| Emission problems have become more and more seriously along with the rapid increase of automobiles. In order to comply with the stricter emission regulations, on-board diagnostic (OBD) system has become necessary in light-duty gasoline vehicles. The fault diagnostic methods and off-line diagnostic technique have been investigated in this thesis, combined with National 863 Project of "OBD and remote diagnostic technology for light-duty gasoline vehicles emission control" during "the Eleventh Five-Year Plan" of China.Firstly, a fault simulation platform which can simulate representative faults in light-duty gasoline vehicle OBD system, such as misfire, malfunction of oxygen sensor, catalyst aging, has been set up. For the sake of meeting the demand of fault diagnose threshold calibration in the new vehicle OBD explore process, researching faulty degree's influence on vehicle emission for different representative faults, and meeting other demands, for example, representative faults diagnostic strategy or method investigation, and new automobile type approval, a general device of misfire fault producing has been designed and developed in view of all kinds of ignition system used in vehicles at present. In this system, many kinds of misfire methods can be realized and rate of misfire can be changed. A suit of oxygen sensor failure simulation device has been designed and developed, which can be applied to most of on-off oxygen sensor used in the vehicles and simulate different kinds of representative oxygen sensor failures. A set of bench system designed for three-way catalytic converter aging test has been set up according to the Chinese national standard?. This system can produce different kinds of sample in the process of catalyst aging by means of fast-aging in this system, which is useful in new vehicles design and matching.Secondly, the diagnostic methods of OBD for representative fault have been investigated. Nowadays, diagnose of misfire in multi-cylinder engine and in the engine condition of high speed and low load has been recognized as difficulty. Fault diagnostic method of two-cylinder misfire has been studied in this thesis. A new misfire fault diagnose arithmetic has been brought forward, based on the hardware, measuring the instantaneous engine speed signal, adopting EMD in decomposing and analyzing instantaneous engine speed signals, introducing the indicator of each cylinder working time, combing mode identify function of back-propagation neural network. By using mean value model for engines, a close loop control model based on oxygen sensor signal of fuel has been set up. The influence of oxygen sensor fault on fuel injection regulation and emission has been analyzed in theory. The diagnostic strategy of different oxygen sensor fault has been brought out. When the oxygen sensor happen to be error, a virtual oxygen sensor-based emission control model has been built up, which can improve the emission level of the in-use vehicle. Due to the oxygen storage and release characteristic of the catalyst, a fault diagnostic method based on a model of catalyst's storage capability of oxygen has been put forward aiming at finding out whether the catalyst is aging or not. Four evaluation indices for the catalyst have been brought out and the parameters in this model have been distinguished and estimated by means of "dense and subtle " experiment. An assessment method of catalyst performance based on its oxygen storage process has been studied by experiment.Finally, the off-line fault diagnostic technique of vehicle has been studied. Since OBD is becoming indispensable to the vehicle control system, off-line fault diagnostic system matching with OBD is also necessary in vehicle's maintenance. A plan of general vehicle fault diagnostic system using personal computer based on OBD has been presented. Specific to K-Line and CAN bus,which are widely used in vehicle's fault diagnose, the vehicle diagnostic communication switching system hardware based on Freescale 16-bit MCU has been designed and developed. Communication protocol stack has been introduced according to the different communication protocol, which realizes the switch of communication between vehicle diagnostic protocol and upper-computer. The upper-computer diagnostic software is developed by using Visual Basic, which can carry out all kinds of diagnose. After one year's development, test, optimization and upgrade, this system can fulfill diagnose of the full series vehicles produced by one automobile corporation in China. |
PDF Full Text Request