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What the heck is a DPF?

By Paul Abelson
senior technical editor

It seems like every time the Environmental Protection Agency comes out with new emissions reduction mandates, truckers have to add more letters to their alphabet soup vocabulary.

The latest addition to the glossary of acronyms is DPF – short for diesel particulate filter. That’s all well and good, but what the heck is that thing?

The short answer is: When it comes to DPFs for heavy-duty trucks it depends on who you ask.

In order to meet the EPA’s 2007 emissions standards that call for a steep reduction in the amount of particulate matter – which is primarily soot and ash – in the exhaust, all engine makers are now using some sort of after-treatment system.

In the heavy truck industry, the entire system burns the soot out of the exhaust and captures the ash particles in the diesel particulate filter.

But some engine makers are actually calling the entire after-treatment system the DPF – such as Volvo’s system, which is called a Compact Diesel Particulate Filter. And, the use of DPF as the name for the whole after-treatment system is gaining in popularity.

Many questions about what the “DPF” actually does, however, are really questions about the entire after-treatment system.

There are generally two common major parts to the after-treatment systems.

First, exhaust passes over a diesel oxidation catalyst. It creates heat to oxidize the carbon soot into carbon dioxide. As a gas, it can pass through the diesel particulate filter.

The second part is a porous ceramic filter – the actual diesel particulate filter. It’s porous to let gases pass through while trapping any unburned soot and particles of ash. It’s ceramic to withstand the high temperatures necessary during regeneration.

Regeneration, simply put, is the cleansing process for the DPF. To regenerate, fuel is injected into the diesel oxidation catalyst portion of the after-treatment system, which heats it and the filter to very high temperatures – for example, as much as 1,100 degrees in the Detroit Diesel after-treatment system. The heat converts the built-up soot to carbon dioxide, clearing the passageways of everything but ash.

“Active regeneration” is the cleaning of the filter on the fly – not to be done while sitting still because of the heat involved. However, if this process is shut down before completion too many times, or not done often enough, some systems, like Detroit Diesel’s after-treatment system, may call for a “stationary regeneration” which takes about 20 minutes.

No matter how many times regeneration – or cleaning – takes place, the DPF portion of the after-treatment system will eventually need to be cleaned. Cleaning out the ash is done when the truck is out of service. Depending on the truck and how it’s used, the filters will need to be cleaned every 200,000 to 400,000 miles.

However, it is important to note, once again, that the term DPF is used too generically and inconsistently within the industry. When purchasing a 2007 and later model truck, you need to familiarize yourself with the after-treatment system and its individual components as well as its care and maintenance.

And, for crying out loud, when a shop tells you that your DPF needs to be replaced, find out for sure if it is just the filter or the entire after-treatment system. There’s sure to be a huge cost difference in the two.

If a shop tells you it needs to be replaced, find out what part and why. The diesel oxidation catalyst will need replacing only if it has become contaminated with higher sulfur fuel – for example, using low-sulfur fuel instead of ULSD.

The ceramic element in the actual DPF will need replacing only if it has cracked. Make sure the assembly is never dropped. When the actual ceramic filter needs to have the ash removed, some dealers may exchange elements rather than have you wait while your filter is cleaned out. Other than that, there should be no need to replace the element for more than a million miles.

Paul Abelson may be reached at