Study Of Key Technology For Sedan On-Board Diagnosis (OBD) System

With the development of automobile industry, the quantities of vehicle increased rapidly, the air pollution stemed from tail gas of vehicle had become one of social pollution. The U.S.A and EU etc. had issued more stringent emission regulation. Although new vehicle can run with lowest emission, incorrect maintenance or defective components will cause increasing tail gas. The function of OBD system is to keep vehicle emission qualified during the whole in-use life.According to materials concerned, China had surpassed France and become the fourth car manufacturing country. The tail gas pollution of car became first polluting source in many cities. Due to so severe environmental pollution situation, the NEPB issued"Limits and Measurement Methods for Emissions from Light-duty vehicles (Phase III, IV) GB 18352.3-2005"on April 17th,2005, and it prescribes that Phase III will be implemented from July 1st,2007 in China, the first class gasoline car will be equipped with OBD system from July 1st,2008.This article studied key technologies concerned of OBD system based on Jetta 1.6l manual transmission vehicle.Design of Jetta OBD systemJetta 1.6l vehicle is equipped with the most advanced SIMOS 7 EMS system from Siemens, engine control software uses torque management to deal with input/output signals concerned. The diagnosis of catalyst efficiency is accomplished via using dual- oxygen sensora to measure OSC of catalyst indirectly. The engine misfire is detected via evaluating engine speed vibration.During nuclear technologies study of Jetta 1.6l OBD system, the calibration system ETAS INCA CCP from ETAS enterprise was used. INCA is compositive software/hardware system integrated with measuring, calibration and diagnosis functions. INCA can provide full-scale measuring frame, evaluate measured data and recorde calibration results accurately.Study contents and results about efficiency diagnosis of 3-way catalyst convertorEfficiency diagnosis of 3-way catalyst convertor is one of most important task for OBD system, the target is to evaluate if emission value of type I test exceeds OBD thresholds under normal conditions."Limits and Measurement Methods for Emissions from Light-duty vehicles (Phase III, IV) GB 18352.3-2005"standard defines OBD thresholds of gasoline car as following table: Category Class Reference massIn the meantimes, the standard prescribes to estimate efficiency of catalyst convertor via monitoring HC tail gas.Taking reliability and costs of monitoring system into consideration, at present the method for efficiency diagnosis of catalyst convertor used extensicely is: utilizing OSC character of catalyst convertor, using upstream/downstream dual oxygen sensors to measure OSC change of catalyst convertor, and establishing statistical correlation between performance of catalyst convertor and OSC, then measuring HC emission indirectly.Type I emission test can be divided into two stages in view of working efficiency of catalyst convertor: non- catalyst light-off phase and catalyst light-off phase. During non- catalyst light-off phase, catalyst convertor doesn't work normally and converting efficiency is very low, so emission results have nothing to do with OSC of catalyst concertor basically, and only are related to calibration strategy and vehicle status. During catalyst light-off phase, convertor works normally, converting efficiency is mainly related to OSC of catalyst concertor catalyst concert when calibration is accomplished. The reasons mentioned above result in high non-linear correlation between OSC of catalyst convertor and HC emission value of type I test, so it is impossible to fix on this kind of correlation accurately during development activities.Because Phase III/Phase IV standard only requires that MIL must be illuminated when HC emission value of Type I reaches OBD thresholds 0.4g/km (+20% plus tolerance), and MIL is allowed to illuminate when HC emission value is lower than OBD thresholds in the meantime, based on these premise, Jetta 1.6l vehicle uses two binary oxygen sensors for on-line diagnosis of catalyst convertor, and compares signal amplitude from downstream oxygen sensor with model signal amplitude, then obtain conclusions about if the catalyst convertor is defective or not. The model signal amplitude comes from borderline catalyst.During study of Jetta OBD system, for efficiency diagnosis of catalyst, this article developed a new numerical arithmetic used for calibration. The new numerical arithmetic uses signal of downstream oxygen sensor for calculation of catalyst converting efficiency, and the signal of upstream oxygen sensor is only responsible for air/fuel control during catalyst diagnosis. The calibration results of Jetta 1.6l vehicle express that the new numerical arithmetic meets requirements of national standard Phase III/Phase