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In recent years, the dimethyl ether (DME) fuel has been attracting attention as an alternative engine in terms of diesel utilization. This is (a) because its cetane number is close to that of diesel fuel, (b) an innovative chemical process has been developed to produce DME efficiently from natural gas and coal, and (c) DME as a fuel has fewer environment-polluting characteristics than diesel fuel. Inasmuch as DME fuel have lower molecular weights, a molecular C-O bond, and are much more volatile or evaporative than diesel fuel, it is possible to control particulate matters much more easily when DME is used instead of diesel fuel. As for NOx, however, even when using DME, there still remain problems under stringent exhaust gas regulations. Developed and optimized accordingly has been the NOx storage-reduction (NSR) system, using the DME engine with a common-rail injection system. The NSR system is coated with an NOx storage catalyst principally comprised of Pt and Rh.

The increase in number of automobiles due to its convenience brought serious increases in environmental load. The rate of attainment of environmental standards for nitrogen dioxide (NO2) and suspended particulate matter (SPM) in urban areas is still low in Japan. Diesel vehicles emit the vast majority of air pollutants from exhaust. Therefore, developing emission measures, particularly for diesel vehicles, is an urgent task for addressing air pollution. Furthermore, at the Third Conference of the Parties to the UN Framework Convention on Climate Change (COP 3) held in Kyoto in December 1997, Japan pledged to reduce greenhouse gas emissions to 6 percent below 1990 levels for the first commitment period of 2008 to 2012. To address vehicle emissions, Japan is gradually introducing increasingly strict NOx and particulate matter regulations.