Diesel particulate filters will incinerate most soot but still need occasional maintenance to clear ash.
Next year, the Environmental Protection Agency’s emissions regulations will force truck diesels into the world of exhaust after-treatment for the first time. And unlike your car’s catalytic converter, diesel particulate filters are going to need some maintenance. We talked to the major engine manufacturers to find out what you should expect.
The diesel engineer’s biggest challenge in meeting the EPA’s new emissions standards is the “NOx/particulate tradeoff.” It’s easy to cut down on the soot and smoke that comes out of the exhaust pipe, and it’s also relatively easy to cure nitrogen-oxides or “NOx.” But it’s really hard to cure both at the same time, because the things that cure one make the other worse.
NOx had to drop so much for the last major change in the standards that engines now sport either water-cooled EGR or Caterpillar’s ACERT technology. In 2007, both emissions will drop – the NOx 55 percent to 2.5 grams per horsepower hour, and the particulate by 90 percent to 0.1 gram per horsepower hour.
To help solve the problem, the NOx will be reduced right in the cylinder, with still higher rates of EGR and a larger cooler. Cat will fine-tune ACERT and use a different form of EGR called “Clean Gas Induction.”
Exhaust gas, especially after it’s cooled, absorbs a tremendous amount of heat. Since NOx is the product of excessive temperature in the cylinder, recirculating exhaust, cooling it with engine coolant and adding it to what’s in the cylinder already reduces the combustion temperature and the NOx.
The problem with doing this is that the exhaust mixed in with the air gets in the way, so the fuel has a harder time finding the oxygen and burning. Engineers help the problem by making sure the engine gets as much air as it has in the past, in addition to the added exhaust. This means cranking up the turbo pressure so it will be even higher than in present engines. They have also redesigned the combustion chamber in the top of the piston to do an even better job of stirring up the air to help the oxygen and fuel find each other.
Still, with all the exhaust in the way and the particulate standard dropping down a notch, the only way to meet the 2007 regulations is to add a filter onto the exhaust system. This will be the “DPF” or diesel particulate filter.
How the DPF works
The DPF will consist of a mesh or grid made of a ceramic material called “cordierite.” It will have a honeycomb structure, but with some of the passages blocked off and small holes interconnecting those passages. This will cause the exhaust to weave through and drop its particles of carbon soot on the crossbars.
Ceramics are materials that can withstand high temperatures and don’t carry heat away like metal does. This is ideal because the soot must be kept hot for the DPF to work. Related piping and housings will be made of stainless steel, so truckers won’t be replacing those parts during a typical truck’s life.
The exhaust in a diesel has a lot of extra oxygen in it that has not been used up in the cylinders – diesel engines have used more air than the fuel ideally would need to burn for a long time, to help burn up the soot. This air comes in handy when the exhaust gets back to the DPF. The DPF grabs the soot out of the air and holds it while the exhaust, with unused oxygen in it, flows by.
Diesel exhaust gets pretty hot when the engine is under load because the burning fuel in the cylinders heats the air. As a result, under normal cruising conditions, with the engine working hard, the exhaust will heat the DPF and keep it hot. The DPF will be located a short distance from the exhaust manifold of the engine to retain exhaust heat.
Most of the engine manufacturers further the heating process by using a DOC, or diesel oxidation catalyst, located just in front of the DPF and part of the same assembly. The DOC contains a noble metal that speeds up burning, just like the catalytic converter in a car. It will accelerate combustion of the hydrocarbons and soot passing through, releasing energy and heating the exhaust beyond its normal temperature as it enters the DPF. The Cummins DPF will have a light coating of precious metals to enhance its performance, as will the Mack design.
In the up to 600-degree F heat and under the stream of exhaust and oxygen flowing past, the particulates burn off as fast as they can accumulate in the filter. This is called “passive regeneration” and will be the more important mode of cleaning the DPF on most trucks.
In the case of Caterpillar, rather than adding a DOC, the entire DPF will be impregnated with a catalyst material to heat up the air and accelerate combustion of the soot, probably because the exhaust in ACERT engines is cooler.
Either way, end of soot problem. Well, almost.
Low load regeneration
What happens when you are crawling through city streets or driving in heavy traffic? Under these conditions, the exhaust will be so cool that soot will not burn up in the DPF but will build up and begin clogging it. Accumulated soot would soon block the flow of exhaust through the DPF.
