How does the "auto" setting on my car a/c work?
August 5, 2012 8:29 AM   Subscribe

Does the "Auto" setting on the air conditioner in my car work the same way as it does in my house?

I have a 2010 Nissan Altima Hybrid, and it has an "Auto" setting for the cabin temperature. I can turn a dial to 76 degrees and the cabin will stay at that temperature. When you first get into the car, the air blows hard and cold, and as the cabin temperature comes down, the force of the air is reduced, and it seems like it blows slightly warmer air, to maintain whatever temperature you set. It works great.

In my house, if I set a temperature of 76, the A/C unit will kick on when the house warms up past 76 and stay on until the house cools down to below 76. The fan and the compressor are both off, and then they're both on, and then they're both off again. I can understand this.

In the car, however, the air constantly blows. Sometimes it blows full blast, sometimes at the lowest setting, but the fan is running. This is nice. My question: Does the auto setting regulate temperature by cycling the compressor on and off, or does it just add a little hot air to the mix?

If it cycles the compressor on and off, I figure that's the more efficient method of temperature regulation. If it leaves the compressor on, but just varies the fan speed and the mix of hot/cold air, I figure that's the worst way to do it, because the compressor is constantly running. If that's the case, I should do what I've done in other cars and just regulate manually by turning the compressor on when I'm warm, and turning it off once I'm comfortable, and letting the fan run constantly.

Can anyone here tell me how it works?
posted by spikeleemajortomdickandharryconnickjrmints to Technology (14 answers total)
 
It cycles the compressor. It doesn't make sense to do it any other way.
posted by Brockles at 8:31 AM on August 5, 2012


Response by poster: I was hoping so. I should add that I can feel the compressor kick on/off in every other car I've driven, but not in this one, so that's why it's hard for me to verify that.
posted by spikeleemajortomdickandharryconnickjrmints at 8:32 AM on August 5, 2012


Note that this is probably not actually the way the auto setting on your house works.
posted by fleacircus at 9:41 AM on August 5, 2012


Really? Unless I am misunderstanding in what way you think it is different, that's precisely how every domestic and commercial air conditioning system I have experienced works. I mean, it stops at the set temperature, rather than goes below it, but the basic principle is the same.
posted by Brockles at 10:08 AM on August 5, 2012


The compressor is going to be a huge power drain on your engine, so I assume any decent design will allow a lot of compressor downtime. It would make very little sense to run the A/C compressor constantly.

I assume the speed of the fan blower is altered (unlike your house) and operates in a threshold as a function of the difference in current air temperature and the set A/C temperature.

A home A/C will usually have the fan blower ON or AUTO (where it will be activated simultaneously with your compressor), and secondary to this will be your temperature setting.
posted by fieldcannotbeblank at 11:19 AM on August 5, 2012


Hybrid cars sometimes have air conditioning systems that run a different way from normal cars. I have no idea if this is an issue in your model of hybrid.

(I'm a mechanical ignoramus, but this post from an auto AC site describes the difference like so: "Some hybrid vehicles use air conditioning compressors that are operated by an electric motor instead of a drive belt. The motor’s insulated windings are immersed in the compressor’s lubricating oil" and thus when servicing the system you must be sure to use the correct type of oil or you can damage the system including causing premature shutdown of the compressor.)
posted by LobsterMitten at 12:22 PM on August 5, 2012 [1 favorite]


It looks like the 2010 Altima Hybrid does use an electric-motor-driven AC compressor.
posted by LobsterMitten at 12:27 PM on August 5, 2012


In a car, there's no reason to avoid cooling the air with the condensor, and then heating it right back up again with the heater. The energy to heat the air is free (it's a byproduct of the internal combustion engine). So the A/C system maintains the condensor temperature at a certain point by cycling the compressor on and off, and then the larger HVAC system manipulates that air to maintain whatever set point you've got by using the variable speed blower and the heater.

In a house, the fan has one speed: on. Further, heating isn't free. Ergo, it makes the most sense to run the house blower when air needs to be cycled, and turn the compressor on and off in order to maintain the correct condensor temperature. Since the blower in a house is moving so much more air, there is less risk of freezing up the condensor, so the net effect is that the house A/C just turns on or off.
posted by TheNewWazoo at 1:51 PM on August 5, 2012


In a car, there's no reason to avoid cooling the air with the condensor, and then heating it right back up again with the heater.

It feels like we're all groping in the dark here (surely there must be an automotive engineer who does interior / HVAC around). However, on this point, my suspicion is that this is exactly what happens anyway - the condensors run continuously to dehumidify the air whether you're applying any cooling or not.

The combined humidity output of 5 people in a small enclosed place means that if you don't run the condensors you will rapidly fog up the interior glass (hot, humid air inside hitting cold glass)

From personal experience, the A/C draws power from the engine (noticable fuel consumption difference) regardless of whether it's in a cooling or heating cycle. The only way to get "free" A/C is to turn the A/C unit off and operate it in passive mode - some cars have this - where if you set the temperature higher than ambient, it will passively run hot air from the engine into the cabin, and if you set temperature lower than ambient, it will run outside air into the cabin, assuming your outside air is cooler than cabin. But running it in this mode can fog up the interior, while if you run the A/C you basically never have this happen.
posted by xdvesper at 7:39 PM on August 5, 2012


In a typical car, the A/C compressor is either off or is driven by the engine. The speed is therefore connected to the engine speed. To keep the car cool even at idle on a hot day, the compressor needs to be large enough to for that situation which means that it's larger than it needs to be in every other situation (IE, the car is moving and/or the engine is turning at a higher RPM than idle). So the auto-A/C function will use a combination of cycling the A/C compressor on and off (since the engine controls the relative speed), rising/lowering fan speed, and re-heating the air slightly with the heater. As an aside, my Altima (a 2005 3.5SE and not a hybrid) uses the A/C even in the winter when it acts as a de-humidifier to help keep the windows from fogging up. I sold Nissan's for a living around 2005 so I can confirm that this was common on all Nissan's made around that time and I "think" it's a common thing on most cars now.

