A Tunable Diode Laser Measurement Technique for the OnBoard Evaluation of Tail Pipe Ammonia Emissions

Ammonia emissions of modern petrol fuelled cars are of raising concern especially in the city. As to heavy duty (HD) diesel vehicles the possible use of urea in the Selective Catalytic Reduction (SCR) of nitrogen oxides can result in ammonia slip.

In the development or evaluation of vehicles equipped with recent or new emission control technology an onboard measuring system capable of determining ppm level emissions of ammonia is used. This system features the latest in laboratory grade Tunable Diode Laser (TDL) analysers. The system consists of a novel Norsk Elektro Optikk TDL analyser with the transmitter and receiver mounted on the opposite sides of a compact measurement cell. This system is mounted on the exhaust piping measuring under full flow conditions. Extensive testing in the laboratory and on-board a HD test vehicle yielded sub ppm zero stability and detection limit.

A real life on-board evaluation is executed on a SCR equipped city bus. An average NO_x reduction of 85% is found. The ammonia slip is below 10 ppm for most of the time with some exceptions depending on engine, urea dosing and catalyst parameters.