Using a MISiC-FET sensor for detecting NH/sub 3/ in SCR systems

Abstract

One way to decrease the emitted levels of NO/sub x/ from diesel engines is to add NH/sub 3/ in the form of urea to the exhausts after combustion. NH/sub 3/ will react with NO/sub x/ in the catalytic converter to form N/sub 2/ and water, which is called selective catalytic reduction (SCR). The amount of NH/sub 3/ added may be regulated through closed-loop control by using an NH/sub 3/ sensor. The metal-insulator silicon-carbide field-effect transistor (MISiC-FET) sensor has previously been tested for this application and has been shown to be sensitive to NH/sub 3/. Here, the sensors have been further studied in engine SCR systems. Tests on the cross sensitivity to N/sub 2/O and NO/sub 2/, 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 NH/sub 3/ in diesel engine exhausts. The addition of urea was computer controlled, which made it possible to add NH/sub 3/ in a stair-like fashion to the system and detect it with the MISiC-FET sensors. The presence of water vapor was shown to have the largest effect on the sensors at low levels and the NH/sub 3/ response was slightly decreased by a background level of NO/sub 2/.