Fuel Economy

[SteveG1988]

Remember in the mountains the octane level is lower, due to the vehicles having altered compression ratios. that is why you never fill up with the lower octane fuels if you are driving through, your car will not run properly. 85 octane vs 87

[corco]

As far as safety features in cars in bad weather go- if you already know how to drive in bad stuff they can really be a pain in the ass, especially stability control.

When I went to the Tulsa meet back in February I rented a Chevy Malibu to drive from Wyoming down. There was a snowstorm in Colorado, and I was getting back on the freeway in C Springs- at that point the conditions were such that the ramps were bad but the freeway itself wasn't, so you wanted to get up to a pretty good clip on the ramps.

I figured- OK, this is a front wheel drive car and an uphill ramp, I'll just give it a little more gas than I normally would on a packed snow/slush surface and if I start to lose traction I'll just take my foot off the gas and instantly regain traction because of the FWD and slope of the hill. So, I start to lose traction, and two things happen that really piss me off:

1) The car chimes and a light comes on and the odometer screens says "LOW TRACTION"- which, no shit. I would hope any driver would realize when they were losing traction. That said, I want to react to the situation, not have to stop to read a light to tell me what I already know
2) I let my foot off the gas to regain traction and start to correctively steer, but as I'm doing that the car decided to jam the brakes because it thought I was trying to turn left while I was going straight. What was a very simple maneuver that I have executed dozens if not hundreds of times in a load of different cars turned into me almost going into the ditch because the car was doing things that were contrary to what I wanted it to do without my consent. Needless to say, I turned stability control off for the rest of the trip.

Stability control might be nice if you have no clue what the hell you're doing, but if you already have pretty good reflexes in traction-loss situations, it seems to be really dangerous. ABS isn't horrible if you recognize its limits and for short distance, low speed braking on ice it can actually be a little bit helpful- that said, if you kick ABS on at more than 5 MPH or so, you're driving wrong.

If those things cost me fuel economy, they're definitely not worth it.
Post Merge: December 15, 2010, 01:53:21 AM

must've been the elevation.  Cars run more lean up in the mountains - less air implies less gasoline to mix with it for the ideal mixture.

My experience has been the same, but I'd assumed it was less air resistance at altitude.

My issue may be that when I lived in Wyoming, I was usually driving up fairly steep hills, which is where gas mileage suffered. Lower horsepower from less air/oxygen made me have to strain the engine more which nuked the fuel economy. I also may not have adjusted for constant head or crosswinds that nuked the gas mileage.

[agentsteel53]

My issue may be that when I lived in Wyoming, I was usually driving up fairly steep hills, which is where gas mileage suffered. Lower horsepower from less air/oxygen made me have to strain the engine more which nuked the fuel economy. I also may not have adjusted for constant head or crosswinds that nuked the gas mileage.

ostensibly, you came down the hills as well.  With a stick shift, I got quite good at taking the car out of gear for extended periods of time.  I once got 51mpg on a tank of gas driving from 9200 feet elevation down to 2000.

[corco]

I think a lot of the issue were the crosswinds in Wyoming- unless the grade is more than 6% or so, dropping the car out of gear will cause it to slow down so you still need gas. There's never a straight tailwind or headwind on I-80 so you have to gas to go downhill and gas a lot to go uphill.

I also don't have a stick right now   - especially on steep hills it seems like a properly-driven stick shift can easily add 5 MPG to a cars performance

[agentsteel53]

the car decided to jam the brakes

define "jam".  To press down completely, or to render unusable?  Either malfunction seems like litigation city in the USA, though... may as well get your piece of the pie.

[corco]

define "jam".  To press down completely, or to render unusable?  Either malfunction seems like litigation city in the USA, though... may as well get your piece of the pie.

It lightly applied the brakes on the right side of the vehicle- "jam" may be too strong of a word. That said, if I'm going at a decent speed up a hill and start to lose traction in a front wheel drive car and I've already begun correctively steering, the LAST thing I want to happen is for one side of the vehicle to unexpectedly start braking!

[J N Winkler]

must've been the elevation.  Cars run more lean up in the mountains - less air implies less gasoline to mix with it for the ideal mixture.

