Want to know what mountain driving does to FE?

Here is a suggestion for calculating the effect of altitude gain/loss on FE. This comes from discussions with a friend who does not visit GreenHybrid (or drive a hybrid). His idea is to calculate a Horizontal Equivalent (HE) distance by converting the change in altitude to an equivalent change in horizontal distance.

A car has a Compensation Factor, CF, which relates to the energy change with altitude to an equivalent horizontal miles per vertical mile. For the Camry Hybrid, I calculate the CF to be around 30. For a 60's vintage muscle car, my friend calculated a CF of 22. I would guess the Prius and Civic Hybrid to be closer to 35. I am debating with my friend now how the CF can be calculated strictly as a function of vehicle weight and a coasting deceleration factor.

The equation is: distance (miles) + (altitude change (miles)) * CF = HE miles

Just multiply the altitude change in feet by the constant CF/5280. Add that number to your trip up miles, and subtract the same number to your trip down before dividing by the gallons of gas used in order to calculate the HE MPG. Up and back should be about the same, except for other FE factors (wind, temp, etc.).

Here is an example. I just returned from a Phoenix to Colorado to Phoenix trip, with a fillup before I left at 1180' in Tempe, again in Silverton, CO at 9250', and again back home. The trip up from Tempe, AZ to Silverton, CO, was through Flagstaff, Four Corners and Durango. The trip back was through Ouray, Cortez, Mesa Verde, Gallop, Hollbrook, and Payson. The AC was used a bit more on the way up than back. The actual FE was:

Up: 563.3 mi / 14.12 gal = 39.9 MPG

Down: 629.5 mi / 13.37 gal = 47.1 MPG

With the HE compensation:

Up: 563.3 mi + (8070 ft / 5280 ft/mi ) * 30 H mi / V mi = 563.3 + 45.9 = 609.2 HE mi
609.2 HE mi / 14.12 gal = 43.1 HE MPG

Down: 629.5 + (-8070/5280)*30 = 629.5 - 45.9 = 583.6 HE mi
583.6 HE mi / 13.37 gal = 43.6 HE MPG

If you have similar trips (from A to B and back with significant differences in altitude, and exact miles driven and gallons use in each direction), please see if this works for you, and perhaps we can determine if the CF is reasonably constant for each model car, or that perhaps the theory is no good!

-- Alan

 

Alan
I have tinkered with this question for several years. The equation I use is the same as yours, but somewhat rearranged:
FC = FC0*(1+slope*C)
where FC is fuel consumption in gallons per mile, FC0 is the consumption you get running at the same speed on the level, slope is (altitude change)/distance and C is a constant which turns out to be your CF. My spreadsheet tracks elevation at each fillup as well as the usual stuff, and using this formula takes a lot of the noise out of my lifetime fuel economy graph. I use a value of 33 for C, although values as high as 60 can be justified.

I have a feeling that C (or CF) depends a lot more on coasting deceleration than vehicle weight or anything else. Weight, vehicle type, etc are all accounted for in FC0.

Dave

 

Hmmm....

On trip last week through Arizona and Utah in my Mercedes B 200, I found that the mountain climbs (to 10,000 feet) were kind to FE and I got 6.79 L/100 km in those conditions, at freeway speeds of 75 MPH, with five aboard plus luggage (GVW around 1850 kg) and no measures used to enhance fuel economy. That surprised me. On the flats in Oregon and Idaho (but with a MONSTER headwind) it was in the low to mid 7 range.

 

I don't know why you are doing this, but I have years and years of evidence ( I call it "proof" ) that mountain driving takes less gas than flat land driving.

A) Higher altitude has less wind resistance: 15% less at 5,000 feet, 32% at 10,000 feet.
B) Higher altitude has less oxygen = less fuel used per RPM
C) many cars today cut 100% of the fuel when "coasting" downhill
D) At high altitude your car is at Wide Open Throttle much more of the time. WOT has less pumping losses.

You don't get much opportunity for C) on the flat lands.
As far as D) goes, say at sea level your engine needs a manifold pressure (Air) of 9.8 psi.
Sea Level air pressure is 14.7 psi. Thus, your throttle is one-third closed, creating energy loss.
At 10,000 feet, the outside air pressure is about 9.8 psi. Thus, your engine is WOT and the losses are reduced.

My FEH gets about 35 MPG highway on the flat lands.

On a 50 mile out and back ( 100 miles round trip ) it works out like this:

On flat ground, say at 5,000 ft ( denver ) I get 35 MPG.
35 MPG = 2.857 gallons of fuel per 100 miles.

On a 50 mile trip up from 5000 ft to 10,000 feet I can get about 25 MPG.
On the 50 mile trip downhill, I can get 120 MPG.

2.0 gallons uphill plus 0.416 gallons downhill = 2.416 gallons per 100 miles.

I have done this too many times to count, and in all types of weather.