OK, you’re in the car, and the air-conditioning/heating dial goes from a frigid blue on the left to a blistering red on the right. This controls how hot-cold the air comes out.
The other controls are the fan (how powerful the blow) and the location (top vents, feet vents, windshield, combos).
You turn the A/C on. Sometimes it’s just too cold on the lowest fan setting with the hot-cold dial all the way to the left (blue/cold).
If I move that dial a little toward the right, what happens? Does the A/C system produce less cold? Or does it mix hot air into the cool air, thus wasting the energy used to produce the cold air?
After all, if you turn the dial into the red, you are now using the heater, even if the A/C button is engaged.
On most cars, they just mix warm air with the cold, so you end up wasting some energy. Cars with “climate control” (where you set the desired temperature) will turn the AC on and off as needed to regulate the temperature and won’t be quite as inefficient.
On most cars, if you turn the AC on in the winter with the defroster on and turn the dial up to heat, it will defrost your windows better than if you had the AC off. Some cars will automatically turn on the AC while the defroster is on because of this.
The air loses some water due to condensation on the evaporator coil of the AC. After that, the air is heated, which increases the amount of water that can be absorbed.
When did the manufacturers start doing the air-mixing thing? Way back when AC was introduced the unit had an electrically operated clutch on the compressor that turned the AC on or off to regulate the temperature.
I’ve had cars from the 60’s that did the mixing. Even the one I had without A/C could mix the outside air with heated air to regulate the temperature of the heat and defroster. The compressor did cycle on and off a lot, but that was due to various pressure switches on the system I rebuilt.
No! Bad-bad-bad! Similarly for cars with climate control?
Have you ever driven a car hard on a hot day, and warm air suddenly started coming out of the vents until you let off on the accelerator? If not, just take my word for it that it’s truly annoying, and screams “low quality.”
A/C compressors don’t consume too much power relative to total engine capacity, and comfort for most people is more important than an insignificant drop in efficiency, especially in cars that are equipped with climate control (i.e., not your little Cobalt).
Mixing air once your desired temperature is reached is far, far more common than cycling the compressor on an off. On manual systems, manually mixing the air is a lot more comfortable than constantly turning the compressor on an off.
But you know what? If I’m using the A/C in my car with the manual system, I find it very, very difficult to mix the air, too. Yeah, despite everything I said above, it just bugs me think that I’ve got the compressor running but I have to mix warm air into the cold! So there’s obviously something more at work here than simple knowledge!
The A/C compressor is driven by the car endine, so it’s speed and cooling capacity is directly related to engine rpm, although it is strictly an on/off control. Variable displacement compressors were used at one time, but I haven’t seen one in a very long time. On all modern automotive A/C systems that I am aware of, the temperature control dial is either a simple linkage to the heating water control valve or an electronic thermostat which accomplishes the same thing by automatically controlling the heating valve to achieve a specific discharge air temperature.
to Balthisar, the A/C cuts out at high rpms (around 4500 in my car) not to save engine power, but to save the compressor from excessive head pressure.
Well no. The older systems ran the compressor until the cooled air out of the unit reached a certain temperature. When you first start up with the temperature control set a certain level the warm air in the car takes some time to cool down to the set temperature. At that point the AC clutch disengaged so the compressor stopped which stopped the refrigeration. The AC air-circulating fan kept running and the air in the car gradually warmed up. At some temperature maybe 3 degrees above the set temperature the clutch reengaged the compressor and the AC cooled the air down again. This is the way most home ACs work except the power to the compressor motor is shut off by the thermostat which stops the compressor. This isn’t possible with an auto AC because the compressor is run from the engine, as you said, so they had to use the clutch to start and stop the compressor.
I agree that the slight loss in fuel economy by running the AC full time and mixing in warm air isn’t worth worrying about, at least as long as fuel isn’t too expensive, but I just wondered when the method changed.
Yeah, that’s what I referred to about hard driving, above. It’s really dependant upon the system. Newer systems are higher pressure systems (the coolant has a lot to do with it), and the higher pressure systems are more efficient, and so on. The ambient temperature and driving style (the high RPM’s) can cause an over pressure in the system, and then you have an expensive repair. On some cars, the compressor switches off at a certain RPM. On others, according to the actual pressure, which is roughly tied to the RPM’s. In either case, the compressor won’t just switch back on until some type of equilibrium is achieved, otherwise it’ll be switching on and off every time your car switches gears during a hard accelleration.
The “low quality” feel I mentioned can be mitigated if there’s enough metal and coolant to serve as a buffer until you let up on the pedal. In order to not feel like a “cheap” system you should get cold air while you’re flooring it from a dead stop until you’re at a reasonable speed – say 80mph. If it’s a Corvette, you don’t need a lot of buffer; you’ll let up on the gas pedal soon enough. If you’re in a PT-Cruiser or something, it’ll take forever to get up to 80 – you’ll feel the hot air coming out of the vents until you let up on the gas pedal.
My '85 van had both the evaporator and heater core within one box with a “blend door” controlled by the dash temperature control cable to regulate temp in those modes where the AC compressor was energized.
::: Sigh:::
OK let’s get a few things straight.
All auto A/C systems have to have some type of temp control for the passenger compartment. Otherwise unless you were in Arizona during Aug you would complain that the system got too cold. They all have to also have safety cutouts for low temp/low pressure and high temp/high pressure.
On every system I am aware of, when you move the temp lever inside the car you are blending some warm air with the cold to arrive at the correct discharge temp. Either you are opening a water valve, or cracking a flap to draw some hot air into the cold air stream. Or both. This is just like when you go to take a shower at home, you have to mix some cold water in with the hot, so you don’t get scalded.
Now let’s talk about the various safety systems that are in the system.
first the low temp control. Now we are talking about the temp of the evaporator, not the temp of the discharge air, there is a difference.
On old systems from the 60’s and before, the low temp control was done by an expansion valve. As the evaporator warmed up the expansion valve opened, and allowed more Freon into the evaporator to cool it down. As the evaporator got colder, the valve closed down some to limit the amount of Freon in the evaporator. The compressor ran all the time on these systems.
Expansion valve systems are still in use today on many cars. Once again, the opening and closing of the expansion valve does not regulate the temp inside the car. This regulation is done to prevent the evaporator from freezing the condensed water on it into a solid block of ice. (yes this can happen! :eek: )
the other main type of compressor control is to use a fixed orifice and cycle the clutch. In this system, the orfice is fixed, the flow is varied by turning off the compressor when the discharge pressure drops to a preset point (meaning the temp of the evaporator has dropped to the designed lower limit) The compressor then turns off, and the temp of the evaporator rises (along with the pressure) When the pressure reaches the cut in point, the compressor re-engages and the flow through the evaporator resumes and it gets colder.
Again the compressor cycling is not for temp control in the car, but to keep the evaporator from freezing.
As far as high-pressure safety goes, back in the day there was a valve that opened and vented the Freon to the outside air. Now a days there is a high pressure switch that cuts out the compressor until the pressure drops back down.
Engineers, being well, engineers, there are many many variations of these two basic systems. There are systems that have an expansion valve and cycle the clutch also.
Also contrary to what has been posted, variable displacement compressors are not dead and gone. I know of one 06 model car that uses one.
Oh, one more thing, compressor cut out during acceleration is not a factor of quality, but rather performance. If the compressor is cuts out during full throttle acceleration, the car performs better. Volvo has been doing this on our cars since 1989, and I don’t think you should refer to our cars as low quality. Particularly you Balthisar We also knock out the compressor in the event of an engine overheat.
We also knock out the compressor in the event of an engine overheat.