By using an electrically driven A/C compressor it can be much smaller (and therefore has a smaller impact on fuel efficiency). The engine can be at idle but the A/C compressor can still be at it's max RPM since the engine doesn't drive it mechanically, they can run at different relative speeds. Heck, the engine can and does turn off completely but the A/C compressor keeps going. It also means that the A/C system has much finer control over the cooling output of the compressor so it's almost certainly just adding that control to it's bag of tricks.

Eventually, most cars should switch to electrically driven A/C compressors but it requires higher voltage electrical systems that carry some other complications. Hybrids need to use the higher voltage system for the electrical motors anyways so it makes more sense to make the A/C compressor electrically driven vs. belt driven.

I would bet that some home HVAC systems work the same way. I know that some higher end systems have variable fan speed and I would bet that some have use variable compressor speeds too if it would increase efficiency.

I found a decent PDF with some info comparing belt driven to electrically driven compressors here. It seems to be geared towards those who might have to do repairs/maintenance on those systems but there is still some useful/interesting info there.

This paper [PDF] implies that the electrically driven compressors (or at least the most efficient ones) are variable speed, "The optimum compressor speeds were discovered to be much slower (i.e., 700 rpm) than typical mechanically-driven compressor systems, allowing the compressor to operate at higher efficiency regimes."

I'm not a mechanic or an engineer or anything but I'm pretty sure I'm on the right track.
posted by VTX at 9:30 AM on August 6, 2012


In a car, there's no reason to avoid cooling the air with the condensor, and then heating it right back up again with the heater.

Yes there is: efficiency. It affects fuel economy and also will affect the life of the AC pump, which is in a pretty harsh enough environment as it is without running constantly. It will run and provide dehumidification (and so runs more at the start of a journey even when in temperate climates) but it will definitely switch in and out.

It feels like we're all groping in the dark here

No we're not. The condensor does not run all the time, be it belt or electrically driven. There is no mystery. Run air conditioning on a belt driven AC car, open the bonnet and watch the engine. You will see and hear a kick and a change in tone (and idle speed momentarily) when the AC compressor kicks in and again when it turns off.

With an electric AC pump the draw from starting the condensor does not directly affect the engine (the 'surge' is damped by the battery but alternator load will increase slightly as a result). So it still kicks in and out, you just don't feel it. You may hear it if you have your head under the bonnet, but are less likely to in other situations - particularly from inside the vehicle.
posted by Brockles at 9:31 AM on August 6, 2012 [2 favorites]


Yes there is: efficiency. It affects fuel economy and also will affect the life of the AC pump, which is in a pretty harsh enough environment as it is without running constantly. It will run and provide dehumidification (and so runs more at the start of a journey even when in temperate climates) but it will definitely switch in and out.
This is true, but every climate control system I've seen requires the user to set the A/C to operating or not (independent of compressor operation). I've not seen one that will turn the A/C system on automatically. The question of efficiency is therefore decided by the user, not the HVAC, and re-heating the air is effectively free.
posted by TheNewWazoo at 12:13 PM on August 6, 2012


The question of efficiency is therefore decided by the user, not the HVAC,

No, the option for additional (maximum) fuel efficiency (over and above the system itself) by allowing the user to disable the AC entirely is available. This doesn't mean efficiency starts and stops with the user. The AC switch is to disable it, and the AC (while operating) efficiency method is to cycle the compressor on and off. The efficiency of the HVAC system itself (and so in turn its drain on the efficiency of the car as a system) is the reason that the pump doesn't run continuously.

and re-heating the air is effectively free.

I'm getting confused as to what point you are making here, to be honest. When the car is cooling the interior, reheating anything is not only not free in energy terms (because you're adding hot air to a system you are trying to cool) but costs you anyway - you have to cool the air down again later. This is why the fan modulates speed and the air con compressor switches on and off to control how much and how cold the air entering the cabin will be. The compressor runs more for maximum cooling and less for minimum cooling.
posted by Brockles at 1:26 PM on August 6, 2012


Okay, so firstly, I've been using the wrong word. I've been intending to talk about the evaporator, not the condensor. Not quite sure why.

Except for details, I think we're more or less in agreement. My main point is that the target temperature of the evaporator (and therefore cooled air temperature for a given blower speed) isn't adjusted by the HVAC computer - it's set, and the compressor is cycled on and off to keep it around that set point, no matter what the desired outlet air temperature. Your statement "The efficiency of the HVAC system itself ... is the reason that the pump doesn't run continuously" isn't first-order correct - the reason the pump doesn't run continuously is to avoid freezing the evaporator up. There are other inputs like throttle position and RPM, obviously, for other reasons but once the A/C is operating, it's on - the air is being cooled (and dried) no matter what the set point is.

The primary difference between a car and home A/C system is that the home system modulates the compressor and the fan in sync, and in direct relation to the set point and current temperature. If the fan is blowing, the compressor is on, and because the interior temperature is higher than the set point. In a car, the compressor operation is independent of the fan or set point. The car HVAC system modulates the A/C to maintain proper evaporator temperature, and the computer manipulates the air flow (fan, vents, heating, routing, etc) to maintain cabin temperature.
posted by TheNewWazoo at 2:50 PM on August 6, 2012


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