# A/C 101: General principles, and the BMW design



## RKT BMR (Sep 7, 2002)

There have been several threads regarding AC performance and loading impact on the board recently, and a quite lively discussion about the topic in general. As such, I thought it would be helpful to share what I know about these systems to help people better understand the topic, and perhaps clear up some questions/misconceptions people might have. So, here we go...

All cooling systems rely on the basic thermodynamic physics of gases. We've all experienced the effect of releasing a gas under pressure, confined to a small volume, and it's drop in temperature. A butane refill cylinder gets cold when refilling a lighter; scuba tanks get cold if you open them up and let the compressed air shoot out; a balloon, filled to capacity, will feel cold when all the air is allowed to escape unregulated.

This is the basic physics behind refrigeration, and is what drives your AC system. A particular chemical compound is carefully chosen as the "working fluid" to be used for creating this cooling effect so that it's boiling temperature is at just the right place under the various pressures used in the system so that it will condense to a liquid while under high pressure, and when pressure is released will vaporize into a gas, cooling tremendously in the process.

A basic AC system is really quite simple. It has the following components:

A compressor, to pressurize the working fluid to high pressures.
A Condenser, to cause the working fluid to condense into a liquid, under pressure.
An expansion valve, to provide a mechanism to allow the high-pressure liquid to expand into a low-pressure environment thereby vaporizing and cooling off in the process of gas expansion
An evaporator, to transfer the cold conditions of the gaseous working fluid to the cabin air.
And that's pretty much it. The entire system is a close loop as shown in this diagram:










Now, of course, all the complication comes from _controlling_ this process to achieve optimal results. However, the basic principle is rather simple:

The compressor compresses the working fluid (in a gaseous state) to high pressure. This heats it up to well above ambient air temperature, even on a hot day.
The high-pressure, hot gas then goes through the condenser, which is just like your engine's radiator, where ambient air passing through the coils and fins cools off the high pressure gas causing it to condense into a liquid (remember, the chemical chosen as the working fluid was chosen specifically to condense and evaporate at the pressures and temperatures the various stages of the system were designed for). It is this stage where the heat extracted from your car's interior is shed to the outside world.
The now liquid working fluid, still at the same high pressure but now much cooler, moves to the expansion valve (which is effectively just a tiny orifice restricting the flow, keeping pressure high on one side, low on the other), where it is allowed to expand enormously in volume into a significantly lower pressure environment. Again, due to the boiling-point characteristics of this material, it vaporizes completely as it expands and pressure drops, and in the process get *VERY COLD*.
The now freezing cold, low-pressure gas then goes through the evaporator, essentially another radiator through which the cabin air is circulated, and this cools the cabin air off, heats the working fluid, thereby transferring heat from the cabin to the working fluid.
The WF then returns to the compressor, where the whole cycle repeats.
Now, if all relevant variables were held constant -- outside air temp, interior air temp, compressor RPM, etc. -- the system would be very simple, as the various pressures and temperatures in the system would essentially remain constant, so things like the orifice size for the expansion valve, the amount of compression the compressor produces, etc., could be designed for these static conditions.

Of course, the real world doesn't work that way. So additional mechanisms have to be designed in to the system to provide for some variability in operating conditions. For example, when the ambient weather conditions are very hot, the rate of heat energy that the AC must remove from the cabin air to maintain temperature is much greater; this will result in higher temperature, higher pressure return gases to the compressor. However, if the overall system were designed specifically to optimize these operating conditions, when it is cold outside and the AC is turned on, we might find liquid returning to the compressor rather than gas (as little heat energy is being removed in the evaporator stage of the system), and having liquid -- which is incompressible -- enter the compressor could be catastrophic!! Good-bye compressor.

So, an AC system employs temperature and pressure sensors to monitor the working fluid on both the high-pressure and low-pressure sides of the system, then adjusts operation in several ways (discussed shortly) to keep these temperatures and pressures within safe, functional margins so the system works effectively under widely varying environmental and load demand conditions.

The classic way of controlling an auto AC system to adjust operation to match load conditions is to simply provide a means to turn the compressor on and off. By doing this, when pressures get out of whack (high side too high, or low side too low), the compressor is just turned off, and gradually the two sides of the system will act to equalize through the expansion valve, lowering pressure on the high side, and raising it on the low side. When pressures get back within a specified operating margin, the compressor is again enabled.

