The perils of DPF cleaning

An unusual failure at ADS serves as a warning to all

Published:  26 November, 2012

MOST OF the time, I find myself writing about success in diagnosing and repairing vehicle faults. With this in mind, I thought it would make a change to discuss a case of failure.

We base our success on experience gained from our willingness to adopt new products and evolution of our techniques. So when presented with a new product from an automotive chemical firm, we had no hesitation in trying it out. The product is designed to treat blocked diesel particulate filters (DPFs). To apply it, you remove the lambda sensor and spray the aerosol directly into the leading edge of the DPF filter housing.

Let's begin with an overview of the need for DPF filtration, current legislation and problems with DPFs that we all know and love. We all know that a DPF's main job is to stop tiny pieces of un-burnt carbon from entering the atmosphere. There are two basic means of improving the combustion of the soot through the DPF system; namely, additive systems where urea is added into the exhaust stream or thermal systems, where an increase in exit temperature is used to burn off residual carbon from within the DPF substrate.

Excessive soot

So what causes excessive soot within the DPF? Can it be incomplete combustion and nothing else? Once excessive soot is produced there are collateral problems that increase the effects of a blocked DPF and cause premature failure. The EGR valve can fail and there can be problems with the intake swirl flap and turbocharger - this brings me nicely to the subject of this month's article.

A BMW was presented to our workshop with restricted performance. An initial examination confirmed a blocked DPF. The extent of the blockage meant we could only treat it chemically.

As previously stated, it is imperative to ensure all critical components function correctly, with this in mind, the EGR valve (which was sticking), the lambda sensor and temperature sensors were pre-cleaned prior to the application of the DPF cleaner directly into the casing, as per instructions.

First, we allowed the chemical product to treat the soot prior to a regeneration drive cycle. We then used the Autologic platform to instigate the regeneration process - of course, this cannot under any circumstances be conducted with outstanding DTCs or system errors.

With the lambda sensor replaced and vehicle at normal operating temperature, my son David took the vehicle on the mandatory drive cycle, all the time monitoring data via the serial platform.

Having covered only a short urban drive cycle, David observed burning embers on the road surface in the rear view mirror - hardly ideal. Returning to the workshop promptly, first indications appeared well, the DPF sensor value had reduced considerably and power had improved too. However, exhaust fumes within the driving compartment suggested that all was not well. Getting the vehicle in the air and examining the DPF casing exposed a critical and worrying condition. During the process of burning off carbon from within the filter, temperatures had reached a critical level... sufficient to actually melt the DPF casing!

Where does this leave us? We thoroughly reviewed our process - nothing had changed from many previous successful regeneration procedures except the application of a direct additive product.

Unlike the previous successful use of intake systems cleaning, where complete control of both function and result is possible, this particular method once applied has no predictability.

The obvious conclusion to which, excluding total thermal failure as with our BMW, is that this method threatens the integrity of the DPF substrate with no means of warning.

Further information

aftermarketonline.net/technical

New laws will change the regeneration game

It is likely manufacturers will adopt additive treatment systems given the current problems in the use of diesel powered vehicles in unfavourable conditions. These include poor and inappropriate servicing regimes, low quality oils and short journey drive cycles.

Additionally, the mechanical functionality of the engine is vital. Correct soot removal from the DPF requires all diesel system components to operate to exact specifications.

Types of regeneration

What are the technical options available to regenerate the DPF filter?

Passive regeneration

During suitable driving conditions, the exit exhaust gas temperature will reach up to 500ºC. This will burn off low and moderate soot deposits without any internal or external stimulus.

Active regeneration

Stimulated by an increase in back pressure, mileage or time driven since last DPF regeneration, a series of active events will take place the outcome of which is designed to increase the exhaust gas temperature even further. We have seen this rise to 800ºC during serial data logging.

Forced regeneration

This process is only available through serial intervention and only then if a strict series of parameters are met. These include all of the previous mentioned components.

Additive treatments

These fall into three categories:

? Fuel tank additives which promote a reduction in the temperature at which carbon can be passively burned off

? Induction additives such as BG products which are injected directly into the air intake system whilst the engine is running. The advantage of this method is control of process and it completes treatment of the entire intake and exhaust system

? Direct additives act as an initiator, creating an environment that promotes an increase in the combustibility of the soot deposits. The problem with this method lies within a total lack of control and predictable reaction once applied.

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