Today, a large portion of the class 6, 7 and 8 trucks on the road have a diesel exhaust filtering system that captures the majority of particulate matter and dangerous nitrogen oxides (NOx). In the last issue of The Cooling Journal, we published an article detailing how certain components of a diesel engine can affect the exhaust and eventually impact the filters that are designed to filter the emissions produced by a diesel engine. A significant contributor to extending the diesel particulate filter’s life is the regeneration a diesel engine produces to help clean the Diesel Oxidation Catalyst (DOC) and the Diesel Particulate Filter (DPF).
When I was a young man, we had exhaust systems on our cars that consisted of two hooker headers and two cherry bomb mufflers with some steel tubing plumbed out the sides of the car. Fuel loaded, filthy exhaust produced by a 650 double pumper Holley carb dumping raw fuel into the engine, which half of it came out the exhaust barely burnt. The cars were almost as bad as the old trucks going down the road billowing black smoke out of the dual chrome exhaust pipes on either side of the cab. Those days are long gone, auto and diesel exhaust today is almost as clean as the air we breathe.
Different Types
There are three types of regeneration that any DPF service and sales personnel should be aware of:
- Passive regeneration
- Active regeneration
- Manual regeneration
Passive regeneration is achieved when a diesel engine produces an exhaust temperature of 350 degrees Celsius or 662 degrees Fahrenheit. Just by the exhaust reaching, this temperature soot trapped in the diesel particulate filter will be burned off. A problem can occur in vehicles who operate at a lower temperature for extended periods of time due to operating conditions or a component malfunction.
Active regeneration occurs when the DPF becomes loaded with soot. Pressure differential sensors located before and after the DPF are responsible for detecting this condition and alert the engine ECU the filter is filling up with soot and needs to be cleaned. The ECU sends a command to the HC (hydrocarbon) dosing valve located before the DOC and DPF. The HC Dosing valve opens allowing diesel fuel to enter the exhaust stream and elevate the temperature of the exhaust to facilitate the burning of the soot collected in the DPF. The Active Regeneration process happens automatically as the vehicle is in operation. There may be a light on the dash that will alert the driver this process is underway.
Manual regeneration is initiated by the operator of the vehicle and only occurs when the vehicle is stationary. The check engine light, MIL, or light similar to a check engine light will illuminate notifying the driver a regeneration needs to be initiated. The vehicle’s operator must set the parking brake and activate the regeneration switch. This manual regeneration process is very similar to Active regeneration as diesel fuel is injected into the exhaust pipe before the DOC and DPF. The only differences between Active and Manual is operator initiated and the vehicle is stationary.
Now that the different types of on vehicle DPF regenerations have been explained, let’s find out what conditions could affect the regeneration process. If regeneration does not occur for whatever reason, the diesel particulate filter will fill up with soot which will cause the engine to derate and or completely shut down. Either way, this is not an ideal situation and will need immediate attention to remedy the problem. It is imperative vehicle operators pay attention to the instrument cluster and any MILs that light up.
Common Causes
Let’s look at some common causes that prevent regeneration or increase the frequency of regeneration.
1.The HD doser should be checked for proper operation. If the hydrocarbon doser fails, diesel fuel will not be injected into the exhaust stream and the engine will fail to regenerate.
2. If either of the DPF pressure sensors malfunction, the aftertreatment control module or the Engine Control Unit (ECU) will receive false readings and could initiate frequent regenerations.
3. Exhaust Gas Recirculation fault codes can interfere with the regeneration process. Any fault codes related to the EGR system will cancel a regeneration.
4. DOC failure, commonly referred to as face plugging, will interfere with the regeneration process. The injected diesel fuel uses the diesel oxidation catalyst to raise the temperature of the exhaust and start the process of burning the soot in the Diesel Particulate Filter (DPF). Face plugging of a DOC can occur when a vehicle is used on short trips and or stop and go traffic. It is important that an engine is allowed to get to the correct temperature to insure DOC face plugging does not occur.
5. Diesel Particulate Filter plugging can happen which will prevent a satisfactory regeneration. Regeneration times may be longer with poor results. This usually occurs when a DPF has not been removed for routine service by either baking and blowing or an aqueous cleaning which restores the filter to at least 95% of its new capacity.
6. Temperature sensors being faulty will stop the regeneration process. The readings from these sensors give an indication of how the DPF system is operating.
7. Variable Geometry Turbo failures can prevent regenerations from being completed as not enough heat is generated to complete the process.
8. Engine fault codes related to temperature, emissions, pressures, turbos, boost, and other components can prevent the regeneration process from occurring.
9. Faulty electrical components like plugs and wires can also prevent regeneration. Sensors may not be malfunctioning but the ECM or other related computers will receive incorrect values due to the wiring harness being defective.
It becomes evident pretty quickly that the newer emissions systems with diesel particulate filters are much more sophisticated than earlier diesel engines. It is imperative that the emissions system be properly diagnosed and repaired when the regeneration process fails. Usually it is less expensive to repair the system before a DOC and DPF are compromised beyond servicing, requiring replacement.
The schematic below will give you an indication of the amount of technology that goes into controlling an exhaust system so it meets current Environmental Protection Agency regulations.