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Old 31st March 2008, 17:59   #61
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Pl.correct me if i am wrong.
For heating only the available heat in the circulating cooling water is used and hence no efficiency loss while operating on heating mode.
Similarly in most of the (small) cars when the temperature control knob is turned for lower cooling only the hot air (taking heat from circulating cooling water) is mixed up to reduce the cooling effect.
As I understand this is the system in Maruti800 and Alto
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Old 22nd April 2008, 09:46   #62
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Cool driving

What is the best way for fuel saving while driving with the AC on
1. To keep switching the AC on and off time and again or
2. To lower the blower speed while keeping the AC on through out.
My reason for query being that i have had severe arguments on this point. In my head its the second way.
Also is keeping the exhaust vent open permanently not advisable. Same reason. Lot of argument.
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Old 22nd April 2008, 12:31   #63
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The home air conditioner should not be switched on and off frequently as it will get damaged - so shouldn't we follow this for the car air conditioner also?

I guess another option is to set the temperature at a comfort zone, rather than at the extreme cold setting - let the thermostat handle the a/c cutting in and out.

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Old 23rd April 2008, 00:25   #64
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Originally Posted by RADHIKADAS View Post
What is the best way for fuel saving while driving with the AC on
1. To keep switching the AC on and off time and again or
2. To lower the blower speed while keeping the AC on through out.
My reason for query being that i have had severe arguments on this point. In my head its the second way.
Radhika, i have moved your post to a more appropriate thread.

To understand how the A/Cs affects FE, first an understanding of how a car a car A/C works is necessary.

Simplified, there are three seperate parts of a car AC system that you should know about :
1. Compressor - Controlled by the A/C on/off button as well as the temperature setting/thermostat.
2. Blower - Controlled by the fan speed selector.
3. Condensor - Its like a smaller radiator, it depends on ambient temp / air flowing across it.

Name:  Car_A_C_Compressor.jpg
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Size:  28.6 KBWhat does the compressor do?
The compressor's job is to compress the liquid refrigerant that flows around in the pipes. Later on, this compressed refrigerant will be allowed to expand, making it very cold, due to the Joule–Thomson Effect.
The compressor is located in your engine bay, and looks similar to your alternator or power steering pump - and is connected the same way, with a belt that runs off the power from the crankshaft(engine).
The compressor however is not always being turned. It has an electro-magnetic clutch on the front of it that enables it to engage when it needs to turn and disengage when it doesnt.

This compressor's clutch is what is controlled by the AC on/off button and the temperature selection/thermostat. When the AC is ON the clutch is engaged until the cabin is cool enough as per the set temperature, at which point the compressor is disengaged till the cabin starts to get warm again.

The compressor has a lot of heavy moving parts - and is connected to the engine. Hence, whenever it is engaged, it is putting more load on the engine, hence obviously affecting fuel efficiency, as well stealing some power (thats why people switch of their AC when going uphill etc).

The compressor is the only part that directly affects the FE, however, the other parts in the system affect how the compressor behaves, hence indirectly affecting the FE.

Name:  catimage.jpg
Views: 10779
Size:  16.6 KBWhat does the blower do?
The blower is basically an electric fan. Whether electric appliances affect FE is a whole other debate, BUT - this blower could affect FE in another way.

Once the compressed refrigerant from the compressor is allowed to expand and get cold, the blower's job is to blow air over it striaght into the cabin via the AC vents.
The refrigerant is housed in a radiator like thing called a evaporator for when the air is blown over it (see pic below).
Name:  Auto_Evaporator.jpg
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Size:  14.6 KB

So the speed of the fan affects how quickly the refrigerant in the evaporator gets warmed up again - which in turn also affects how soon the compressor might have to come on again.
The faster the fan, the more air flowing over the evaporator, the more heat being exchanged the sooner the compressor might have to turn on; and vice-versa.

Name:  Auto_Condenser.jpg
Views: 10781
Size:  11.6 KBWhat does the condensor do?
The condensor works the same way as a car's radiator. It takes the compressed refrigerant from the compressor and runs it through a housing designed to cool it do the airflow / ambient temperature.

Naturally, the more airflow over the condensor and the cooler the ambient temperature - the better the condensor will work, cooling the refrigerant even further and hence due to the series of events that follows, requiring the compressor to work less.

So now that you have understood how the system works, you will see that its very hard to say what affects the FE the most, as there is no solid answer - and it changes from situation to situation depending on all the variables.

Here is how the system ties together :
A/C and fuel economy & How to use aircon optimally.-airconsystem.jpg

Originally Posted by RADHIKADAS View Post
Also is keeping the exhaust vent open permanently not advisable.
If the AC is set to "recirculate", the blower takes air from inside the cabin and blows it back over the evaporator.
Since the air inside the cabin is probably already cooler than the outisde air, this would promote cooling.

If you set the AC to "fresh air" it takes in air from the outside and mixes it with the air being blown over the evaporator. The advantage of this is that for long drives etc - you will be breathing fresher air (more oxygen!) as compared to the air that might have been re-re-recirculating in the cabin for a long time already.

