Using a MISiC-FET sensor for detecting NH3 in SCR systems

One way to decrease the emitted levels of NO_x from diesel engines is to add NH₃ in the form of urea to the exhausts after combustion. NH₃ will react with NO_x in the catalytic converter to form N₂ and water, which is called selective catalytic reduction (SCR). The amount of NH₃ added may be regulated through closed loop control by using an NH₃3 sensor. The MISiC-FET (metal insulator silicon carbide field effect transistor) sensor has previously been tested in this application and has been shown to be sensitive to NH₃. Here, the sensors have been further tested in engine SCR systems. Cross-sensitivity tests and studies concerning the influence of water vapor have been performed in the laboratory. The difference between Ir and Pt films, with regard to catalytic activity, has also been investigated. The sensors were found to be sensitive to ammonia in diesel engine exhausts. The addition of urea was computer controlled, which made it possible to add NH₃ in a stair-like fashion to the system that could be detected by the MISiC-FET sensors. The presence of water vapor was shown to have the largest effect on the sensors at low levels and the ammonia response was slightly decreased by a background level of NO₂.