Why do warmer air temperature's increase MPG?

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 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

 

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.

 

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

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

 

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.

 

Not Prius specific but good answers here

Scientific American

 

<div class='quotetop'>QUOTE(Tideland Prius\";p=\"105688)</div>

What the biggest factor is depends on your own driving conditions.

Trips at highway speeds will be affected mainly by temperature affecting drag.

City driving will be affected by engine running extra to heat the catalyst and the fact that there is almost no battery operation at 0 degrees. My car begins to use the battery after 1/2 hour but most of my trips are less than that.

AC use on the highway can be kept manageable. In city traffic it becomes very costly, in hot weather and will be the main factor.

And very short trips it is the low mileage rich mixture warmups.

 

<div class='quotetop'>QUOTE(bobjeri\";p=\"105702)</div>

You are so lucky! Where I live the roads are too big, full of traffic, and fast, plus my commute to work is 10-15 minutes. I went on a trip recently to Central MA and got into hills and smaller roads I could drive at 35-55 on. My mileage was great! 59 or more instead of my usual summer 55 mpg

For me ideal temps are about 80 to 85 degrees, because I do use AC and stay comfortable. The air density to there seems to pay for the AC usage and I get a net mileage increase over a 70 degree day.

Temps of 40 are still not too bad even on my short commute. I'd get more like 50 mpg at that temp. But when it gets down to 20 or less, and my battery virtually shuts off, and I never reach stage 4 warm up, is where my mileage really plummets, sometimes to under 40 mpg.

 

dont foreget that most areas use diff formulas for winter and summer gas. winter gas has anti freeze components added which do not burn but take up space effectively reducing the actual percentage of gas in a gallon

 

<div class='quotetop'>QUOTE(DaveinOlyWA\";p=\"105766)</div>

Right, and l took a look at my mileage records and looks like I got summer fuel toward end of April. Looks like that was maybe 4 mpg of the difference from my summer and winter mileages.

Anyone know for sure what the energy content difference is summer to winter?

 

actual difference runs from 6-14% depending on location i believe. also some select COLD areas may go through two formula changes

the actual difference in real world will vary because of the reasons mentioned. also have to consider that the optimum operating temperature is pretty high for a car. also ethanol isnt used here for winter formula since we use ethanol year round here. from what i understand, the additives for winter formula do not burn at all or rather i should say dont add any energy.

 

Thanks for the gas info. So my mileage jump of 4 mpg that I could not otherwise explain may be the final summer fuel switch. We may have the second switch too, but that one was probably too hidden by all the other super cold weather effects. I remember getting like 35 mpg on 0 degree days last winter. Lucky it didn't stay that bad for a whole tank.

 

well ya an nah...

you should see more like a 10-20% in jump and only about half of that will be because of the formula switch. the rest is caused by the things mentioned in this forum.

 

er...

Let the Texan speak to this issue...

Outside temperature = 104 degrees

AIR CONDITIONING - Man!

little doubt that it's hitting my mileage. I can get the car cooled down then switch off my AC (for a few minutes at least) and, all else being equal see a significant jump in MPG with the AC switched off.

For the terminally curious among you I'm about to head off to the lab here at 11:06 on a Sunday morning to get things ready for tomorrows 'first day of class' (I'm a professor). I'll try that process and report in later on just how big a change it makes. Better yet, seeing is believing, just go try it yourself, assuming you have a 104 degree day to go play in like I do.

$0.02

Terry