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

I think the discrepancy comes from working with the reciprocal of what we
really want to measure: fuel per increment. For example, suppose it requires
5 gallons of gas to move the car 100 miles with the windows rolled up and
A/C off. That is 20 mpg. If the window drag at some speed consumes 0.2
gallons in 100 miles at that speed the economy drops to 19.2 mpg for a loss
of nearly 5%. If the drag is the same in a vehicle that requires 2 gallons
to move the car 100 miles (50 mpg) the window drag drops it to 45.5 mpg, a
nearly 10% hit. I'm not a big fan of the "liters per 100km" measurement but
it works a lot better than mpg here.

So, here's an example of how it goes - theoretically - with the window and
A/C. Assume (for illustration) that at 50 mph the window drag consumes 0.1
gallon per hundred miles. Similarly, assume the A/C consumes 0.1 gallon per
hour. For our hundred mile trip that would mean the windows would use 0.1
gallon and the A/C would use 0.2 gallons at 50 mph. If we increase the speed
to 100 mph the window drag, increasing with the square of the speed, becomes
0.4 gallons for the one hour the trip takes while the A/C loss drops to 0.1
gallon for the hour instead of 0.2 gallons for two hours.

For different vehicles the numbers would change; the A/C losses are higher
for a larger vehicle with more glass and the window drag will certainly vary
with body style. In the end, all that will change is the speed at which the
A/C is more economical than windows down.

 

My bad on the dates. 7.2% loss between air on and air off with windows
closed, with almost identical standard deviations with decent sample
sizes.

It looks like with windows partially open is just about the same as them
closed (.4% difference probably insignificant due to standard test
error) until you get to the 6" opening in the rear (though there is only
1 sample), which is just about the same as with them wide open (again,
small sample size). 6" to wide open the same as with the air on and
closed.

Probably could use more "open window" numbers just to verify, but it
makes sense if you look at some simple HVAC calculations for orifice
flow....

A 14" diameter that has about 154 sq/in of area (semi close guess to a
car window, I haven't actually measured one) with a round edge discharge
co-efficient has a flow rate of 31K CFM at 1 PSI, 70k CFM at 10 psi and
almost 100K CFM at 20 psi (sharp edge drops numbers by 40%).
I'm not sure about the effects of the angle of the flow on the opening
and I haven't measured the air pressure entering a car through an open
window moving at 70 mph, but I'm sure it would be quite high (and then
multiple the CFM x 4!) That is a lot of drag......

I'll take the 7.2% loss without the wind howling in my ears!

Thanks for the clarifications. Good real world application.

 

My bad on the dates. 7.2% loss between air on and air off with windows
closed, with almost identical standard deviations with decent sample
sizes.

It looks like with windows partially open is just about the same as them
closed (.4% difference probably insignificant due to standard test
error) until you get to the 6" opening in the rear (though there is only
1 sample), which is just about the same as with them wide open (again,
small sample size). 6" to wide open the same as with the air on and
closed.

Probably could use more "open window" numbers just to verify, but it
makes sense if you look at some simple HVAC calculations for orifice
flow....

A 14" diameter that has about 154 sq/in of area (semi close guess to a
car window, I haven't actually measured one) with a round edge discharge
co-efficient has a flow rate of 31K CFM at 1 PSI, 70k CFM at 10 psi and
almost 100K CFM at 20 psi (sharp edge drops numbers by 40%).
I'm not sure about the effects of the angle of the flow on the opening
and I haven't measured the air pressure entering a car through an open
window moving at 70 mph, but I'm sure it would be quite high (and then
multiple the CFM x 4!) That is a lot of drag......

I'll take the 7.2% loss without the wind howling in my ears!

Thanks for the clarifications. Good real world application.

 

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.....).

 

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.....).

 

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.

 

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.

 

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

 

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

 

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

 

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

 

Amen.

 

Amen.

 

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

 

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

 

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

 

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

 

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