For the future application of large diesel engines constructional and operational measures are necessary to reduce the pollutant emissions within the regulated air quality limits. A clear decrease in the nitrogen oxide emissions can be achieved by injecting a reducing agent (e.g., ammonia or urea solution) into the exhaust gas stream. For individual high-power engines Selective Catalytic Reduction (SCR) facilities are connected at the engine outlet, for exhaust post-treatment by reducing agent injection. This approach suffers from high capital and opearating costs as well as substantial additional space requirements for the catalyst plant.

In an innovative procedure the reducing agent is injected directly into the combustion chamber at the end of the combustion cycle, using therefore the engine cylinders as reaction chambers and thus avoiding the additional catalyst chamber. However, the strong corrosion damages resulting in the engine, due to the aggressivity of the reducing agent, prevented so far the practical application of this procedure in reducing-agent injection systems.

A test stand with a Common Rail injection system was developed, which enables direct injection of reducing agents and water mixtures at approx. 1,200 bar. In extensive test runs the causes of the strong corrosive and erosive engine wear were examined and remedies proposed.

The experiments suggest that potential NOx reduction rates of the order of 40 to 50% may be obtained, however the expenditure in reducing agent quantities remains high. To explain this unexpected phenomenon a complex simulation was performed. It was proved that, due to the inhomogenous conditions within the combustion chamber oxidation reactions between oxygen and the reducing agent take place, which increases its consumption substantially.