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I was just wondering if anybody knows if the density of the air has anything to do with getting better mileage when it is warmer out. I know that helicopters have to be careful when temperatures rise because they have to deal with alltidude density. so the warmer the temp the less altitude can be achieved, because the air is thinner when it's warmer. so does this correlate. the warmer it is then the less drag is created by the car. Or am i way of base and it has more to do with the engine being colder or something else. Just curious.

You are correct. I figured it up once and discovered that the air is approximately 20% denser at -20F when compared to 80F. It does indeed make a difference!

You got me looking, #9 on this page discusses air density, and concurs that denser air is better for fuel economy and performance:

http://www.driverside.com/auto-library/top_10_factors_contributing_to_fuel_economy-317

The rest of the list is interesting too.

They're wrong. They're following the commonly-accepted, but incorrect, assumption that higher power must also equal higher mpg because you use less throttle to make that power. If you follow that logic to its extreme, a 10 liter V17 engine would be the most efficient. We know that's false, since the best mileage cars are the ones with tiny engines. (1.0 L Insight, 1.0L Metro, 1.0 L Smart, 1.5L lean-burn Civic VX)

Less dense air means that you open the throttle more to make the same power. That reduces the restriction at the throttle, so the air can go in more freely. THAT helps mpg.

Or to think of it another way, less dense air is basically making the engine smaller. If the air is 20% less dense, that's 20% less oxygen that can go in there. The ECU matches that with 20% less fuel.

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I also question point 6 - intake/exhaust restriction. The Insight has one of the most twisted and restrictive intakes in existence.

Also point 10, for reasons listed above. They're advocating cold air intakes for mileage, while I say warmer intake air is actually better. (within reason, of course)

^-- Ditto.

My post should have included, "This is one of the reasons mileage is worse during winter temperatures." It is easy to see in coast down distances.

An adjunct to this is the effect that relative humidity has on air density and mileage. Since water molecules are less dense than any of the other atmospheric gases, the higher the humidity, the less dense the air. Also the water molecules displace part of the oxygen that would be sucked into the engine, reducing your engine's power output and making it run more efficiently. I've noticed the effect of high humidity on improving coasting performance when coasting on dry roads just outside the wet area within the humid air mass spilling out from an afternoon thunderstorm.

I've seen the same effect. Comparing morning commutes, with the same temperature, the days when there is dew all around average 7% better mileage than dry mornings.

My data shows that coasting losses are linear to temperature changes. Cold weather slows down the car faster than warm air.

That's 'splain why I get better mileage on foggy mornings and right before thunderstorms.

They're wrong. ...
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I also question point 6 - intake/exhaust restriction. ...
Also point 10, for reasons listed above. They're advocating cold air intakes for mileage, while I say warmer intake air is actually better. ...

I also challenge point 9, Altitude. Excluding the steep climbs/descents that require extra braking coming down, I always get better FE at higher altitude. And I don't have the turbo that they claim changes things.

You do get better FE at higher altitudes, airplane pilots have known this for ages. The thinner air at altitude reduces aerodynamic drag and also provides less oxygen to the normally aspirated motor. Once the engine is leaned to the proper fuel/air mixture for that altitude, its fuel burn rate decreases and, with the throttle plate open wider and exposed to a lower outside air pressure, has reduced pumping losses for increased engine efficiency. I get better mileage driving on the reasonably flat stretches of I-26 at 2000 ft in NC than I can on the lower altitude stretches of this interstate in SC.

MPG will be better with lower ATM pressure-all other things being equal.We have driven from New Orleans to Flagstaff(3000 miles round trip) and back many, many times in many different vehicles(going from sea level to 7000 feet). There is no question that our mpg is better once we get past Amarillo.The last 600 miles-Amarillo-Flagstaff probably averages about 3500-4000 feet, and we get the best mpg during these stretches despite climbing about 5000 feet.
In our old(1998) worn (210,000), heavy 5200 lbs, blocky vehicle we get a good 22+ mpg on this stretch with the CC set at 68-70mph. The rest of the trip yields 20-20.5 mpg with the stretch nearest home dropping to high 19's.