Originally Posted by king_julian View Post
The home air conditioner should not be switched on and off frequently as it will get damaged - so shouldn't we follow this for the car air conditioner also?
Im not sure about this, but i think that the bad thing about switching a home AC on and off a lot is due to the huge electrical and torque load placed on the starter coil of the compressor electric motor. In the case of cars, the engine and compressor-clutch take care of this starting load.


Source of images :
1. Auto A / C Compressor DY - 508
2. Auto Car Parts
3. Auto Evaporator
4. Shop for Blower Motor at 1A Auto
5. MB Auto Air Conditioning - How Does it Work?

Last edited by Rehaan : 23rd April 2008 at 00:28.
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Old 23rd April 2008, 08:05   #65
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Hello to all TBHP-Members.

Rehan Sir has done a exceptional job in explaining the car A/C refrigeration cycle.
I will try to do some more explaining which will clear out some more doubts.

Rehan sir has left out a very important component - THE EXPANSION VALVE.
-The gas which is cooled in the condenser- Note that when the gas leaves the compressor it is in a gas state.In the condenser the gas is converted into liquid state.The gas then goes to the receiver if present .The receiver acts as a storage for the cooled /liquid gas.This cooled liquid gas then goes to The expansion valve .

The expansion valve acts like a spring loaded valve (tap) like the one we find in water dispensers,just like water on demand.Also note that the expansion valve gets an temp feed back of the air temperature from the car cabin.This feed back actually controls the lift of the valve and allows the correct quantity of gas to go to the evaporator.The gas enters the expansion valve in liq state and passes through the open valve which acts as a orifice.As soon as the gas passes thro the orifice there is a pressure drop and the gas expands/evaporates.Any liquid including water which evaporates leads to cooling.The gas exists the expansion valve in a gas state.If the expansion valve does not work properly there could be liquid carry over in the evaporator and more so to the compressor with disastrous effects.
In the evaporator the cooled gas gives away the cool-ness to the passing blower air.Now the more the speed more the transfer of coolness more the efficiency .Whats the use if we dont take away all the coolness from the gas by keeping the blower speed less.
kindly note that in any refrigeration cycle the over-all efficiency is not more than 45 percent.May it be an house A/C or a refrigerated container.
All the components have to work in unison to get the max out of the system.Note that even if one has some grease or gummy deposit on the bulb of the expansion valve ,then it will affect the opening of the valve and the valve will open more than required .This will again affect cooling, more gas will pass when not required in the evaporator and then if the fan speed is less then more cool gas will go in the compressor ,so eventually one is using the system under utilized.
The compressor which is given so much importance in this forum is not a very important component as far as the refrigeration cycle is concerned.It is only a machine which keeps the gas in circulation.But the fact that it consumes max power,has the max complicated construction and is the only component having moving parts, makes it important.Technology has optimized other components of this system leaving only the compressor to work on.Please note that cutting off the compressor will increase efficiency and less fuel consumption.In cars having climate control once you set the temp and if the temperature in the cabin reaches the desired value the compressor will cut off and when the temp rises will cut in saving fuel.
One more thing one should keep in mind in order to keep the fuel consumption low is ,not to set the Ac at lower temp.set it at a reasonable temp to allow the compressor t to cut off.

In case of home Ac the compressor should not be switched off and started often because for every start stop the Ac draws in heavy current leading to sparking in all the contacts right from the starting switch to the compressor terminals.But note that the compressor does cut in and cut off when the required temperature is attained and only this reduces the electricity bills

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Old 23rd April 2008, 08:36   #66
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Thanks Rehaan. That was a fairly informative description of a car ac.
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Old 23rd April 2008, 08:36   #67
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Nice write up rehaan, very informative for anyone.
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Old 23rd April 2008, 10:51   #68
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Many Thanks Rehaan. Your explanation has cleared many of my doubts. Please continue the good work.
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Old 14th April 2010, 12:26   #69
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i have a logan diesel. vehicle has run 11700 kms so far. i get an fe of over 19 running the car with ac on always. running is usually city / mumbai , bumper to bumper and once in a month to pune on the expressway.

however one thing which i have observed is that the compressor never cuts off. in my darling maruti 800, setting the temperature to a little high would cut the compressor which could be found out by the lower rpm and a katch katch sound.

however in my logan, the temperature reaches chilling levels - around 19 degrees - onboard watch with temperature gauge :-) - but i have never seen it cut off. the rpm always stays around 850 / 900. when switched off it is 650 / 700. it will chill even during the daytime but will never cut off. engine temperature is always constant at 4 notches. never overheated.

however, kindly note that all the glass in the car has been filmed. i feel that they also have a positive effect on the cooling. the dash remains cool.
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Old 9th November 2011, 11:45   #70
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Default Questions about Air conditioner Load

Hi, Forgive me if I am asking something really Stupid...

1. If I reduce the a/c control... does it decrease the load on the engine?

2. If I changed from cold to Hot air but still leave the a/c switch in on position, does it still load the engine?

I am reffering to cars that do not have an automatic temp control.

