5 April 2000

Selective catalytic reduction (SCR) of NOx with urea has been successfully used to control NOx emissions from stationary sources. Currently, a lot of work is being done in Europe to adapt this technology to mobile diesel engines. Even though a number of problems still remain to be solved, notably urea distribution system and legal compliance issues, SCR catalysts are likely to be commercialized for Euro IV/V heavy-duty diesel engines.

Increased interest in the SCR technology can also be seen in the USA. Recently, Mack Trucks completed the first US urea-SCR demonstration program on a heavy-duty highway truck. The objective of the program was to evaluate the feasibility of SCR for meeting the anticipated US-2007 standard, which is expected to call for a 0.5 g/bhp-hr NOx limit from heavy-duty diesel engines.

The demonstration was conducted on a Class-8 truck weighing 16,500 lbs with a loaded trailer of 53,500 lbs, for a total GVW (gross vehicle weight) of 70,000 lbs, which is typical for US interstate carriers. The truck was equipped with a heavy-duty, 12 liter, 6 cylinder diesel engine rated for 350 bhp at 1800 rpm.

A Siemens SINOx SCR catalyst system including 63-liter catalyst, an on-board urea solution tank, and a control system were installed on the vehicle. Test cell evaluations using an improved 45 liter catalyst were also conducted. The test truck is currently being upgraded with this new catalyst for further over-the-road evaluations.

The truck was operated on US highways for a test period of 1200 miles. NOx emissions were tested both over-the-road, utilizing a prototype NOx sensor, and on an engine test bench. Over-the-road measurements showed an average NOx reduction of 65%. Engine tests on the FTP and ESC cycles showed NOx reductions between 55 and 85%. NOx levels of 1.55 g/bhp-hr were measured on the hot FTP test with the SCR system calibrated for conservative urea dosing levels to minimize ammonia slip.

The results of the project were presented during the SAE Congress in Detroit last month and have been published as an SAE paper #2000-01-0190.

The SCR system will be optimized for improved NOx performance. An extended, 10 truck SCR program, recently approved by the EPA under the Consent Decree, is expected to be launched by Mack later this year.

For more information contact Will Miller, will.miller@macktrucks.com.