Development of Urea-SCR System for Commercial Vehicle - Basic Characteristics and Improvement of NOx Conversion at Low Load Operation - 2003-01-3248

Although diesel engines have high thermal efficiency and excellent reliability, legislation in locations worldwide are calling for further reductions in nitrogen oxide (NOx) and particulate matter (PM). One possible method of compliance is a urea-SCR catalyst system to reduce NOx. It is widely known, and has been demonstrated in stationary engines, that there is a significant NOx reduction effect when a sufficient catalyst activation temperature is obtained. Recently several commercial vehicles have been outfitted a urea-SCR system to and characterized their NOx reduction effects. This report outlines the basic characteristics of the urea-SCR system, and evaluation of the effectiveness when used for heavy-duty diesel powered commercial vehicles.

First, in order to investigate the basic characteristics of the urea-SCR catalyst, NOx reduction characteristics were measured using model gas equipment. Since it is known that the NOx reduction rate changes greatly with NO₂ in the urea-SCR catalyst, tests were carried out to evaluate reactions of NO only, NO₂ only, and a 1-to-1 ratio of NO:NO₂. Results showed that the 1:1 mixture of NO:NO₂ had the highest NOx reduction rate.

Thus using modified heavy-duty diesel powered commercial vehicles, transient testing was carried out, following the Japan domestic 13 mode and city mode test cycles, followed by investigation of NOx reduction characteristics of the urea-SCR system. In the 13 mode tests, the exhaust temperature was relatively high at medium and high loads, and a significant NOx reduction effect was obtained, however the low load portions were associated with a low exhaust temperature and the NOx reduction effect decreased. Additionally, in the transient tests simulating the city mode test cycle, the catalyst temperature was low and sufficient NOx reduction effect was not obtained. It was found during the transient testing, that even at these low temperatures, the NOx reduction rate improved in case NO₂ rate improved with the oxidation catalyst and the catalyst temperature could be controlled at higher values. In doing so, even in the transient city mode tests, it was possible to remarkably reduce NOx, to values under 10ppm in the NH₃ slip in our tests.

Next, vehicle testing was carried out using the same urea SCR system. An on-board NOx measurement system was used when measuring the NOx reduction rate during driving tests. This confirmed that a sufficient NOx reduction effect was obtained in the vehicle. These tests showed that even during low temperature city mode type operation, the urea-SCR system resulted in a sufficient NOx reduction effect.