On a hot day, the low pressure side will generally stay at a higher pressure, on average, than it will on a cold day, because of all the heat in the cabin air that is heating up (thereby increasing the pressure) of the WF in the evaporator. So, on average, the compressor will run longer, and it's disengagement periods will be shorter, than on a cold day, when the low-side pressures will tend to stay low and for longer due to the _lack_ of heating by cabin air. Engagement of the compressor is achieved via an electrically actuated clutch mechanism on the compressor drive pulley in most systems, and this is what you hear clicking on and off in some cars (particularly older model vehicles) when the AC is on.

You also probably have noticed that when it cycles on, there is an unmistakable parasitic load on the motor, and the vehicle is sluggish feeling as compared to when it is off. This is because in such fixed-compressor designs when the compressor is on, it is compressing at it's maximum capability at all times. This amount of compression needs to be sufficient to provide cooling under the most demanding of conditions (say, Phoenix AZ in the summer at 115°F), with some design margin above that to allow for system wear and other degradation over the life of the car.

This means that, the vast, vast majority of the time, for the vast majority of people, the compressor is over-achieving when it's engaged, by a large margin, and loading the engine down far more than it needs to. Of course, this is compensated for by the cycling behavior; however, that doesn't mean a hill of beans to you, the driver, if the compressor just happens to be in the engagement phase of it's cycling when *you* need a short burst of temporary power to pass someone or something. You're just out of luck under those circumstances, and you're paying a heavy price for all those nutcases that choose to live in little patches of hades here on the earth's surface  (Hi Todd, aty, and a whole bunch of the rest of you -- just kidding of course)

One compensatory strategy to deal with this that is rather common is to disengage the compressor when the driver indicates they are demanding motive power from the car, i.e. during Wide Open Throttle (WOT). Many, if not most cars today that employ fixed compressors disengage them when the pedal is put to the floor. This makes a big difference, and goes a long way to reducing the noticable parasitic effects of this compressor design.

Still, a fixed, cycling compressor has some substantial drawbacks, chief among them relatively low efficiency, and therefore a big hit on gasoline consumption. Also, even with the WOT adaptation, there is still noticable and annoying parasitic power losses when not flooring it, which you're not most of the time. Finally, as some of you may have noted, the air blows cold, then not so cold, then cold, then not so cold, on and on, as the compressor cycles. This makes it harder to design effective automatic climate control systems, and can affect the resulting comfort in the cabin -- especially when it's really hot outside -- for anal retentive, OCD types like myself.

A much better solution would be to _vary the compression_ depending on the demand on the system, maintaining high and low side pressures within as tight margins as possible, and thereby maintaining a relatively constant temperature drop across the evaporator. This way, if you set your climate control to 70°F, there might be 50°F air coming out of the evaporator more or less reliably, and the control system can then depend on this and better control mixing with cabin or outside air to maintain temperature in the cabin.

This is exactly the strategy used in the E46 and all late-model BMWs to manage the cooling output of the AC system. The compressor uses a more complex technology called a _variable thrust-plate_ design, which allows the amount of compression to be varied by adjust the piston travel in the compressor. It is described here:



















This design matches the load on the engine to the actual cooling demand, rather than loading the engine in an all-or-nothing fashion as the cycling strategy does. As a result, most of the time under most conditions the compressor load is modest, the impact on fuel economy small, and the cooling output of the system much more constant so that the climate control system can manage cabin temperatures more effectively.

Because of this, parasitic losses are far less noticable, especially when ambient temperatures are relatively mild. One can leave the AC on to gain the benefits of dryer air and controlled cabin temperatures even when the outside temp is in the sixties or seventies, and yet the impact on acceleration performance is almost unnoticable. As with the fixed-compressor design, BMWs employ the WOT cut-out strategy as well to ensure that the driver's intent is being served.

Overall, this design, while more complex, produces a far more efficient system, resulting in better cooling performance, better load management, and better fuel economy. It also enables the driver to simply leave the AC on all the time if they desire, so that cabin air is always controlled and comfortable, without much concern about acceleration performance or fuel economy impact -- the system matches itself to the needs of the driver and the ambient conditions. Those times when we would have left the AC off for fear of trashing milage and suffering unnecessary parasitic power losses for driving -- i.e. when outside air is mild or cool -- and instead suffer the problems with open windows or too-humid outside air in "vent" mode on the climate control system, can now just leave the thing on all the time and not worry about it.

