Circuit/System Description

The exhaust gas temperature sensors are variable resistors that measure the temperature of the exhaust gas at the inlet and outlet of the diesel oxidation catalyst as well as the inlet and outlet of the Diesel Particulate Filter (DPF). The Engine Control Module (ECM) supplies a bias 5 volt signal to the exhaust gas temperature sensor signal circuit and supplies a ground to the low reference circuit. The exhaust gas temperature information is used by the ECM during the particulate filter cleaning process.

The selective catalytic reduction (SCR) system reduces oxides of nitrogen (NOx) emissions by injecting a metered amount of diesel exhaust fluid (DEF) or reductant into the exhaust gas stream entering the SCR catalyst. Within the SCR catalyst, the reductant reacts with the NOx to form nitrogen, carbon dioxide and water vapor. Upstream and downstream NOx sensors provide the engine control module (ECM) with engine-out and tailpipe-out NOx levels. The ECM varies the amount of reductant added by varying the reductant injector duty-cycle in response to changes in engine-out NOx levels. The ECM supplies voltage to the reductant injector on the injector high control circuit. The ECM turns On the reductant injector by grounding the low control circuit of the injector. The reductant injector temperature is modeled based on internal injector resistance of the injector coil and is used to monitor the temperature of the injector to prevent damage.