Cummins Inc. says it will use selective catalytic reduction on its 2010-model heavy duty engines, reversing a previous announcement that it would avoid the urea-dependent aftertreatment by using enhanced exhaust-gas recirculation. Cummins executives said SCR will deliver a 3 percent to 5 percent fuel economy improvement while helping to clean the exhaust of its ISX diesels to required levels.

The effectiveness of Cummins' version of SCR means it no longer needs a 16-liter version of its ISX to cover high-horsepower ratings, executives said during a phone-in press conference Aug. 13. Starting in January 2010, when the next round of federal limits on diesel exhaust takes effect, the heavy duty range will include a 15-liter ISX, similar to the current product, and a new 11.9-liter version, which will replace the 11-liter ISM. Both heavy and medium engines will use a common electronic control module and software, which will enable adoption of common on-board diagnostics, another government mandate.

In its announcement one year ago, Cummins said it would use SCR in its medium duty diesels because they run mostly locally and refilling their urea tanks would not be a problem. Future availability of the solution (also called diesel exhaust fluid) along rural routes was questionable at that time, so using enhanced EGR for its heavy over-the-road engines seemed a better idea. Since then, much progress has been made at planning a urea-distribution infrastructure in North America, reducing users' concerns.

But SCR's ability to improve fuel economy, especially at today's high fuel prices, was the most compelling reason for the turnabout, executives said.

Cummins thus joins Detroit Diesel, owned by Daimler Trucks North America, and Volvo Powertrain, which produces engines for Volvo and Mack, in choosing SCR as the way to meet 2010 diesel exhaust limits, which reduce oxides of nitrogen (NOx) levels to virtually zero. Only Navistar International will avoid SCR on both its medium duty and upcoming big-bore diesels, and rely instead on advanced combustion and fuel systems. It's not yet clear which path will be taken by Paccar, which will begin building its own heavy diesels next year.

Cummins said it will use a special catalyst with a new material to cleanse NOx from the exhaust stream. The material, copper zeolite, naturally forms microscopic tunnels that help trap pollutants. It is "very, very efficient at reducing NOx, more so than any other material we've tried in recent years," said Steve Charlton, Cummins' vice president, heavy duty engineering. It is used as a filtering material in the medical field and became available for automotive applications about a year ago.

"We have some pretty extensive SCR experience, with 200,000 units in use in Europe," Charlton said to reassure any wary customers. Cummins Environmental Solutions, which makes the European NOx catalysts using a different material, will also make the North American catalyst. It will have a honeycomb substrate coated with copper zeolite. Cummins holds no patent on the material's use and other builders could use it if they wish, he said.

Cummins will still use cooled EGR on its 2010 engines as it does now, he said, but "we'll dial it back to a lower rate" because of the copper zeolite catalyst's effectiveness. Less exhaust gas entering the cylinders not only allows more efficient combustion, but also lowers the amount of heat that has to be transferred to and dissipated by the truck's radiator, which simplifies the cooling system. An XPI common-rail fuel system, developed with Scania of Sweden, is another feature of the '10-model diesels.

Increasing fuel economy compared to today's levels will save the typical over-the-road customer thousands of dollars a year, said Jeff Jones, vice president, sales and market communications. Economy improves because the urea solution effectively displaces diesel fuel at the rate of about 2 percent. Although urea's price has almost doubled in Europe, where SCR is widely used, it is still less than diesel, he said. And even if the two were priced the same, and if diesel prices were to fall to pre-crisis levels, the fuel economy improvements would still be a plus.

Topping off a 25-gallon urea tank on an over-the-road heavy tractor should be required about once every two to three weeks, Charlton estimated. Cummins will follow Environmental Protection Agency-prescribed procedures that call for a warning lamp on the dash to illuminate when the urea level reaches 25 percent. Then, as the fluid level drops further, an audible alarm will sound and finally a power derate will occur. (A complete engine shutdown will not be required because it could strand a truck in a travel lane, threatening safety.)

Cummins will use some EPA credits to certify high-mileage ratings of the ISX with more economical performance by means of minutely dirtier, but still legal, exhaust. Credits are being earned by the cleaner-than-required performance of the ISB-based Cummins Turbo Diesel it makes for Dodge Truck; that engine already meets 2010 limits.

Cummins' urea system will consume some of a truck's frame space and weigh about 150 pounds, executives acknowledged. But some of the weight will be offset by a lighter fuel system and cylinder head for the ISX. They would not discuss how much more a 2010 engine will cost, saying prices are set by truck manufacturers.