When turning the A/C on at idle I notice a rise in RPM's, however when driving along at 65mph I can turn the A/C and the tachometer will not move at all, the same thing goes for turning the A/C off while at speed. This leads me to believe that the only time the A/C affects gas mileage is when sitting at idle at a stop light, parked with A/C on, etc. Anyone know if this is true or not?

yes

when the clutch on the ac compressor locks in the pulley is no longer free wheeling, it def robs some power (or more like takes power to turn)
the rpm's only move at idle because the computer adjsuts for the added drag you just put on the serpentine system.

Any pulley on that belt will use some potential power, if that pulley is used to pump or generate it will take even a bit more.

At the lower RPM not enough energy is stored in the flywheel. When the compressor clutch kicks on, the PCM senses this and ups the rpm at the idle speeds. At higher rpm there is sufficient inertia that the rpms don't noticably drop on the tach. At WOT the compressor clutch opens.

As 410 says, yes, any accessory extracting power from the engine is preventing that power from reaching the drive train.

At the lower RPM not enough energy is stored in the flywheel.

WTF? Flywheel has zero to do with the ac compressor, they are on opposite side of the engine. Flywheel doesn't store energy either.

I loose 1.5-2 MPG with the ac on.

When I turn the ac on at idle, the RPM's will rise slightly and then drop back down to normal.

I loose 1.5-2 MPG with the ac on.

I loose no MPG with the AC on during highway driving.

Flywheel doesn't store energy either.



Sure about that??

A flywheel is a heavy rotating disk used as a storage device for kinetic energy. They come as an alternative energy storage device. Flywheels resist changes in their rotational speed, which helps steady the rotation of the shaft when an uneven torque is exerted on it by its power source such as a piston-based, (reciprocating) engine, or when the load placed on it is intermittent (such as a piston-based pump). Flywheels can also be used by small motors to store up energy over a long period of time and then release it over a shorter period of time, temporarily magnifying its power output for that brief period. Recently, flywheels have become the subject of extensive research as power storage devices; see flywheel energy storage.

A momentum wheel is a type of flywheel useful in satellite pointing operations, in which the flywheels are used to point the satellite's instruments in the correct directions without the use of thrusters.

The kinetic energy stored in a rotating flywheel is

E = \frac{1}{2} I \omega^2

where I is the moment of inertia of the mass about the center of rotation and ω (omega) is the angular velocity in radian units. A flywheel is more effective when its inertia is larger, as when its mass is located farther from the center of rotation either due to a more massive rim or due to a larger diameter. Note the similarity of the above formula to the kinetic energy formula E = mv2/2, where linear velocity v is comparable to the rotational velocity, and the mass is comparable to the rotational inertia.

The flywheel has been used since ancient times, the most common traditional example being the potter's wheel. In the Industrial Revolution, James Watt contributed to the development of the flywheel in the steam engine, and his contemporary James Pickard used a flywheel combined with a crank to transform reciprocating into rotary motion.

I loose no MPG with the AC on during highway driving.
Thats what I assumed to be true. It only affects idle consumption.

Thats what I assumed to be true. It only affects idle consumption.

I don't believe that's possible. The A/C compressor has two modes. One with the clutch disengaged and one with it engaged. There is NO WAY that the compressor requires the same HP to drive an engaged compressor as it does to turn the freewheeling pulley on the end of the compressor. The engine will use more fuel and you will have less available HP at the rear wheels with the compressor simple physics.

I stand corrected. The flywheel does store some energy. It is more of a issue on smaller engines. The flywheel has stored enough energy at low rpm's though. If it did not, the engine might possibly stall when you turned the compressor on at idle.

And yes you will lose MPG with the ac on at highway speeds. It is less than around town driving, but it is still at least 1 mpg.

And yes you will lose MPG with the ac on at highway speeds. It is less than around town driving, but it is still at least 1 mpg.

I beg to differ. I have tested this many times.

