Fuel System Description (K07UFM)

Fuel System Description (LC8\K07\UFM)

Fuel System Overview

The liquid propane injection system is a 3 tank system located at the rear of the vehicle. The liquid propane injection system is a direct replacement propane fuel injection system. It replaces the gasoline fuel injection system and works the same as a gasoline fuel injection system with the exception it injects propane, in a liquid state, into the intake port. The gasoline system electronic engine management stays the same and controls the LPG system just as it did the gasoline injection system. Engine control diagnostics remain unchanged so the same scan tool and diagnostic approach remains equal to a gasoline system. The LPG control module will use the Wait to Start indicator to display flash codes if a fault is present.

The liquid propane injection system consists of three main components: the tank assembly, the fuel lines and the injectors. The tanks are located to the rear of the vehicle and the lines are routed forward to the engine compartment where the injector rail assemblies are mounted in the same position as the original gasoline injector rails were installed. The fuel tank is the most complicated area of the system. It includes an internal electric fuel pump, fuel supply & bypass valves, a baffle that keeps the pump submerged in liquid propane, fuel level float assembly, pressure relief valve, overfill prevention device, liquid and vapor service valves. The LPG fuel pump increases or boosts the tank pressure by 275-414 kPa (40-60 psi). No matter what the propane tank internal pressure is, the pump boost pressure remains the same. This is how the propane stays a liquid throughout the liquid supply section of the system. The fuel is supplied to the injectors and whether the injector is open or not fuel passes through a cooling bushing in the injector and is returned to the tank. This is called a refrigeration cycle and also aids in maintaining the fuel in a liquid state throughout the supply passageways in the system. Because propane easily vaporizes, when the refrigeration cycle stops (when the engine is turned off) or if the bypass valve malfunctions closed, the propane will vaporize and cause a loss in power or hard hot restarting. To help in hot restarting, the system goes through a purge cycle for 1 to 20 seconds, depending on temperature and pressure, before every start up attempt. This strategy is built into the system's LPG control module. During the purge cycle the "Wait to Start" indicator will be illuminated.

Fuel Tanks

The fuel tanks meet American Society of Mechanical Engineers (ASME) design for a working pressure of 2154.6 kPa (312.5 psi) and a burst pressure of 8618 kPa (1250 psi). Baffles are built in the tank to keep the fuel pump submerged in the liquid propane. At the front of the tank assembly is a bulkhead that provides access to the fuel pump.

Overfilling Prevention Device

The overfilling prevention device is a mechanical float-actuated valve that stops the tank from being filled more than 80%. By code, you should not fill a propane vehicle tank more than 80% full of liquid, to allow room for the liquid propane to expand when it gets warm. There is also a fixed liquid level gauge. This is a small valve located at the 80 % liquid level on the tank. To fill a tank using the fixed liquid level gauge open the valve before filling. Propane vapor will hiss out. When the liquid reaches the height of this valve the liquid propane will show up as a white mist. When the mist is present stop filling the tank and close the valve.

Over Flow Valves

Every inlet or outlet valve on the liquid propane tanks has a built in over flow valve. If propane tries to exit the system at a higher rate then the calibrated amount the difference in pressure closes the over flow valve and restricts the flow. The valve closes down to a 2 mm (0.080 in) diameter orifice. Once the difference in pressure is equalized the over flow valve will open.

Pressure Relief Valve

If the pressure in the fuel tank exceeds 2155 kPa (312.5 psi), the pressure relief valve (PRV) will vent propane vapor to the atmosphere. The pressure will not get this high unless the tank has been overfilled, or unless the tank is hotter than 60°C (140°F). When the PRV vents, the sudden pressure drop significantly cools the remaining liquid, because the boiling of the propane absorbs heat.

Fill Filter

The fill filter is located on the frame rail between the fuel tank and the fill valve. The fill filter traps particles larger than 3 microns to help reduce any contamination from the filling equipment and is required to be replaced at the proper maintenance intervals.

Fill Valve

Clean Fuel vehicles use a standard pro-pane vehicle refueling fitting (Sherwood 1855 series) with integral back-check valve.

