Burner-assisted DPF overcomes cold-temp, [NO.sub.2] limitations

Diesel Fuel News, Nov 11, 2002 by Jack Peckham

San Diego--A fuel-burner-assisted system for regenerating a diesel particulate filter (DPF) not only overcomes problems with "too cold" duty cycles for "passive" DPF systems, but also avoids "[NO.sub.2] slip" problems with systems using nitrogen oxides (NOx) to regenerate catalyzed DPFs.

The system, first developed in the 1990's in Germany by ArvinMeritor partner Zeuna Staerker for low-temp/low-load applications such as garbage trucks and street sweepers, might enjoy a renaissance due to the growing need for DPFs in wider diesel applications. Both retrofit and original equipment maker (OEM) applications are seen.

Example: Euromot and Engine Manufacturers Association (EMA) just released a research report (see Diesel Fuel News 9/30/02, p7) that terms burner-assist as the "ideal" system universally applicable for non-road engines that can't be 100% assured of high-enough-temperature duty cycles for "passive" DPF regeneration.

Now, Zeuna Staerker and ArvinMeritor see potential for the burner-assisted DPF to compete with "passive" DPFs not only on total life-cycle system costs, but also for potential integration with NOx traps.

The system taps a small amount of compressed air typically found on heavy-duty diesel vehicles (for air brakes) to atomize diesel fuel for the burner. Additional air is injected via a pump to ensure soot-free, oxygen-rich combustion in a burner just upstream of a DPF.

A carefully programmed electronic control unit (ECU) and pulse-width modulation ensure the correct temperature window for DPF soot oxidation, while avoiding runaway exotherms that could crack or melt a DPF. This is achieved "irrespective of the exhaust gas temperature and the delivery rate" of exhaust through the DPF, the researchers explained in a paper (SAE 2002-01-2787) to Society of Automotive Engineers Powertrain & Fluids conference here.

This scheme also allows the use of a lower-cost cordierite DPF, described here as a "robust cordierite" variant that's "more exotherm-tolerant."

In tests for the "VERT" DPF certification process (see Diesel Fuel News 10/28/02, p15), the system developers put this new cordierite DPF under "worst-case" conditions at idle, when low exhaust flow minimizes heat transfer from a DPF undergoing soot oxidation.

The burner control program can be altered to dampen such exotherms by reducing fuel injection quantity.

The system also employs backpressure sensors to avoid soot over-loading. A fast heat-up rate allows DPF regenerations of only a few minutes, rather than hours as can occur with "passive" catalyzed soot filters. Resulting fuel penalty is typically less than 2%.

Recent initiatives to reduce total system cost include moving the ECU development work in-house, slashing the control-box size, and employing a relatively low-cost cordierite DPF substrate.

Fleet testing of the latest burner-DPF system is now in progress in Europe and further tests are now being planned for North America. VERT certification of the product is expected early in 2003.

Future research on possible combined burner-DPF/NOx trap systems are under investigation.