Okay, let's compare a round trip at 70MPH with no wind to a round trip at 70MPH with 20MPH head/tail wind.

Most of the factors in the equation are constant, so we can ignore them. The formula boils down to Du=V^2 where DU is Drag Units. Wind speed is W.

On the first trip we get Du = V^2 * 2 = 70^2 * 2 = 9800.
On the second trip we get Du = (V+W)^2 + (V-W)^2 = 90^2 + 50^2 = 8100 + 2500 = 10600.

So the drag on the second trip increased by 10600/9800 - 1 = .08 = 8%.

In the worst case you'll get 8% worse FE. But since other factors consume energy, like mechanical friction in the engine and drive train, road friction, acceleration and braking, the hit to FE will be less than 8%.

 

Another interesting comparison is the effect of speed on drag (in the absence of wind).

Using the same formula:

drag at 60MPH is (60)^2/2*Cf*A*r = 1800 * Cf*A*r
drag at 70MPH is (70)^2/2*Cf*A*r = 2450 * Cf*A*r

So, the drag is (2450-1800)/1800 = 36% higher

Based on my commute tests (incomplete at this point, I still need the ride home @ 70), the FE difference is:

FE at 60MPH = 51.7
FE at 70MPH = 46.2

So the FE is (51.7-46.2)/46.2 = 12% lower, 1/3 of the drag difference.

 

Hi Nasagineer,
Looks exactly like the numbers I got. Wonder if anyone knows the Cd for the HCH?? Because the air is much denser in the winter (cold) this probably has a slight effect on FE, also altitude. I'm on the coast of NE so no luck there. 200-400 ft above sea level at best.

 

Well you forgot that while drag may have increased 36%, speed increased by 17% which compensates partially.

Remembering MPG = MPH / GPH, the increase in drag forces your GPH to increase to maintain speed, lowering MPG, but since your MPH increased, that helps raise MPG a bit. Specifically, using our naive and almost certainly wrong math, the change would be sort of like 1.17 / 1.36, or 86% of the original value. 51 * .86 = 44.

So then going from 70 to 80 is +14% speed, +30% drag, so 1.14 / 1.30 = ~88%, and 88% of 46 (your figure) is about 40. Well we'll see how you do.

By the way, one source of imprecision is the fact that there are other resistances at work. Rolling resistance should increase similarly to drag but drivetrain loss might not be affected as much and there are probably other things I'm not thinking of. These would reduce the impact from driving faster, and indeed you got 46 mpg rather than the calculated 44.

 

True, and my point was that you can't look at drag alone to predict FE, as you also pointed out. Besides the factors you mentioned, there's also the fact that the ICE FE with respect to RPM's is non-linear, so you can't simply extrapolate.

I'm guessing that your estimate of 40 is pretty close, maybe a bit better if the trend continues (41 or 42?)

 

I believe rolling resistance increases linearly with speed. Drivetrain resistance is where things get tricky, and would depend more on the car than anything. I understand this is why hybrids take more of a FE hit at high speeds than other cars, because the downsized ICE must rev a lot higher to overcome the drag, you're leaving your engines high efficiency range, and incurring a lot more pumping losses etc. I suspect that the extra horsepower, and all the measures to reduce engine friction are the main reason why the '06 Civic does better than the '03-'05 over 70mph-- it doesn't need to rev as high, and the amount of friction per revolution is reduced.

 

Thsi formula implies that fuel flow would be constant at any speed if it were not for drag. I doubt that's true.




Once you reach a high enough speed the drive train is in top gear. From that point on, at least, any increase in speed translates directly into an increase in the drive train's rotational speed. Thus, from a certain speed up, drivetrain frictional losses should increase (at least) linearly with vehicle speed. The part of the drive train between the transmission and the wheels always rotates in direct proportion to vehicle speed, with proportional friction loss.

The other thing you're missing is the kinetic energy of the vehicle. Even if the vehicle was travelling in a vacuum with no friction of any kind, it would take energy to accelerate it. That energy is eventually burned off as heat when you brake. In a hybrid some of that energy is recaptured.

 

It assumes nothing. Simple algebra: MPH / GPH = (m/h) / (g/h) = (m/h) * (h/g) = m*h / g*h = m/g. At any given moment, your mileage is a function of your speed divided by the rate of fuel consumption. Likewise, if you're curious what your fuel consumption is over time, just take your speed and divide it by your mileage. If I get 40 mpg at 80 mph, my fuel consumption at that point in time was 2 gallons per hour. Getting 50 mpg at 60 mph, my fuel consumption is 1.2 gallons per hour.

 

I agree that there are many other forces involved in calculating MPG but I think that air resistance (aka drag) is a bigger factor than most of the others when higher speeds are involved. The HCH is already very low to the ground so "ground effect" is probably the best it will get without scraping off the front grill every time you hit a pebble... The biggest factor for me right now is the cold weather, rain/snow/wind, here in new england for the next 5 months. My MPG's are down to 47 ish from the mid 50's of summer.. Mostly short trips under 15 miles lately as well..

 

I just scanned most of the replies so sorry if this is a repeat.

Tires, tires, tires. I just replaced my original tires with new (suposedly LRR tires, a tad larger (195 vs 185) but nearly identical weight). I was getting 50-51 in previous tanks - first tank on the new tires was about 45. I'll be posting all the data once I get through about three tanks. Same time as I swapped the tires, a cold spell hit, and that certainly has something to do with it (see the thread I started on my seasonal results - https://www.greenhybrid.com/discuss/...onal.4695.html)

However, I can say as an absolute fact that the new tires were much more "sticky". Parts of my commute where I could cruise with 80MPG on the bar were suddenly at 40, and struggling. I'm through the first 500 miles now and the car is rolling much better, but not up to what is was before.

Be patient - tires will break in (1-2K miles usually). As your weather improves, things will go up too.