increases the pulse width to the injectors to provide extra fuel during acceleration. This is also known as power enrichment. The ECM determines the amount of fuel required based upon the TP, the ECT, the MAP, the MAF, and the engine speed.
Deceleration Mode
When the driver releases the accelerator pedal, air flow into the engine is reduced. The ECM monitors the corresponding changes in the TP, the MAP, and the MAF. The ECM shuts OFF fuel completely if the deceleration is very rapid, or for long periods, such as long, closed-throttle coast-down. The fuel shuts OFF in order to prevent damage to the catalytic converters.
Battery Voltage Correction Mode
When the battery voltage is low, the ECM compensates for the weak spark delivered by the ignition system in the following ways:
* Increasing the amount of fuel delivered
* Increasing the idle RPM
* Increasing the ignition dwell time
Fuel Cutoff Mode
The ECM cuts OFF fuel from the fuel injectors when the following conditions are met in order to protect the powertrain from damage and improve driveability:
* The ignition is OFF. This prevents engine run-on.
* The ignition is ON but there is no ignition reference signal. This prevents flooding or backfiring.
* The engine speed is too high, above red line.
* The vehicle speed is too high, above rated tire speed.
* During an extended, high speed, closed throttle coast down-This reduces emissions and increases engine braking.
* During extended deceleration, in order to prevent damage to the catalytic converters
Fuel Trim
The engine control module (ECM) controls the air/fuel metering system in order to provide the best possible combination of driveability, fuel economy, and emission control. The ECM monitors the heated oxygen sensor (HO2S) signal voltage while in Closed Loop and regulates the fuel delivery by adjusting the pulse width of the fuel injectors based on this signal. The ideal fuel trim values are around 0 percent for both short term and long term fuel trim. A positive fuel trim value indicates the ECM is adding fuel in order to compensate for a lean condition by increasing the pulse width. A negative fuel trim value indicates that the ECM is reducing the amount of fuel in order to compensate for a rich condition by decreasing the pulse width. A change made to the fuel delivery changes the short term and long term fuel trim values. The short term fuel trim values change rapidly in response to the HO2S signal voltage. These changes fine tune the engine fueling. The long term fuel trim makes coarse adjustments to the fueling in order to re-center and restore control to short term fuel trim. A scan tool can be used to monitor the short term and long term fuel trim values. The long term fuel trim diagnostic is based on an average of several of the long term speed load learn cells. The ECM selects the cells based on the engine speed and engine load. If the ECM detects an excessive lean or rich condition, the ECM will set a fuel trim diagnostic trouble code (DTC).