SCR technology (SCR = selective catalytic reduction) reduces the concentration of nitrogen oxide in raw exhaust gas. It works by using a reducing agent and catalysts and also optimises the engine's fuel consumption. The reducing agent is converted catalytically (using urea, for example) or thermally (e.g. using ammonium carbamate) to ammonia and then used in combination with appropriate catalysts to convert nitrogen oxides to the harmless substances nitrogen and water. Since 1995, HJS has developed and trialed different systems for reducing nitrogen oxides in combination with different ways of producing the reducing agent. Processes for using ammonium carbamate as the reducing agent were developed, which are particularly useful for applications where package space is limited.

Currently in the commercial vehicle sector, SCR systems are being fitted on some new vehicles that use urea as a reducing agent to decrease the level of nitrogen oxides. A combination of particulate filters filters and SCR technology can be used to achieve a reduction in both particulate matter and nitrogen oxides.