Physics fail to apply to your vehicle Al?

study has shown it does affect MPG but it's very minimal (as long as there are no other variables).. windows down cause an aerodynamic drag that's almost worse than having a/c on..

I have read a study somewhere that showed the aerodynamic drag by windows down was a greater energy robber than AC with the windows up. I'll see if I can find it... but I swear I read that somewhere. Doesn't make it TRUE, but it looked legit to me.

Mythbusters tried it and the vehicle with windows down got better economy than the vehicle with the A/C on and windows up. Their test used identical vehicles with the fuel line attached to a graduated cylinder filled with fuel. Others have mentioned that Mythbusters tested at 40mph and that the drag from rolled down windows wouldn't become apparent until 50mph or greater.

However if you are just testing the parasitic effect of the A/C compressor at highway speed you would need to run the no A/C test with the windows rolled up to eliminate that variable.

I've been a pilot for almost 40 years, so have had an interest in things aerodynamic for a long time. I had a college buddy studying to be an Aerodynamic engineer. I learned a lot of neat stuff through him as a result of my interest and his chosen field. I was very interested in parasitic drag and icing potentials related thereto. One thing I remember learning was that parasitic drag does not really start to mean much until a certain airspeed is reached, and it increases exponentially thereafter (well maybe not exponentially but not linearly). 40 MPH is not a fair test of parasitic drag from windows down on a car IMHO. I think what I read was a test done at freeway speeds. Around town I think there is no doubt AC uses more fuel than windows down. In a 500 mile interstate trip... I doubt it does. But then, what the h*ll do *I* know.

ps. it was 102 and muggy in Sacramento today. I drove around town. Wanna guess which I chose? AC or windows down?

DING DING DING we have a winner!

I have read a study somewhere that showed the aerodynamic drag by windows down was a greater energy robber than AC with the windows up. I'll see if I can find it... but I swear I read that somewhere. Doesn't make it TRUE, but it looked legit to me.

This is true; I have tested it. However, only at 70 MPH in my testing.

However if you are just tneedesting the parasitic effect of the A/C compressor at highway speed you would to run the no A/C test with the windows rolled up to eliminate that variable.

That is exactly how I have evaluated it.

One thing I remember learning was that parasitic drag does not really start to mean much until a certain airspeed is reached, and it increases exponentially thereafter (well maybe not exponentially but not linearly).

Glacier991 is correct - drag is nearly linear at low speeds where the Drag Force is =~ -b*v, where b is a constant dependant on the fluid and v is velocity. At higher speeds, it does increase exponentially where drag force = -0.5*p*v^2*A*Cd. The v^2 indicates the exponential effect of speed on drag. I'm not sure of the transition point for an Explorer, but 40 mph definately qualifies for the high drag equation. Stick your hand out the window and this will give you an idea of how drag force changes with speed.

When the AC is on, it has to have a negative impact on gas mileage, otherwise it defies every law of physics. Whether or not one is using equipment sensative enough to detect it is another story. As we all know, the compressor has to do work in order to recompress the fluid. It gets this energy from the engine, therefore some energy that would be used to move the wheels is now going to run the compressor. Unfortunately, you can't get something for nothing - you can only use as much as you generate, no more.

Whether or not it is more efficient to run AC or have the windows open is going to depend on the vehicle drag properties and the evaluation speed. Assuming temperatures and humidity, etc. are all constant, the energy required by the compressor will be constant at all speeds (independent of speed). Drag, on the other hand, is going to increase with speed, so the net impact of having the AC on is going to decrease with speed. That's why you are going to notice a greater affect at lower speeds than highway speeds. Either way, I think most people would rather be comfortable than slightly more efficient.

When the AC is on, it has to have a negative impact on gas mileage, otherwise it defies every law of physics. Whether or not one is using equipment sensative enough to detect it is another story

Your theory is sound. The ability to measure minute changes in gas mileage would require some elaborate testing equipment. In all of my testing by the usual method of calculating gas mileage, I have not observed any difference. I have made many long 300-500 mile highway runs testing this.

