| Although recent progress in vehicle technology and regulation has improved the environmental performance of new model vehicles, the continuing use of old, high-polluting vehicles contributes to air quality issues. While scrappage programs attempt to reduce emissions from old, high-emitting vehicles, life cycle assessment (LCA) studies show that scrapping old vehicles and manufacturing new vehicles also account for significant life cycle emissions. The expected median lifetime of automobiles increased from 12.5 years for model year 1980 to 16.9 years for model year 1990. But it is unclear whether this trend is optimal from an energy and environmental perspective. This study combines the LCA method with mathematical tools such as dynamic programming to determine optimal vehicle replacement/retirement policies.; A life cycle optimization (LCO) model is developed and applied to mid-sized generic cars based on driving 12,000 miles annually, over a 36-year time horizon (between 1985 and 2020). For CO, NMHC, and NOx, automobile lifetimes ranging from 3 to 6 years are optimal for 1980s and early 1990s model years, while optimal lifetimes are 7 to 14 years for model year 2000s and beyond. On the other hand, a lifetime of 18 years minimizes cumulative life cycle energy use and CO2 emissions.; In addition, both ideal and practical fleet conversion policies were investigated from a life cycle perspective. According to the simulation results, accelerated scrapping policies increase greenhouse gases but reduce regulated emissions. These results are consistent with the results of the LCO model.; Maintaining old vehicles in good condition would be another effective strategy for reducing emissions from high-emitting vehicles. The inspection and maintenance programs (I/M) have been used to identify and repair high-emitting vehicles. Based on Fault Tree Analysis (FTA) of emission control systems together with IM147 test records in the Arizona area, this study identifies limitations of specific repairs (e.g., catalytic converter repairs) for I/M improvement. Benefits of I/M programs and policy scenarios combining scrappage programs are also explored.; The models used in this study can help policy-makers, manufacturers, and consumers understand the importance of issues regarding vehicle retirement decisions and guide decision-makers to environmentally sound fleet conversion strategies. |
