The Effect of Ambient Temperature and Humidity on Measured Idling Emissions from Diesel School Buses

The New Jersey Department of Transportation (NJDOT) is currently sponsoring a research study at Rowan University to develop strategies for reducing diesel emissions from mobile sources such as school buses and class 8 trucks. One such source of diesel emissions results from unnecessary idling of school buses, which is a typical practice that occurs in the mornings to warm up engines and in the afternoon while bus drivers wait to pick up children for their afternoon routes. To quantify emissions and fuel consumption during idling, three school buses equipped with an International T444E, an International DT466E, and a Cummins 5.9L B series engine were instrumented and tested in an environmental chamber. To simulate a wide variety of idling situations, tests were conducted at four different ambient temperatures (20°F, 40°F, 65°F and 85°F) and relative humidity ranging from 37 to 90%. Exhaust gas emission measurements were made using a Semtech-D mobile emissions analyzer to measure CO, CO₂, NO₂, NO and unburned hydrocarbons, along with a Sensors PM-300 to measure PM. In addition to quantifying school bus emissions during idling conditions, another objective of this study was to develop a NO_x humidity correlation for use in mobile school bus emissions testing currently being performed in conjunction with the idling study. From the results of this study, a new NO_x humidity/temperature correlation was developed using a similar technique to that found in SAE standard J1243. Three sets of engine specific coefficients for the J1243 NO_x correction equation were calculated and applied to the idling data collected in the chamber.