Air Conditioning 101

Black_Dog · **Posted:** Thu Jun 11, 2009 11:42 am

Auto gurus,

I have been searching around the Web trying to gain a better understanding of how a car’s a/c system works vs. how a home a/c system works. So far I haven’t found answers that satisfy me.

First, I understand the basics: compressor forces hot refrigerant through the condenser coils where it cools and liquefies; water/water vapor gets absorbed in the dryer; an expansion valve or orifice tube controls the pressure of the liquid refrigerant as it passes into the condenser where it absorbs heat from the cabin space while it depressurizes. Finally, the very hot low pressure refrigerant vapor returns to the compressor to begin the cycle again.

I also understand that sensors in the valve measure pressure, and in some types of systems, temperature, cause the compressor to cycle on and off to keep the valve from freezing shut or the pipes from bursting open.

My question is in regard to what factors influence this systemic cycling on and off of the compressor. What role does ambient temperature play in this? Does the system cycle off more often on a cooler day than on a hot one and does that have an impact on gas mileage? In other words, if you run the A/C on an 85-degree day, does the a/c cycle off more often (resulting in better fuel economy) than on a 100-degree day? Or even more dramatically, since the A/C of a Jeep Liberty is programmed to run anytime the heater (vent to foot) or either defroster settings, does the A/C cycle off more often on a 20-degree night (resulting in better fuel economy) than on a hot summer day?

Furthermore, does the temperature dial (I’m not calling this a thermostat because I don’t think it works the same as a real thermostat) on the dashboard play a role in how often the compressor cycles on and off? My understanding is that when you turn the dial toward heat it simply introduces heat extracted from the engine coolant even when the A/C is running. So a car would pay the same MPG penalty while running the A/C with the dial turned 75% toward cool as it does then it is dialed in at 100% cool. Am I right or does this also (actively or passively) impact how often the A/C compressor cycles off?

I look forward to your insights. Thanks.

Wasatch15 · **Joined:** Wed Jul 13, 2005 7:52 am **Posts:** 242

The orfice or valve controls the flow of liquid refrigerant into the evaporator. As it leaves the control device it FLASHES into gas. The is the process which removes heat from the evaporator. In most systems I have seen, there is a pressure switch in or near the evaporator that senses pressure and controls the ON/OFF cycles of the magnetic clutch of the compressor. There is a direct relationship between the pressure of the refrigerant and the temperature of it. Too low of pressure in the evaporator and it freezes up, causing restricted airflow. You don't want your evaporator to operate near 32F. As outside air temperature rises, it gets harder to release heat from the condenser so your AC system has to work harder. I would say yes, there is more OFF time on a cooler day because heat is removed faster. Anytime the AC is on your mileage will decrease. I have heard as much as 30 to 40 hp is required from the engine to run the compressor. The temperature dial controls what temperature the operator wants the cabin and will call the compressor ON if pressure in the evaporator is high enough. If pressure is low, the compressor will cycle OFF.

Jeepjeepster · **Posted:** Fri Jun 12, 2009 3:23 pm

The compressor will kick in LESS on a cooler day since the coils have a higher chance of freezing over. On a hot day the compressor will stay kicked in longer since more heat is being moved over the evap.

Yes, that dial will change how long it stays kicked in because that dial changed the amount of air moving over the evap. Less air=more kicking on and off for the compressor.

As far as I know a compressor in a house hold unit cannot kick on and off. If the filter inside gets too clogged up the higher pressure side will kick the unit off. The pressure goes up since heat makes a liquid or gas expand(as far as I know thats why it goes up).

tjkoen01 · **Posted:** Fri Jun 12, 2009 11:08 pm

the temp control in the jeep controls the temp of the air coming out, it does not affect the compressor at all. it controls the mixture of air going over the heater core and the evaporator, blending the the two to achieve the temp at the vent. turning the temp up with the a/c on only wastes gas in the sense that you are still cooling the air, but then adding heat back into it (you are still paying to cool the air even though it is coming out warm).

from the fsm:
"The heater and optional air conditioner are blend- air type systems. In a blend-air system, a blend door controls the amount of unconditioned air (or cooled air from the evaporator on models with air conditioning) that is allowed to flow through, or around, the heater core. A temperature control knob on the A/C Heater control panel determines the discharge air temperature by controlling an electric actuator, which moves the blend door. This allows an almost immediate control of the output air temperature of the system."

the cycling of the compressor on a vehicle is driven by a couple of factors:
1. outside ambient temp
2. inside ambient temp (if in recirc mode)
3. engine rpm
4. amount of refrigerant in system

1. the hotter it is outside, the more heat you are trying to remove from inside and the tougher it is to move the heat outside. therefore the higher the temp, the less cycling of the compressor (less time off means more work)

2. if you have it in recirc mode, you are removing less heat (the inside air is cooler than the outside air) so the compressor has to work less and will cycle if the outside temp is low enough or engine rpm is high enough. this will save gas vs. not being in recirc mode and will provide cooler air.

3. the higher the rpm's, the more refrigerant the compressor can move/compress. the cycling is controlled by pressures. if the high side gets to high, it cuts out; if the low side gets to low, it cuts out. if the system is operating correctly, the low side switch controls the compressor. at higher rpm's the compressor is able to achieve lower pressures (suction) and begin to cycle. the accumulator is present to help even out the fluctuations in pressure on the low side caused by fluctuating rpm's (and also catch any liquid refrigerant before the compressor...it also contains desicant to remove any moisture that is in the system).

