Cummins Westport Inc. says it has begun development of an ISB6.7-G, a midrange 6.7-liter natural gas engine designed for medium-duty trucks and C-class school buses. It will enter production in 2015.

The upcoming product will meet increasing demand for engines that run on low-cost natural gas, and will join existing products and others planned for 2013 and 14, the joint venture firm said. It will be based on the Cummins ISB6.7 diesel and will use Cummins Westport's proven spark-ignited, stoichiometric cooled exhaust gas recirculation combustion design.

Exhaust aftertreatment will be provided by a simple, maintenance-free three-way catalyst. The engine will run on compressed natural gas (CNG), but fuel may be stored on the vehicle in liquefied natural gas (LNG) state or as CNG. Power and torque ratings are yet to be determined.

"The addition of the ISB6.7 G will round out our family of high performance natural gas engines," said Jim Arthurs, president of Cummins Westport. "It joins the 8.9-liter ISL-G, with over 16,000 engines in service, and the 11.9-liter ISX12-G, which will start production in 2013, to give our customers a broad range of natural gas engines for on-highway applications."

An ISX15-G, also using spark ignition, had previously been announced for initial use in 2014. Cummins Westport currently offers a dual-fuel HD engine, based on the 15-liter ISX.

The ISB6.7-G will be designed to meet Environmental Protection Agency and California Air Resources Board regulations in force at the time of launch in 15, the company said.