Sensors will measure “backpressure,” which is how much pressure the exhaust flow creates in the DPF. When soot builds up to the point where backpressure is higher than normal, a sophisticated system will start to pump and meter a tiny bit of finely atomized fuel in to heat the exhaust. Most of the engine makers will add the fuel right after the turbo. When the fine fuel mist and warm exhaust meet the DOC or catalyzed Caterpillar DPF, the fuel will ignite, heating the exhaust to almost 1,000 degrees F.
Under these light-load conditions, there is even more oxygen available to burn off soot, even allowing for the fuel the regeneration system adds. The combination of this hot exhaust and even more oxygen will cause accumulated soot to burn off much faster than it enters the DPF, effectively cleaning the soot out of it, resulting in “active regeneration.” You won’t have to worry about active regeneration or switch anything on to make it happen, though some trucks may activate a light to let you know it’s happening.
The amount of fuel required to accomplish this is so small that all the engine makers say you will find fuel economy to be the same as today in nearly all applications. Even when regeneration is needed frequently, the impact should be very small.
Caterpillar’s engineers tell us the system will monitor the regeneration event to make sure the DPF operates at the right temperature.
Detroit Diesel will add an intake throttle valve, which will accelerate regeneration by reducing airflow through the engine, thus increasing exhaust temperature with a minimum of added fuel.
In extreme light-load conditions where normal regeneration won’t keep up with the soot, a stationary regeneration process will be needed, says Liane Bilicki, senior communications manager at Detroit Diesel. The driver will have the option of stopping the vehicle for 20 minutes at a convenient time so ideal regeneration conditions can be created.
Clean Gas Induction
Caterpillar ACERT engines cool the intake air and favorably affect the combustion process, reducing NOx. But the 2007 standards are so tight, Cat will add a different form of EGR. Clean Gas Induction will pipe a smaller amount of exhaust than regular EGR systems back into the engine. CGI will not require an exhaust gas cooler on the engine, and it will take the exhaust from behind the DPF through a pipe integral with that assembly after removal of most of the soot.
DPF size and weight
The DPF will add from 50-60 pounds, depending on the engine size. The Cummins DPF will be about 12 inches in diameter and 3 feet long. Other DPFs will be about that size, though the length and width may vary. DPFs will be located either along the frame rails or at the bottom of the exhaust stack.
Volvo offers a “compact DPF” as its primary option. This mounts under the cab on the right side and allows a simple, vertical straight pipe that may allow the trailer to be moved closer for better fuel economy.
One thing that helps the size and weight situation is that the DPF filter is a lot like a muffler in the way it affects exhaust flow. This means it quiets the exhaust at the same time it’s removing soot. International’s Matthew Tyo, product engineer-aftertreatment development, says just designing the International DPF made the exhaust as quiet as it should be. Other DPF assemblies may incorporate design features that help the filter itself to reduce noise. But either way, no separate muffler will be needed.
Unfortunately, the DPF story doesn’t end there. The problem is ash in the exhaust, which does not burn but will accumulate slowly in the DPF.
Ash doesn’t come from fuel but from the engine oil. It’s there only because of TBN (Total Base Number) additives. These are antacids like Alka Seltzer that are in the oil only because the sulfur that’s naturally in fuel creates acids. TBN “neutralizes” the acids, or counters their effects, so they won’t destroy the engine’s internal parts. One thing that will help here is Ultra Low Sulfur Diesel.
Because sulfur can poison a DPF, the oil industry will be lowering the sulfur content of fuel to 15 parts per million before 2007 arrives (see Special Report on ULSD on page 12). This way, the DPF will likely last the life of the truck.
Removing more sulfur will make fuel cost a little more and slightly decrease energy content and miles per gallon, but it will help the ash situation. Less acid is good for the engine anyway.
The 2007 engines will also use a new grade of diesel engine oil called CJ-4. It will be a better oil in every respect, and it will contain much less ash because, with less sulfur, there will be much less acid in the oil for the TBN additives to neutralize. Since all highway trucks will be running the same ULSD, the oil will be “backward compatible,” working fine in older engines.
Even the small amount of ash left in the new oil will eventually leave a fine coating on the DPF. Backpressure will result, and since the ash won’t burn even when extremely hot, the unit will have to be cleaned. Those sensors will detect the gradual increase in backpressure and warn the driver when the unit has to be cleaned.
