Design And Research On Diagnosis System Of Urea-SCR For Diesel Engine

With the growing emphasis on environmental pollution problems, the regulations on harmful exhaust emissions of different countries become increasingly stringent. Against the NOx emissions of diesel models, there are mainly two types of technical routes, one is In-engine processing+DPF and the other one is In-engine processing+SCR, Considering the good fuel economy and not sensitive to fuel sulfur content of SCR technology, SCR will become the main technical route for our country to meet China IV and V emissions regulations. Meanwhile, the increasing complexity of automotive electronic control systems caused by rapid development of automotive electronic technology brings new challenges to fault diagnosis technology of automotives. Therefore, the development of SCR fault diagnosis system can not only detect emission-related faults, make alerts and take appropriate measures when NOx emissions exceed limits, but also can help the after-sale service personnel to locate the fault area quickly and accurately to reduce troubleshooting time.This paper first introduces the composition, working principle and reaction mechanism of SCR system, and analyzes the possible faults according to the OBD regulations. Based on these, the fault diagnosis strategy is researched, including sensor diagnostic strategies, actuator diagnostic strategies and hardware-related diagnostic strategies. At the same time, the diagnostic management system is developed, diagnostic management system is composed of three functional units: fault monitoring unit, fault storage and deletion unit and diagnostic communication unit. Fault monitoring unit monitors the working status of SCR system and applies time-debouncing method or statistics-debouncing method to realize the fault state transition of recognized fault path. Fault storage and deletion unit realizes the storage and deletion of fault information and reacts to the fault correspondingly. Diagnostic communication unit manages the communication between SCR control unit and universal OBD-II diagnostic tool and realizes the access control mechanism to DCU.Second, against coexisting of ISO14230and ISO15765diagnostic protocols, a universal fault diagnosis system is developed. Against the limited functions and low efficiencies of traditional hand-held diagnostic tool, the combination of PC diagnostic software and VCI common fault diagnostic interface are used. The development process of fault diagnosis system are analyzed detailedly, including the hardware design of VCI, software design of VCI and design of PC diagnostic software. The communication protocol stack based both on K-line and CAN bus are designed to realize the low-level protocol conversion of physical layer, data link layer and network layer according to the OSI network layer model; PC diagnostic software is designed with modular construction method, different modules of PC diagnostic software collaborate with each other to realize the generation and output of diagnostic services.Finally, bench test of the developed diagnosis system are executed, which not only validate the quick and efficient implement of diagnosis system, and the diagnostic strategies and diagnostic communication process satisfies the design requirements, but also shows the good performance of the diagnosis system.