A study performed by the University of California, Riverside’s College of Engineering and Center for Environmental Research and Technology (CE-CERT) examines the benefits of a low-emissions Cummins natural gas engine versus a standard diesel engine.
The majority of heavy-duty commercial vehicles on the road run on diesel fuels, the report states. Federal clean air standards adopted in 2010 reduced oxides of nitrogen (NOx) emissions by 90 percent from the previous levels, but by 2023 the aim is to achieve another 90 percent decrease. That, the researchers add, is going to be a difficult task to achieve as today's diesel vehicles regularly exceed the 2010 EPA standards of 0.20 g/bhp-hr of NOx as their duty cycles progress. The study recommended another look at newly available natural gas engine options.
The research sponsored in part by the California Energy Commission, the South Coast Air Quality Management District and the Southern California Gas Company, found that NOx levels emitted by a Cummins ISL G NG (near zero) 8.9 liter engine remained below the proposed standard of 0.02 g/bhp-hr even during increased duty cycles in driving conditions typical of the South Coast Air Basin near Los Angeles. Notably, “the NOx emissions (g/bhp-hr) decreased as the duty cycle was decreased, which was the opposite trend for the diesel vehicles (where emissions increased as duty cycle decreased),” researchers wrote. They concluded that it “is expected NG vehicles could play a role in the reduction of the South Coast NOx inventory problem given their near zero emission factors demonstrated.”
“Stoichiometric natural gas engines with three-way catalysts tend to have better low-duty cycle NOx emissions than diesel engines with SCR aftertreatment systems. Thus, a real NOx success will not only be providing a solution that is independent of duty cycle, but one that also reduces the emissions an additional 90% from the current 2010 standard,” said the study.
Researchers examined Cummins ISL G performance while driving in the city, near ports, in refuse cycles, and central business districts. Of interest to student transporters was how diesel and NG engines respond to decreased duty cycles in the form of stop-and-go traffic, slower speeds and idling. Because of the ultra-low level of NOx emitted, a trace-level analyzer and real-time ambient second-by-second corrections were used in addition to measuring the exhaust flow in real time.
Emissions during the first 100 seconds of a diesel truck engine’s startup were 2.2 g/bhp-hr in a cold test and 0.006 g/bhp-hr in a hot test. In both systems, the emissions output stabilized at 0.05 g/bhp-hr. Depending on the driving location, SCR was inactive for 60 to 100 percent of the decreased duty driving cycles, which resulted in NOx emissions “as much as 10 times higher than the 2010 standards,” researchers said. Additionally, efforts to reduce NOx output resulted in increased fuel consumption.
The NG engine’s NOx emission levels averaged between 0.014 and 0.002 g/bhp-hr in hot starts. In cold starts, an increased emission value was responsible for about 1/7 of the total cycle, after which the level went down to hot start values; the resulting weighted average was 0.0181 g/bhp-hr NOx emissions. However, the study said that this spike only occurred once in the NG engine because “once the catalyst performance is achieved it remains at this high performance unlike the diesel SCR equipped engines where low duty cycle will cause the NOx emissions to increase again.”
Researchers said the study found that gradual acceleration resulted in lower emission release, revealing the impact of driver behavior. Lower methane levels were measured, which the study said “may be a result of the closed crankcase ventilation system.”