The development of technology suitable for meeting the CARB Ultra-low- emission-vehicle (ULEV) legislation has now become a main focus for vehicle manufacturers worldwide. This proliferation of interest is mainly a result of the increasing number of eastern US-states currently considering the adoption of CARB legislation and the indication that emission legislation in Europe and Japan for the turn of the century is likely to be of the same severity as CARB ULEV legislation.
Current three way catalyst (TWC) emissions control technology suffers from low catalytic conversion efficiency of HC, CO and NOx pollutants during cold operation i.e. before catalyst light off. Cold start emissions generally contribute up to 70% of HC and CO tailpipe emissions during an FTP test. However, in some cases even early light-off of the catalyst, similar to hot operation is not sufficient to achieve catalytic conversion over a test cycle to reach ULEV emissions levels. This depends on the base vehicle characteristics, the base emissions levels and the configuration of the exhaust and after-treatment sytems.
By adopting a “total system” approach to ULEV technology development, this paper demonstrates that there are promising routes to achieving ULEV levels even in difficult cases of medium-to-large high performance vehicles.
Exhaust system design with “thermal management” objectives, custom design of catalyst systems incorporating an electrically heated catalyst (EHC) and secondary air injection,with advanced air-fuel-ratio (AFR) control for optimum catalyst performance have been successfully combined using a European specification 3.0 litre V6 vehicle.