Exhaust System Thermal Management: A Process to Optimize Exhaust Enthalpy for Cold Start Emissions Reduction

摘要

Future vehicle North American emissions standards (e.g., North American Tier 3 Bin 30 or LEVIII SULEV 30) require the exhaust catalyst to be greater than 80% efficient by 20 seconds after the engine has been started in the Federal Test Procedure. Turbocharged engines are especially challenged to deliver fast catalyst light-off since the presence of the turbocharger in the exhaust flow path significantly increases exhaust system heat losses. A solution to delivering cost effective SULEV 30 emissions in turbocharged engines is to achieve fast catalyst light-off by reducing exhaust system heat losses in cold start, without increasing catalyst thermal degradation during high load operation. A CAE methodology to assess the thermal performance of exhaust system hardware options, from the exhaust port to the catalyst brick face is described, which enables compliance with future emissions regulations. In addition to close-coupling of the catalyst, wastegate strategies, directed wastegate flow, engine displacement, and scroll configuration were studied with respect to the magnitude and preservation of exhaust enthalpy delivered to the catalyst face. The concept of total sensible enthalpy available at the catalyst face, and a measure of exhaust thermal efficiency is introduced as a means of quantifying the likelihood of successful catalyst light-off prior to vehicle drive-away. Boundary condition definition, numerical requirements, and correlation to measured data are discussed.