Influence of window opening vs. A/C use on fuel economy

"L Alpert" > wrote
> Good real world application.

Ditto.

What Gordon found appears to be consistent with some study
(or a summary of a study) I read not too long ago. Tom and
Ray of "Car Talk" may have cited it at their web site, if
memory serves. Namely, even when moving at highway speeds,
not using the A/C and leaving windows open for some cooling
is best.

Plenty of studies on the net on this. E.g.
---
Several sources claim that closing windows and using a car's
air conditioner will provide better fuel economy at freeway
speeds than leaving the windows open without the air
conditioner. However, FSEC tests showed this in not the
case. In repeated evaluation at 65 miles per hour, our test
car experienced 11% better fuel efficiency with no A/C and
the windows open than using the air conditioner. We also
found that closing windows at freeway speeds improved fuel
efficiency by 2-3%.

http://www.fsec.ucf.edu/Pubs/energynotes/en-19.htm
---

The above site has a nice chart for freeway speed driving.
The test vehicle was a 1986 VW GTI.

OTOH, the damned people at FSEC.UCF appear to misquote a
study done by edmunds.com, indicating that there's no
difference at highway speeds.

I suspect this all depends on the model and its wind
resistance characteristics more than we'd like to admit.

Perhaps the best approach is to perform one's own
experiments with one's own car.

"L Alpert" > wrote in message
...
>
>
> The net effect will be determined by environmental conditions and choice.
> My preferences are usually determined by speed of travel and
> temperature/humidity conditions. <90° and lower humidity while driving on
> back roads at <50 mph would be open windows for me for the most part
> (general rule of thumb). >90° would most likely be AC all the time.
>
> Highway speeds would either be AC or vent, depending on the preferred
> comfort level at that specific time. I am not very concerned about the
> differences in fuel economy, only in comfort level (although it is always
> nice to know.....).
>

That's undoubtedly the bottom line. I used to live in Phoenix, and the
windows don't open wide enough to make 117 F comfortable!

Mike

"L Alpert" > wrote in message
et...
> Gordon McGrew wrote:
>> On Sat, 9 Sep 2006 16:41:34 -0700, "Michael Pardee"
>> > wrote:
>>
>>> Vehicles with lower fuel economy are less sensitive to differences,
>>> because the losses are already pretty high. In the Toyota Prius fora
>>> people are shocked to find that running the heater can decrease the
>>> in-town fuel economy 10 mpg... the finer the edge the more quickly
>>> it dulls.
>>
>> Running the heater? Isn't this just waste heat anyway?
>
> I guess it has to do with the amount of juice needed to run the
> fan.......????
>
The effect on the amount of time the engine runs can be pretty radical. The
car has a display for the mpg over 5 minute intervals, so the first winter
we had it I decided to see just how much the heater would drag it down. I
opened the windows and turned the heater on full while driving in town. The
previous five minute bars had been something like 40 or 45, while with the
heater on full it dropped to 25! Even at 75 mph with the A/C on full and
five adults in the car it never went below 30 in real life.

Mike

Michael Pardee wrote:
> "L Alpert" > wrote in message
> ...
>>
>>
>> The net effect will be determined by environmental conditions and
>> choice. My preferences are usually determined by speed of travel and
>> temperature/humidity conditions. <90° and lower humidity while
>> driving on back roads at <50 mph would be open windows for me for
>> the most part (general rule of thumb). >90° would most likely be AC
>> all the time. Highway speeds would either be AC or vent, depending on the
>> preferred
>> comfort level at that specific time. I am not very concerned about
>> the differences in fuel economy, only in comfort level (although it
>> is always nice to know.....).
>>
>
> That's undoubtedly the bottom line. I used to live in Phoenix, and the
> windows don't open wide enough to make 117 F comfortable!
>
> Mike

Amen.

Michael Pardee wrote:
> "L Alpert" > wrote in message
> et...
>> Gordon McGrew wrote:
>>> On Sat, 9 Sep 2006 16:41:34 -0700, "Michael Pardee"
>>> > wrote:
>>>
>>>> Vehicles with lower fuel economy are less sensitive to differences,
>>>> because the losses are already pretty high. In the Toyota Prius
>>>> fora people are shocked to find that running the heater can
>>>> decrease the in-town fuel economy 10 mpg... the finer the edge the
>>>> more quickly it dulls.
>>>
>>> Running the heater? Isn't this just waste heat anyway?
>>
>> I guess it has to do with the amount of juice needed to run the
>> fan.......????
>>
> The effect on the amount of time the engine runs can be pretty
> radical. The car has a display for the mpg over 5 minute intervals,
> so the first winter we had it I decided to see just how much the
> heater would drag it down. I opened the windows and turned the heater
> on full while driving in town. The previous five minute bars had been
> something like 40 or 45, while with the heater on full it dropped to
> 25! Even at 75 mph with the A/C on full and five adults in the car it
> never went below 30 in real life.
> Mike

Amazing. That's not something they advertise.......

