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Old 19th August 2008, 11:10   #1
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Default AC running query

I recently bought an ALTO, primarily because of its FE. Since i plan to drive interstate, fuel efficiency was the deciding factor.

Since this is my first car, my questions might sound noobish :P but please bare with them.

I have a small query about the running the AC. As you all might have noticed, the ACs we use at our homes have a built in thermostat that simply turns off the compresser after a while (when the ambient temprature reaches the temperature set by the user). At times, only the blower works while the compresser runs intermittently. Isn't there a similar mechanism in the cars ?

I noted that the car AC runs infinitely. Since Alto is a small car, AC simply hampers even the small kick, the new engine is having. What I did while driving from my way from punjab to gurgaon, I simply replicated the same scenario by manually turning off the AC after a while and then when starting it again, hoping that it will save fuel and it really adds to the drive quality (marginally though).
I am not sure if its the right way but it works.

Now all I want to confirm from you guys is:
1) Whether it is a preffered way ?
2) Am I really saving fuel by doing this ?
3) Do I save any fuel by running the AC on fan speed 1 or 2 or 3 ? (I dont know the blower ratings so not sure the power i draws demands any big fuel consumption or not). I mean to ask "does the fan speed have any MARGINAL effect on the fuel consumtion" ?
4) Does the continuous starting and shutting off the compresser mars its life ?

Please advice exhasutively. I would like to gather as many thoughtas as possible. Thanks.

Last edited by nitin.dangayach : 19th August 2008 at 11:14.
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Old 19th August 2008, 15:33   #2
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1) no it is not preferred
2) not as much fuel as much damage you can cause by doing so
3)no way fan speed doesnt have effect on fuel as compressor is running . blower only blows air at different speeds
4) turning on and off compressor ruins compressor life
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Old 19th August 2008, 16:18   #3
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Originally Posted by harry2223 View Post
1) no it is not preferred
2) not as much fuel as much damage you can cause by doing so
3)no way fan speed doesnt have effect on fuel as compressor is running . blower only blows air at different speeds
4) turning on and off compressor ruins compressor life
And how do you, then, advocate the fact that normal home ACs work on the same principle. They do shut down the compressor after a while.

So how do they carry forward that thing since even I have a notion that turning the compressor on and off reduces its life.
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Old 19th August 2008, 16:38   #4
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1) Whether it is a preffered way ?
May be or maybe not.
But I do it all the time.

2) Am I really saving fuel by doing this ?
Yes, because you are not running the AC.

3) Do I save any fuel by running the AC on fan speed 1 or 2 or 3 ? (I dont know the blower ratings so not sure the power i draws demands any big fuel consumption or not). I mean to ask "does the fan speed have any MARGINAL effect on the fuel consumtion" ?
Yes it does.
The air flowing exchanges its heat with the AC system.
So as more air flows thrus the AC systems its coldness also goes away.
So the compressor needs to work more to keep its system colder longer.


4) Does the continuous starting and shutting off the compresser mars its life ?
I dont think so. But I don't mean that you switch it on and off every 2 mins.
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Old 19th August 2008, 16:50   #5
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Agree with Bblost. Even i don't use the AC running all the time as i feel claustrophobic with windows up all the time, if its a free running time, i prefer my windows down and if i am stuck in traffic, then AC is the way to go for me.

However, if you do this on/off thingy keeping FE in mind, then it would not really help because the difference would be so marginal, you may have to use a binoculor to your calculator to get the right saved figure.

And, yes switching on and off does not harm the compressor but, do it maybe 3-4 times in a 20km city drive ( just a ballpark that came to my mind)
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Old 19th August 2008, 16:56   #6
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IMHO the milage at 60~80 kmph will be the same if
AC is on.
or if
Windows are down.

As the car goes faster the milage with AC on will be better than with windows down.
and of course vice versa.
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Old 19th August 2008, 17:10   #7
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Quote:
Originally Posted by bblost View Post
4) Does the continuous starting and shutting off the compresser mars its life ?
I dont think so. But I don't mean that you switch it on and off every 2 mins.
But my home AC does so. I have seen it switching off and on the compressor very frequently sometimes.

And the other thing, since I drive at more than 60 on highways for long hours (when i go back my home from gurgaon to punjab or back), will running the AC impact me less than while switch the AC on on slower speeds ? I dont know why but i am not really convinced.
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Old 19th August 2008, 17:14   #8
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Nitin, Think about this, do you concentrate on the road or the AC.

what I do is:
Keep AC on.
When you start feeling cold, switch it off. but keep the blower on.
When you start feeling the need for cooler air switch it back on.


and sorry I dont have hard numbers to maintain my claims or opinion.
so will just let it be.