Okay, that's it, class. Hope this was helpful, and interesting. I'll now open it up to questions


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## EdCT (Mar 14, 2002)

RKT BMR said:


> . I'll now open it up to questions


Can I have my windows open with the A/C on? 

Ed


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## RKT BMR (Sep 7, 2002)

EdCT said:


> Can I have my windows open with the A/C on?


No.

The variable thrust-plate compressor will jump out of the engine compartment and leap through the window, then a snaky tentacle will emerge from the back of the unit and actuate the window buttons to close them.

Then, in a manner reminiscent of Dr. Octavius in _Spiderman 2_, more tentacles will emerge from the compressor and it will work it's way back in to the engine compartment through the cooling ducts and reinstall itself on the motor.

It's all really quite impressive. You should check it out.


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## EdCT (Mar 14, 2002)

RKT BMR said:


> No.
> 
> The variable thrust-plate compressor will jump out of the engine compartment and leap through the window, then a snaky tentacle will emerge from the back of the unit and actuate the window buttons to close them.
> 
> ...


Great, I'll tell my wife next time she opens the windows with the A/C on, she hates anything with tentacles (she's kosher) AND she hates spiders. :rofl:

Ed


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## kdshapiro (May 1, 2003)

Interesting. Thanks for taking the time to write it up.


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## Magna (Jan 4, 2002)

*What does the HOT country setting do?*

I live in Boston while not as hot as Phoenix, we do have many days that are hot, humid, and hazy that are in the league of Houston's humid weather so I had the AC programmed under Car Memory at the HOT country setting. What does that setting do differently compared to the standard AC setting?


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## dingfwen (Jun 2, 2004)

Wow, amazing writeup. Thanks for the good information. I appreciate it much more than the armchair Mechanical Engineering that happens here sometime.

I'm one of those nuts that cycled the compressor by hand to achieve desired temp and match performance needs. Needless to say, it was a royal pain in the butt. That was in my Jeep. With my new bimmer I didn't notice any performance drop so I just left it on all the time, but felt really guilty about it until now. Thanks for clearing that up.


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## RKT BMR (Sep 7, 2002)

Magna said:


> I live in Boston while not as hot as Phoenix, we do have many days that are hot, humid, and hazy that are in the league of Houston's humid weather so I had the AC programmed under Car Memory at the HOT country setting. What does that setting do differently compared to the standard AC setting?


It's almost embarrassingly simple: All it does is subtract an offset from the displayed setting for the actual, true temp that the climate control system is trying to maintain, and change the vent flap behavior to favor the dash vents more than otherwise in controlling the system. I don't know offhand how much it is, but it's around 2-4°C. So, if you set your system to 20°C indicated, it will actually be trying to maintain around 17°C.


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## KatanaPilot (Mar 25, 2003)

How about the MINI? I don't feel the compressor in my MINI cycling, so I suppose it is probably the same as the BMW's.

I wish the system could be designed to place a smaller load on the engine at low RPMs. According to the dscription of the system, at low RPMs, compressor pistons have a longer stroke. In theory, iti s not supposed to rob much power, but in the MINI, the amount of power loss is downright dangerous. It is bad enough that I got into the habit of turning A/C off when coming to an intersection. Is there any computer settings the dealer could change to make it less dangerous for us MINI owners?


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## Hattori Hanzo (May 13, 2004)

Interesting acticle. I am a newb, so here goes my dumb question. I live in BAKE-rsfield CA where the temps get over 100. Is it ok to drive my Cic with the top down and the air on? What negative affect will it have on my car?

TIA


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## RKT BMR (Sep 7, 2002)

Hattori Hanzo said:


> Interesting acticle. I am a newb, so here goes my dumb question. I live in BAKE-rsfield CA where the temps get over 100. Is it ok to drive my Cic with the top down and the air on? What negative affect will it have on my car?


No problem at all. I do it all the time when the temps are in the 90s here in the Bay Area.

The biggest "problem" this causes is just a slightly greater loss of fuel economy due to the increased aerodynamic drag with the top down. It doesn't stress the AC system in any sort of harmful way.