Altitude is your friend, especially on interstates where you usually keep the altitude once you get it. On Podunk local roads-up and down-might be a different story, since you never get back all of what you lose climbing.

Yeah for altitude when traveling. I wonder if we have anyone here on the forum who lives on a high plain in a rural area-say east of Denver or some such. They could produce good numbers. City dwellers-Denver Flagstaff etc- can't do quite as well because of the city driving, and hills.
Charlie

PS Prevailing winds help also-W>E we get better mpg also. The winds blow from the SW usually on our trip(slightly downhill also, but wind is more important).I gotten 24 mpg tanks W-E on the trip home.

I wouldn't go near that site with a 10 foot keyboard. Too much wrong info.

Whoa boy, it's interesting to see the pluses & minuses of where I live.

Pros: generally higher temperatures, high humidity (in summer).
Cons: high temps make me use my A/C, which counteracts the high temps.
(not as bad while driving, but doing a FAS for a long red light at 2pm can get rather sweaty in July).

I've been noticing that my commute to work in the morning (no A/C needed) has gotten better over the past couple of months. Coming home it's lousy, as it's a lot hotter (yes, less dense) but I'm not FASing at the red lights because I don't want to be sitting in a pool of my own sweat. There's actually a ~8mpg difference between morning and afternoon. Traffic is approximately the same.

I actually prefer driving at night for this reason. I can usually shut down at a long light, and it's easier to see when it's about to turn green.

-Mike
-Mike

;) It's only a con if you USE the AC. ;)

;) It's only a con if you USE the AC. ;)
What is AC?;):D

I hear this mention of A/C every so often...is there something you guys need to tell me? What are you hiding??? :D

I agree that lower pressure would help FE and hurt power. Power is all about how hard you can push something, whereas with FE we are talking about sustaining just enough pressure to keep the car moving at some speed with the minimum energy input. The lower pressure will assist in lowering the drag of the car as well as opening the throttle more (as pointed out by PaleMelanesian).

The other factor at play here is the warmer weather. In warmer weather your fuel injectors actually spray better and create smaller droplets, which means the fuel can burn more quickly. This will allow for greater energy to be pushed into the piston and down the line to the road, which ultimately leads to less fuel needed. This would give us the minimum explosion needed to counteract all the forces of drag, friction, and so on.

The power is better for engines with a colder intake air. This means that with the higher air concentration and lower temperature before the compression stroke would give us the maximum bang (Look up PV=nRT) This shows that the bigger the temperature change also helps increase the pressure (power) of the engine.

Hope this helps to answer some questions.

A/C that's for EVs, right? When recharging at the house. :D - Dale

AC? Bah... I don't use anything in my Ranger anymore.

I just found out there is only one setting (vent) where the AC compressor doesn't come on. Heat, Defrost, AC, it doesn't matter.

I've taken to wearing an extra pair of socks on cool mornings and the vent was the most I normally used in the summer anyway.

Warmer air = less density, that's why a warm air intake is so effective on improving FE.

An adjunct to this is the effect that relative humidity has on air density and mileage. Since water molecules are less dense than any of the other atmospheric gases, the higher the humidity, the less dense the air. Also the water molecules displace part of the oxygen that would be sucked into the engine, reducing your engine's power output and making it run more efficiently. I've noticed the effect of high humidity on improving coasting performance when coasting on dry roads just outside the wet area within the humid air mass spilling out from an afternoon thunderstorm.

I was kind of watching the Indy 500 mini marathon and it had been raining here for a few days prior. They said that running in humid weather like we had here is good for running because there is more oxygen in the air.

One of my buddies told me yesterday that some WWII planes had an emergency water bladder that could be punctured by jamming the throttle all the way in. this water would flow into the combustion chamber and in effect would raise the compression in the chamber. this would give them some extra power for about thirty seconds or so. Not exactly sure if this is true. if it is that maybe would help to explain an increase in mpg during wetter weather.