Does the A/C actually cause poorer gas mileage?

[uh60james]

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?

[410Fortune]

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.

[shamaal]

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.

[KPT]

Originally Posted by shamaal

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.

[aldive]

Originally Posted by JL 3x12 Xplorer

I loose 1.5-2 MPG with the ac on.

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

[Rick]

Originally Posted by JL 3x12 Xplorer

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.

[uh60james]

Originally Posted by aldive

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

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

[Rick]

Originally Posted by uh60james

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.

[KPT]

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.

[aldive]

Originally Posted by JL 3x12 Xplorer

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.

[Rick]

Physics fail to apply to your vehicle Al?

[gensic11]

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..

[Glacier991]

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.

[Rick]

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.

[Glacier991]

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!

[aldive]

Originally Posted by Glacier991

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.

[aldive]

Originally Posted by Rick

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.

[Homerexplorer]

Originally Posted by Glacier991

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.

[aldive]

Originally Posted by Homerexplorer

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?

[aldive]

Originally Posted by Rick

Physics fail to apply to your vehicle Al?

No, however real world testing does.