What can be easier or simpler than adding LPG to a diesel engine? Isn't it as simple as tossing a 20 lb barbeque cylinder in the back of a pickup truck and running a hose from the cylinder's regulator to the engine?
Yes, it can be that simple BUT that it can also be a shortcut to destroying your engine or to killing yourself or others around you. From a safety aspect, most people attempting this type of fuel system are often oblivious to the requirements of the gas code (NFPA 58 and CSA B149.5), which regulates the use of propane fuel in automobiles. These regulations were developed over many years through industry experience and responses to accidents and are akin to the National Electric Code for home wiring. You know that, if you cut corners with wiring, it's only a matter of time before your house burns down or someone gets electrocuted. Cutting corners with home-brewed propane or natural gas conversions can be just as risky.
On a BTU basis, propane must be less than 65% of the price of diesel to save money on fuel costs. CNG is typically priced in terms of GGE (Gasoline Gallon Equivalent) so its price must be less than 90% of the price of diesel to save money on fuel costs. Conversely, you can multiply the price of propane by 1.541 to get its Diesel Gallon Equivalent (DGE) price. Similarly, you can multiply the price of CNG by 1.115 to get its DGE price. Because the retail DGE price of propane is usually more than the price of diesel, diesel-propane systems are often used to boost power rather than to save money on fuel. CNG, on the other hand, is usually much cheaper than diesel (on a DGE basis) so it can potentially significantly reduce overall fuel costs. Since CNG and LPG burn so much cleaner than diesel, the regen cycle times on post-2007 vehicles should also reduced with an additional boost in savings. In addition, CNG has a higher octane number than propane so it can be used in higher relative amounts than propane. However, because CNG is a supplemental fuel, power gains can potentially be similar to those of propane.
Detonation (knocking) in diesel dual fuel systems is a very real concern which is very well explained in Dual Fuel Fumigation and all commercial systems address this issue to some degree. Due to the limited capability to control fuel mixtures to maximize fuel economy while minimizing detonation, budget diesel dual fuel systems have a limited ability to reduce fuel costs. Generally, the more expensive the system, the more sophisticated the operation so that there is a greater potential for reducing fuel costs and risk to your engine. The cost of the system becomes less important with the increase in fuel savings and reduction in risk. However, there are a lot of commercial, low-priced diesel dual fuel systems already installed on vehicles and their owners seem quite pleased with them. This article is an attempt to explain the operation of each system to help buyers make an informed decision.
Whatever system you decide to use, be sure to have the installation inspected and approved by a licensed LPG or CNG inspector in your jurisdiction BEFORE you put any fuel in it. Better yet, work with a local conversion shop on your project so that it meets both the spirit and letter of the regulations.
NEVER use a 20 lb cylinder as your propane motor fuel tank and NEVER run fuel lines inside the passenger compartment of your vehicle.
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.
For Diesel-CNG systems, many people often first think of the Natruell CNG Injection System, which was popular in 2008 but has since disappeared. A review of the Natruell CNG system that appears from the Diesel Power Magazine article (Natruell CNG Conversion Kit - CNG Injection) describes it as a system developed with Bully Dog using Prins CNG system components. The article doesn't go into details about the controller used to control CNG flow so it appears that the Natruell CNG System too uses a simple control strategy, much like the Bully Dog propane system. A search for Natruell on the internet reveals very little detailed information and many people suggest the Diesel Power Magazine article as a reference. Although the Natruell Diesel-CNG system uses Prins CNG components, it is completely different from the Prins Dieselblend system.
DeLuca Fuel Products' "The Fuel Stretcher" is also available in a CNG version. It uses the same VFF30 and Model J as the propane version but adds a high pressure CNG regulator. Because CNG is a high pressure gas rather than a liquefied gas, the converter doesn't need to be heated but hot water keeps the regulator from frosting. Heating the regulator will also help to keep the fuel density more constant for better flow control. Downstream of the converter, everything remains the same as the LPG system. As with the LPG version, CNG flow is adjusted with the choice of secondary regulator springs (blue, orange, red) and the flow control valve. A installation of The Fuel Stretcher is documented in MrTruck.Net: CNG, is it the Answer for the Fuel Crisis?
Although not mentioned on their web site, Performance Diesel Inc. makes (made?) a CNG fumigation system very much in function to the ATS Torque Pro system. Rather than using an Impco Model J converter, PDI uses a regulator similar to the Impco Model EV with a custom cover assembly. Motortopia has a review of PDI's CNG system in their Green Diesel article. PDI includes a Hobbs pressure switch to the intake manifold so the system activates when boost pressure reaches the switch's setpoint. Described as being a negative-draw system in the article, the PDI CNG system includes a venturi to generate sufficient vacuum in the intake duct upstream of the turbocharger. The Model EV can either supply fuel at -1.5" WC (Blue Spring) or at -0.5" WC (Orange Spring) and I suspect that it more likely uses the orange spring. Very little is shown of the venturi as it would be a proprietary design but I would expect that it incorporates a manually-set valve (load block) to minimize overfueling. The Green Diesel article reports that PDI claims CNG usage reduces DPF regen cycle times as well.
Even when supplied with vapor only (both LPG and CNG), if the gaseous fuel system is adjusted to supply too much fuel, the engine could still suffer from detonation and very expensive repairs. DeLuca reports on his web site that he was able to substitute 96% CNG for diesel under light load but found that CNG substitution rates over 70% had detonation under heavy loads. Generally, stationary engines in power generation applications substitute as much as 90% CNG for diesel and greater amounts of substitution (closer to 95%) have diesel injector cooling issues. Because the system draws fuel in response to intake duct vacuum, constant adjustment of the manually-set fuel control control valve would be required to maintain the optimum CNG flow under varying engine speeds and loads. Again, DeLuca Fuel Products has a new microprocessor-based system in the works that addresses the risk of detonation.
Automotive CNG system installations are governed by NFPA 52 and CSA B109 and it is extremely foolish to cut corners with a fuel stored at 3000 or 3600 psi in your vehicle.
Since public CNG stations are less common than propane motor fuel stations, there area other CNG Fuelling options available.
Although still relying on vapor-withdrawal, another relatively inexpensive diesel dual fuel system to consider is the Red Rooster Diesel-Propane or Red Rooster Diesel-CNG system. Unlike the simple mechanical systems, the Red Rooster system uses electronically controlled injection to control fuel flow. With user-adjustable start pressure, maximum pressure, and flow settings, the LPG/CNG flow begins at the start pressure and tops-out at the maximum pressure with a linear (straight-line) flow increase with boost pressure. Fuel flow is determined by the regulator pressure and affects the entire range of operation.
The better diesel dual fuel systems are injection systems that deliver fuel based on a load-based (boost pressure vs RPM) fuel map. The fuel map allows fuel flow to be non-linear so that relatively more flow is delivered at low loads (even at idle) and cuts back fuel flow as loads increase to ensure that detonation never occurs. The best fuel-map systems (like the LPG EcoDiesel System and CNG EcoDiesel System) deliver fuel to the intake manifold rather than upstream of the turbocharger, which eliminates control issues from fuel transport delay. The EcoDiesel System also has a self-calibrating base fuel map feature, which conservatively maximizes LPG/CNG use and is fully laptop-programmable. The design of the EcoDiesel System also prevents any surging issues sometimes prevalent with budget systems.
As mentioned earlier, the budget systems do work and the people that use them seem to be pleased with their operation. They're all different in configuration so they'll each work differently. Make sure you understand how each system works and don't just rely on testimonials before laying out your hard-earned money. Caveat Emptor!