The Effect of Ambient Temperature, Humidity, and Engine Speed on Idling Emissions from Heavy-Duty Diesel Trucks

A significant fraction of diesel emissions can be attributed to heavy-duty diesel vehicles at idle conditions during which power is being used for systems such as cabin heating or cooling. Although, a variety of low emission, auxiliary power solutions already exist for HDDV trucks, they are not in wide spread use. Moreover, very little work has been done to date to quantify the total emissions and fuel consumption from truck idling. Accordingly, the U.S. Environmental Protection Agency, in collaboration with the New Jersey Department of Transportation, the U.S. Army Aberdeen Test Center, Oak Ridge National Laboratory and Rowan University has initiated a study to quantify the idling emissions and fuel consumption rates for HDDV trucks. Testing was performed in an environmental chamber on five different class 8 trucks with model years ranging from 1990's to 2001. To simulate a wide variety of idling situations, 38 tests were conducted at three different ambient temperatures (0°F, 65°F and 90°F), relative humidity ranging from 22 to 90% and idle speeds from 600 to 1200 RPM. Each test was conducted for approximately 3 hours during which HC, NO_x, CO, CO₂, O₂ and PM emissions were monitored. This paper focuses on the effect of ambient humidity and temperature on HDDV truck idle emissions. The test results show that the emission rates are a function of both the inlet temperature and engine load. For example, a Detroit Diesel engine idling at 600 RPM produced an average NO_x emission of 54.8 g/hr for 0° F ambient temperature (with cabin heater activated) to 105 g/hr at 90° F (with cabin air conditioner activated). The effect of humidity was evidenced through a 15 to 20% decrease in NO_x concentration when relative humidity increased by a factor of three.