POWERTRAIN CONTROL SYSTEM
PCM I/O Devices
Whenever possible the OEM production I/O devices are retained during the LPI conversion.
A list of the I/O devices that are different from the OEM vehicle can be found in Table 2.
Table 2: I/O Changes
PCM Control System Strategy and Calibration
The PCM calibration consists of a modified version of the OEM strategy and a LPG specific calibration.
Control System Strategy
The ROUSH CleanTech LPG strategy is built off of the OEM strategy with the addition of new logic to drive and monitor the fuel rail pressure control solenoids. Because the strategy is based off of the OEM strategy, the OEM PCM retains all fuel control and diagnostic functions. No modifications are made to the scan tool interface features of the logic except to add the LPG specific DTC’s. The LPG DTC’s are stored in the PCM memory and are cleared via the OEM strategy’s fault state controller.
Changes to the OEM calibration are made in the following areas:
- Modified I/O scaling as required
- Modified spark and fueling functions as required by physical properties of propane
- Disabled purge feature
- No changes required
- Disable purge monitor and related component monitors
SYSTEM OPERATION & PERFORMANCE
One of the key benefits of the ROUSH Clean Tech LPG system is that vehicle operation and performance is virtually transparent as compared to that of the base gasoline vehicle. One of the reasons for this is the fact that there is no loss in engine performance with the ROUSH Clean Tech LPG system. The following graph shows the engine performance comparison of the Ford 5.4L 3V gasoline engine as used in the 2010MY F250/350 vehicle both before and after the ROUSH Clean Tech LPG conversion.
Even though ROUSH Clean Tech re-calibrates the PCM and has the ability to take advantage of the higher octane rating associated with LPG by employing a more aggressive spark advance calibration, we systematically choose to match the torque curve of the base gasoline engine in order to preserve the overall powertrain and vehicle systems durability. This strategy serves our targeted fleet customers well where the need for vehicle longevity far outweighs the need for maximum performance.
Cold Start Performance
All liquid injection systems must be able to keep the propane fuel in the liquid state for proper operation. This is a particular concern during an engine cold start event and/or after extended soak periods (engine shut down) due to vaporization of the LPG fuel. Liquid must be injected at all times, or the engine will fail to run. This is accomplished in the ROUSH Clean Tech system by circulating excess LPG fuel throughout the injectors and fuel rails while using the fuel tank as a heat sink. Utilizing the Injection Pressure and Temperature Sensor (IPTS), located on the fuel rail, the PCM determines the state of the fuel based on a thermodynamic table embedded in the unique ROUSH Clean Tech control system software. If the propane is in the gaseous state, as determined by the Roush software, then the PCM will activate the fuel pump and open the flow control solenoid purging the vapor from the fuel rails until the IPTS readings indicate that liquid fuel is present. During this period the ignition is suspended and a warning light in the instrument cluster is illuminated indicating to the driver/operator that the vehicle is purging. Once the system is fully purged of vapor the vehicle start sequence continues and the engine is commanded by the PCM to fire and run.
This entire engine start and vapor purge sequence is computer controlled and accomplished by a single “Key On/Run” start input command from the driver. The engine start sequence typically requires between 3 to 5 seconds to complete and can vary depending on vehicle and environmental temperatures. ROUSH Clean Tech cold start performance is validated at all environmental temperature and barometric pressure extremes including minus -40 degrees to 120 degrees Fahrenheit.