How Does Air Conditioner Works in Cars?

[yoscenario2000]

If you drive the car in a lot of traffic like me you will have had this experience. Standing in traffic with the foot on the brake, there is a sudden drop in engine rpm and the brake pedal sinks in. This happens as a energy saving feature in an extended idle where the engine drops from 1200rpm or so to 700. But it also happens when the aircon disconnects when a set cabin temperature is reached. The load on the engine drops and the ECU reduces the rpm. Then the vacuum applied to the brake system reduces and the brake pedal sinks in


If compressors didnt cutout off, their life would be very short. In an auto climate car, the cabin temperature and humidity is just controlled more accurately.
The real question is, if I set the aircon dial to a lesser amount of cooling- being winter now, will that save any fuel- will the compressor run for a shorter time or is hot gas from the engine used to reduce supply temperature to the cabin?

Ameyam

Spot on..

I'm experiencing the same situations daily as you have mentioned.

FYI.. Dude I drive my car on JVLR in Mumbai and I don't think there is any need to explain the driving conditions at JVLR to you as mentioned that you also drive in a lot of traffic.

 

[ameyam]

Which car do you drive by the way?

Ameyam

 

[Kichu]

I too have maruti Ritz but the compressor run's always until i switch off the ac.

As for too my knowledge manual ac has no temperature sensor's to monitor the AC system.

Hi Kannan,

AFAIK, even in manual AC there is a temperature sensor called Thermistor(Its a cheap temperature sensitive resistor).

I believe, then blower fan speed really playing a vital role in FE of a vehicle ?

Hi friend,

Even I believe so..

My observations with the car A-star goes like this.

Speed1: Compressor Works for 4-6secs and Stops for 12secs.
Speed2: 8-10secs and 6 secs
Speed3: 16-20secs and 4 secs
Speed4: Even after 70secs it continued.
These are the rough number of secs that I calculated with my mind when using AC

So according to the above values, the total number of secs that the compressor would work at different speed levels go some what like this.
Speed1: Compressor works for 17 secs in a min
Speed2: For 36 secs in a min
Speed3: For 50 secs in a min
Speed4: For 60 secs in a min I guess

I do also believe that at any of the speed level, when the thermistor senses the temperature is too low(dont know the temperature limit) then the compressor stops working until the temperature rises above the limit.

Regards.,

 

[ameyam]

Spot on Kichu. Thats my experience too

Adding in another experience with my old Wagon R. The first week I drove to calculate average, I got 9kmpl tank to tank. So next week I drove with the dial at a lower cooling setting. I got 11.8kmpl. Not sure this has got a lot to do with my aircon setting or with my driving style or the traffic encountered

Ameyam

 

[Kichu]

Hi Ameyam.,

Not sure whether it helps in reducing Fuel consumption by adjusting the temperature knob. I think the knob just helps in mixing warm and cold air.

My observation is, even if I keep the knob to the hottest position still the AC compressor works(If the AC button is switched ON.) So I don't think adjusting the knob will helps in reducing the FC. Sometimes you may had lesser traffic Or your temperature knob setting of your vehicle must be an intelligent one. May I know which car do you own?

Regards.,

Edits:
You already mentioned your vehicle, sorry I missed it
I think we guys are having almost similar kind of AC setup, except for their capacity.

 

[Sidd7hartha]

The hot and cold knob of AC has two half arcs.They are indicated by blue & red arcs. The blue arc is longest. What is the effect if I place knob at minimum level of the blue arc? From very beginning the AC knob is set at the extreme end of the arc.I control the cooling effect by setting blower speed

 

[ameyam]

Thats what we have been discussing sidd7harth. It seems that setting the knob to a lower cooling setting simply supplied mixed air without slowing the compressor. So modulating the temperature by seitching airco on & off is better

Ameyam

 

[Figoian]

This happens as a energy saving feature in an extended idle where the engine drops from 1200rpm or so to 700.

