(244d) Low-Temperature Emission Control: Challenges and Aftertreatment Options

The next round of automobile emission regulations would mandate a drastic reduction of hydrocarbon (HC) and nitrogen oxide (NOx) exhaust emissions.  Since a large fraction of the total exhaust emissions of HC and NOx occurs during cold-start periods (i.e., a few minutes after an engine cold start), it is critically important to effectively control exhaust emissions at low temperatures in order to meet the stringent future emission control requirements.  The introduction of fuel-saving technologies, such as lean burn, turbocharging and advanced combustion, generally lowers exhaust temperatures, making low-temperature emission control even more challenging.  In addition to developing low-temperature catalysts, recent efforts have also been directed toward cold-start emission control strategies which involve combining storage function with catalytic conversion and electrical heating of the catalyst.  Although improved low-temperature catalytic activity can be obtained by increasing noble metal content in current catalytic converters, this approach can lead to providing only diminishing emission benefits at excessively high catalyst costs, highlighting the need for breakthrough catalyst technology development.  Current HC/NOx adsorber systems can store HC & NOx reasonably well at low temperatures, but exhibit a tendency to release these stored species before the catalyst becomes fully operational in converting them into harmless species.  Thus, it is necessary to discover/develop storage materials with higher release temperatures and/or catalysts with lower light-off temperatures to improve the effectiveness of adsorber systems.  Previous studies on electrically-heated catalyst (EHC) technology have demonstrated significant promise in the control of cold-start emissions from conventional gasoline engines, and its potential applications to diesel engines and hybrid vehicles present new challenges and opportunities.  This presentation will discuss in more detail the capabilities and limitations of each of these advanced cold-start emission control technologies as well as the critical issues that need to be resolved for their successful implementation on future vehicles.