Whether or not it is more efficient to run AC or have the windows open is going to depend on the vehicle drag properties and the evaluation speed.

Have you actually tested this?

Physics fail to apply to your vehicle Al?

No, however real world testing does.

Your theory is sound. The ability to measure minute changes in gas mileage would require some elaborate testing equipment. In all of my testing by the usual method of calculating gas mileage, I have not observed any difference. I have made many long 300-500 mile highway runs testing this.

That's my point.

Have you actually tested this?

Not the effect of AC vs. windows down. Yes, I have studied and tested the effects of velocity on drag under different conditions. The nice thing about the physics involved is that they apply across a broad range of circumstances.

Long winded as my answer was, the purpose was to explain why one would notice a larger difference at lower speeds, not necessarily to question your real world experience. No intent to offend your findings.

the purpose was to explain why one would notice a larger difference at lower speeds, not necessarily to question your real world experience. No intent to offend your findings.

None taken. I apprecialt your intellegent input.

Al, I told you you should get an independent verification of your incredible fuel economy feats. Since you have what appears to be a one of a kind vehicle I think the auto manufacturers would love to find out why you get approx 10 MPG more than others with the same vehicle.... Now a couple MPG may be attributed to driving style, but 10MPG?

Even EPA test cycles which do not use A/C, and average 48 MPH with a max speed of 60 MPH can't come close to your 30+ MPG claims.

Sorry to be such a PITA, but claims like yours need to be verified.

Al, I told you you should get an independent verification of your incredible fuel economy feats. Since you have what appears to be a one of a kind vehicle I think the auto manufacturers would love to find out why you get approx 10 MPG more than others with the same vehicle.... Now a couple MPG may be attributed to driving style, but 10MPG?

Even EPA test cycles which do not use A/C, and average 48 MPH with a max speed of 60 MPH can't come close to your 30+ MPG claims.

Sorry to be such a PITA, but claims like yours need to be verified.

Thank you for your opinion.

No problem... so when are we going to have the EF fuel economy proof in the pudding run?

Holey Moley! Ask a simple question....

On a stock 1st gen explorer you will loose MPG with the AC on & at HW speeds. I've had 2 & I've tested them on 1200+ mile road trips.
You are talking about a 12+ year old vehical. A 160HP one that weighs 4K lbs. AC increases the engine load, now you need to compensate by more gas, more gas = less MPG. Anything that old you can't compare to newer cars, or freaks of nature like aldive's X.
It does depend on the car. If you have 300hp the AC isn't going to make a difference. My acura that has 300hp gets the same milage with or without the ac. I've had other new cars that didn't see a difference & some that did.

No, however real world testing does.

Real world testing fails to apply to your vehicle?

May wanna reword that.

But I will take real world physics over your testing, sorry to say. Would it be fair to say that the difference in consumption with ac on/ off is small enough to have slipped through your calculations.

But I will take real world physics over your testing, sorry to say. Would it be fair to say that the difference in consumption with ac on/ off is small enough to have slipped through your calculations.

I already posted that the measuring w/o elaborate testing instruments might not be able to discern a difference. See post # 19.

Your theory is sound. The ability to measure minute changes in gas mileage would require some elaborate testing equipment. In all of my testing by the usual method of calculating gas mileage, I have not observed any difference. I have made many long 300-500 mile highway runs testing this.
Have you actually tested this?

I thought about this more and re-read my original post - my post was intended to support your findings (to some extent), but may not have been as clear as I originally thought so let me try again...

The the 3 possibilities being discussed
(1) windows up w/out AC
(2) windows down w/out AC
(3) windows up with AC

Option (1) will always be the most efficient because drag is minimized and the compressor is not drawing any energy from the the engine. I don't think anyone is arguing this point.

