Day Hiker wrote:As usual, your post inspired some thought. So here are a few ideas I have had regarding this topic:

The only way the accessories would be "powered from gravity" is if gravity is the sole driving force on the engine during the descent. But this is obviously not the case if the transmission is in neutral. And even in gear and using engine braking, fuel is being fed to the engine, albeit a small amount.

Anyway, a hundred watts or two of accessories is small compared to the several thousand watts used to overcome wind drag at freeway speeds, so for rough calculations, the accessory load can be ignored.

For stored potential energy to be fully returned on the downhill, no engine or other braking can take place. Any braking is energy being lost. So hills that have descents with steep grades are not as efficient as those with grades shallow enough to allow coasting in neutral.

By the way, the combination of downhill and neutral is illegal in some states (not that it ever stopped me from doing it).

oh yeah I screwed that up, I didn't mean that the potential energy would be returned...but at least that no additional energy should be required for the downhill. Of course in hybrids, they are doing something to attempt to return at least a portion of that energy.

2 cases for a non-hybrid (energy absorption).

1) 80 miles flat at V velocity

2) 40 miles up X% grade and 40 miles back down. lets assume downhill costs 0 energy so its 40 miles uphill X% grade at Z velocity

gas consumption in case 1 is largely driven by the velocity^2. driving fast costs a lot.

in case 2, grade is the main factor I imagine. velocity Z will be much lower than in case 1 and will be a function of the grade. higher grade -> lower velocity -> less drag cost, but obviously higher cost due to climbing.

question is, how is gas consumption related to grade? linear, squared, etc...? I looked a bit but didn't find anything.

It could be that some grades grades could be beneficial to gas consumption by cutting down the velocity, but other grades could be detrimental

Yeah, right ...

Could you please repeat that in English ???

Arthur Digbee wrote:I'd love to know if the thinner air at altitude has enough of an effect on drag to affect mileage. Any engineers?

There absolutely must be less drag at lower air densities, although I don't know whether or not wind drag is proportional to air density.

I suspect air density is a significant factor in vehicle efficiency at different elevations, since at only 3000 feet, the air pressure is already only around 90% of that at sea level. At 6500 feet, it's only around 80%. (Generally speaking, there would be a lower temperature associated with the higher elevation which would affect density in the other direction, but it's a smaller effect compared to the change in pressure: At 3000 feet, P has decreased by 10%, while T has decreased by only around 2%. The effect of these two parameters on density can be seen with the formula n/V=P/RT, where n/V is density.)

First it's language and correct grammar usage, and now it's Physics. I swear that Dayhiker should be a teacher.

Ze wrote:1) 80 miles flat at V velocity

2) 40 miles up X% grade and 40 miles back down. lets assume downhill costs 0 energy so its 40 miles uphill X% grade at Z velocity

in case 2, . . . velocity Z will be much lower than in case 1

You need a car with a better engine.

Ze wrote:question is, how is gas consumption related to grade? linear, squared, etc...? I looked a bit but didn't find anything.

You already know how how to calculate power versus grade, I believe. The power equals the change in potential energy divided by the time. So at a given horizontal speed, the power (work per time) is proportional to grade. At normal road grades, the road speed is roughly equal to horizontal speed, so we could say that work per time is essentially proportional to grade at a given road speed as well.

To get the fuel usage, here is a bit of info: At a given engine speed, the graph of fuel usage (y-axis) versus power output (x-axis) is roughly linear, as in y=mx+b. The y-intercept is the fuel usage at no load; it takes fuel to spin the engine, of course. The slope will tell you how much additional fuel is required to handle additional load.

The actual values depend on the engine, and I don't have a bunch of example values to offer, unfortunately. The value for b would typically be smaller for a smaller engine, since it takes less power to spin a smaller engine.

I think we can keep things simple by talking about moderate road grades, not ones on which we need to downshift to maintain a safe, time-efficient 90mph. So the comparison between level and different uphill grades can be made simpler by assuming a constant speed (so the same wind drag) and a contant, top gear (so the same engine speed).

Grades of zero to a few percent do not require downshifting (unless the car is junk), and the roads in the western Great Plains would generally not exceed this range. But the 6-percent uphills from Denver to Eisenhower Tunnel do require some downshifting, except maybe for a decent sport bike.

Typically air drag is ~proportional to density.

MoapaPk wrote:Typically air drag is ~proportional to density.

Thanks. That was my guess, but I couldn't confirm it with my lazy version of Internet searching.

So it would seem that the lower air pressure when driving at higher elevation would provide one obvious reason to have better mileage in the West, since the wind drag would be substantially lower than in most of the Flat East.

I think the answer is obvious. Bob is depressed being back on the east coast, so he fillS the back of truck with more booze.

But at higher altitude, the engine is less efficient per volume of air brought into the cylinders.

I'm working with the faulty odometer.

THE AIR IS THINNER. Less wind resistance. Remember, air pressure is 14.7 psi at sea level... and halves every 15,000 ft up one gues. Air pressure on top of a 14,000 ft mountain is only 7.5 psi.

Going up to 5,000 ft (like at Denver) means air density drops (approximately) 20% and hence better mileage on highway roads when driving at CONSTANT high speeds.

Also... RAIN and very wet parvements are murder on gas milage. Raindrops hitting cars must be brought up to speed... and that saps the car's forward momentum. Very wet pavement, wheels picking up water and throwing it off tires takes energy.

The model's mpg estimates are usually lower than actual. Most all cars get better mileage that the posted dealer's sticker. It's something to do with government regulations... manufactors don't want to get in trouble with the govenment if their mileage predictions don't match their claims.

MoapaPk wrote:But at higher altitude, the engine is less efficient per volume of air brought into the cylinders.

At full throttle, I think the limiting factor is indeed the amount of O2 in the cylinder; any more fuel than a stoichiometric quantity is just wasted. But partial throttle is a reduction in fuel, not necessarily so much a reduction in air into the cylinder. So at partial throttle, the limiting factor is the amount of fuel in the cylinder, and there is a surplus of O2, if I am not mistaken.

If, due to altitude, the air becomes so thin that there is not enough O2 in the cylinder to burn all the fuel, there is a reduction in power, and the vehicle would not maintain its speed. (And a fuel-injected engine would reduce the fuel to no more than a stoichiometric ratio.) But then we no longer have the conditions for a comparison at equal vehicle speeds.

So as long as the vehicle is maintaining speed, we know there is enough air in the cylinder to burn the fuel necessary to produce the power to maintain that speed. I don't see how less-dense air would increase the consumption of fuel.

As long as you don't drive the car over 15000', you are probably OK.

In Albuquerque at ~5200' above sea level, it was fairly imperative to adjust the fuel system for the altitude... bet that may have been mainly to reduce emissions. But I think the gas mileage was pretty much what one would expect

MoapaPk wrote:In Albuquerque at ~5200' above sea level, it was fairly imperative to adjust the fuel system for the altitude... bet that may have been mainly to reduce emissions.

Was that a carbureted engine (not manufactured since . . . forever ago)? I am surprised that a modern fuel-injected engine would not automatically adjust for the air pressure.

Good point; the first had a carburetor; after 1988 they were fuel-injected.