IV for efficiency diagnosis of catalyst.To make OBD borderline catalyst is one of the most important jobs for efficiency diagnosis of catalyst. Based on traditional methods of making borderline catalyst, this article developed a new method for making borderline catalyst, which combines oven aging with engine bench aging, and obtained the best oven aging temperature, misfire rates and time of misfire aging for Jetta 1.6l manual transmission vehicle. Test results express that new making method of borderline catalyst is high efficient and reliable, and meets requirements of national standard Phase III/Phase IV. Study contents and results about diagnosis of engine misfireAccording to requirements of Phase III/Phase IV for misfire diagnosis of engine and basic physical principle of misfire diagnosis, this article detects engine misfire indirectly through monitoring fluctuation of engine speed. In the meantime, the concepts of measure segment period and engine roughness etc. are introduced. Based on this premise, this article comes up with a numerical arithmetic about engine roughness. The numerical arithmetic includes three parts:Static component, it compares the current segment with the following one. When the result drops below a threshold in negative side, a misfire is detected.Dynamic component, it caculates time delta between current measured period of certain segment and pre- measured period of this segment, the static component has to be corrected by means of the dynamic component, and influences of acceleration and deceleration are eliminated.Curvature component, it caculates time delta between post- measured period of certain segment and pre- measured period of this segment, the remaining deviations in case of extremely non-stationary processes, for example, running on rough road, are corrected.The test results expressed that this kind of numerical arithmetic is accurate and reliable. During study of misfire diagnosis function for Jetta OBD system, in order to eliminate the influence of system errors, for example, manufacturing tolerance and eccentric assembly etc., to measuring accuracy of segment period, adaption arithmetic of segment period is developed. Engine adaption speed is divided into three kinds of range, which are low engine speed range, middle engine speed and high engine speed, so accuracy of segment period is guaranteed. The test results expressed that adaption arithmetic of segment period can increase reliability of misfire diagnosis effectively.Based on the criteria CARB A (the misfire rate which could lead to catalysts overheating causing irreversible damage) / CARB B (the misfire rate which would result in emissions exceeding the OBD limits in the type I test), the mappings of misfire rate related to CARB A/ CARB B are acquired for Jetta 1.6 manual transmission through quantities of tests. The test results expressed that the calibration concerned is successful.Based on the characters of ignition system of Jetta 1.6l engine, this article acquires detecting misfire ways for single cylinder and dual cylinders, and the mapping of engine roughness threshold concerned. The test results expressed that engine misfire can be detected accurately using this mapping.Fleet test of OBD systemIn order to ensure the reliability and applicability of OBD system during the whole in-use life, the automobile industries of U.S.A and Europe use full-scale fleet test to investigate software and hardware reliabilities of OBD system. FAW-VW also uses fleet test to check reliability of OBD system developed.According to the premises including requirements of national OBD regulation, fuel status and special areas (only Beijing and Guangzhou) where OBD regulation is implemented, this article divides OBD fleet test into three parts: in-use fleet in Changchun, in-use fleet in Beijing and OBD road test fleet in China. For these three fleets, test specification, test fuel and running road are designed.The test results of fleet test express that test specifications of fleet test are reasonable and effective. The fleet test can detect software/hardware problems effectively during development of OBD system, and offer the evidences for solving problems. In other words, fleet test is essential process for development of OBD system. In the meantime, fleet test results also testify that the general scheme of Jetta OBD system is reasonable, and can meet requirements of national standard Phase III.This article also studys the influence from sulfur content, olefin content and vapor pressure of fuel to vehicle emission during fleet test, and draws the primary conclusions:With raising of fuel vapor pressure, the emission results of type I test become betterWith raising of sulfur content in fuel, the emission results of type I test become worseWith raising of Olefins and aromatics contents in fuel, the emission results of type I test become worse...