Southwest Research Institute (SwRI), as part of the United States Department of Energy's (DOE) Next Generation Natural Gas Vehicle Program, has developed a low-emissions, heavy-duty natural gas engine that more than meets stringent US Environmental Protection Agency (EPA) 2010 emissions standards. The next-generation engine emits reduced levels of oxides of nitrogen (NOX) and particulate matter (PM), allowing production of heavy-duty trucks that restrict release of pollutants to the atmosphere.
SwRI engineers modified a standard production Mack E7 natural gas-fueled engine to reduce regulated emission levels. Test results with a “degreened,” or seasoned, catalyst showed NOX emissions at 0.049 gram per brake horsepower hour (g/bhp-hr), less than one-quarter of the 2010 standard, and PM at 0.002 g/bhp-hr, one-fifth of the future standard. A degreened catalyst is one that has been operated briefly before testing to achieve a stable emissions reduction.
The DOE's Next Generation Natural Gas Vehicle Program funds developing technologies for alternative fuels. The National Renewable Energy Laboratories and the South Coast Air Quality Management District (a California air pollution control agency), through separate agreements, financed the program with Mack Trucks Inc. to develop the next-generation, stoichiometric heavy-duty natural gas engine and to evaluate the prototype in a fleet of refuse hauler trucks. As subcontractor to Mack, SwRI was tasked to develop the engine and perform emissions evaluations.
Other changes to the engine included adding a cooled exhaust gas recirculation (EGR) system and modifying an engine controller with algorithms to control the EGR and to maintain proper air-fuel ratio control with varying EGR rates. Use of the EGR improved engine efficiency, lowered temperatures, reduced engine-out NOX emissions and decreased the tendency to knock.