When you do this, however, I would strongly suggest putting the climate control system in to a manual mode, however, setting the vents to the dash and floor, recirculation, and pump up the fan to the highest setting. Center vent control set to full cold (three blue dots). Orient the dash vents so that they are not blowing upward at all. Put all the windows up, and your windscreen (you *do* have a windscreen, don't you?  )

Leaving the system in automatic will do no harm, but it will not function properly since it is impossible to reach and maintain the set temperature under these conditions. So you'll wind up with less than optimal settings of the vents, etc.

If you don't have a windscreen, the effectiveness of this strategy is reduced considerably.


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## RKT BMR (Sep 7, 2002)

KatanaPilot said:


> How about the MINI? I don't feel the compressor in my MINI cycling, so I suppose it is probably the same as the BMW's.
> 
> I wish the system could be designed to place a smaller load on the engine at low RPMs. According to the dscription of the system, at low RPMs, compressor pistons have a longer stroke. In theory, iti s not supposed to rob much power, but in the MINI, the amount of power loss is downright dangerous. It is bad enough that I got into the habit of turning A/C off when coming to an intersection. Is there any computer settings the dealer could change to make it less dangerous for us MINI owners?


My apologies... I don't know any specifics about the system in Minis. However, what you described sounds like a fixed-compression system, not a variable compressor design.


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## Moolji (Jun 25, 2003)

RKT BMR said:


> We've all experienced the effect of releasing a gas under pressure...


:rofl:

Seriously, thanks for the write-up!:thumbup:


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## Technic (Jun 24, 2002)

Excellent write up!! :thumbup: :thumbup:


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## dev (Dec 23, 2003)

Excellent, thanks for educating me...

I have some questions about the A/C controls... I recently purchased a 330i but haven't been able to form my own "mental model" of how the A/C controls work. Maybe you could answer some of my elementary questions?

1. If I have the A/C in the auto-off mode, and change the temperature adjustment, does the air really blow at the temperature indicated? How does that work? For instance I turn off Auto mode, and crank up the temperature to 90 degrees but hot air doesn't start to come out of the vents. 

2. Do I need to have the snowflake button turned off to get heat? Does the snowflake button indicate cooling only?

3. What do each of the 3 buttons do for controlling where the air comes from? I get the one that is for dash vents but what about the other 2? I couldn't tell the difference at first glance. 

So far I've mostly used the A/C in the non-auto mode, because I like choosing the fan speed either to reduce noise or the amount of air in my face. Also I like to choose where the air comes from, dash vents and at my feet. But if the temp control doesn't work in non-auto mode then I guess I'm using the A/C wrong. 

Thanks for any help you can provide in enlightening me on this topic.

Dev


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## dev (Dec 23, 2003)

Ok so I found a good thread addressing many of my questions about A/C controls...

http://www.bimmerfest.com/forums/showthread.php?t=56532&highlight=a/c+control

In case anyone else wants to know.


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## Magna (Jan 4, 2002)

RKT BMR said:


> It's almost embarrassingly simple: All it does is subtract an offset from the displayed setting for the actual, true temp that the climate control system is trying to maintain, and change the vent flap behavior to favor the dash vents more than otherwise in controlling the system. I don't know offhand how much it is, but it's around 2-4°C. So, if you set your system to 20°C indicated, it will actually be trying to maintain around 17°C.


I recall there was another car memory setting regarding vent behavior, also there's a temp. offset car memory setting. HOT country must play off those two separate memory settings.


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## Hattori Hanzo (May 13, 2004)

RKT BMR said:


> No problem at all. I do it all the time when the temps are in the 90s here in the Bay Area.
> 
> The biggest "problem" this causes is just a slightly greater loss of fuel economy due to the increased aerodynamic drag with the top down. It doesn't stress the AC system in any sort of harmful way.
> 
> ...


 Thanks RKT. Its 97 here right now. The wind deflector is great and is up all the time...probably a MUST have accessory for a vert. I will certainly heed your suggestions.


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## Frank Rizzo (Aug 2, 2003)

When I got out the Army in '87 I got a job as an commercial A/C mechanic, because that was one (of many) jobs I had while in service. I did this for a few years ...... untill I had to go on a service call at a Sizzler....... in Indio, CA......in August......on a galvanized metal roof. It was 142 degrees ambient - - - I decided then on a new career path.

We used to use variable capacity A/C compressors at our military computer facilities. There were some potential issues with power spikes from the compressors cycling on and off - so they ran constantly and dumped hot gas into the suction line.


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## Artslinger (Sep 2, 2002)

That write up was a little vague RKT BMR could you go into a little more detail.


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