Drop of 500 RPM? I don't think that's possible. Sure that the RPM varies with the Compressor, however, not to this level. The idle RPM that I know in Figo is 850 & 750 with & w/o AC. I will try to capture it with the Digital RPM and post here soon.

Hi Kannan,

AFAIK, even in manual AC there is a temperature sensor called Thermistor(Its a cheap temperature sensitive resistor).

Speed1: Compressor Works for 4-6secs and Stops for 12secs.
Speed2: 8-10secs and 6 secs
Speed3: 16-20secs and 4 secs
Speed4: Even after 70secs it continued.
These are the rough number of secs that I calculated with my mind when using AC

Yes, Thermistor controls the temperature.

The blower level will have "some" effect on the compressor switching, yet not much. If I am right, this is how it works. Experts to clarify if not.

1. Compressor Works to either Cool or Heat up the cabin as the Temperature setting (Knob, Blue or Red).
2. Blower sets the speed of the fan, irrespective whether the compressor is on or not.
3. In re-circulation mode, the AC pulls the cabin air, cools it and blows back. This cycle should work until the air in the cabin reaches the desired temperature level set by the user. Once the cold air hits the thermistor, and if that's the desired level, the compressor should go off.
4. That would obviously raise the temperature level in the cabin in few secs depending on the ambient temperature, load, number of passengers etc. and would restart the compressor once the temperature varies from the reqd level.

So, how blower's speed affects this process? Maybe, maybe not or a little, but not much.

 

[Highway Ranger]

Yes, Thermistor controls the temperature.

1. This cycle should work until the air in the cabin reaches the desired temperature level set by the user. Once the cold air hits the thermistor, and if that's the desired level, the compressor should go off.

The catch is that the user cannot set the desired temperature. That is preset.

The only way the user can control the temperature is by mixing the cold air with normal air via the Damper controlled by the knob with the Blue and Red marking.

 

[Figoian]

The catch is that the user cannot set the desired temperature. That is preset.

The only way the user can control the temperature is by mixing the cold air with normal air via the Damper controlled by the knob with the Blue and Red marking.

Thanks HR. If so, then why does the compressor goes off? That's the open question.

 

[ameyam]

Compressors in any king of aircon system are not meant to operate continuously. If they did, their lifespan would be very short. HR explained it very correctly- supply air temperature can only be controlled by mixing cold and hot air. That has nothing to do with the compressor switching because the thermistor controlling its operation is located in the return air stream- the one coming back from the cabin. So the compressor switches off when the cabin temperature reaches the preset. HR made it quite clear in that the manual aircon cars cant set cabin temperature by driver, the compressor simply uses the preset.
Regarding rpm, I don't have a digital tach, so I am just reading it from the dash. I know that 1000-1200 rpm setting is there and the drop to 700 rpm is in prolonged idle. It was a figure also confirmed by the MASS when he was explaning me the car. Now whether its 1000rpm or 900 or 1100 is a matter of debate and setting I guess

Bottom line is that setting the aircon in the white zone doesn't save fuel as just mixed air is delivered. But does setting the knob higher in blue zone save any fuel?

Ameyam

 

[Kichu]

The blower level will have "some" effect on the compressor switching, yet not much. If I am right, this is how it works. Experts to clarify if not.

1. Compressor Works to either Cool or Heat up the cabin as the Temperature setting (Knob, Blue or Red).
2. Blower sets the speed of the fan, irrespective whether the compressor is on or not.
3. In re-circulation mode, the AC pulls the cabin air, cools it and blows back. This cycle should work until the air in the cabin reaches the desired temperature level set by the user. Once the cold air hits the thermistor, and if that's the desired level, the compressor should go off.
4. That would obviously raise the temperature level in the cabin in few secs depending on the ambient temperature, load, number of passengers etc. and would restart the compressor once the temperature varies from the reqd level.

So, how blower's speed affects this process? Maybe, maybe not or a little, but not much.

