Exhaust gas aftertreatment refers to the processes that clean exhaust gases mechanically, catalytically or chemically after they have left the combustion chamber.
Aftertreatment is carried out to convert the pollutants produced during combustion into harmless exhaust gases. Exhaust gas aftertreatment components include catalytic converters and particulate filters. Nowadays, both components can be installed on a direct-injection petrol engine as well as on a diesel engine.
The following systems, for example, can be installed in the exhaust system to reduce pollutants:
Petrol engine
Diesel engines
Depending on the vehicle manufacturer and system, various particulate filter regeneration procedures can be carried out.
Passive regeneration
Passive regeneration takes place as soon as the exhaust gas temperatures in the particulate filter reach a value of 350 - 500°C on motorway journeys at increased speeds.
Active regeneration
Active regeneration is carried out by the engine management system. When the load limit of the particulate filter is reached, the exhaust gas temperature is specifically raised to 600-650°C via the engine control unit in order to burn off the soot particles.
Forced regeneration
This type of regeneration can be carried out by a workshop using a diagnostic device in accordance with specified instructions.
The NOx absorber is used in diesel and direct-injection petrol engines. The catalytic converter has a catalytic layer of substances such as potassium oxide or barium oxide that bind nitrogen oxide molecules. As soon as the absorber has reached a certain absorption capacity, the engine control system greases the air-fuel mixture, increasing the exhaust gas temperature. The changed exhaust gas composition leads to regeneration, which sees the nitrogen oxides (NOx) reduced to nitrogen (N2) and water (H2O).
Modern exhaust gas aftertreatment systems not only consist of the components of the exhaust system, but also require various sensors to monitor the exhaust gas composition and pass on their information to the engine control unit.
Before starting control unit diagnostics on the vehicle, a visual inspection of the entire exhaust system should be carried out first. External damage can usually be detected when the noise behaviour changes and can be caused by cracks or rusting through at pipes, connections or mufflers. Noises coming from inside the system components can be localised by shaking or knocking on the relevant component. Of course, the tight screw connections, radiator plates and rubber mountings should also be checked as part of this. The exhaust gas sensors should not be forgotten either. They may be mounted over the entire course of the system. Wiring or electrical plug connections may have been damaged here due to environmental influences such as dirt, water or road salt.
The function check can only be carried out on the injection system or the exhaust gas aftertreatment system with a suitable diagnostic device.
The function of the individual exhaust gas aftertreatment components is monitored via sensors and transmitted to the relevant higher-level system control unit. Any errors that occur are stored in the engine control unit's error memory and can be read using a suitable diagnostic unit. Depending on the vehicle and on the system, additional functions, such as parameters or actuator tests, can be selected and displayed or executed on the diagnostic device. The data from control unit communication forms the basis for actual troubleshooting and for successful repair work. In addition, the exhaust gas values can be checked and evaluated using a tailpipe measurement.
The following diagnostic information uses a Mercedes-Benz E350 24V CDI (212) and a Volkswagen Golf 5 Plus as examples.
In this function, the error codes stored in the error memory can be read out and deleted. In addition, information on the error code can be called up.
In our case study, a defective NOx sensor was detected and, as a result, the error code P220317 was stored in the error memory.
In this function, current measured values such as engine speed, temperature or the status of individual exhaust gas components can be selected and displayed.
System-specific information can be taken from vehicle information and used for troubleshooting purposes. Here, for example, a system overview of the exhaust gas aftertreatment system can be used for further troubleshooting.
