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In the United States, most school buses are powered by diesel engines. Some have advocated replacing diesel school buses with natural gas school buses, but little research has been conducted to understand the emissions from school bus engines. This work provides a detailed characterization of exhaust emissions from school buses using a diesel engine meeting 1998 emission standards, a low emitting diesel engine with an advanced engine calibration and a catalyzed particulate filter, and a natural gas engine without catalyst. All three bus configurations were tested over the same cycle, test weight, and road load settings. Twenty-one of the 41 “toxic air contaminants” (TACs) listed by the California Air Resources Board (CARB) as being present in diesel exhaust were not found in the exhaust of any of the three bus configurations, even though special sampling provisions were utilized to detect low levels of TACs.

Two of the types of particulate trap systems that have evolved to control exhaust particulate matter include the catalyzed trap system and the additive-regenerated trap system. Exhaust emissions from these two types of trap systems have been characterized and quantified as completely as possible. The two vehicles evaluated in the study included a 1986 Mercedes-Benz 300 SDL, which utilizes a catalyzed trap system, and a prototype Volkswagen, which utilizes an additive trap system. The vehicles were tested using a chassis dynamometer, a dilution tunnel, and a constant volume sampler. The exhaust emissions were evaluated as to driving cycle, presence of particulate trap, engine condition, trap condition, and fuel aromatic content.