4. if the system is low on refrigerant, it will cycle more at all engine speeds b/c it is easier for the compressor to achieve the low pressure to cut the compressor off. this is why you loose your cool with low refrigerant.

how is this different from a household a/c? the biggest difference is the engine rpm's. in a household a/c, the compressor is electric and it's speed does not vary. this takes away one variable from the thermodynamic equation, which will make it more efficient have have less to compensate for. that leaves only the inside & outside temps and the amount of refrigerant in the system as variables. also, the thermostat in a house does not control the temp of the air coming out of the vents, it only controls when it turns on and shuts off...or in other words, when it cycles.

i hope this helps

Black_Dog · **Posted:** Mon Jun 15, 2009 2:14 pm

Perfect. Thanks.

Jeepjeepster · **Posted:** Mon Jun 15, 2009 8:46 pm

tjkoen01 wrote:

the temp control in the jeep controls the temp of the air coming out, it does not affect the compressor at all. it controls the mixture of air going over the heater core and the evaporator, blending the the two to achieve the temp at the vent. turning the temp up with the a/c on only wastes gas in the sense that you are still cooling the air, but then adding heat back into it (you are still paying to cool the air even though it is coming out warm).

It may not directly effect it but there is a temp gauge on the evap that makes sure the evap doesnt freeze over. If there isnt as much air moving over the coils then it will freeze up but the comp is kicked off to keep that from happening.

And I agree with the wasting gas part. I either have it all the way on cold or its off. If its too cold in my Jeep with the fan on speed 1 and the temp on the coldest then I should have the windows down anyway.

tjkoen01 · **Posted:** Mon Jun 15, 2009 9:06 pm

Jeepjeepster wrote:

tjkoen01 wrote:

the temp control in the jeep controls the temp of the air coming out, it does not affect the compressor at all. it controls the mixture of air going over the heater core and the evaporator, blending the the two to achieve the temp at the vent. turning the temp up with the a/c on only wastes gas in the sense that you are still cooling the air, but then adding heat back into it (you are still paying to cool the air even though it is coming out warm).

It may not directly effect it but there is a temp gauge on the evap that makes sure the evap doesnt freeze over. If there isnt as much air moving over the coils then it will freeze up but the comp is kicked off to keep that from happening.

And I agree with the wasting gas part. I either have it all the way on cold or its off. If its too cold in my Jeep with the fan on speed 1 and the temp on the coldest then I should have the windows down anyway.

some systems may have a temp switch, but not ours (or any that I have see and I have seen my fair share)

from the fsm:

"The a/c low pressure switch contacts are open when the suction pressure is approximately 141 kPa (20.5 psi) or lower. The switch contacts will close when the suction pressure rises to approximately 234 to 262 kPa (34 to 38 psi) or above. Lower ambient temperatures, below approximately -1 °C (30 °F) , will also cause the switch contacts to open. This is due to the pressure/temperature relationship of the refrigerant in the system."

Pressure, temperature and volume of a gas are directly related (Pv = nRT). Volume is fixed and by controlling the pressure, you control the temp; therefore there is no need to have a temp switch. The only way for your evap to freeze up is if your low side pressure is too low which would indicate that your low pressure cut off switch is not opening when it should.

if you have low air flow over the evap, then the expanding gas does not pick up as much heat and therefore does not build as much pressure. this allows the compressor to overcome the expanding gas, drawing the pressure down below the cut off pressure and shuts the compressor off. this is why it cycles more on cooler days.

Jeepjeepster · **Posted:** Mon Jun 15, 2009 9:23 pm

Gotcha. I had read some time back that the kj had a temp gauge. I wont mention who posted it.

Jeep Dawg · **Posted:** Mon Jun 15, 2009 9:42 pm

OK here is a super short way to tell you how it works and you will understand 100%....

you are hot, you turn your AC on and you get cool.......

tjkoen01 · **Posted:** Mon Jun 15, 2009 10:12 pm

Jeep Dawg wrote:

OK here is a super short way to tell you how it works and you will understand 100%....

you are hot, you turn your AC on and you get cool.......

you hope so!

flair11111 · **Joined:** Mon Dec 01, 2008 1:00 am **Posts:** 25

good info here. so to save more gas with ac on at any outside temp, if you have to use the ac, use the recirc position...correct?

tjkoen01 · **Posted:** Sat Jun 20, 2009 10:15 pm

flair11111 wrote:

good info here. so to save more gas with ac on at any outside temp, if you have to use the ac, use the recirc position...correct?

correct

CRDMiller · **Posted:** Sun Jun 21, 2009 12:58 am

I am pretty sure my liberty blows much colder and, a larger volume of air, when it's not on recirculate.

I attribute this to restrictions on the return path, and thus less efficient heat transfer.

tjkoen01 · **Posted:** Sun Jun 21, 2009 9:34 pm

CRDMiller wrote:

I am pretty sure my liberty blows much colder and, a larger volume of air, when it's not on recirculate.

I attribute this to restrictions on the return path, and thus less efficient heat transfer.

check your owners manual....that ain't right. if in fact that is what's happening you have something wrong.

when you have it on recirc, you are taking the presumably cooler air in the cab and sending it back thru the evaporator (for example, cooling 70 degree air from inside verses 90 degree air from outside). it should be louder b/c you are opening a damper right by the fan to let it suck in the inside air (less restriction as well). you typically get more air b/c you are not trying to force air into a relatively air tight vehicle, just recirculate it.

Air Conditioning 101

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