Large vehicles like trucks, vans, and buses (both gasoline and diesel) are excellent candidates for propane conversions. Besides using a lot of fuel, which greatly accelerates the payback time for a conversion, there is generally enough space somewhere on the vehicle to mount the LPG tank. Although conversion kits are readily available for modern trucks, vans, and buses, conversions only make economic sense if the price of propane is significantly less than the price of gasoline or diesel fuel. Just on an energy basis, this means that propane should cost much less than 73% of gasoline and much less than 66% of diesel fuel.
Gasoline-Powered Vehicles
Being fleet-type vehicles, alternative fuel system manufacturers have developed conversion kits specifically for the GM 6.0L & 8.1L engines, the Ford 6.8L & 5.4L engines, as well as the Dodge Hemi and V-10 engines. So that these vehicles can meet EPA and CARB regulations, propane conversions are generally bi-fuel systems that invariably start on gasoline and switch over to LPG at a pre-programmed temperature or time interval. All emission-controls remain in place and fully functional. Ignition timing is also not modified by the conversion but advances automatically as programmed by the OEM due to the lack of engine knock from propane's 104 Octane Rating. Generally, the only thing standing between a non-certified system and an EPA/CARB certification is the very expensive and onerous emission test procedure. Because the low demand for EPA/CARB-certified systems, very few systems actually have emission certifications.
Diesel Powered Vehicles
The turbo-diesel engines installed in pickup trucks (Duramax, Powerstroke, and Cummins) may easily be converted with diesel-propane dual fuel system. Since these systems do not supply more than 25% of the fuel required by the engine, these systems are not regulated by EPA or CARB. Diesel-propane dual fuel systems need not (and cannot) be certified as propane is NOT the primary fuel. Diesel-propane dual fuel systems save money for the vehicle owner by substituting a cheaper LPG energy for higher priced diesel energy. There is also a small increase in thermodynamic efficiency due the the combustion of a homogeneous fuel mixture. There may also be an increase in fuel economy in 2007 and newer vehicles due to reduced regeneration cycle time of the diesel particulate filter.
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