TECHNOLOGY: Packing a Diesel-Like Punch

Like many waste management companies in California, Norcal Waste Systems, San Francisco, faces community pressure to reduce its truck emissions. Doing its part, the company has purchased several heavy duty trucks that run on natural gas.

Supplied by Peterbilt, Denton, Texas, the trucks are equipped with natural gas engines specially designed by Cummins Westport Inc., a joint venture between Cummins Inc., Columbus, Ind., and Westport Innovations Inc., Vancouver, British Columbia, Canada.

The 15-liter ISX-G engines are a modified version of Cummins' heavy duty ISX diesel truck engine equipped with Westport's high-pressure direct-injection (HPDI) natural gas injection technology. According to Cummins Westport, the technology is unique in that it produces diesel-like torque and fuel efficiency.

HPDI technology relies on late injection of high-pressure natural gas into the engine's combustion chamber. The injection occurs after the compression stroke, when air is compressed so intensely that it ignites the diesel fuel without a spark plug. Late injection allows the ISX-G to preserve the diesel engine's high compression ratio, which delivers low-speed torque and high fuel efficiency, according to Cummins Westport.

This differs from other natural gas engines that cannot preserve the diesel cycle's high compression ratio. This is because natural gas engines rely on the early introduction of an air-fuel mixture into the engine cylinder, prior to compression.

To make the cycle work, the manufacturer needed to acknowledge that natural gas is more difficult to ignite than diesel fuel. Natural gas does not always automatically ignite from the hot compressed air in the engine cylinder. Therefore, for every combustion event, the injector squirts a small amount of diesel fuel into the engine immediately before the natural gas. The diesel fuel, which ignites when it touches the hot compressed air, acts as a pilot to ignite the natural gas.

Also, the cycle uses a single-injector device to deliver the pilot and the natural gas. The injector fits into the cylinder head in the same location as the diesel injector. No modifications to the engine are required, according to Professor Philip Hill, who developed the technology during the past 10 years at the University of British Columbia, Vancouver, Canada. Because it uses late injection, the engine can operate with variable qualities of natural gas. This is an important reliability consideration because natural gas is not a refined fuel and its composition can change by region or season.

The result is a clean-burning engine that emits no more than 2.5 grams of nitrogen oxide per brake-horsepower-hour, according to Cummins Westport. Depending on the test cycle, nitrogen oxide emissions are reduced by between 35 percent and 50 percent. Particulate matter emissions are reduced by 50 percent to 70 percent because of the lower carbon content of the natural gas per unit of energy and its lower propensity to form soot, the company says. Also, the ISX-G engine will produce 20 percent less carbon dioxide emissions than diesel fuel.

These results may enable engine purchasers in California to receive financial incentives from the state. The engines have obtained low-emission certification from the California Air Resources Board of Sacramento.

To date, Sanitary Fill Co. drivers seem pleased with the ISX-G's performance, as they haul waste 60 miles from San Francisco to a landfill, the company states.

Reduced costs and maintenance may be additional benefits to using the cleaner engines. The company is testing extended oil change intervals with Norcal trucks to define oil life during on-highway operations.