Effects of Vehicle Speed and Engine Load on Motor Vehicle Emissions

Abstract

Laboratory studies have provided a foundation of knowledge regarding vehicle emissions, but questions remain regarding the relationship between on-road vehicle emissions and changes in vehicle speed and engine load that occur as driving conditions change. Light-duty vehicle emissions of CO, NO x , and NMHC were quantified as functions of vehicle speed and engine load in a California highway tunnel for downhill and uphill traffic on a ∼4% grade. Emissions were measured throughout the day; average speed decreased inside the tunnel as traffic volume increased. Emissions of CO were typically 16-34 g L -1 (i.e., grams of CO emitted per liter of gasoline consumed) during downhill driving and ranged from 27 to 75 g L -1 during uphill driving. Downhill driving and moderate-speed uphill driving resulted in similar CO emission factors. The factor of 2 increase in CO emissions observed during higherspeed uphill driving is likely evidence of enriched engine fuel/ air ratios; this was unexpected because uphill driving observed in this study occurred at moderate engine loads within the range experienced during the city driving cycle of the U.S. emissions certification test. Emissions of NO x (as NO 2 ) were typically 1.1-3.3 g L -1 for downhill driving and varied between 3.8 and 5.3 g L -1 for uphill driving. Unlike observations for CO, all uphill driving conditions resulted in higher NO x emission factors as compared to downhill driving. NO x emissions increased with vehicle speed for uphill driving but not as strongly as CO emissions. Emissions of CO and NO x are functions of both vehicle speed and specific power; neither parameter alone captures all the relevant effects on emissions. In contrast to results for CO and NO x reported here and results for NMHC reported previously by Pierson et al. (Atmos. Environ. 1996, 30, 2233-2256, emissions of NMHC per unit of fuel burned for downhill driving were over 3 times greater than NMHC emissions for uphill driving. Emission rates of CO and NO x varied more with driving conditions when expressed per unit distance traveled rather than per unit fuel burned while NMHC emission rates normalized to distance traveled were approximately constant for uphill versus downhill driving during peak traffic periods.

Analysis of model-based PM2.5 emission factors for on-road mobile sources in Mexico

“…An analysis of the data inputs used for estimating the PM2.5 emissions from mobile sources in the 2005 MNEI indicates that the average vehicle speed and temperature had the highest sensitivity on the MOVES2010a PM2.5 emission factors estimates. In general, PM2.5 emission factors were higher for higher average vehicle speeds, consistent with studies indicating higher PM emission rates observed under high engine load conditions (i.e., vehicle Table 1. speeds) (Kean et al, 2003;Cadle et al, 2007;Zavala et al, 2009b). Similarly, recent studies are consistent with the observation that fine PM emissions increase with engine load for diesel-powered engines and that volatile fraction of fine PM decreases with increasing load (Ziemmann et al, 2002;Kwon et al, 2003).…”

Trends in on-road vehicle emissions of ammonia

“…In Figure 2, the normalized emission factor decreases gradually from 2 to 6 PM and then increases rapidly to a maximum at 9:30 PM. Also included in the figure are vehicle speeds through the tunnel in 2001 (Kean et al, 2003). The increase in ammonia emissions with increasing vehicle speed and thus engine load is apparent, as we have previously reported for CO and NO x (Kean et al, 2003).…”

Are Emissions of Black Carbon from Gasoline Vehicles Underestimated? Insights from Near and On-Road Measurements

“…Determination of such EFs is advantageous because they are relatively simple to determine under real world conditions and may vary considerably less than travel-based emission factors for some emissions as driving modes change. 32,33 A fuel-based emission factor can be given as:…”