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In TECHMAIL, we answer your motoring queries…
Please explain the difference between a diesel-particulate filter (DPF) and a catalytic converter. Is the DPF intended for diesel engines only and the catalytic converter for petrol engines, or are they compatible with both?
My son has just experienced a costly exercise with his diesel Honda CR-V that saw the car lose power. The DPF had to be cleaned/removed at a cost of around R8 000 and is not covered in the service plan or warranty. Are all cars now fitted with these filters, or only certain cars?
I will definitely be asking questions around guarantees for these filters when looking for another vehicle. Can you get throwaway-type paper filters?
Brian Carter – Hillcrest, Kwazulu-Natal
As the name suggests, the diesel-particulate filter catches soot particles from diesel combustion. Catalysts are used for lowering harmful emissions gases and are used on both petrol and diesel engines. In Europe, current Euro6 emissions standards are strict on particulate emissions, requiring all diesel passenger vehicles sold there to be fitted with DPFs. South Africa is only at Euro2 emissions level, so a DPF is not necessary.
Many diesel vehicles sold here are therefore not fitted with a DPF. However, many imported vehicles that do not have the option of DPF deletion will consequently come with one.
The unit goes through a cleaning cycle roughly every 600 to 1 000 km depending on driving style. In this process, the normal 400 degrees Celsius exhaust temperature is raised to around 600 degrees Celsius (with additional diesel fuel by post-injection) to burn the captured soot and clean the filter.
It’s possible for the filter to become full without a chance to clean itself if the vehicle is used only in stop-start conditions and the engine never gets up to temperature. The ECU measures the state of the DPF with a pressure sensor pre-and-post the DPF and a level is set for regeneration to start. When this level is exceeded (no regeneration could take place), a warning light on the instrument cluster informs the pilot to drive on a motorway to get the engine up to ideal operating temperature so that the filter can be cleaned.
If the driver ignores this and the filter exceeds the safe limit, reduced-engine-power mode (“limp-home mode”) is implemented to encourage the owner to take the vehicle to a dealer. There, the filter can be cleaned with a service tool plugged into the vehicle. The Honda will be static, but the engine speed will be raised and a regeneration operation can commence.
The highest soot level means the filter is completely blocked and it won’t be able to regenerate; a new filter needs to be fitted.
Unfortunately, the DPF is a canned ceramic-type brick that looks like a catalyst, so no throwaway option exists.