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The Hot, Hot truth about DPFs – Diesel Particulate Emissions


Over the last couple of years there has been talk of increased emissions for diesels. CARB mandated new legislation in 2007, now in effect, making all diesel manufacturers responsible for “cleaning the air”

Bob Greenwood


This means no more black smoke and “soot” from exhaust systems. One of the methods to reduce the harmful emissions from exhaust diesel is to eliminate particulate matter by installing a type of particulate filter (DPF) to catch the soot as the engine runs. These filters will be as commonplace in diesel exhausts as catalytic converters are on gasoline-powered cars. The goal is to reduce diesel particulate emissions to about 80% but the technology’s not without problems. Trucks that are equipped with a DPF are required to be operated in certain conditions to clean the filter using a process called regeneration or “regen” for short. This regen process will heat the filter to a point that the soot from the diesel fuel is burned. If the filter becomes blocked because the regen process cannot burn off the soot on the filter, a warning lamp can illuminate and in more severe conditions, cause the vehicle to “shut down” due to a blockage in the exhaust.

How do they work?

DPFs or ‘traps’ do just that. They catch bits of soot in the exhaust. As with any filter (think of the bag in your vacuum cleaner) they have to be emptied regularly to maintain performance. OTC offers a solution for cleaning filters off the vehicle. You can view their video at www.otctools.com for more complete information. DPF that takes care of the soot while driving is regenerated – the accumulated soot is burnt off at high temperature to leave only a tiny ash residue. Regeneration may be either passive or active.

Passive regeneration

Passive regeneration takes place automatically on highways or extended drives. It runs when the exhaust temperature is high. Many trucks don’t get this sort of use, so manufacturers have to design-in an ‘active’ regeneration where the engine management computer (ECU) takes control of the process and forces a regeneration process.

 

Peugeot accepted a new technological challenge: to race the Le Mans 24 Hours, with a car powered by an HDi diesel engine equipped with a diesel particulate filter system (DPFS)

 

Active regeneration

When the soot loading in the filter reaches a set limit (about 45%) the ECU can make small adjustments to the fuel injection timing to increase the exhaust temperature and initiate regeneration. If the driver’s style is a stop/start and there is little chance for a cycle of regeneration the warning light will illuminate. It should be possible to start a complete regeneration and clear the warning light simply by driving for 10 minutes or so at speeds greater than 60 kph.

If you ignore the warning light and keep driving in a relatively slow stop/start pattern, particulate soot loading will continue to build up until around 75% when you can expect to see other warning lights illuminate too. At this point driving at speed alone will not be sufficient and the vehicle will have to go to a dealer for regeneration. This can be expensive and with some cases a tow will be required.
If warnings are still ignored (or bypassed) and soot loading continues to increase then the most likely outcome will be a complete new DP filter or engine damage will be the result.

How it works

How does the computer know when to start the regeneration process? Typically a computer monitors one or more sensors that measure back pressure and/or temperature, and based on pre-programmed set points the computer makes decisions on when to activate the regeneration cycle. The additional fuel can be supplied by a metering pump. This extra fuel is used to “light” the catalyst and the heat generated is used to burn the soot. Running a cycle too often will keep the back pressure in the exhaust system low, which is good but will use extra fuel to do this. The opposite is also possible. Potential engine damage and/or uncontrolled regeneration and possible DPF failure could be caused by regeneration that does not happen often enough. Quality regeneration software is a necessity for longevity of the DPF system.

Diesel particulate matter combusts when temperatures above 600 degrees C are reached. This temperature can be reduced to somewhere in the range of 350 to 450 degrees by use of a fuel- borne catalyst. The actual temperature of soot burn-out will depend on the chemistry employed. The start of combustion causes a further increase in temperature.

In some cases, in the absence of a fuel-borne catalyst, the combustion of the particulate matter can raise temperatures above the structural integrity threshold of the filter material, which can cause catastrophic failure of the substrate and destroy the DPF. Various strategies have been developed to limit this possibility.
A gasoline engine after combustion will have typically less than 0.5% oxygen in the exhaust gas stream before the exhaust emission control device(s). Many diesel engines run above 15% oxygen before the DPF. While increased amount of available oxygen makes for a fast regeneration of a filter possible, it also can contribute to a runaway regeneration problem (blowtorch effect). The most common type of DPF is within an integrated oxidizing catalytic converter that is located very close to the engine where exhaust gases will still be relatively hot, so that passive regeneration is possible.

There’s not always space close to the engine for the DPF so some manufacturers use a different type of DPF which relies on a fuel additive to lower the ignition temperature of the soot particles so that the DPF can be located further from the engine. The additive is stored in a separate tank and is automatically mixed with the fuel whenever you fill up. Tiny quantities are required, so a liter of additive should treat around 2800 liters of fuel, enough to cover 20,000 km. With this type of DPF, regeneration can be initiated by the ECU about every 400 km, (depending on vehicle use) and will take 5-to-10 minutes to complete. You shouldn’t notice anything other than perhaps a puff of white smoke from the exhaust when the process is completed.

Nova Scotia Community College is continuing its work with DPF systems and other after- treatment systems for diesels that are available on OE vehicles. If you would like more information on DPF training or information with these systems please contact NSCC, Transportation Center at 1 902 491 4933. If you wish to register for the Spring 2010 program or require further information, contact NSCC’s Annette Cooper or Dave Giles at 902 430 2951.

Dave Giles is a technical trainer with Nova Scotia Community College (NSCC) and facilitates high tech training sessions for technicians throughout Atlantic Canada. To find out more about a technical training session in your area please contact Dave at NSCC directly at 902-430 2951, or Email him here

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