Diesel particulate filter regeneration: what Cummins is learning in field tests

Diesel Fuel News, Oct 28, 2002 by Jack Peckham

San Diego -- Diesel particulate filters (DPFs) might work "passively" in some applications, but growing field experience shows that many DPFs will need sophisticated engine management or other schemes to avoid failures, Cummins field experience shows.

Example: In tests with 17 "passive" DPFs in a New Jersey Transit (NJT) test program, five cracked or melted, seven failed emissions requirements and seven caused excess backpressure to the engine, Cummins engineer Bruce Bunting explained to the Diesel Engine Emissions Reduction (DEER) workshop here.

Data-logging for a separate project for the U.S. Navy likewise showed that only 30% of vehicle applications would allow DPFs to regenerate passively, and a New York City Transit project with diesel-electric hybrids also showed the need for special heat-boosting measures to ensure DPF regeneration.

"Cold," low-load operations cause excessive soot loading in a DPF. If followed by high-load operation, "uncontrolled" regeneration can crack or melt a DPF.

For the Navy project -- all Thomas school buses with different engines -- data logging indicated that typical duty cycles should have allowed for passive regen, with exhaust temps at 400[degrees]C 10% of the time. Yet in its field tests, Cummins found excessive backpressure, Bunting said.

More issues: DPFs not only must be designed to be able to regenerate in any application, regardless of duty cycle, but also provide "self-monitoring" for problems or service. These systems also must be "easily applied to any application with a minimum of special mapping and test work," he said.

Among the possibilities for fail-safe regen are diesel fuel injection into an upstream catalyst (prior to DPF); injection into an upstream fuel burner to boost DPF temps; DPF electrical heater schemes; an exhaust brake scheme; and engine management (including post-main injection, timing, air/fuel ratio changes, exhaust gas recirculation) combined with an upstream pre-catalyst.

Using an upstream catalyst to oxidize NO to [NO.sub.2] (for soot oxidation) can employ both "passive" and engine management schemes in an exhaust temperature range of about 200 to 400[degrees]C, Bunting showed. By contrast, "catalytically enhanced oxidation" could work at temperatures over 300[degrees]C, while "direct oxidation" (using hydrocarbon injection) works above 450[degrees]C, he showed.

Given the advantage of wide effective temperature range, the engine management/upstream catalyst combo seems to be a logical path for future development. However, integration of combined PM/NOx reduction systems could lead to even more clever schemes, and certain diesel applications might favor special technologies.

Example: In a separate investigation, Cummins tested the Rypos electrically-regenerated soot filter for a gen-set application. This four-month operations test involved two 72kW mobile generator-sets at Edwards Air Force base, with emissions tests by University of Utah.

The Rypos system includes four uncatalyzed sintered metal fiber cartridge elements housed in a metal can. Electric heat regenerates the cartridges sequentially. Electric power for the oxidation conveniently came from the Cummins B9 185 horsepower gen-set itself, although secondary (or extra-large) alternator power could be used in a mobile application.

This first-generation system only filtered 62% of particulates smaller than 10 microns, as it suffered from gasket leaks and a failed cartridge. This has led to an improved design. An "overly aggressive regeneration strategy" also may have penalized system efficiency, he said. So, Cummins aims to test the second-generation system for PM reduction performance and energy efficiency.