Transient Performance of a Urea deNOx Catalyst for Low Emissions Heavy-Duty Diesel Engines

A urea SCR deNOx system for heavy-duty diesel engines was developed comprising a SCR catalytic converter combined with a low sulfate formation oxidation catalyst. Urea is injected upstream the SCR catalyst system by means of an open-loop controlled injection system.

Test bed investigations with the urea deNOx system applied to a 315 kW EURO 2 baseline heavy-duty diesel engine demonstrated a NOx emission reduction potential down to a level of less than 3 g/kWh. Heavy-duty diesel engines equipped with such a deNOx converter are able to comply with the expected EURO 4 NOx emission limits, independent from the ultimately effective test cycle. The particulate emission level of the EURO 2 engine was found not to be influenced by the SCR catalyst. The combination of SCR and oxidation catalyst results in a particulate increase of up to 25%, depending on the utilized test cycle.

Over the FIGE transient cycle the influence of the oxidation catalyst stage on ammonia and N₂O emissions was investigated. It was found that a reduction of the already low ammonia slip after the SCR stage down to a level of less than 3 ppm is achieved. However, the N₂O emission increases over the oxidation catalyst to levels comparable to current passenger cars. Besides that, CO, HC including unregulated emissions, and irritating exhaust odor are considerably reduced.