Fuel Filter

The fuel filter at the fuel pump a sock type filter is mounted to the inlet of the fuel pump assembly inside the main fuel tank. There is also a fuel filter mounted to the frame rail in the fuel supply line between the fuel tank and fuel injector rail. The frame mounted fuel filter is required to be replaced at the proper maintenance intervals.

Fuel Pump

The LPG fuel pump is mounted inside the fuel tank and is used to increase line pressure of the liquid propane by 275-414 kPa (40-60 psi) over the internal tank pressure to insure the liquid state of the propane is maintained. The inlet to the LPG fuel pump is submerged in liquid at all times by a baffle in the tank assembly.

Fuel Lines

The fuel lines are Type III LPG approved hoses with minimum permeability in order to pass evaporative shed testing. The hoses are rubber coated stainless steel braided to protect against chaffing and have a burst pressure rating of 12,066 kPa (1,750 psi).

Fuel Level Sensor

A float and arm type fuel level sensor is used in the fuel tank. As the level of fuel increases in the tank the float arm raises reducing the resistance across the sensor.

LPG Cut-Off Solenoid

The LPG cut-off solenoid valve is mounted to the outlet port of the fuel tank. The normally closed valve opens when the fuel pump relay is energized. The LPG cut-off solenoid valve has a built in over flow valve that will reduce the flow of LPG if the pressure difference between the inlet and outlet of the valve is greater than the calibrated amount. There is also a manual shut-off valve integrated into the LPG cut-off solenoid.

LPG Bypass Solenoid

The LPG bypass solenoid is mounted to the return port of the fuel tank. The normally closed valve opens when the LPG control module energizes the LPG bypass relay. When energized, fuel bypasses the fuel pressure regulator and flows directly into the fuel tank. This reduces the time to purge all vapor from the system during start up. The LPG bypass solenoid has a built in over flow valve that will reduce the flow of LPG if the pressure difference between the inlet and outlet of the valve is greater than the calibrated amount. There is also a manual shut-off valve integrated into the LPG bypass solenoid.

LPG Pressure and Temperature Sensor

The combination LPG pressure and temperature sensor is located at the back of the right hand side fuel rail. The LPG control module provides a reference voltage and ground to the sensor and receives the fuel pressure and temperature signals from the combination sensor. The signals are used to calculate the amount of purge time required for start up.

LPG Control Module

The LPG control module is mounted under the center of the dash above the engine cover. It controls the amount of time the wait to start indicator is illuminated and the amount of time the LPG bypass solenoid valve is energized. The LPG control module provides a 5 v reference voltage and ground to the fuel/temperature sensor. The return signals from the sensor are used to determine if the system needs to be purged and for how long the system needs to be purged. The LPG control module also controls both the wait to start indicator relay, bypass relay, and evaporative emissions vacuum pump. The LPG control module monitors the signal and control circuits for proper operation. If a fault is detected the LPG control module will command the wait to start indicator to flash for 0.5 second for a total equal to the fault number. There will be a 3 second pause between flashes or a 6 second pause if multiple faults are present.

Fuel Injector Rails

The fuel injector rails are built of billet aluminum for minimum heat transfer. Fuel is supplied to the injectors through dedicated supply passages and the return fuel is returned to the tank through separate return passages.

Fuel Injector

Each fuel injector has a supply passage and a return passage. The passage in the injector from the supply section to the return section is restricted by a cooling bushing. As liquid propane passes through the cooling bushing, a pressure reduction takes place, which causes the propane to vaporize and effectively cools the area around the supply section. This is called a refrigeration cycle and aids in maintaining the fuel in a liquid state for all driving conditions, regardless of the outside temperature. The design of the injector compensates for changes in the fuel pressure so a specific fuel pressure is not required.

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 LPG control module for 30 seconds. While this voltage is being received, the fuel pump relay supplies battery voltage to the primary fuel pump and LPG cut-off solenoid to allow liquid propane to circulate through the entire system. This is done to evacuate all propane vapor from the fuel system. 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 24:1.

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).