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Tests are carried out to assess the potential of using SCR DeNOx catalysis, for achieving ultra-low NOx emission levels with heavy-duty diesel engines. A prototype DeNOx catalyst system has been developed which consists of a moderately sized SCR catalyst, upstream of which urea is injected through a heat-shielded nozzle. Downstream of the SCR catalyst a low sulphate oxidation catalyst is used to ensure any ammonia slippage from the SCR stage is oxidized. The system was tested on a standard 2-valve EURO 2 engine over several steady-state and transient test cycles currently under discussion for European use. Over these test cycles NOx emission values of about 2 to 2.5 g/kWh are accomplished, which are sufficient to fulfill future European heavy-duty NOx emissions legislation to be expected around the year 2005.

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.