Fuel System Description (LC8K07UFP)

Fuel System Description (LC8\K07\UFP)

Fuel System Overview

The LPEFI(R) system is a 4 tank system, 3 secondary tanks at the rear of the vehicle and 1 main tank near the center of the vehicle. The LPEFI(R) 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 LPEFI(R) system just as it did the gasoline injection system. Onboard diagnostics remain unchanged so the same scan tool and diagnostic approach remains equal to a gasoline system.

The LPEFI(R) system consists of three main components: the tank, the fuel lines and the injectors. The tanks are located to the rear and middle 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 & filter, fuel supply & return valves, baffle that keeps the pump submerged in liquid propane and various other valves, fuel level float assembly, pressure relief valve, overfill prevention device, liquid and vapor service valves. LPEFI(R) vehicles are fitted with two tanks, a main tank, which controls all fuel delivery to the injectors and an optional tank, which only transfers fuel to the main fuel tank. The main fuel tank fuel pump increases or boosts the tank pressure by 35 to 50 psi. No matter what the propane tank internal pressure is, the pump boost 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 return 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 10 to 15 seconds before every start up attempt. This strategy is built into the system's liquid propane control module. During the purge cycle the "Wait to Start" lamp 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 end of the main tank is a bulkhead that provides access to the fuel pump, fuel filter, and overfilling protection device.

Overfilling Prevention Device

The overfilling prevention device is a mechanical float-actuated valve that stops the tank from filling 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 of 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 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. After the PRV has vented for the first time, it is a common practice in the propane industry to drain the tank and replace the PRV.

Fill Filter

The fill filter is located between the fill valve and the fill hoses that go to the fuel tanks. The fill filter is a 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

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

Fuel Pump Filter

The fuel pump filter is mounted to the inlet of the fuel pump assembly inside the main fuel tank.

Fuel Pump

The fuel pump is mounted inside the main fuel tank and is used to increase line pressure of the liquid propane by 35-50 psi over the internal tank pressure to insure the liquid state of the propane is maintained. The inlet to the fuel pump is submerged in liquid at all times by a baffle in the tank assembly.

Fuel Transfer Pump

The liquid propane control module monitors each of the fuel level sensors. When the liquid propane control module detects a difference in fuel tank levels it supplies power to the fuel transfer pump and secondary fuel supply solenoid. The liquid propane is pumped from the secondary tanks to the main tank to ensure adequate fuel is supplied to the engine.

The main fuel tank has a scavenge pump that operates when the main fuel pump is energized. The scavenge pump ensures the baffle area of the main tank is full of liquid propane.

Fuel Lines

The fuel lines consist of two flexible hoses, one inside the other, in a concentric arrangement. The nylon inner line supplies liquid propane to the injectors while the area between the outside of the inner line and the larger outer hose is the fuel return passage. The concentric fuel line design has a number of benefits:

* Cuts the number of possible leak points in half

* Reduces vapor-lock in the supply line by using the return fuel passage as insulation,

* Postpones the vapor-lock that occurs after a hot engine is shut off,

* Shortens the purge cycle time needed to restart a hot engine.

Fuel Level Sensors

A float and arm type fuel level sensor is used in the main and front secondary fuel tanks. The liquid propane control module monitors each sensor and provides an output to the engine control module. If the difference in fuel level between the tanks going out of the calibrated range the liquid propane control module will turn on the secondary tank fuel pump.

Liquid Propane Delivery Module

The liquid propane delivery module is mounted to the outlet end of the main fuel tank and houses the supply and return fuel solenoid valves. The fuel supply and return solenoids are 12 V electro-mechanical valves that close when not energized. The fuel supply solenoid is energized whenever the vehicle is running or in purge mode. The return solenoid is only energized during purge mode.

A secondary liquid propane delivery module is mounted on the front of the secondary tank assembly. When the liquid propane control module senses a difference in fuel level between the primary and secondary tanks power is supplied to the secondary liquid propane delivery module. This opens the secondary supply valve and powers the secondary fuel pump. Liquid propane is then pumped to the primary tank.

Liquid Propane Control Module

The liquid propane control module is mounted on the left frame rail near the main fuel tank. The liquid propane control module controls the voltage supply to the liquid propane delivery module and liquid propane fuel pump relay. When the ignition is initially energized the liquid propane control module turns on the wait to start lamp and starts the purge mode. During purge mode the primary fuel pump, fuel supply and return solenoids are energized to allow liquid propane to circulate through the entire system. This is done to evacuate all propane gas from the fuel system. The liquid propane control module determines fuel level between the two input signals from the fuel level sensors and will enable the secondary fuel pump when the difference between the main and secondary tanks are outside the calibrated range. The liquid propane control module also supplies the engine control module with a fuel level signal.

For safety, an impact switch is mounted inside the liquid propane control module. If an impact greater the 12 g's occurs the liquid propane control module will go into shutdown mode. When this occurs all valve and pumps will be disabled, sealing the system. After the fuel system has been inspected for damage the control module may be reset by disconnecting the power to the liquid propane control module for several seconds.

Fuel Injector Rails

The fuel injector rails have the same concentric design as the fuel lines. The inner line supplies liquid propane to the injectors while the area between the outside of the inner line and the larger outer rail is the fuel return passage. This concentric design helps to maintain a liquid fuel state.

Fuel Injector

Each fuel injector has a supply passage and a return passage. The fuel injector rails have the same concentric design as the fuel lines. 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 fuel pressure regulator 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 liquid propane control module for XX minutes. While this voltage is being received, the liquid propane control module provides power to the primary fuel pump, fuel supply and return solenoids to allow liquid propane to circulate through the entire system. This is done to evacuate all propane gas 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).