Exhaust pollutants of diesel engines have to be drastically reduced. The oxides of nitrogen (NOx) and particulate matter (PM) form the main challenge for diesel exhaust cleaning. The fuel economy of the engines must also be kept at an adequate level to prevent the increase in CO2 emissions and operational costs.
For diesel engines, there are two main strategies for the reduction of NOx emissions. One is to increase the volume of cooled exhaust gas recirculation (EGR). The other is to use selective catalytic reduction (SCR) for NOx removal. For SCR, urea derived ammonia may be exploited but NOx can also be catalytically reduced by means of hydrocarbons (HC) originating from engine fuel. This latter system is termed HC-SCR.
In the present study, an Ag-alumina based prototype HC-SCR catalyst was studied for NOx removal in a turbocharged, intercooled direct-injection non-road diesel engine. The fuel post-injection of the engine common-rail injection system was optimized to produce a sufficient amount of hydrocarbons into the exhaust gases. The efficiency of the catalyst was then investigated in a few sets of engine operating conditions.
At its highest, a NOx removal efficiency of approximately 70% was obtained. This is not yet enough when compared with urea-based SCR catalysts. In the HC-SCR concept, however, no liquid additional to engine fuel is needed and problems of urea injection can be avoided.