Originally Posted by 08 EPA Tech documentation
A third factor which can be quantitatively estimated is the effect of wind. Wind affects fuel economy by changing the road load of the vehicle. Wind can affect both rolling resistance and aerodynamic drag. Rolling resistance is primarily affected by a side wind, which pushes the vehicle sideways. The driver must compensate by turning the steering wheel into the wind. This increases the drag caused by the tires on the roadway surface. However, the effect of wind on aerodynamic drag is far the larger of the two effects.
Aerodynamic drag is generally assumed to be the product of three factors: 1) the frontal area of the vehicle, 2) the air speed going by the vehicle squared, and 3) the “drag coefficient or Cd.” A headwind increases the speed of the air going by the vehicle directly (i.e., a 10 mph wind increases air speed 10 mph). A tailwind decreases air speed by the vehicle. Even if the frequency of a headwind and a tailwind is the same, total aerodynamic drag increases, due to the fact that drag is proportional to air speed squared. For example, 40 mph squared is 1600 mph2. Given a headwind of 10 mph, air speed increases to 50 mph and 50 mph squared is 2500 mph2. Given a tailwind of 10 mph, air speed decreases to 30 mph and 30 mph squared is 900 mph2. The average of 2500 and 900 mph2 is 1700 mph, which is more than 6% greater than 1600 mph2.
Thus, even a randomly directional wind will increase total aerodynamic drag and decrease fuel economy.
An even greater effect of wind, however, is that it changes the drag coefficient, Cd, and increases the effective frontal area of the vehicle in the direction of the wind. For example, as large as the frontal area is of a semi-tractor trailer combination, its area from a side view is 5-10 times as large. The greater the side wind relative to vehicle speed, the more the truck is actually driving sideways down the road as far as aerodynamic drag is concerned. With respect to cars and light trucks, their body shapes are designed to reduce aerodynamic drag when traveling into the wind. Front and rear ends are sloped. Spoilers and other rear end shapes are designed to minimize the creation of vortices behind the vehicle which “pull” the vehicle back as it is driving forward. However, as soon as a significant side wind occurs, these benefits start to diminish.
For the 1984 label adjustment rule, EPA estimated that wind reduced onroad fuel economy by 3% for a small car and 2% for a large car.29,35 These estimates were based on several estimates made by the Department of Transportation: 1) the effect of 10, 15, and 20 mph winds on aerodynamic drag at a constant speed of 55 mph as a function of wind angle, 2) the effect of increased aerodynamic drag on 55 mph fuel economy, and 3) a distribution of onroad VMT as a function of wind speed (with the national average wind speed being 9 mph). EPA applied these estimates directly to highway fuel economy, but reduced the fuel economy effect by 80% for city driving. This reduction was based on the fact that roughly 20% of the FTP test is at speeds near 55 mph.
We reviewed this methodology in detail to determine if any improvement could be made. Two areas were identified. The first area was the fact that the effect was estimated only for cars, as that was the focus of the study. The second area was the assumption that wind had no effect on fuel economy at vehicle speeds below roughly 55 mph. While aerodynamic drag is much lower at city driving like speeds than highway speeds, wind speed is a higher fraction of vehicle speed at low vehicle speeds. The effect of wind on a vehicle’s effective drag coefficient increases as the effective angle of the air speed increases. Thus, the effect of a side wind can be significant, even at low vehicle speeds.
In order to expand the previous analysis, we developed a model of aerodynamic drag and its impact on fuel economy as a function of wind speed and angle. We broke down the speed of the vehicle through the air in terms of its x and y coordinates (i.e., parallel and perpendicular to the direction of the vehicle). The parallel component is the speed of the vehicle plus the cosine of the wind angle times wind speed. The perpendicular component is the sine of the wind angle times wind speed. We then calculated the net angle of the air flowing past the vehicle and its speed from these two x-y components. The net angle of the air flowing past the vehicle is the arctangent of the ratio of perpendicular air speed to parallel air speed. Net air speed is the square root of the sum of the square of the perpendicular air speed and the square of the parallel air speed. Aerodynamic drag is the square of the net air speed times the vehicle drag coefficient.
DOT estimated that the vehicle drag coefficient increased 1.5% for every degree increase in yaw angle, or angle of net air flow past the vehicle. Using this estimate, we were able to reproduce the estimates of the change in aerodynamic drag as a function of wind speed and direction on a vehicle traveling at 55 mph, which were presented Figure 26 of the EPA report.
