Research on OBD of Modern Automobile Diagnostic System
With the increasing number of cars in the world year by year and the enhancement of human awareness of environmental protection, the pollution of car emissions has also attracted more and more attention from the world, and the emission regulations formulated by countries around the world are becoming stricter. Because environmental agencies require more accurate methods to detect engine performance problems that cause increased emissions, OBD (On-Board DiagnosTIc: on-board diagnostics) systems came into being. The newly launched second-generation on-board diagnostic system is one of the effective means to monitor vehicle emissions. The most important point of the second-generation on-board diagnostic system is that the system is designed to detect whether the exhaust emissions of vehicles, such as HC, CO, NOx, and fuel evaporation pollution, exceed the emission limits. The OBD system will not replace regular emission testing, but will function as an on-board emission monitor. As a responder to the EU's automobile emission regulations, China will definitely implement EOBD (European On-Board DiagnosTIc: European on-board diagnostic system) in the near future.
1 OBD system overview
1.1 Development history of OBD system
The OBD system has gone through two stages: OBDâ… (first-generation on-board diagnostic system), OBDâ…¡ (second-generation on-board diagnostic system), and EOBD. OBDâ… was first used by California, USA in 1991. Its function is relatively simple, mainly to diagnose the complete failure of emission-related parts. There is no unified standard for OBDâ… , and the content of OBD connector socket, fault code, communication protocol and other forms are very different, which brings a lot of inconvenience to the fault diagnosis and maintenance of electronically controlled vehicles. The second stage OBDâ…¡, EOBD system is very complicated. In addition to the complete failure diagnosis of the emission-related components, OBDâ…¡ and EOBD also have to diagnose the excessive emissions caused by aging and partial failure of the components. Therefore, the OBDâ…¡ and EOBD systems are the real means to realize the emission control over the entire service life of the vehicle in use. OBDâ…¡ and EOBD use a unified standard, as long as one instrument can be used to diagnose and test various vehicles, which provides great convenience for global automobile maintenance testing.
Because the United States and Europe have adopted two different emission regulations, the second-generation on-board diagnostic system has two forms: OBDâ…¡ and EOBD. The United States implements OBD II, while countries that adopt European emission regulations implement the EOBD system. From the root, the OBDâ…¡ system in the United States was implemented earlier and the standards were stricter. The U.S. Environmental Protection Agency stipulates that the electric control systems of cars and light trucks (loads below 6.5t) produced after 1996 are required to be equipped with the OBDâ…¡ system, and on January 1, 2000, all cars and light trucks produced by car manufacturers The OBDâ…¡ system must be configured. Canada began to implement the OBDâ…¡ system in 1998. In Europe, the EOBD system has been gradually implemented since 2000. In 2001, all newly-produced cars in Europe (with a load of 2.5t or less) were limited to gasoline engines equipped with the EOBD system. For diesel engine cars, the EOBD system must be mandatory by 2004. There are no OBD regulations in the Euro â…¡ standard currently issued in China, but with the implementation of the Euro â…¢ standard, the use of EOBD is bound to be on the agenda.
1.2 Composition of OBD system
The OBD system is very complicated. The California Air Resources Board (California Air Resources Board: CARB) OBD II system regulations include more than 70 pages of detailed regulations and hundreds of pages of detailed SAE and ISO standards. The OBD system is functionally implemented by software and hardware. OBD's software? Take the source of the risk of the Xia Xia Xian Xie? Ye Ping Xuan ?? â’ ?? Xie Xian Zhong Huang emu hook burning? Ye â’ ?? Zu Portuguese low car Na Zeng ?? T æ° é¡“ 湫 secretly â’ ?? Liu Portuguese low orange ???? Check the BD part of the code accounted for half of the entire software content, there are more than 150 possible fault codes. Typical EOBD software includes 60,000 lines of code and 15,000 calibrations. The hardware of OBD is mainly composed of subsystems related to engine exhaust control, such as sensors, ECU (Electronic Control Unit), OBD connector socket, fault indicator, actuator and circuit.
