Why do warmer air temperature's increase MPG?

[bobjeri]

I've noticed a number of statements that higher air temps can result in observable higher mpg.

My understanding from the 50's and 60's of the theory for carberators was that colder-denser air contained more oxygen per volume so that more usable energy was created during combustion - could be wrong but the old manual and auto chokes were engaged only during engine warm up. I recall adjusting the air-fuel mix for various reasons including altitude and desert climates.

Modern doo dad compersation devices no doubt optimize numerous aspects of combustion.

I assume that warmer-thiner air resistance is essentially a microscopic non-issue and frequent stops are not an issue so warm up is virtually equivalent.

So why?

[silentak1]

I've been "taught" that warmer air helps the engine run cooler and thus slightly increasing your performance and MPG. Sounds too simple to be 100% true though.

[bobjeri]

'warmer air helps the engine run cooler'

Say what?

I'd have to still be ON the turnip truck to believe that one without a good explanation.

[priusham]

bobjeri and silentak1,

I don't know where to start ...

First, you have to define "warmer temperatures." The question is, "warmer than WHAT?"

Second, "warmer air helps the engine run cooler." That's the kind of statement of fact that makes my teeth spin around in my mouth and crew through my spine.

The truth is the Prius suffers a hit to the MPG anytime that little ICE is running. Simple enough?

If you're so cold that you need to turn on cabin heat, or run a defroster/defogger, you're going to be asking the ICE to supply the heat. That means the engine has to run - a lot - to keep the hot antifreeze flowing across the heat exchanger. Even at stop lights this winter, my little ICE was merrily running and keeping me warm and getting 40 MPG.

That explains the science, so why are people claiming that the Prius gets better mileage at 80F than at 50F?

I don't know. BUT, my theory is that the people saying this are relatively new owners (< 1 year) and they are actually seeing the car's mileage improve through "break-in" as well as their own driving habits improving. Once you start driving "Prius Style," every car you drive will experience greater fuel efficiency.

Of course, unless you are totally anal and track the mileage over time, you’ll never really know. But who would do that? http://www.w8kc.com/priusmileage.htm

[tomdeimos]

Warm air is thinner so has less drag. Helps same as high altitudes.

I generally expect winter driving at 55 mph is equivalent to summer driving at 70 mph. The curves were posted here somewhere or on yahoo.
Here's another set already posted: http://priuschat.com/forums/drag-vt7205.ht...rag+temperature

I notice increases in mileage of 1-2 mpg for every 10 degrees from like 40 degrees to 80 degrees. Above 80 things may change depending on how you set the AC, since that load is going up fast. Plus it gets complicated by the battery needing cooling too.

Below 50 there is the added effect mentioned above of the engine running more. And longer warm up times start to hurt unless a trip is quite long, like over 30 minutes.

I recently have been exceeding 55 mpg on each tank. Never would see much over 50, if temps were under 60 degrees on my commute. And in winter I feel lucky if I can keep over 40 mpg. All the same kind of driving and distances.

[kirbinster]

<div class='quotetop'>QUOTE(tomdeimos\";p=\"105639)</div>

Warm air is thinner so has less drag. [/b]

BUT warmer air generally holds more humidity so it is actually denser than colder air and creates more drag.

[ammiel]

The car has to heat and keep warm the cat converter. The only way to do this is with gasoline. The tires are stiffer when cold. I beleive these are the main reasons.

[smasho]

Not Prius specific but good answers here

Scientific American

[bobjeri]

Special thanks to Smasho.

Scientific American is my favorite mag and as always it made perfect sense. Had no idea air density varies so much with temperature or that cold tires provide so much rolling resistance.

My thoughts on heating the cat converter etc -- heat is a major by-product of combustion so that there should be plenty to go around at normal temps say 40-90 deg and normal speeds say 45-65 mph.

Rememer when they would say that when your car is over heating--turn on the heater.

Thanks to all -- an interesting thread -- at least to me.

[Tideland Prius]

I guess the biggest factor is a significantly reduced warm-up period, summer fuel and if you mean warmer as in moderate temperatures, then neither A/C nor the heater is needed.