Feel the burn

| July 02, 2006

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.

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