Actually, the terms "lean," "rich," and "stoichiometric" refer to the mix ratio, rather than the absolute amount of gasoline consumed.  A stoichiometric mixture has less gasoline at high altitude than it does at sea level simply because there is also less air in the mixture.  In both cases the mix ratio is precisely the same.  The tradeoff at high altitude is that the thinner air results in reduced power, but if the car still has enough power to remain in top gear for long distances (it doesn't really matter in this case whether the transmission is manual or automatic operating in lockup overdrive), the thinner air also means that less gasoline is consumed per turn of the crankshaft.  The reduced aerodynamic drag that results from thinner air also partially offsets the reduced power output at high altitude.

The worst kind of high-altitude driving is a combination of very high altitude (over 10,000') and long steep upslopes.  I-70 in Colorado (long lengths above 10,000', 6% upslopes) requires a large number of gear changes to maintain a steady speed, to the extent that cruise control is almost useless.  As a result of these frequent changes into lower gear, fuel economy on I-70 is about the same as you would expect on a flat Interstate with a similar speed limit at a lower altitude.  On the other hand, if you choose a relatively flat mountain crossing which is also at very high altitude (US 160 La Veta Pass comes to mind), fuel economy goes way up.  My personal best fuel economy in Rosie involved a considerable amount of 55 MPH driving, including a crossing of La Veta Pass:  550 miles on a 15.5-gallon tank (35.4 MPG).

My experience has also been that the fuel economy penalty from driving in very cold weather correlates fairly closely with the increase in air density one would expect from the ideal gas law (PV = nRT).

I disagree with the advice to buy gasoline at higher octane when passing through higher altitudes.  There is no meaningful difference between 85-octane gasoline at high altitudes and 87-octane gasoline at low altitudes; the difference in octane rating simply reflects the difference in test results at high altitude since pump octane has a MON component.  If your car is designed to use 87-octane gasoline at sea level, it will be just fine with 85-octane gasoline in the mountains.  On the other hand, if it is not designed to exploit the enhanced antiknock properties of midgrade or premium gasoline at low altitude, why would you expect it to do that in the mountains?

[corco]

I think maybe what he means by "passing through the mountains" is that you're in the mountains for now, but by the time your tank is empty you won't be any more.

I used 85 for 2 years in my Jeep Liberty at 7000 feet with nary a knock to be found. That said, if I were headed to Nebraska or some other low elevation state, I'd put in 87 because I knew by the time my tank ran out in North Platte or wherever, I'd be at a low enough elevation that my car would need 87.

But yeah, there's a reason the only place you can buy 85 octane gasoline is in high elevation places- because it works!

I have known people to need 87 down in Denver (5000 feet), however- I had a friend with a completely stock 99 Honda Civic that would knock with 85 down there- it was okay up in Laramie (7000 feet), but it had problems down there. I'm not sure why this was.

[agentsteel53]

(PV = nRT)

since V is directly proportional to fuel consumption (optimal stoichiometric ratio and all that), we get V=nRT/P. 

20 degrees fahrenheit is about 88 percent of 80 (given that T has to be expressed starting from absolute zero, we convert to Kelvin first), so we expect a vehicle to give about 12% less miles per gallon in 20 degree weather than in 80.

atmospheric pressure at one mile is about 850 millibars (sea level is defined as 1 bar), so we actually expect 18% better gas consumption at elevation.

so, figuring the two together, you're getting about 3.5% better gas consumption (1/.85*.88 = 1.035 or so) driving around on a 20 degree day in Denver, as compared to an 80 degree day in San Diego.

the moral of the story is to go high into the mountains... but don't get cold.  (why there is a strong correlation between elevation and cold climate, I will leave as an open question for the meteorologists among us to answer.)

[agentsteel53]

I have known people to need 87 down in Denver (5000 feet), however- I had a friend with a completely stock 99 Honda Civic that would knock with 85 down there- it was okay up in Laramie (7000 feet), but it had problems down there. I'm not sure why this was.

I always used 85 in New Mexico if I knew I'd be staying at altitude.  If I wasn't sure that I wouldn't end up in El Paso or something, I'd put in 87 just to be safe.

[Alps]

(PV = nRT)

since V is directly proportional to fuel consumption (optimal stoichiometric ratio and all that), we get V=nRT/P. 

20 degrees fahrenheit is about 88 percent of 80 (given that T has to be expressed starting from absolute zero, we convert to Kelvin first), so we expect a vehicle to give about 12% less miles per gallon in 20 degree weather than in 80.

atmospheric pressure at one mile is about 850 millibars (sea level is defined as 1 bar), so we actually expect 18% better gas consumption at elevation.

so, figuring the two together, you're getting about 3.5% better gas consumption (1/.85*.88 = 1.035 or so) driving around on a 20 degree day in Denver, as compared to an 80 degree day in San Diego.

the moral of the story is to go high into the mountains... but don't get cold.  (why there is a strong correlation between elevation and cold climate, I will leave as an open question for the meteorologists among us to answer.)

