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Research has shown that there are many factors that affect the long-term performance of nitrogen oxides (NOx) control systems used in diesel engine applications. However, if the NOx emissions can be accurately monitored, it might be possible to restore performance by making adjustments to the control systems. This paper presents results from a study that tested the durability of 25 NOx sensors exposed to heavy-duty diesel exhaust for 6,000 hours. The study, conducted by the Advanced Petroleum-Based Fuels - Diesel Emission Controls (APBF-DEC) project, tested the sensors at various locations in the exhaust stream.

The use of compressed natural gas as an alternative to conventional fuels has received a great deal of attention as a strategy for reducing air pollution from motor vehicles. In many cases, regulatory action has been taken to displace diesel fuel with natural gas in truck and bus applications. Emissions results of heavy-duty transient FTP testing of two Cummins L10-240G natural gas engines are presented. Regulated emissions of non-methane hydrocarbons, total hydrocarbons, CO, NOx, and particulate were characterized, along with emissions of formaldehyde. The effects of air/fuel ratio adjustments on these emissions were explored, as well as the effectiveness of catalytic aftertreatment in reducing exhaust emissions. Compared to typical heavy-duty diesel engine emissions, CNG-fueled engines using exhaust aftertreatment have great potential for meeting future exhaust emission standards, although in-use durability is unproven.