In order to reduce diesel NOx emissions, aftertreatment methods including LNT (Lean NOx Trap) and urea SCR (Selective Catalytic Reduction) have been researched. One of the shortcomings of urea SCR is its NOx reduction performance degradation at low exhaust gas temperatures and possible emission of unregulated byproducts.
Here, a new type of a urea-dosing device to overcome these shortcomings is studied. This dosing device actively produces ammonia without depending upon the exhaust gas temperature, and designed for onboard application.
The device incorporates an electrically heated bypass with a hydrolysis catalyst. An injector supplies urea solution into the bypass. The bypass is heated only when thermolysis is needed to produce ammonia (NH3). The hydrolysis catalyst further assists in the production of NH3.
The ammonia gas obtained is then mixed with the main exhaust gas flow. A method for achieving near uniform gas distribution with acceptable pressure loss and dimension was pursued by gas flow measurements and simulation. Swirl flow combined with radial gas distribution was found to be best. NH3 concentration variation was less than 10% with a short mixing length (L/D < 2) for onboard application.
Static engine test results show that the new device reduces NOx under a 200°C-exhaust gas temperature, with some operating points achieving nearly a 100% NOx reduction rate.