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.