The U.S. Tier 2 emission regulations require sophisticated exhaust aftertreatment technologies for diesel engines. One of the projects under the U.S. Department of Energy's (DOE's) Advanced Petroleum Based Fuels - Diesel Emission Controls (APBF-DEC) activity focused on the development of a light-duty passenger car with an integrated NOx (oxides of nitrogen) adsorber catalyst (NAC) and diesel particle filter (DPF) technology. Vehicle emissions tests on this platform showed the great potential of the system, achieving the Tier 2 Bin 5 emission standards with new, but degreened emission control systems. The platform development and control strategies for this project were presented in 2004-01-0581 [1].
The main disadvantage of the NOx adsorber technology is its susceptibility to sulfur poisoning. The fuel- and lubrication oil-borne sulfur is converted into sulfur dioxide (SO2) in the combustion process and is adsorbed by the active sites of the NAC. As a result, they are unavailable for NOx storage. This leads to a drop in system performance. In order to recover the active sites, the bound sulfates need to be released. The necessary conditions for sulfate release are relatively high temperatures in combination with rich exhaust gas conditions. This process is often referred to as desulfurization.
After the completion of the development phase, the emission control system was aged with 15-ppm sulfur diesel fuel to a full useful life (120,000 miles) condition. During the course of this aging, intermittent desulfurizations were carried out. The desulfurizations were preceded and followed by evaluation procedures to allow a detailed analysis of the desulfurization effects.
Desulfurizations were performed every 150 hours up to 300 hours, every 100 hours up to 1,600 hours, and every 50 hours to the end of aging at 2,200 hours. This paper analyzes the effects on gaseous emissions, before and after desulfurization, over transient driving cycles such as the light-duty Federal Test Procedure (FTP-75).
The results indicate a high degree of recovery, maintaining a low level of tailpipe emissions over the useful lifetime (120,000 miles) of the emission control system.