Err, not really.  PV=nRT defines an ideal gas.  Gasoline is not only non-ideal, it's not even a gas.  You're combusting vapors, yes, but that's in the piston chamber itself, where the temperature is much higher than 20 or 80 degrees by the time the gas gets there (pre-spark).  In fact, that temperature is relatively constant regardless of ambient temperature except for the first minute or so that your car is running.  Therefore, air pressure dominates and you should get better fuel economy regardless of temperature, for the same RPM.  Now, given that more gas is required to achieve the same power, you have a whole new set of calculations.

[J N Winkler]

Err, not really.  PV=nRT defines an ideal gas.  Gasoline is not only non-ideal, it's not even a gas.  You're combusting vapors, yes, but that's in the piston chamber itself, where the temperature is much higher than 20 or 80 degrees by the time the gas gets there (pre-spark).  In fact, that temperature is relatively constant regardless of ambient temperature except for the first minute or so that your car is running.  Therefore, air pressure dominates and you should get better fuel economy regardless of temperature, for the same RPM.  Now, given that more gas is required to achieve the same power, you have a whole new set of calculations.

Neither Jake nor I were talking about the ideal gas law as it applies to what happens in the combustion chamber.  Instead, we were talking about aerodynamic drag.  This becomes the most important frictional loss at high speeds and is directly proportional both to the density of the air and (for speeds below 100 MPH) the square of the speed of the vehicle.  So, when speed and ambient temperature are held constant, a car will register better MPG at higher altitudes than at sea level because the air is less dense.  At constant altitude, it will do the same at warmer temperatures than lower ones because, again, the air is less dense.

There are other factors which impair efficiency, so density is not a perfect predictor of MPG, but my own experience has been that there is usually about a 10% to 15% variation in MPG between high and low altitude, and between high and low ambient temperatures, which can plausibly be attributed to changes in density.  I used to take short winter vacations in New Mexico and tended to observe MPG figures as low as 27 MPG when working my way through Kansas and Oklahoma, which then recovered to 30 MPG or even a little better when I got to parts of New Mexico which were warmer and at higher altitude.

I believe that wind direction is also important because it massages the velocity component of the drag equation.  The standard approximation is to treat wind speed and direction as a vector, and to add the component parallel to the car's direction of travel to the car's speed on the ground, but I am not sure how well this works in practice.  (As an example, if you are driving 70 MPH into a 40 MPH headwind, the speed value used in the drag equation becomes 110 MPH.)

[agentsteel53]

the wind is certainly important.  Crosswinds may be doubly so because you are having to drive in a non-forward direction, causing undue drag at the tires as well.  I definitely remember getting 31 mpg while driving I-25 in southern Colorado, tacking into the wind so hard that the steering wheel was rotated to two o'clock just to maintain a straight course on the road. 

In still air, I would get 40-41 mpg on that stretch of road.

[Duke87]

I always figured that the dip in winter fuel economy was because lower ambient temperature = engine loses heat faster = more energy is expended getting it hot and keeping it hot.

[Alps]

Neither Jake nor I were talking about the ideal gas law as it applies to what happens in the combustion chamber.  Instead, we were talking about aerodynamic drag.

AH.  That explains why I noticed worse fuel economy when I filled up today.  Thanks.

[Scott5114]

I always figured that the dip in winter fuel economy was because lower ambient temperature = engine loses heat faster = more energy is expended getting it hot and keeping it hot.

Heat is a waste product in engines, is it not? The force from the explosion is what moves the pistons, not the heat, right?

[Brandon]

I always figured that the dip in winter fuel economy was because lower ambient temperature = engine loses heat faster = more energy is expended getting it hot and keeping it hot.

Heat is a waste product in engines, is it not? The force from the explosion is what moves the pistons, not the heat, right?

Yes.

[mightyace]

In the winter, the heat exchanger transfers waste heat from the exhaust into the air going into the cabin.  So, your car heating is essentially "free" other than the power to the blower motor(s).

In a place like Middle-TN where you often need the AC all summer, your overall mileage is better in the winter because the AC generally is more of a drain than the cold weather vis-a-vis warm weather and no AC.