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Open Windows or Air Conditioning?

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#21 · (Edited)
"The a/c has to always be loading the engine when you turn it on, as far as i knew."

Sorry, but obviously you don't know far enough. I'll give ya' some education though. Some terms:

clutch-cycle pressure switch: A pressure activated switch that controls the air-conditioner-compressor clutch action to prevent evaporator icing.

clutch-cycle switch: An electrical switch, pressure or temperature actuated, that cuts off the air conditioning compressor at a predetermined evaporator temperature.

WOT Switch: An electrical switch that cuts off the air conditioning when the engine is at Wide Open Throttle (WOT).

These all affect whether the compressor is engaged or not, resulting in:

clutch-cycle time (total): The time between when an air-conditioner clutch engages and when it disengages, then reengages; a time equal to one on and one off cycle.

So the Compressor will shut off when the evaporator reaches a pre-determined low temperature. And this is affected by how much warm air is flowing over it and into the car, i.e., faster air flow, more heat transfer, the longer the compressor is "on," and the shorter the off period between "on" times.

Here's an activity for ya' After stopping and with the A/C running on a low fan speed, let the car idle, open the hood, and watch the A/C pulley/magnetic clutch for a few minutes. You will see it cycling on and off.
 
#23 · (Edited)
"what difference would it be in mpg highway?"

Depends on the ambient temperature and other things, as no one can even give you a precise answer as to the MPG impact of having it run on MAX COOL. But, as I said, it all depends on the amount of heat you are having the system extract. I would estimate that at the lowest setting the MPG impact is probably 20% of what it is at the highest setting, e.g., if the MPG impact is 3 MPG at Max Cooling, it would probably be around half an MPG at lowest cooling.

But if you were in Florida with 95 degree humid air, the difference would be less, as even at the lowest speed you are already putting a lot of heat into the evaporator, e.g., once you have "maxed out" the system, additional fan speed or temperature mix setting will not impact it a lot, as you are allready at the system's capacity (of course, this also means the turing the fan speed high will not make the car any cooler either, except for the cooling effect/sensation of more moving air over the skin). When it gets really hot and the A/C is maxed-out and cannot "keep up," I find sometimes a speed slower than the highest speed keeps me more comfortable. Why? Well, if it is so hot and humid that the A/C system is maxed-out at fan speed two, then the air coming out of the vents will actually be warmer at higher fan speeds. I find having a directed flow of cooler air go right onto me at these times preferable to having a faster, but warmer, stream of air flowing.
 
#24 ·
CAIS95 said:
that show MYTH BUSTERS on discovery channel tested this....and their results showed almost no difference with either a/c on or windows down...they ran the same car with a full tank of gas around a track at the same speed ...once with the a/c on, and the 2nd time with the windows down and no a/c and they ran out of gas after the same amount of laps.......i do both myself...depends on the damn new orleans humidity....
Click to expand...
Wrong, they got an additional 30 laps with the windows down and the AC off. Their first run was done with an OBD-II scan and estimated fuel economy based on the MAF readings and it indicated the same mileage on both vehicles. Also bear in mind that this was on an Explorer at sub-highway speed, so that could easily equate to 1.5 to 2 gallons of gas saved with the AC off...and at Focus MPG rates that could easily turn into an additional 50-60 miles per tank with the windows down and the AC off.
 
#26 ·
I have no AC so the decision is easy for me. Window always down except for on freeway, and the fan helps a little when it is really hot. During summer when the average is in the 80s I have to suffer a bit, expecially while getting in the car when the temp can climb about 100 degrees
 
#29 ·
"keep your RPM's in the low end of the tach and the AC won't effect your milage"

Well, while it is true that lower RPM can reduce the impact of having the A/C on, it is just because you are limiting the output of the A/C by limiting the output of the A/C compressor. Simply keeping the A/C turned down would have the same effect, as the cycle time of the A/C compressor would increase, effectively giving the same net effect as having a longer cycle time with lower compressor output due to low engine RPM.

Thermodynamics says you can't get something for nothing, and the average HP the compressor requires is directly related to how much heat is extracted from the passenger compartment, the more heat extracted, the more average HP it takes, and the greater the impact on your MPG. Whether this is due to the A/C compressor being "on" 25% of the time with high compressor output, or 90% of the time with low compressor output, the net impact on MPG will be essentially the same.
 
#30 ·
penguin said:
"The a/c has to always be loading the engine when you turn it on, as far as i knew."

= car, i.e., faster air flow, more heat transfer, the longer the compressor is "on," and the shorter the off period between "on" times.

f.
Click to expand...
actually the slower the airflow over the evaporator, the more heat it transfers. That is why if you want to see the absolute coldest your a/c will get, you put fanspeed on the lowest setting at the coldest temp.
 
#31 ·
> actually the slower the airflow over the evaporator, the more heat it transfers.


You are confusing heat with temperature -- they are two different things, sort of like confusing amperage with voltage. With higher air flow the temperature of the air coming out will be higher than on lower speeds, but the heat transferred will actually be larger because of the greater mass of air that the system is putting out. At the highest fan speed the A/C compressor will be "on" a greater percentage of the time.
 
#33 ·
On my Ford Freestyle station wagon, the automatic temperature control always turns on the A/C, summer or winter. I know with that system, the compressor doesn’t run when the outside temperature is below freezing. I’ve never felt any drag on the engine in that car. But our Foci have that tiny little 4 banger under the hood. On my Focus, I still use the A/C most of the time. I only open the windows to go through the drive-thru or to communicate with local law enforcement. I drive about 170 miles a day (mostly highway) and would be deaf in my left ear by now if I kept the windows open all the time. Also, I like it freezing cold all the time. When the Focus is cruising above 65 or 70 MPH there really isn’t any noticeable drag on the engine.
 
#34 ·
> Fuel economy will be the same on or off.


No, that is incorrect. Anything that is taking HP from the engine will hurt fuel economy.

Typically an automotive A/C compressor will take from 4 HP to 8 HP when it is engaged, depending upon the size and type of the compressor.
 
#36 ·
Lysaof said:
At speed on the highway. It doesn't matter the drag on the motor is the same as the drag created by the windows open. Should not be a noticeable difference. Ive read on it a few times.
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You actually have other factors to consider when on the highway, like wind and turbulence caused by other vehicles, which could affect your MPGs with the windows down. Also hills and curves affect the performance of the engine when the A/C is on. You will experience MPG changes depending on those factors form day to day. One day may be better than the next.

Generally on the highway there is little difference on MPGs with running either.
 
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#37 ·
lol 3 year old thread and "penguin" still is where he left off...
 
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