Last edited by bblost : 19th August 2008 at 17:17.
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Old 19th August 2008, 17:17   #9
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Quote:
Originally Posted by bblost View Post
Nitin, Think about this, do you concentrate on the road or the AC.

what I do is:
Keep AC on.
When you start feeling cold, switch it off. but keep the blower on.
When you start feeling the need for cooler air switch it back on.
LOL yes, thats what I am trying to do. But since the thing cools the ambience in pretty less time, i turn it off and then the same coolness goes away early.....so i end up doing the same thing pretty frequently. Thats what I wanted to know :P
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Old 19th August 2008, 17:25   #10
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Quote:
Originally Posted by nitin.dangayach View Post
But my home AC does so. I have seen it switching off and on the compressor very frequently sometimes.

And the other thing, since I drive at more than 60 on highways for long hours (when i go back my home from gurgaon to punjab or back), will running the AC impact me less than while switch the AC on on slower speeds ? I dont know why but i am not really convinced.
Nitin, you need to use the round switch on the right side of the ac console. The one which has markings from blue to white to red, blue stands for cold and red for hot. Keep the knob somewhere in between, at a setting comfortable to you. I usually keep it at 10 on the clock position. This controls the thermostat and automatically switch on/off the compressor when the optimal temp. is achieved.

The round dial switch on the left is the blower speed switch, you can keep it on whatever speed you prefer.

Using the thermostat will definitely save you fuel as the compressor would be engaged only when required.

Hope this helps.

Last edited by cyclops : 19th August 2008 at 17:30.
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Old 19th August 2008, 17:25   #11
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Hi,

> I noted that the car AC runs infinitely.

It would be difficult to guage when the compressor is on or off in a moving vehicle.

If you have a heating part also in your Alto, then move that lever / knob away from the cold position as per your need for the moment. You can adjust comfort by increasing / decreasing the blower speed.

If the temperature knob is at the coldest position, the compressor would run continuously.

Sorry I am not familiar with the Alto dashboard or controls.
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Old 19th August 2008, 17:29   #12
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1) Whether it is a preffered way ?

I don't know. But yes I do switch it on and off as required. (overtaking, feeling cold whatever reason)

2) Am I really saving fuel by doing this ?

Maybe, but can't be much. Since switching off can warm up the cabin maybe its doing more harm than good.

3) Do I save any fuel by running the AC on fan speed 1 or 2 or 3 ? (I dont know the blower ratings so not sure the power i draws demands any big fuel consumption or not). I mean to ask "does the fan speed have any MARGINAL effect on the fuel consumtion" ?

Again there is no connection b/w the fan and the temperature control, but as pointed out elsewhere in the thread more speed can improve airflow and that can lead to a more "effective" so lesser load on the compressor.

4) Does the continuous starting and shutting off the compresser mars its life ?

Any device mechanical, electrical or whatever will have less life if switched on/off frequently.
So an A/C compressor is no different.

In any case the FE difference will be very minimal.
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Old 19th August 2008, 17:30   #13
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Quote:
Originally Posted by trrk View Post
Hi,

If the temperature knob is at the coldest position, the compressor would run continuously.

I dont think so.
There are two inputs to the air that finally exits the blower.
One comes from the condensor part which is cold.
The other from the engine side which is hotter.

The Hot-Cold lever controls the % of these two inputs.
So when kept at the coldest setting only the cold air is sent out.
as you move it to the hotter setting more and more hot air and less and less cold air is being mixed.

The amount of work the compressor has to do does not change.

but this is my understanding.
Would love to learn more.
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Old 19th August 2008, 17:52   #14
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Quote:
Originally Posted by cyclops View Post
This controls the thermostat and automatically switch on/off the compressor when the optimal temp. is achieved.

Using the thermostat will definitely save you fuel as the compressor would be engaged only when required.

Hope this helps.
Quote:
Originally Posted by trrk View Post
.

If the temperature knob is at the coldest position, the compressor would run continuously.
A car HVAC system is designed to operate till it reaches a pre-determined temperature. At this point the compressor cuts-off.
The temperature control knobs do not control the thermostat. This controls the amount of hot air that is mixed with the cold air. This essentially means that you are not reducing the load on the compressor.
Search for threads in the technical section. This issue has been discussed at length.

Bblost - you are right in your understanding

Last edited by viper_711 : 19th August 2008 at 17:54.
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Old 19th August 2008, 18:19   #15
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Quote:
Originally Posted by bblost View Post
I dont think so.
There are two inputs to the air that finally exits the blower.
One comes from the condensor part which is cold.
The other from the engine side which is hotter.

The Hot-Cold lever controls the % of these two inputs.
So when kept at the coldest setting only the cold air is sent out.
as you move it to the hotter setting more and more hot air and less and less cold air is being mixed.

The amount of work the compressor has to do does not change.

but this is my understanding.
Would love to learn more.
I second bblost. I too have a similar understanding and feel that the red and blue "supposedly " the temperature knob for the ac just controls the hot-cold air mixing percentage.
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