In order to broaden the estimate to include light trucks, we obtained estimates of the effect of wind angle on a vehicle’s drag coefficient from Gillespie.36 Gillespie presents the estimated absolute increase in drag coefficient as a function of wind speed for four vehicle designs: pick-up trucks, station wagons, family sedans, and sports cars. The results are presented in Table III.A-24 below in tabular form. Gillespie did not present estimates for sport utility vehicles (SUV). We estimated the effect for SUVs by averaging the impacts for pick-up trucks and station wagons.
Table III.A-24. Effect of Wind Angle on Vehicle Drag Coefficient
Wind Angle (Deg) Pick-Up Truck Station Wagon Family Sedan Sports Car SUV Average
0 0 0 0 0 0 0
5 0.045 0.015 0.010 0.010 0.030 0.025
10 0.0120 0.050 0.040 0.025 0.085 0.070
15 0.195 0.090 0.080 0.050 0.143 0.121
20 0.240 0.110 0.125 0.070 0.175 0.155
We estimated a fleet average change in the effective drag coefficient by averaging the estimates for the five model types. We averaged the estimates for the three types of passenger cars equally (33/33/33), the two estimates for light trucks equally (50/50) and then averaged the averages for car and light trucks equally (50.50). For a wind angle of 20 degrees, the average change in drag coefficient for cars is 0.102 and 0.208 for light trucks. Assuming average drag coefficients in still air of 0.30 for passenger cars and 0.40 for light trucks,25 these changes represent increases of 1.7% and 2.6% per degree of wind angle. The figure for cars matches the DOE estimate from 1974 quite well, while that for light trucks is much larger. We performed a regression of the change in drag coefficient versus wind angle in degrees and found the following relationship:
Change in Drag Coefficient = -0.00376 + (0.006815 * Wind angle) + (0.000065 * (Wind angle)2 )
We also performed a similar regression used a linear model. The linear model yielded larger average increase in vehicle drag coefficient. Therefore, we retained the non-linear model.
In order to expand the estimate to include city, as well as highway driving, we again used PERE.25 Using PERE, we estimate that a 10% increase in aerodynamic drag or drag coefficient decreases city fuel economy by 0.93%. Likewise, highway fuel economy decreases 3.11%. Implied in the DOT estimate of the effect of wind speed on 55 mph fuel economy is a decrease of roughly 4%. Thus, PERE estimates a somewhat lower effect of wind speed on fuel economy even at highway speeds.
We then applied our model using the DOT estimates of the national average distribution of wind speeds, which is shown in Table III.A-25 below. We assumed that the average wind speed within a range of wind speeds was the average of the lower and upper limit of the range. We assumed that the average wind speed for winds above 25 mph was 27.5 mph.
Table III.A-25. Frequency of Wind Speeds in the U.S.
Wind Speed (mph) Assumed Average Wind Speed (mph) % of National VMT
0 – 3 1.5 16%
4 – 7 5.5 28%
8 – 12 10 30%
13 – 18 15.5 18%
19 – 24 21.5 6%
25 27.5 2%
Using an average vehicle speed of 19.9 mph for city driving and 57.1 mph for highway driving (from Draft MOVES2004), the vehicle drag coefficient increases by 73.5% and 15.9%, respectively, on average. Assuming average city and highway fuel economy of 18.8 and 25.5 mpg (from our database of 615 recent model year vehicles without any factor for non-dynamometer effects) and city and highway VMT weights of 43% and 57%, respectively, composite fuel economy is 22.1 mpg in still air and 20.8 mpg with a typical distribution of wind. Thus, taking wind into account reduces onroad fuel economy by 6%. This is more than twice that estimated for the 1984 label adjustment rule.
Roughly 60% of this 6% increase is due to the increase in drag coefficient during city driving. This portion of the estimate is likely the most uncertain, due to the large wind angles which can occur at relatively low vehicle speeds (e.g., 45% or more). This means that the figures taken from Gillespie are being extrapolated to a significant degree. We are not certain that the drag coefficient would continue to increase beyond 20 degrees wind angle at the samerate as below 20 degrees. However, the effective frontal area of the vehicle would continue to increase. Rolling resistance is also likely to increase, as the vehicle must be driven increasingly sideways to travel in the direction that the vehicle is pointing (i.e., down the road). It is unlikely that either the DOT or Gillespie estimates consider an increase in rolling resistance, as they were likely developed in wind tunnels where the vehicle is standing still. Thus, it is likely that the estimate for the effect of wind on onroad fuel economy is more uncertain than those for fuel quality or tire pressure. Still, the effect of wind appears to be very significant and likely larger than either of the other two factors.
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