On to the bone of contention: which is more efficient (2) or (3)? It depends on speed. Remember, drag is exponentially increased as speed increases so the faster you go the more drag you experience and the more engine work you lose to drag. Opening windows increases drag - I hope everyone can agree on this so I won't go into it. Suffice to say that the increase in noise when the windows are down is an indicator that you are losing energy to drag.

Unlike drag, the energy required by the compressor is (for all intents) constant, unaffected by speed. At some speed, the energy required to run the compressor is the same as the energy lost by the increased drag of the open windows (remember drag is increasing and compressor energy is staying the same). At this speed, neither open windows nor AC are any more efficient than the other. As speed increases beyond this point, drag causes more wasted energy than the compressor and the AC (3) becomes more efficient. At what speed this situation occurs is completely dependant on the car, and will change from car to car - ie, there is less drag on a Formula 1 race car at 70 mph than there is on the Explorer (pretty close to the drag characteristics of a 4x8 sheet of plywood) ;) . That being said, Al may have found a speed (70 mph) at which the compressor "drag" is less than or equal to the window "drag." To compare his results to those of a '93 Explorer are like comparing apples to oranges - different cars, different drag coefficient. If one is waxed, that too, affects the drag and would yield different results.

I'm very surprised at the conclusions that have been drawn by some of these high profile tests (specifically, Consumer Reports and Mythbusters). I like both of them, so I was really surprised that neither of them made this distinction and generalized their findings across all cars at all speeds. Their findings will only pertain to the car they tested, at the speed they tested. I hope this explanation makes that apparent. If one knew the actual energy required to run the compressor (can be calculated, knowing the efficiency of the compressor, etc) and the drag of the particular vehicle, one could determine the speed at which the two are equal.

Hope this makes sense. Great discussion.

When turning the A/C on at idle I notice a rise in RPM's, however when driving along at 65mph I can turn the A/C and the tachometer will not move at all, the same thing goes for turning the A/C off while at speed. This leads me to believe that the only time the A/C affects gas mileage is when sitting at idle at a stop light, parked with A/C on, etc. Anyone know if this is true or not?

Not derail the thread by returning to the original question ...

I think uh60james was speculating that because the tach dropped when the A/C was at idle and the A/C clutch engaged, and it did not appear to change when the when the engine was at a higher speed and the A/C clutch engaged, then the A/C engaging only affected gas milage at idle.
At idle, my car dips 200 rpm at idle before coming back up.
I believe that the stored engine energy in the flywheel, drivetrain, accessories, etc; at higher (2000) speeds prevent the compressor load from showing on the tach. The load on the engine is the same.
Not being a racing person, do race cars have A/C, other than the ones with open cockpits of course?

This leads me to believe that the only time the A/C affects gas mileage is when sitting at idle at a stop light, parked with A/C on, etc. Anyone know if this is true or not?

To answer this question directly: Having the air conditioner on will always be less efficient than having it off, at any speed, at any time of the day, any day of the year, any temperature (assuming the AC system is working) ;) . No matter what, the AC system requires energy to operate and hence requires gas to run it - indirectly but true none the less.

As was said before, the relative drain on the system at idle is larger than at highway speed, that's why it is more noticable (tach moves). At highway speeds there is enough energy being produced/stored where the drain from the AC is not really noticeable.

At highway speeds there is enough energy being produced/stored where the drain from the AC is not really noticeable.
On a low HP, high weight car you will notice it.

This leads me to believe that the only time the A/C affects gas mileage is when sitting at idle at a stop light, parked with A/C on, etc. Anyone know if this is true or not?

This just shows than none of have read your thread closely. Your gas mileage when sitting at a stop light or parked is 0.0 mpg, no matter what is turned on or off, including the engine :) . Problem solved.

On a low HP, high weight car you will notice it.
Point taken. I guess it will always be "noticeable", it just depends on how closely and what instruments you are using for your observation.