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Old 9th November 2011, 15:49   #71
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Default Re: Air conditioner Load

Increasing or decreasing the speed does not increase or decrease the load on the engine, it only increases or decreases the speed of the Fan. On the other hand increasing or decreasing the temperature does impact the load on the engine as the AC compressor has to work that much harder/realized depending on the temperature setting in the car.

You can feel this while driving and changing the settings and feeling the drag on the car.
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Old 9th November 2011, 16:05   #72
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Default Re: Air conditioner Load

Originally Posted by vu2pmc View Post
1. If I reduce the a/c control... does it decrease the load on the engine?

2. If I changed from cold to Hot air but still leave the a/c switch in on position, does it still load the engine?
The moment you switch the a/c on, be it to cool or heat, the compressor is switched on. And that will put a load on the engine.

And yes, blower speed will put a load too; ultimately, it is the engine that powers the blower when the engine is running.
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Old 9th November 2011, 16:24   #73
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Default Re: Air conditioner Load

For point 1: What do u mean by Reduce A/c Control: Do u mean by Temperature Setting or reducing blower Speed.
1.a. For Temperature setting, surely A/C will be switched on based on thermostat setting. If u keep it warm (middle) of temperature knob, then definetly A/C will switched-on for less period of time during its operation and hence reduce the load on the engine.
Conversly if A/C Temperature Setting is set to Coldest Setting, then definetly A/C is switched-on for more period of time for maintaining temperature and hence more load on engine.
1.b - Regarding blower speed, i dont think it will put more load on engine. Since blower is electrically powered. But blower will certainly put electrical load on battery and charging system (alternator). But i dont think Basic Entry level hatchbacks and sedans, have got smart-alternator to switch-off the alternator when battery is fully charged. So, normally in most of cars(except for few with smart charging system) alternator and charging system would run irrespective of electrical loads.

2. Swtiching from Cold to Hot Air, will defenitely switch-off A/C compressor ,henceforth reduce load on engine. But switch-on heater (heater is again electrically powered). and some condition as above 1.b would apply.
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Old 9th November 2011, 17:48   #74
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Default Re: Air conditioner Load

I'll explain how the cooling/heating system in a car works. (Same explaination also hold true for home air conditioner)

Basic components and their functions:
1. Fan/Blower: It just blows the air coming in from cooling coil or heating coil. It is adjusted by the 'speed' control on your dash.
2. Cooling Coil + Compressor: These 2 components are what cool the air. They don't have speed or level. They always cool at the same rate when turned ON.
3. Heating Coil: It is basically a small radiator, which takes hot coolent fluid from the engine gasket andheats up the air which flows through it. This air is then fed into the cabin.

In a manual AC (no climate control):
  • The fan switch controls the fan speed
  • The AC switch controls the Compressor
  • The Cool/Hot switch controls whether the wir wil be taken only from compressor OR only from heating coil OR mixture of both
All these 3 are independent of each other. So, even if your setting is on full hot. Compressor will still be running if your 'cooling switch' is ON.

Also, as i mentioned before, the rate of cooling of a compressor does not change. So, fan speed setting actually does not affect the rate at which your cabin will cool, it will only affect the distribution of this cool air and the perceived cooling effect because fast moving air will be flowing over your skin (same principle of a normal ceiling fan)

Now for Climate control systems:
  • There is a thermostat which is controlled by the temperature setting the user selects. Ex. If user has selected 23 degrees. The compressor will be turned ON and kept ON untill temperature reaches 22 degrees. Then, compressor will be turned OFF, and the temperature is allowed to rise upto 24 degrees at which point compressor is turned ON again. This cycle keeps repeating.
  • Also, depending on the temperature selected and outside temperate, it will automatically select cold air or hot air or mixture.
  • Also, on full auto mode, the temp. selected will affect fan speed. It will be higher when target is far from current temperature and will progressively reduce as target tamp. comes closer. (This does not affect rate of cooling but just because of higher air speed, human body feels comparitively less heat)
  • (Extra point: Contrary to popular belief, lowering the temperate won't make the AC cool faster, it will just keep compressor ON for more time to achieve that target temperature, Ex.: If it iakes 5 minutes to reach 23 degrees, even if you set it at 18 degrees, it wil still take 5 minutes to reach 23 degrees; but only difference would be that the compressor will remain ON and cool till 18 degrees
    Please make note of this important point and save electricity in homes and also fuel in cars by setting it to whatever temperate you need and not lower temperature)
Now, regarding load on engine:
  • As long as the compressor remains ON, there will be load on engine, irrespective of temperature OR air source (hot/old) selected.
  • Also, the fan speed will affect loading on the engine indirectly via the alternator, but its effect is very small compared to that of the compressor
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Old 9th November 2011, 17:49   #75
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Default Re: Air conditioner Load


^^ I was thinking why the compressor needs to be on when the heater is switched off? Can a set up be made where compressor is not required? Or is there any specific reason for the compressor to come on.

I am referring to vehicles without automatic climate control.


Last edited by SPIKE ARRESTOR : 9th November 2011 at 18:07.
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