4WD MODIFICATIONS - POWERTRAIN
A diesel particulate filter (DPF) is part of a diesel 4WD’s exhaust system. We all know about DPF regeneration cycles, but what’s not widely understood is that deeper cleaning is periodically necessary for all DPFs.
A DPF is fitted to nearly every modern diesel engine, regardless of size. We’ve explained in Tech Torque that the function of a DPF is to trap engine combustion soot particles that would otherwise be emitted into the atmosphere.
Obviously, this trapped soot needs to be eliminated from the filter and that’s done automatically by the 4WD’s inbuilt DPF regeneration system that literally burns the soot out of the ceramic filter channels.
However, as any of us with a home fireplace know only too well, even an efficient slow-combustion fire produces plenty of soot. It also suffers from a buildup of non-combustible, pinkish-white ash that needs to be physically removed. A 4WD DPF is no different.
Ash accumulates in a DPF over extended use and progressively occupies a large portion of the filter volume, usually lining the filter channels and plugging the ends. There’s then less space for soot accumulation, reduced gas flow through the filter medium and restriction in the exhaust system.
That restriction increases fuel consumption, requiring more fuel injection for a given output and also forces the engine into more regeneration cycles that also use more fuel. The likely fuel consumption penalty may be around 10-percent.
In extreme ash-clogging cases, the ash becomes the primary filtering substrate! If the DPF is a ‘catalysing’ type, the ash effectively insulates the emission-reducing catalyst that coats the ceramic tubes and it can’t do its intended job.
Engines with high oil consumption have accelerated amounts of DPF clogging, in terms of ash and soot.
Off-vehicle DPF cleaning
DPF ash removal frequency depends on DPF type and engine vocation. Linehaul trucks operating at high engine loads may not need DPF ash cleaning before 600,000km or more, while a lightly-loaded, stop-start tradie’s ute may have a clogged DPF in as little as 90,000km.
The best way to remove ash from a DPF is by approved off-vehicle licensing processes, operated by specialist companies who have invested in purpose-designed equipment. DPFs are removed and sent off to these companies for cleaning.
The thermal cleaning process is commonly referred to as ‘bake and blow’ procedure that involves baking the DPF filter in an oven, to oxidise any remaining soot and using compressed air to force the ash out of the filter.
The ‘aqueous’ process uses a surfactant to surround the ash particles and enable water flow to wash the ash out of the substrate of the DPF. The DPF is then dried in a special drying cabinet.
Other methods include an ultrasonic technique, where the DPF is dipped in a high temperature water tank and ultrasonically generated micro-bubbles combine with high-pressure to dislodge soot and ash. However, there is a risk of damaging the filter through these generated vibrations.
Regardless of the cleaning method, the procedure should begin with weighing the DPF. Experienced cleaning companies will know from experience the degree of ash buildup, but it also provides evidence of how much residue gets removed during the cleaning process.
The next step is inspection of the DPF for obvious mechanical damage that may sideline it. If the casing is intact, it should then be tested for rate of flow, using a meter and examined internally with a probe light.
Some cleaning companies that deal mainly with small DPFs cut the casing open to check the condition of the substrate, rather than wasting time trying to get a small probe inside. The cut casing is TIG-welded following the cleaning procedure.
Inspection may reveal that the DPF core is melted or cracked, or that it’s irrevocably blocked, due to poor engine maintenance or ignored DPF warning indicators.
A reusable DPF is then cleaned, using one of the above procedures, before a final inspection, flow-rate testing and clean weight measurement.
On-vehicle DPF cleaning
The internet is chock-full of on-vehicle DPF cleaning procedures, but the heath and environmental risks of DIY cleaning mean that major companies just don’t do it. There are literally dozens of DPF cleaning products in the marketplace, either those that are added to the fuel tank or those that are sprayed into the DPF directly.
Some of these compounds are toxic, so great care is ended when handling them. Also, the waste products from the DPF clean-out process should be disposed of as carefully as any other hazardous waste.
Invariably, these chemicals claim to eradicate built-up soot and none that we could find were aimed at removing ash buildup. If engine maintenance is kept up and regen is done when indicated, your DPF shouldn’t have excessive soot buildup in the first place. But even then, periodic ash removal will be necessary and spray-in chemicals and fuel-tank additives can’t do that.
A sure-fire way to optimise DPF life is to use the right oil for your engine, to ensure it produces as few particles as possible.
A clogged or faulty EGR valve can cause it to remain open for longer periods of time, increasing the soot amount that is fed back into the engine and into the DPF.
Unfortunately, a DPF is a legal necessity, to reduce harmful emissions, but given the necessary regeneration, ash removal and eventual replacement cost, it’s understandable that many people illegally delete DPFs. However, there are hefty fines for doing so.