The regeneration of the particulate filter takes place in the following stages:
- Passive regeneration
- Heat-up phase
- Active regeneration
- Regeneration trip by customer
- Service regeneration
During passive regeneration, the soot particles are continuously combusted without engine management system intervention. This usually happens when the engine is subjected to high loads, such as during expressway driving. This causes exhaust gas temperatures of 350 °C to 500 °C. The soot particles react with the nitrogen dioxide and are converted to carbon dioxide.
If the oxidizing catalytic converter and the particulate filter are cold on engine start, the ECM triggers up to two additional secondary injections immediately after the main injection. This brings the oxidizing catalytic converter and the particulate filter up to operating temperature as quickly as possible. The fuel injected during these targeted post-injections burns in the cylinder and increases the temperature level of the combustion. The heat that it generates passes through the exhaust flow to the oxidizing catalytic converter and the particulate filter, and heats them up. The heating phase is complete as soon as the operating temperature of the oxidizing catalytic converter and the particulate filter has been attained for a certain length of time.
In most of the engine operating ranges, the exhaust gas temperatures are too low for passive regeneration. Since soot particles are not burned, the soot accumulates in the filter. As soon as a specific level of soot has accumulated in the filter, active regeneration is induced by the engine management system. The soot particles are burned at an exhaust gas temperature of 550 °C to 650 °C and form carbon dioxide.
Active Regeneration Function
The soot load of the particulate filter is calculated by two pre-programmed load models in the ECM. One load model is determined from the user driving style and values from the exhaust gas temperature sensor and lambda probe signals. Another soot load model is the flow resistance of the particulate filter. It is calculated using data from the G450 Exhaust Pressure Sensor 1, the exhaust gas temperature sensors, and the calculated exhaust mass flow from the ECM.
Actions to Raise the Exhaust Temperature During Active Regeneration:
- The intake air supply is regulated by the throttle valve module
- Shortly after a “retarded” main injection, one or two post-injections are introduced to increase the combustion temperature
- Further post-injection is introduced long after main injection. The injected fuel does not combust in the cylinder, but evaporates in the combustion chamber
- The uncombusted hydrocarbons contained in the fuel vapor are oxidized in the oxidizing catalytic converter. The heat generated during this process passes to the particulate filter and increases the exhaust gas temperature upstream of the particulate filter to approximately 620 °C
- For the calculation of the injection quantity for the retarded post-injection, the ECM uses the signal from G495 Exhaust Gas Temperature Sensor 3
Regeneration trip by customer
- The charge pressure is adapted so that during regeneration torque does not change noticeably for the driver
When driving only extremely short trips, the exhaust temperatures are not high enough to regenerate the filter. If the load condition of the diesel particulate filter reaches the upper limit, the diesel particulate filter warning lamp in the dash panel insert lights up. This signal instructs the driver to perform a regeneration drive cycle.The vehicle must be driven for a short period at a steady speed without turning off the engine. This creates a high exhaust gas temperature. Successful regeneration can occur.
If the regeneration trip was not successful and the load state of the diesel particulate filter has reached 40 grams, the glow plug warning lamp illuminates and the diesel particulate filter warning lamp both illuminate.The text “Engine malfunction - workshop” appears in the MFI and the driver is requested to seek the next dealership service department. To avoid damage to the particulate filter, the ECM will not attempt active regeneration of the diesel particulate filter. The particulate filter can only be regenerated in a workshop using the vehicle diagnostic tester.
With load condition of 45 grams, a service regeneration is no longer possible, because the risk of destroying the filter is too high. The filter must be replaced. During engine operation, a small amount of engine oil is always burned. Part of the burned engine oil is collected as ash in the particulate filter. This “oil ash” cannot be removed, even during active regeneration. For efficient operation, the ash limit should be checked with the diagnostic tester during an inspection. If this limit value is exceeded, the diesel particulate filter must be replaced.
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The following chapter, taken from an official VAG technical document of a CRBC engine, explains the phases of the regeneration process in an EA288 TDI engine. The same principles can be used for all recent VAG TDI engines.