Toyota Motor Corp. has developed the core technology for an innovative diesel emis-sions purification system called DPNR (Diesel Particulate - NOx Reduction System). The new system can simultaneously and continuously reduce particulate matter (PM) and nitrogen oxides (NOx) in diesel exhaust gas, based on TMC•fs NOx storage reduction three-way catalyst technology.

TMC has long worked on developing technologies for cleaner diesel engine emissions and achieved notable reductions in HC, CO, NOx, and PM by using oxidation catalysts and improved combustion technologies such as direct fuel injection, electronically controlled exhaust gas recirculation (EGR), and electronically controlled fuel-injection systems.

To accomplish more reductions in NOx and PM, DPNR has been developed. The principal attribute of DPNR is to reduce PM and NOx simultaneously and continuously with a simple and compact catalytic converter system. DPNR will realize the new way of emissions reduction with a direct-injection diesel engine featuring the latest common-rail, electronically controlled fuel-injection technology.

DPNR features a newly developed fine porous ceramic filter coated with a NOx storage-reduction three-way catalyst that was designed for use with lean-burn gasoline engines. During lean-mixture operation, PM is oxidized by active oxygen released in the NOx storage process and by excess oxygen in exhaust gas. Then, when the engine momentary switches to stoichiometric (rich) operation, PM is also oxidized by active oxygen released in the process of reducing the stored NOx.

DPNR shows a conversion efficiency of greater than 80% in both PM and NOx in the initial stage of operation, compared to the permitted level of exhaust emissions from a 1.8-t (2-ton) diesel truck under 1998 Japanese regulation limits.

By switching the direction of the exhaust gas flow in the catalytic converter, the system increases its PM oxidation capability. However, DPNR requires fuel with low sulfur content to maintain a high conversion efficiency for a long duration. Also, DPNR is not suited for retrofitting onto in-use vehicles because it requires the precise control of injected fuel quantity realized by a common-rail fuel-injection system, which is not available for such vehicles.

TMC plans to introduce vehicles featuring DPNR starting in 2003, after thoroughly developing the system in terms of durability and reliability.

Jean L. Broge

AEI October 2000