Thread: Physics and MPG

I've read about, and experienced personally, much improved fuel mileage at higher altidudes. There have been many explanations proposed for this but, I haven't been convinced by any of them until it hit me.

I don't know of any really high elevations that aren't really hilly and I believe that the hills might work to great advantage in the typical internal-combustion engine setup....even more so on a hybrid.

My theory is, say my miata that normally gets thirty MPG at seven hundred feet altitude, normal highway driving, probably gets around twenty going up a long steep hill but, going downhill using the engine to slow the car with no throttle, I'm getting sixty MPG or better. A round trip gives equal uphill/downhill values so, with the above stated assumptions for MPG, I'm now averaging forty MPG!

I've never seen this theory posted and wonder what you guys think and how, if this is valid, might we apply this bit of physics to improved efficiency.

No, the math doesn't work out that way, even if your mpg assumptions are correct.
Let's say you go 60 miles on the flat. That's 60 miles at 30 mpg, or 2 gallons.
Now on the up slope 30 miles at 20 mpg is 30/20=1.5 gallons. Downslope it's 30/60 or 1/2 gallon. Your total is still 2 gallons.

My mpg goes to hell when driving through mountains.

If it weren't hilly, then you would get increased efficiency due to a lesser coefficient of drag from the lower air density.

Originally Posted by Harold14370

No, the math doesn't work out that way, even if your mpg assumptions are correct.
Let's say you go 60 miles on the flat. That's 60 miles at 30 mpg, or 2 gallons.
Now on the up slope 30 miles at 20 mpg is 30/20=1.5 gallons. Downslope it's 30/60 or 1/2 gallon. Your total is still 2 gallons.

Ok, I can't argue with your math, and my uphill/downhill numbers are speculative but, I know that the computer on my car cuts the fuel completely when the throttle isn't sensed and engine is turning thus, infinite MPG at times during downhill. I live near the mountains and every time I do a trip up and down, I always average over 34 MPG. Others on the west coast in the bigger mountains tend to claim even better and I don't believe it can be explained completely by lower wind resistance due to thinner air.....if anything, the thinner air should make combustion less efficient.

So, as long as I do better than fifteen MPG going up, I'll do better than my average 30 MPG total.

This is speculative, but...
Thinner air should have a lower viscosity therefore there should be greater penetration of the fuel stream into the combustion chamber, hence more efficient combustion. The thinner air will also require less fuel to meet the stochiometric ratio for efficient combustion. In essence thinner air should equate to a smaller bore but slightly more efficient engine. Less horses, more range.

Originally Posted by Suspiciousmind

So, as long as I do better than fifteen MPG going up, I'll do better than my average 30 MPG total.

This would be true if you can coast down the hill without burning any fuel, because at 15 mpg you would use exactly half as much fuel to go halfway, as to go all the way at 30 mpg. I don't think you can do that though, unless you are going down a very steep hill or going pretty slowly. I know that I have to give my car some gas even to cruise downhill at highway speed.

We can look at it from an energy conservation perspective. If your engine is equally efficient going uphill as down, and if you are not wasting any energy by braking, then you will use as much gas either way. A certain amount of energy is needed to gain potential energy going up. That energy is recovered going down. If you use mechanical brakes on the downhill run, some of that energy will be dissipated and lost as heat. Dynamic braking on an electrical vehicle can recover some of that wasted energy.

The only way the hilly terrain would help would be if your engine is more efficient at the higher torque going uphill. However, on the flat, you should be able to shift to a higher gear and get into the more efficient range.

I really think that if you are getting better gas mileage it would have to be due to the lower air resistance. What causes you to doubt that theory?

I doubt the lower air resistance as being the reason because the miata has a relatively low drag coefficient and the effect should be minimal unless we're talking really high speeds...over75 mph it would start to factor in. My 30 mpg typically comes from speed averages of around 50 mph which is typical for me on mountainus roads too.

I should note that the car is a manual transmission and likely the mpg would be worse than normal with an automatic tranny but, definitely no throttle is required for downhill runs....usually using emgine engaged to lower gears to control descent speed.

I think you are mistaken about the 75 mph. The graph here shows that for a typical car the fuel efficiency starts to drop off rather sharply at speeds above 60.
http://www.fueleconomy.gov/feg/drivehabits.shtml
Could it be that you are slowing down in the mountains, and that is why fuel efficiency goes up?

Originally Posted by Harold14370

I think you are mistaken about the 75 mph. The graph here shows that for a typical car the fuel efficiency starts to drop off rather sharply at speeds above 60.
http://www.fueleconomy.gov/feg/drivehabits.shtml
Could it be that you are slowing down in the mountains, and that is why fuel efficiency goes up?

The miata isn't too typical...this is the first generation model which is very lightweight and really small. If you happen to see one, compare it to the vehicles around it. I start to notice a decline in mpg on mine when I do long runs on the interstate involving 75 mph or above....under that, about 30mpg is typical. I could, and have, made runs at relatively constant speeds around 55 mph and get mpg of around 34 but, that's not my driving style and definitely not mountain driving.

I slow some in the mountains but, I speed up a lot too, enjoying the lateral G's in the curves and doing a lot of shifts and high revs....I'm driving less efficiently than normal for sure....rarely is the car in fifth gear where it's going to be most efficient....that's why I really am trying to understand the better mpg....it doesn't make sense but, it's real and, there's an explanation....I think it has to be the downhill efficiency significantly outweighs the uphill inefficiency.

