Archives

Has anyone measured the impact of wind on FE? We seemed to get significantly better FE with a tail wind.

Wind has a huge impact. Not just head and tail winds, but crosswinds too.

The effect of winds is fairly straightforward. For head or tail winds, you add or subtract the windspeed to your groundspeed to see its effect on your FE. If you are traveling at 55mph into a 25mph headwind, your car is seeing the aerodynamic drag of traveling at 80mph (lousy FE). Change it into a tailwind and your car is seeing the aero drag at 30mph (great FE). For crosswinds it gets a little more complicated, since a lot of cars have a drastically lower Cd when the wind is approaching at an angle rather than from straight ahead. Its effect on your FE would depend on the crosswind component of the wind and your car's Cd at that wind angle.

Hi Bear15:

___Have not seen a better study then that created by Wayne Brown over at Hybrid Synergy Simulator (http://privatenrg.com). The following link in particular is particularly brilliant imho.

Wind assist screens. (http://privatenrg.com/#WindAssistScreen)

http://privatenrg.com/index.17.jpg

___Good Luck

___Wayne

Hi Xcel ~ I could not get this link to work. Any suggestions?

Hi Bear15:

___I blew the url tag on the first link. The second should have worked however? Both should work for you now. Sorry about that :(

___Good Luck

___Wayne

Wind Speed Effect on Fuel Economy

How much effect does wind direction have on fuel economy using a 2006 Honda Civic Hybrid (HCH II)?

Methodology
We completed 3 sets of testing. We drove one mile into a headwind of 5 miles per hour (MPH) or more, turned the car around, recorded Trip A miles per gallon (MPG) meter at precisely a distance of one mile, reset Trip A MPG meter, and drove the same distance in a tailwind and then record the MPG. We traveled from 0 - 35 MPH keeping the revolutions per minute (RPM) just under 2000 until we reached 35 MPH and maintained this speed for the remainder of the one mile stretch. We also recorded how long it took for us to go from 0 – 35 MPH and how much time to complete the total 1 mile stretch.

We recorded: MPG, Wind Speed based on Kestrel 3000 Pocketweather meter, Temperature based on reading recorded from car’s computer readout, car’s MPH, Time from 0 – 35 MPH and total 1 mile distance (under 2000 RPM).


Materials and Equipment

-Kestrel 3000 Pocketweather meter
-Stop Watch
-Chart for recording
-Compass
-2006 Honda Civic Hybrid (HCH II)
-Writing Utensil

Conditions
Fairly level road, headwind of 5 MPH or more, sunny, relative humidity 34%, approximately 1:00 PM central standard time, traffic was not a factor, tire pressure was 55 pounds per square inch (psi), all windows were up and fans were off, the gasoline tank was half full with regular unleaded, and location was rural Plainfield, IL.

http://www.cleanmpg.com/photos/data/500/Data_Collection_Chart.JPG


Odometer Start: 8,746.7

Trip A Start: 381.3

Trip A MPG Start: 52.8

Odometer End: 8,753.5


Results

Average MPG with headwind: 58.233 MPG

Average MPG with tailwind: 69.133 MPG

Outcome: There was an increase in Fuel Efficiency of 18.7% with tailwind

WOW. I new there was a benefit, but I didn't think the difference was so large.

Thats only with a 5-7mph wind. 15-20mph winds could be dramatic.

so the more areodynamic the car is, the bigger benifit for the tailwind?

I think we are starting to see two items. 1) the head wind does have a significant impact on FE, however 2) we did not see a significant impact on the amount of time it took to travel from 0 - 35 mph in our HCH II.

I embarked on a business trip of about 100 miles this past Thursday, with little or no wind. I got about 65 MPG. I made the return trip today with brisk headwinds from a strong front pushing through. Despite my best hypermiling efforts, I barely eked out 50 MPG. The return route was different and it was intermittently rainy, so admittedly it's not the best comparison. But the two routes' characteristics were similar enough to convince me that most of the difference was attributable to the wind.

so the more areodynamic the car is, the bigger benifit for the tailwind?

No, an aerodynamic car is better overall. If put a sail on your car with a strong tailwind that would be the best :)

I would assume an aerodynamic car would benifit most from headwinds

2006 vs 2007: Has anyone measured the impact of wind on FE?

I was out of the state today on business and passed a Honda dealer that had FIVE HCH II out in front (2 - 2006, 3 - 2007)-- I was shocked! I had a few min. so I stopped in. I explained to the the salesman that I was a member of this group and wanted to know if he would let me compare the differences btw. the 2006 & 2007. He said sure as business at this dealership seemed very slow.

One of the 2006 was used with 2,300 miles and the window sticker was folded neatly in the glove box (the other 4 were brand new). We took the window sticker out, walked it over to the 2007 window sticker and compared it. They were almost identical. The price on the 2007 was approx. $500 more, the cost of fuel per year was more as they must have figured a higher cost per gal. of fuel, and one of the lines describing the safety cage was a bit different.

If you go to the honda website you will find a few other minor changes, however, both the 2006 & 2007 looked and drove identical to me.

After 10,000 miles, we are extremely pleased with our HCH II -- Silent Opec Nightmare.

The great part is I'm sure you will feel the same at 100K miles.

Well, we like the car so much at this point, we are planning to drive it for as long as possible-- maybe 200K?
:flag:



The great part is I'm sure you will feel the same at 100K miles.

Hi All:

___There are some very detailed measurements and data conclusions in the EPA’s “Final Technical Support Document - Fuel Economy Labeling of Motor Vehicle Revisions to Improve Calculation of Fuel Economy Estimates” paper some here may be interested in as posted below …



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.
___Good Luck

___Wayne

This is also important information to consider when looking at ture FE.