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Going back to the addition of LPG to a diesel engine, the DIY approach is often risky for personal safety as well as the well-being of your engine. You are much better off buying a commercial system and there are many such systems available, several of which are designed specifically for adding power to a diesel-powered pickup truck. The simple mechanical, vapor-withdrawal systems typically use a pressure switch to start propane flow. The Bully Dog Diesel Propane Kit and the Diesel Performance Products PowerShot 2000 System (eCo-shot LPG Injection System) are similar in that they use boost pressure to determine when propane flow starts. In the case of the Bully Dog Propane System, no fuel flows until boost pressure reaches 7 psi (no mention of adjusting this switch) and the regulator includes a straight-forward flow adjustment. In the case of the PowerShot 2000 System, there are both pressure and flow adjustments, with the recommended setting for improved fuel economy is 2-3 psi below the average flat-grade cruising boost pressure. Because the propane can be set to come on at a lower boost level, the PowerShot 2000 System should have a better potential for improving fuel economy. The low-pressure regulator of the PowerShot 2000 System is boost-referenced, which means that as boost pressure increases, propane flow increases in proportion. However, propane flow will drop off as boost pressure approaches the high pressure regulator's setting (40 psi). The Bully Dog advertises a power gain of 80 hp for their Propane Kit while Diesel Performance Products advertises a power gain of 25-30% and a net fuel economy gain of 1-3 mpg. The availability of an in-cab, remote control adjuster for the PowerShot 2000 system suggests that it is difficult to achieve both maximum fuel economy and maximum power with a single adjustment on the low-pressure regulator.
The risk with LPG vapor-withdrawal systems is the very real danger of detonation from over-fuelling. Because these systems rely on only vapor being withdrawn from the propane tank, a good jolt to the tank could splash a slug of liquid in the vapor line. When propane is converted from a liquid to a gas, it expands 270 times and this additional fuel can have catastrophic results in a diesel engine. The Bully Dog regulator is water-heated, which keeps propane at a relatively constant supply temperature to the engine and prevents the regulator from frosting. It should also reduce the possibility of a slug of liquid getting past the regulator. Both the Bully Dog and PowerShot 2000 installation instructions have recommendations about fuel tank orientation to address this concern.
Another popular system is "The Fuel Stretcher" by DeLuca Fuel Products and was featured in Diesel Power Magazine: Diesel Natural Gas Injection - CNG Power . This system is different from the Bully Dog and PowerShot 2000 propane systems because the fuel is delivered in response to vacuum developed in the intake duct between the turbocharger and the air filter. Rather than using a positive pressure regulator, the DeLuca Fuel Products uses a conventional negative-pressure liquid-withdrawal propane converter (Impco Model J) and fuel-lock (Impco Model VFF30), commonly used with Impco air-valve mixers. To boost vacuum, a venturi is created in the duct with the addition of the system's fuel supply nozzle. Because air flow is so low at idle, the vacuum developed is below the vacuum supply threshold of the converter (either -1.5" WC for a Model JB, -0.5" WC for the Model JO, or -0.2" WC for the Model JR) and no gaseous fuel flows at idle. A vacuum level of -0.5 WC (Water Column) is the same amount of vacuum to draw water up in a drinking straw 1/2". However, because vacuum increases with the square of air flow, a manually-set flow control valve is used to keep LPG/CNG flow low enough to prevent detonation. Because "The Fuel Stretcher" is a liquid-withdrawal system, there is no risk of overfueling from a slug of liquid getting into a vapor line. However, the flow valve adjustment that would maximize fuel economy would likely lead to detonation at high loads. DeLuca Fuel Products has a new microprocessor-based system in the works that addresses the risk of detonation. DeLuca Fuel Products advertises a power gain of up to 100 hp.
The ATS Diesel Performance "Torque Pro Propane Injection System" appears to be in many ways very similar to "The Fuel Stretcher" as it too uses a liquid-withdrawal propane regulator. Their installation instructions don't show any sort of flow control valve and the system is activated when boost pressure reaches 5 psi but this setting is user-adjustable. Torque Pro Propane Injection System installation instructions may be found at the ATS Diesel Performance Product Instructions and, from the instructions, the Torque Pro also appears to use a propane converter very similar to the Impco Model J. Since the converter cover doesn't appear to have any pressure adjustment ability, it appears that the Torque Pro also uses a negative-pressure converter. In this case, without a venturi to boost duct vacuum, fuel economy improvements are likely conservative. ATS Diesel Performance doesn't advertise any gains in power or fuel economy for their Torque Pro system.
Another economical Diesel-Propane system is the SuperChips DPI system, originally developed by MSD, with an advertised power gain of up to 75 hp. . It is a relatively sophisticated liquid withdrawal system that uses an electronically controlled solenoid to control fuel flow. The DPI system is vehicle-specific and allows the user to make limited customizations to the program. The DPI System takes into account a number of factors (throttle position, boost pressure, RPM, propane temperature & pressure) to determine the amount of propane supplied. There is an RS-232 port on the DPI ECU for minor programming changes and data acquisition with a laptop computer. Interestingly, neither the MSD nor the SuperChips websites have any information on the DPI system although SuperChips Forum discusses the DPI system in several topics. The DPI system appears to be based on the Impco Model J converter and appears to modulate the output LPG pressure of the converter with a controller much like the discontinued Dual Curve 5952 (Digital Fuel Controller). An orifice in the LPG output nozzle fine tunes the flow. Installation instructions may be found on DPI distributor product pages. Although the DPI system supplies propane at zero boost, since the systems are vehicle-specific and described as supplying safe amounts of propane, fuel economy gains are likely conservative as well.