"L Alpert" > wrote in message
...
> Michael Pardee wrote:
>> The effect on the amount of time the engine runs [in the Toyota Prius]
>> can be pretty
>> radical. The car has a display for the mpg over 5 minute intervals,
>> so the first winter we had it I decided to see just how much the
>> heater would drag it down. I opened the windows and turned the heater
>> on full while driving in town. The previous five minute bars had been
>> something like 40 or 45, while with the heater on full it dropped to
>> 25! Even at 75 mph with the A/C on full and five adults in the car it
>> never went below 30 in real life.
>> Mike
>
> Amazing. That's not something they advertise.......
>
Nope - and I wouldn't if I were them, either. I think it will be
increasingly common as we see more efficient cars, and especially if
electric cars make inroads into everyday use. We all know it takes a lot of
energy to make heat but in cars we assume there's always an excess of heat.

Mike

"Michael Pardee" > wrote in message
.. .
> "L Alpert" > wrote in message
> ...
> > Michael Pardee wrote:
> >> The effect on the amount of time the engine runs [in the Toyota Prius]
> >> can be pretty
> >> radical. The car has a display for the mpg over 5 minute intervals,
> >> so the first winter we had it I decided to see just how much the
> >> heater would drag it down. I opened the windows and turned the heater
> >> on full while driving in town. The previous five minute bars had been
> >> something like 40 or 45, while with the heater on full it dropped to
> >> 25! Even at 75 mph with the A/C on full and five adults in the car it
> >> never went below 30 in real life.
> >> Mike
> >
> > Amazing. That's not something they advertise.......
> >
> Nope - and I wouldn't if I were them, either. I think it will be
> increasingly common as we see more efficient cars, and especially if
> electric cars make inroads into everyday use. We all know it takes
> a lot of energy to make heat but in cars we assume there's always
> an excess of heat.
>
The old air-cooled Bugs didn't have an excess of heat either, so one option
was a gasoline fueled cab heater. That doesn't sound real economical, but
I'll bet it's more economical than having to fire up the engine to get heat.
It doesn't sound real safe either, but if you maintained it it wasn't too
bad.

Earle

After reading through the posts I keep missing the key point...


A/C is more efficient on longer trips when you "DRIVE TO COMFORT". I
worked for Parker Hannifin for 7 years designing Thermal eXpansion
Valves (TXV's) for Auto/Truck use. I started in the research lab and
eventually ended up supporting road trips where manufacturers would
take their soon to be model to the hottest place know to god and test
the A/C. I miss doing that...

Did you notice in the Mythbusters "test" that they had their jacket
on???? I't called drive to comfort. Once you're confortable, you turn
the A/C down. Once the A/C is turned down and the clutch on the
compressor starts to cycle... BANG, there's your savings over driving
with the windows down on a longer (10+ miles) trip.

PS: I drive with the A/C on ALL the time, 1 mile or more... hehe


--
mikeyb420
------------------------------------------------------------------------
mikeyb420's Profile: http://www.automotiveforums.com/vbul...?userid=246171
View this thread: http://www.automotiveforums.com/vbul...d.php?t=616264

http://www.automotiveforums.com

On Thu, 28 Jun 2007 19:55:13 -0700, mikeyb420
> wrote:

>
>After reading through the posts I keep missing the key point...
>
>
>A/C is more efficient on longer trips when you "DRIVE TO COMFORT". I
>worked for Parker Hannifin for 7 years designing Thermal eXpansion
>Valves (TXV's) for Auto/Truck use. I started in the research lab and
>eventually ended up supporting road trips where manufacturers would
>take their soon to be model to the hottest place know to god and test
>the A/C. I miss doing that...
>
>Did you notice in the Mythbusters "test" that they had their jacket
>on???? I't called drive to comfort. Once you're confortable, you turn
>the A/C down. Once the A/C is turned down and the clutch on the
>compressor starts to cycle... BANG, there's your savings over driving
>with the windows down on a longer (10+ miles) trip.
>
>PS: I drive with the A/C on ALL the time, 1 mile or more... hehe

Kind of a late reply...

I didn't see the Mythbusters test. My van doesn't cycle the AC - you
are supposed to blend in some heat for comfort. In moderate weather,
I will turn it on and off manually rather than blending in heat. In
really hot weather I run it full time for comfort so I don't think an
automatic control would save much there.

According to my results, rolling down the window only costs about 2%
on fuel mileage, so the AC would have to get down to a 33% duty cycle
to break even. Perhaps newer cars are more efficient on AC.