Development of NOx Storage Reduction System for a Dimethyl Ether Engine 2004-01-1832
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. Rich conditions are created by injection of DME fuel into the exhaust stream. Stationary NOx-reduction tests have been performed from low load to full load conditions and at variable EGR rates.
In the DME engine, high injection pressures using multi-hole injectors are effective for enhancing combustion. The power output of the DME engine using a high pressure of 30 MPa with a common-rail injection system is almost identical with that of a diesel engine(1). In the NOx reduction system, it was possible to obtain high reduction rate of NOx emissions from the DME engine by applying the NSR catalyst. To obtain, however, a high reduction rate of as much as 80% of the NOx, the temperature at the catalyst inlet must exceed 300 degrees centigrade. Moreover, it was possible to reduce the amount of DME injection by applying the EGR rate because of decreased NOx in emission.