Closed particulate filter (Wall Flow)
This type of filter doesn't allow almost any microsoot particles to pass through. The cells that are plugged at one end and open at the opposite side, force the exhaust gas to flow through its porous walls and trap the particulate matter. If we were to unfold these cells on a passenger car filter the overall area would be bigger than 2.4 m3 (similar to the size of a vacuum cleaner filter bag). The exhaust gas flow restriction is very low. If the engine is OK, it produces a very small amount of smoke (soot generation) anyway and only in driveaway modes.
The task of closed particulate filters:
On modern high pressure injection engines the higher the injection pressure the smaller the soot particles are. This means that these engines generate the smallest and thus the most dangerous exhaust particles, that can penetrate from the lungs to the blood and even to the brain. The particulate filters function is to trap the soot generated mainly during city and cold engine operating modes and to burn them off almost completely in the filter during periodic intervals (around 600 km). The filter together with the catalytic converter not only traps the particles but also removes them here in the most environmental friendly way. This is if everything works as it should.
Regeneration methods (two basic methods)
1. Dry method - the particulate filter is burned out by increasing the exhaust temperature to 570 - 700 °C, by activating post injections when the piston position is after TDC (or by injecting fuel into the exhaust by an independent injector) without any other support.
2. Wet method - an additive is injected into the fuel tank during every refueling (e.g. EOLYS or SATACEN), that is soluble with diesel fuel and the generated soot from this has the ability to burn rapidly at temperatures around 420 °C. The additive reduces the soot combustion temperature by about 150 – 200 °C. If there is no additive in the fuel tank the soot will start to burn at the same temperatures as under the "dry method".
Double task of the oxidation catalytic converter
1. To reduce the emissions of CO and HC and soot in the exhaust gases under normal operating conditions of the engine. The oxidation catalytic converter reduces the volume of soot travelling with the exhaust gases to the DPF filter by almost 30 %. This is under the circumstances that the converter is working correctly, is not contaminated, thermally deteriorated or that the converters "wash coat" hasn't shrunken (gamma alumina > alfa alumina).
2. Increase the temperature of the exhaust gases flowing into the particulate filter during regenerations. This occurs when the ATDC injected fuel contacts the catalytic coated ceramic (metal) grid of the oxidation catalytic converter. During a chemical reaction when HC and CO is converted to CO2 to H2O a large amount of heat is generated (about 450 °C in front of the converter and about 700 °C behind the converter).
Saturated particulate filter symptoms:
– increased fuel economy
– increased engine oil level caused by diesel fuel contamination due to ongoing unsuccessful regeneration attempts
– frequent regenerations
– frequent warning light illumination for high saturation levels of DPF filter
– engine limp-in home condition
– illumination of DPF filter warning light, sometimes together with the OBD MIL light
Main issues with particulate filters
– unsuitable vehicle usage driving short distances (e.g. permanent driving only in cities)
– deteriorated oxidation catalyst (doesn't generate the required temperatures during regeneration)
– using unsuitable chemicals (fuel additives, DPF chemical cleaners - forbidden)
– refuelling petrol insead of diesel (even 3-5 liters is dangerous)
– defective exhaust pressure and temperature sensors, intake valve closed position sensor
– excessive smoking engine (faulty injectors, intake system leak after the turbocharger, chiptuning, worn out engine
Logged DTC's:
– too high exhaust pressure
– particulate filter – soot (ash) high saturationl rate
– filter efficiency is below threshold
How to diagnose the system is described in article DPF filter – How to diagnose problems
Other problems caused by tampered systems
Modifications of the vehicle are always a problem. Removal of the DPF filter without reprogramming or using an emulator devastates the environment because regenerations occur every 800 km (time preventive regeneration excluding the exhaust system pressure value). Because the values in the control unit are shared with other control units and if a value is changed in one unit the formula will be incorrect and the calculated values will be wrong and can cause unpredictable behaviour. The results can be problems that are almost impossible to solve.
Chiptuning, odometer manipulation, filter removal and control unit reprogramming so that the particulate filter is not seen, including emulators of exhaust pressure and temperature signals can create great diagnostic problems on modern cars with future multiple DTC problems, that can't be corrected without returning the systems to their original condition.
The less complex systems on MY 2000 - 2006 vehicles usually worked when the filter was removed and with the modified software. Nevertheless these system can have problems with diagnostics (missing DTC's). On the most current systems the software is much more interconnected, so for instance odometer manipulation (illegal) can influence the DPF filter saturation level calculation so much that the system will report a full DPF filter after driving only several hundred meters. This can occur even when the exhaust pressure is not at the level of natural regeneration. On these modern systems it is almost impossible to remove the particulate filter without any negative influence on the vehicles behaviour. The particulate filter removal will at some stage show up, with the vehicle having problems like bad engine performance, MIL illumination or limp-in home mode with restricted power.
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