1.3 Working principle of OBD system
When the car is in normal operation, the signals (voltage or current) of the electronic control system of the car will change regularly within a certain range; when the signal of the electronic control system circuit is abnormal and exceeds the normal change range, And this abnormal phenomenon will not disappear within a certain time (3 consecutive strokes), the ECU judges that this part has a fault, the fault indicator lights, and the monitor stores this fault in the form of code in the internal RAM ( Random Access Memory: Random Access Memory), the stored fault code can be read through the fault display lamp or OBDâ…¡ scanner during maintenance. If the fault no longer exists, the monitor will instruct the fault display lamp to go out after receiving the relevant signal for three consecutive times. After the fault indicator is off, the engine warm-up cycle is about 40 times, and the fault code will be automatically cleared from the memory.
1.4 Main features of OBD system
Compared with OBDâ…¡, the biggest improvement is that OBDâ…¡ has a unified standard, which provides a lot of convenience for the fault diagnosis, inspection and maintenance of electronically controlled vehicles. In 1988, SAE (American Society of Engineers) created the first fault diagnosis connector socket and a set of fault codes as a standard promotion. The US Environmental Protection Agency adopted most of SAE standards and recommended it to be used uniformly worldwide. All OBDII or EOBD equipped cars must include the following standardized hardware and software: standardized data diagnostic interface (SAE-J1962), standardized decoder (SAE-J1978), standardized electronic communication protocol (kW2000, CAN, CLASSII, ISO9141, etc.), standardized diagnostic trouble codes (DTC, SAE-J2012), standardized maintenance service information (SAE-J2000).
2 Detection objects of OBD system
OBD II requires inspection of any component or system related to emissions. Focus on the detection of fuel and air measurement system failures, ignition system failures or engine intermittent shutdown failures, exhaust control auxiliary device failures.
3 Limitations of OBD system
(1) The OBD system cannot measure vehicle emissions such as CO, NOx and HC, but only acts as a vehicle-mounted emission monitor. Therefore, if it is necessary to accurately analyze the vehicle exhaust emission status, other monitoring methods or other exhaust gas analyzers are needed.
(2) The reliability of the OBD system is affected by the operating environment of the vehicle. In certain working situations, such as poor operating conditions and abnormal working environments, the OBD system may be in error. At this time, the work of the OBD system is generally suspended. And wrong fault indication will reduce the user's credibility of the OBD system, so that some users ignore it after the OBD issues a fault warning, making the OBD's proper function impossible to realize.
(3) The OBD system cannot indicate how to repair the vehicle, it can only monitor the vehicle in real time, store the detected faults in the memory in the form of codes, and notify the driver of the faulted location or by lighting the fault lamp. It indicates that there is a diagnosed fault, and the driver is reminded to repair the vehicle.
(4) The OBD system cannot diagnose all faults in the automotive electronic control system, it can only detect 70% to 80% of the faults in the automotive electronic control system. Relying only on the failure indicator light cannot effectively judge the deterioration of the automobile system.
(5) The OBD system poses a huge challenge to the software. OBD software is about half of the entire electronically controlled car software. Any one of these software errors can lead to false fault indications or violations. In terms of software accuracy, even 99.9% accuracy will still cause many system problems.
In addition, there are many difficulties related to OBD system software and detection, such as too sensitive, too insensitive, and inaccurate detection.
4 Conclusion
OBD is a very complex self-diagnostic system used to detect failures of components and systems that affect vehicle emissions. The purpose of implementing the OBD system is to determine whether the emissions exceed the maximum value due to the failure of parts or subsystems. At present, the domestic automobile emission control system basically adopts foreign mature products directly, and domestic research in the field of OBD is still in its infancy. As vehicle emission regulations become stricter, OBD will be mentioned as a very important position. With the continuous improvement of China's emission regulations, the Chinese government will surely formulate strict OBD regulations to control the emission levels of in-use vehicles. Although the OBD system still has some limitations, it is the real emission control for the entire service life of the car in use. With the implementation of China's Euro III emission regulations, OBD has become a hot topic in the automotive industry.
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