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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.

Diesel fuel properties influence diesel exhaust emissions. This study determined the effect of fuel cetane number and aromatic content on emissions from a heavy-duty diesel engine meeting 1994 emissions standards. No exhaust aftertreatment devices were used in this study. The fuel set was selected to separate the cetane effects from the aromatics effects, since otherwise these effects can be easily confused. Other fuel property effects, such as density, were also considered in the selection of the fuel set. This study found that increasing cetane number reduces all regulated diesel emissions species. Reducing aromatic content reduces NOx and particulate emissions, as was found in the previous Amoco/Navistar study, but this time the aromatics effect was less and the cetane effect was greater. When compared to the CRC VE-1 Phase 2 study, aromatic effects were almost identical and cetane effects were similar on all emissions.

A prototype 1991-model diesel engine was tested using EPA transient emissions procedures to determine the effect of fuel properties on combustion characteristics and exhaust emissions. The eleven test fuel set focused primarily on total aromatic content, multi-ring aromatic content, and cetane number, but other fuel variables were also studied. Hydrotreating was used to obtain reductions in fuel sulfur and aromatic content. Increasing cetane number and reducing aromatic content resulted in lower emissions of hydrocarbons and NOx. Particulate emissions were best predicted by sulfur content, aromatic content and 90% distillation temperature. Multi-ring aromatics showed a greater significance than total aromatics on hydrocarbon and particulate emissions. Combustion parameters were highly dependent on fuel cetane number.