Universal products for compression ignition (diesel cycle) engines.
The type of operation that results when a high octane fuel is supplied to a diesel engine so that a premixed fuel-air mixture enters the combustion chamber similar to an Otto Cycle engine. This is commonly done for stationary engines used for power generation when natural gas is used as fuel premixed with air.
It is possible to use LPG in an automotive CI engine but this is very dangerous to the life of the engine if it is not done properly. The critical compression ratio (CCR) of a fuel is the minimum CR which will cause the fuel to self-ignite from the heat of compression. The CCR for propane is ~12:1 and for natural gas (methane) is ~15:1. Since automotive diesel and turbo-diesel engines are easily higher than 16:1, the amount of propane or natural gas must be supplied at a rate that is too lean to self-ignite with automotive diesel compression ratios. Diesel fuel may safely be substituted up to 25% by volume (i.e., 1 gallon of propane consumed with 4 gallons of diesel).
The most practical automotive fuel for diesel dual fuel systems at this time is propane (LPG) because it can be transported in useful quantities and because it is commonly available. There are a variety of diesel dual fuel systems on the market today and all can improve the power of diesel engines. The claim that they save fuel is only half correct because they substitute one fuel for another. On an energy basis, only when propane is 65.5% of the price of diesel fuel is a dual fuel system cheaper to operate than straight diesel. While manufacturers of these systems claim to improve the overall fuel efficiency of the engine with improved combustion, we have not seen collaborating third party tests to confirm these claims. We prefer to promote these products as cost-saving products rather than fuel-saving or power-adding products.
Another benefit of using a gaseous fuel in conjunction with diesel fuel is the reduction of particulate emissions (soot). The reduction in particulate emissions is dependent upon the amount of diesel fuel being substituted at any given time. Although modern diesel engines have diesel particulate filters (DPF), the use of a gaseous alternative fuel can potentially reduce the fuel consumed during a DPF cleaning cycle.