At higher altitudes it is claimed that the cylinder pressure is reduced. That's why they sell 85 octane gas here in Colorado but the lowest grade available to flatlanders (so I'm told) is 87. Since fuel is ratioed to the air I don't believe there should be any loss or gain of MPG at altitude, but there is a loss of power. There could be some gain in MPG due to reduced air resistance during high speed driving on the level at higher altitudes.

I say it is claimed because it seems to me that if the throttle is less than fully open then ambient air pressure should be compensated for by throttle position, so the cylinder pressure should only be lower if the throttle is maxed out. However, there are also flow resistances that change with altitude in the exhaust system and at the tailpipe exit that have to be considered, and the obvious answers may well not be correct.

I considered that maybe, the pressure differential between atmosphere and combustion chamber could be the difference but intuitively, I don't think that's enough. The first time I experienced the mpg boost, I was really braced for poor mpg and thought it had to be a fluke like a generous pump but, it happens every time.

If you want to play with some drag scenarios, send me you e-mail address and I can share a spreadsheet that has quite a few variables to tweak....I only see about ten pounds force difference at sixty mpg between sealevel and 5000 feet on my miata.

Please don't take this as a stealth advertisement.

One thing I've noticed over the years is that most people drive a lot more car than they need to get from point A to point B.

I think that most people have observed that the vast majority of the drivers they share the road with are alone, ensconced in the vehilce of their choice.

I drive a two seat vehicle for work and for 99% of my other vehicle needs. It has an empty weight of ~1,800 lb and a capacity of 500 lb (passenger plus cargo). It's powered by a 1000 cc, 70 hp gasoline engine.

If I do a lot of highway driving (at ~60 mph) I get 40-42 mpg. If I do a lot of city driving (stop and go), I generally get 37-39 mpg. The altitude here in Cincinnati, Ohio is ~500 ft above sea level.

There are a lot of hills in Cincinnati and I have yet to find one that I couldn't get up (or down) with ease. Likewise, I've never had a problem "blending in" with 80 mph traffic on the highways here (even though the speed limits on our highways are posted for 65 mph). Needless to say, once on the highway I move into the right lane and let all the "maniacs" pass me. These are the same people, no doubt, who are cursing both the high price of gas and the "slow pokes" who are making them change lanes.

My car is not a "stripped down" cheap vehicle. It's well made and well designed. It has air conditioning, electric windows, abs, traction control, cruise control, plenty of leg room and (God bless the engineers) heated seats. It's all the car I need.

If you have children my advice is that you get a "family wagon" for those inevitable road trips with the kids and their friends. If you need a "work car", buy one that's designed to get you to work and back (or to the store and back) safely and comfortably. You don't need a 6-passenger, 250 hp vehicle to do that.

Chris

Originally Posted by CSMYTH3025

One thing I've noticed over the years is that most people drive a lot more car than they need to get from point A to point B.

If getting from A to B with one or two people in fine weather is your only requirement then, yes, a little two seater is fine. Most of us, I suspect, compromise to get a vehicle that can be used for commuting as well as a variety of other activities. I have just bought a Subaru Outback because it has all wheel drive and capacity for four (five if you're very friendly) with backpacks, snowshoes and other outdoorsy kit. I waited several years for Subaru to bring out their diesel boxer and/or hybrid but waited in vain. Gas mileage is not great so I ride the bus to work (mostly) and use the car for other activities.

Originally Posted by Bunbury

Originally Posted by CSMYTH3025

One thing I've noticed over the years is that most people drive a lot more car than they need to get from point A to point B.

If getting from A to B with one or two people in fine weather is your only requirement then, yes, a little two seater is fine. Most of us, I suspect, compromise to get a vehicle that can be used for commuting as well as a variety of other activities. I have just bought a Subaru Outback because it has all wheel drive and capacity for four (five if you're very friendly) with backpacks, snowshoes and other outdoorsy kit. I waited several years for Subaru to bring out their diesel boxer and/or hybrid but waited in vain. Gas mileage is not great so I ride the bus to work (mostly) and use the car for other activities.

I acutally stumbled upon a great deal on the 1990 miata at the ripe old age of 55 after being loyal to toyota pickups for the last twenty-five years or so. I never had a "sports car" before and didn't quite know what its purpose would be for me but, I soon became addicted to its fantastic mpg and "fun factor" compared to the tacoma and now, the truck only gets duty when a truck is needed. I've been looking for ways to curb my gas usage and this is a step in the right direction. I drive the miata for my daily work commute and whatever errands needed and enjoy every trip I make in it except in the rain, especially at night. I sorf of feel like I'm recycling too because not many people wouldn't consider the car "transportation" but to me, it's almost perfect...lightweight, perfectly balanced, well engineered. maneuverable, efficient, simple, reliable, and basic-no frills.....pure transportation.

I've looked for ways to squeeze more efficiency from the stock setup but, there's not much that can be done to the existing configuration. I'm thinking how cool it would be to drop a little diesel in it and try for 70+ mpg....maybe gas prices will go up enough to make me follow through one day.

Sorry to venture off from the original point of my post....why better fuel mileage in the hills?....but, these discussions need to happen too I think....we all need to be looking for ways to use less energy everywhere we can and in that regard, I guess it's pertinent.