A Study of a New Aftertreatment System (2): Control of Urea Solution Spray for Urea-SCR

The urea-SCR system is one of the most promising aftertreatment systems for future automotive diesel engines. We developed a urea dosing device with twin urea injectors for onboard applications, to enhance the NO_x reduction performance at low exhaust temperatures and to lower the electric power consumption of the SCR system. The injectors operate with a single-phase urea solution, without air assist. Of the injectors, one is used to supply urea to a bypass passage routing the exhaust, during low exhaust temperatures. The other injector is located on the wall of the main exhaust duct, directly supplying urea to the exhaust. This direct injection method has a uniform spray distribution problem. A set of impact plates were used to distribute the spray. Impact plates have a high potential for deposition, but use of film boiling was considered. A thermal analysis was conducted and as a result, deposit conditions were theoretically derived. This was confirmed through experiments. Engine test results showed a satisfactory NO_x reduction performance. Post inspection revealed no significant deposit formations.