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The availability of a natural gas fueled engine in a North American heavy or medium duty truck pretty much depends upon whether the truck manufacturer offers Westport products in its engine options. Westport has 4 business units offering alternative fuel products:

For these vehicles, the choice is between dedicated CNG/LNG engines (Cummins Westport) and dual fuel CNG/LNG engines (Westport HD).

The latest spark-ignited, natural gas engine offered by Cummins Westport is the ISL G (based on the Cummins ISL9 8.9L engine) but the Cummins Westport C Gas Plus (based on the Cummins ISC 8.3L engine) and the Cummins Westport B Gas Plus (based on the Cummins ISB 5.9L engine) are also available. The Cummins Westport ISL G engine achieves low emissions with stoichiometric combustion with cooled EGR and a three-way catalyst (TWC).

The latest dual fuel, diesel-natural gas engine offered by Westport HD is the ISX G, which is based on the Cummins ISX15 15L engine). The ISX G engine uses direct injection of gas into the combustion chamber and relies on a pilot injection of diesel fuel to ignite the natural gas. Although not specified exactly (and likely varies with engine speed and load), the ISX G uses extremely low amounts of diesel fuel (~5% on an energy basis) for the pilot injection. From Cummins' emission brochures, the ISX G engine also uses cooled EGR but with Select Catalytic Reduction (SCR) and Diesel Particulate Filters (DPF).

The incremental price of the ISL G and the ISX G engines options are not readily available, nor are their expected fuel consumptions. In his thesis paper " The Role of Natural Gas as a Vehicle Transportation Fuel ", Paul Murphy reports that the LNG ISL G engine adds about $40,000 to the price of a diesel truck while the LNG ISX G engine adds about $100,000 (see p 48). Since the ISL G operates with a throttle and has a lower compression ratio than the ISL9 diesel engine, its diesel-equivalent LNG fuel economy should likely be lower than the diesel-only ISL9.