Highly-Balanced NOx and CO2 Reductions from Light-Duty Diesel Engines, 03-9467

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Principal Investigators
Shizuo Sasaki
Yiqun Huang
Gary D. Neely

Inclusive Dates:  04/01/04 – 04/01/05

Background - Light-duty vehicles (e.g., passenger cars) face stringent emission legislation in the near future. Diesel-powered light-duty vehicles enjoy much popularity in many European countries because of the superior fuel economy, durability, and drivability characteristics. In the United States, diesel vehicles enjoy a much smaller percentage of the market. However, if fuel prices remain high, the diesel-powered passenger car will be an increasingly desirable option because of the diesel's superior fuel consumption compared to gasoline vehicles. However, because of the limited use of exhaust treatment catalysts, emissions from diesel vehicles are much higher than gasoline engines. The biggest problems for diesels are NOx and particulate matter (PM).

To solve the emissions problem, exhaust treatment systems have been studied. Oxidation catalyst has been used in diesel vehicles in Europe and Japan. Diesel particulate filters (DPF) have also been used in some vehicles to dramatically reduce PM emissions. For NOx emissions, some advanced technologies have been developed to reduce NOx directly in-cylinder. However, in many cases, lowering engine-out NOx causes high fuel penalties for only small reductions in NOx. Therefore, some NOx exhaust treatment systems have been developed recently. Lean NOx trap (LNT) is one of the successful solutions as a NOx aftertreatment system.

Approach - In this study, a four-way catalyst system with LNT was applied to a four-cylinder, 2.0L Euro 3 diesel. A method to purify the engine-out NOx and to regenerate the accumulated sulfur was investigated to optimize the trade-off relation between NOx emissions and fuel economy. The test was performed at five steady-state engine operating points to estimate the emission and fuel consumption of the FTP-75 test cycle. To minimize the fuel penalty, post injection was not used, and two new combustion technologies, including low-temperature combustion (LTC) and premixed controlled compression ignition (PCCI), were applied. LTC lean operation was used at light load to maintain catalyst temperature and activity. LTC rich combustion and PCCI rich combustion was used.

Accomplishments - Estimated FTP-75 NOx emission was lower than Tier II Bin 5 emission regulation, and the NOx conversion efficiency of LNT was more than 90 percent. The estimated fuel penalty was 1.7 percent. The fuel penalty to maintain catalyst activity was 1.4 percent, while 0.3 percent of the fuel was used for rich pulse control. With estimating desulfation conditions, high exhaust gas temperature and rich combustion were realized. The catalyst bed temperature was more than 600°C with slightly rich air-fuel ratio at all test points except idle (idle was approximately 550¥C). 

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