Research Of The SCR System And Its OBD Function For The Common Rail Diesel Engine

Compared to the gasoline engine, diesel engine has higher thermal efficiency and lower fuel consumption rate, so the proportion of diesel cars becomes heavier. Because of the diesel engine combustion with high-temperature and rich-oxygen, it makes large amounts of NOx, at the same time, the PM emission value is high. With the emissions regulations become more and more stringent, various emission limits are getting lower and lower. The purification measures inside of the diesel engine can’t meet the emission requirements, it is necessary to use the purification measures outside of the diesel engine. Nowadays, there are two routes, the first route-European route, it use the combustion optimizing and SCR system. We call it European route because it is widely used in European. The second route is U.S route, it use the EGR+DPF. Based on our regulations and the circumstances, we select the European route.The foreign researched the SCR system early, so their technology is more mature. The beginning of research of SCR system in our country is late, so the technology is poor. Today, our institutions of higher learning and Yuchai engine plant are doing some research work of the SCR system. This article describes the SCR system at home and abroad, including the composition and the working principle, and proposes the control strategy of the SCR system.The NO.4emission standards set a low emission limits, as well as retrofitting of OBD.Accordance with the requirements of HJ437-2008, this article designs the OBD structure based on the SCR system, including diagnosis, diagnostic management system and testing.Diagnosis is mainly the fault diagnosis strategy. This article proposes the diagnosis strategy for that will cause the NOX excessive the limit, including the diagnosis of the sensors and other components, the diagnosis of the system circuit fault and the functional diagnosis.The diagnostic system includes three aspects of the fault recognition, fault storage and communication.The communication refers to communication between the ECU and the external equipments, such as calibration software and the standard diagnostic instrument. On the basis of the analysis of J1939, ISO14230and ISO15765, this article select the ISO15765as the communication protocol for the platform.Finally, this article verifies the reliability of platform by the test and raises some questions to be improved.