The EPA has protected the truck owner from excessive cleaning by requiring that it must be needed no more often than every 150,000 miles. Nearly all the engine makers estimate that under favorable operating conditions (trucks getting good fuel economy and not suffering from high oil consumption), the cleaning interval will be about twice that long.
Cleaning will almost always be done with high-velocity air under pressure. Air will be forced through the DPF from back to front, and the ash will be collected and disposed of. A specially designed, high-powered vacuum device is needed. Some of the devices may be available on the market and within the financial reach of small fleets.
The standard cleaning process will normally involve removing the DPF. Although heavy enough to require a floor jack for handling, most will be installed using band clamps that can quickly be removed, allowing the unit to be slid out. The cleaning will take as little time as an oil change on most vehicles, which means it probably won’t cost much more than a half hour’s labor at your nearest truck or engine dealer or repair shop.
Having aftertreatment for the first time on your truck should be a change you can handle without too much pain. Slodawske pointed out that it’s essential to use only CJ-4 oil, as this will make a significant contribution to minimizing ash in the exhaust and resultant cleanings.
Also, it’s more important than ever to take care of your engine because any abnormal increase in oil consumption would mean more ash and more frequent cleanings of the DPF.
The procedures and requirements for DPF cleaning differ by engine manufacturer.
Caterpillar: Caterpillar engineers told us, “The Caterpillar DPF is unique in that it can be cleaned without removal from the chassis. Technicians will just need to hook the Caterpillar DPF cleaner to the inlet and outlet of the DPF and turn it on.” The total time required for the service will be as long as needed for an oil change, and the two operations can be done simultaneously.
They add, “The Cat DPF cleaner can be purchased and can be used by a single individual. However, most users may not want to invest in a piece of equipment necessary only about every two years.” The Caterpillar target for cleaning intervals will be twice the EPA requirement for most applications.
Cummins: Company sources told us, “The ash cleaning machine runs on 110-volt power and uses standard shop air. Attachments will be available to fit every size of particulate filter from Cummins and most other engine manufacturers.” This will help control costs because the repair shop will need to buy only one cleaning machine.
The cleaning, “on average, should take less than 30 minutes. The total procedure should take 90 minutes or less, depending on how accessible the mounting of the DPF is on the vehicle.” It will be possible to exchange Cummins DPFs between vehicles or replace them with a Cummins Recon DPF.
Detroit Diesel: Detroit Diesel’s DDEC IV will predict when DPF cleaning will be needed, allowing the truck driver or owner to plan ahead for the job. Detroit’s studies showed the best cleaning would occur if the filter was removed, so the engineers designed the unit so removal and replacement would take only 30 minutes. Detroit requires that the DPF first be regenerated on the vehicle. Total air cleaning will take two to three hours. Detroit has also designed its DPF so it can be washed with a liquid. Remanufactured DPFs cleaned in this manner will be available as Detroit Diesel remanufactured parts.
International: Warren Slodawske, manager of emissions certification and compliance, says the International DPF will be cleaned at 150,000 miles or 4,500 hours, whichever comes first. It will be easy to remove – “Just loosen the mounting bolts and slide it out.” Cleaning with the proper equipment will be essential if you are to get the best possible fuel economy, so take it to the dealer. International has timed the introduction of the ProStar so that it coincides with the 2007 emissions change because the truck’s “modern aero design” should help ensure fuel economy will be at least as good as, if not better than, earlier Internationals.
Mack: Mack’s DPF will, like most of the others, have band clamps for quick removal. Dave McKenna, product manager-marketing, reports that cleaning should take less than 30 minutes. Mack’s system will trigger active regeneration automatically, but the driver will also be able to initiate it. A vertical back-of-cab system maximizes frame rail space, or a frame-mounted design maximizes back-of-cab space and helps the unit stay hot enough to regenerate passively when you run a sleeper.
Volvo: Volvo estimates that DPF cleaning will cost about $150 and says its dealers will be trained to perform the process. Its DPF can be serviced with a standard transmission jack, and customers with the proper tools and training will be able to service it.
For further information, please contact the following:
Detroit Diesel Corp.
International Truck and Engine Corp.
Mack Trucks, Inc.
Volvo Trucks of North America