Study of Improvements in NOx Reduction Performance on Simultaneous Reduction System of PM and NOx

Performance improvements were studied for the diesel particulate and NOx reduction system (DPNR), a system that simultaneously reduces NOx and Particulate Matter (PM) from diesel engine exhaust gas. The experimental system (hereinafter called the "dual DPNR") consists of two DPNR catalysts arranged in parallel, each provided with an exhaust throttle valve downstream to control the exhaust gas flow to the catalyst, plus a fuel injector that precisely controls the air-fuel ratio and the catalyst bed temperature. The fuel injector is used to supply a rich mixture to the DPNR catalyst, and the flow of exhaust gas is switched between the two catalysts by operating the exhaust throttle valves alternately. Tests were conducted with the engine running at steady state. The results indicated that the NOx reduction performance dramatically improved and the loss of fuel economy from the NOx reduction reduced. This was achieved by using the exhaust throttle valve to lower the Specific Velocity (SV) ratio on the catalyst therefore maintaining a rich atmosphere for the optimum length of time. The dual DPNR system is also expected to improve NOx reduction performance during forced PM oxidation, which is difficult for a conventional system, and also to improve recovery performance from catalyst poisoning due to sulfur. The main issues for commercialization are dealing with transient operation. In this paper, the durability of the catalyst is also reported as the major issue of DPNR in the APPENDIX. Specifically, a long-term endurance evaluation equivalent to 250,000km running was carried out using a mass-production engine. This was to clarify the effects of the sulfur concentration in the fuel by reducing the amount of NOx stored in the catalyst, as well as the effects of low-ash oil that clogged the catalyst. Research has yielded basic data on the use of DPNR catalysts in commercial vehicles which require better catalyst performance and durability.