Hi Figoian,

Depending upon different AC systems, sometimes the blower level may not have any effect. I think in your case you didn't find much. (Frankly, other than mine, I dont know about other AC systems)
But according to the AC system that I have in my Maruti A-star, it does have "major" effect.. and I believe these customers are really lucky since we can reduce the FC when the Blower speed is not at high.

Hey, does the AC compressor work as a heater for heating the cabin???I never heard about it.

Actually while compressing the air, it will produce heat and that heat can be used for heating the cabin.
BUT is that the way used for most of the vehicles for heating the cabin? Again I dont think so, its a wastage of fuel since we already have a source to get enough heat ie from the engine.

In my A-star, we got a heater called "HEATER CORE" the core though which the coolant flows in and out, this heater core is used for heating the cabin. When we adjust the knob to higher temperature a passage opens and the air blown from the blower flows through this heater core and the air gets hotter and the hot air flows out of the vents to the cabin.

If you wanna check whether you have a Heater core or not, do this simple exercise.
*After your drive, shutoff your engine.(Purpose is to get a warm engine)
*Now set the knob to the hottest position.
*Now switch ON your blower (With the engine OFF)

If you are getting warm air, then you are equipped with the HEATER CORE.

Hey, "compressor works as a heater" is that your guess?

Regards.,


Edits:

Thanks HR. If so, then why does the compressor goes off? That's the open question.

My observations with the car A-star goes like this.

"Speed1: Compressor Works for 4-6secs and Stops for 12secs.
Speed2: 8-10secs and 6 secs
Speed3: 16-20secs and 4 secs
Speed4: Even after 70secs it continued.
These are the rough number of secs that I calculated with my mind when using AC"

My answer is, Compressor's working duration is made to depend upon the blower speed level and the Thermistor resistance.

 

[Highway Ranger]

Yes, Thermistor controls the temperature.

You are probably correct. I only said that the thermistor is not user adjustable like our fridge or AC.
It is either that or some sort of high pressure/low pressure cutouts within the system.

I remember in the 70s, the imported cars had user adjustable Thermostats similar to the once used in Window ACs.

In fact, I wanted to fix one for my previous Zen. I even got one second hand AC thermostat. But was lazy to actually do it.


Regarding the blower speed and the compressor cut-out/cut-in sequence... if the thermistor is sensing the cooling coil temperature

Low speed = less amount of air pass through the cooling coil. Result - the cooling coil temperature drops faster. (transfer of heat is less)

High speed = more amount of air pass through the cooling coil. Result - the cooling coil temperature drops slower. (transfer of heat is more)

Ideally, if the cabin temp has reached the lowest and the car is in a shade with no external heat source heating up the car (cool night), the speed of the blower should not matter much.

 

[ameyam]

Hi Figoian,

Hey, does the AC compressor work as a heater for heating the cabin???I never heard about it.

Actually while compressing the air, it will produce heat and that heat can be used for heating the cabin.
BUT is that the way used for most of the vehicles for heating the cabin? Again I dont think so, its a wastage of fuel since we already have a source to get enough heat ie from the engine.

What you are talking about is a Heat Pump. In a heat pump, both heating as well as cooling effect can be obtained from the same compressor. Not getting into details. No, the automobile industry is not there yet with using compressors as heat pumps

My personal observation, the lower the fan speed, the sharper the cooling. That is because, manufacturers for the smallest evaporator coil they can fit into the car. So if you run at low fan speed, you get much cooler air and vice versa because the air doesn't have the time to contact the refrigerant in the coil at higher speeds. Running the fan at higher speed causes the throw of air to be increased so the entire cabin cools faster (rear seaters get cooling faster) where as at lower fan speed the front seats cool faster. The higher fan speeds increases fan static pressure and hence throws the air further. Also creates more noise in the process

Ameyam

 

[Kichu]

Not in that sense..

AFAIK, heat pumps are only used for Electric Vehicles. Do we have any other car with heat pump over here?

Actually, here I was not talking about heat pump. Since the member Figoian said about the working of a normal AC compressor as a Cooler as well as a Heater, I was just saying its possibility.