[dhanushs]

Originally Posted by mayankjha1806

My understanding is in a normal AC it does not sense the temperature inside and keeps blowing cold air without cutting it off ever. Is my understanding wrong?

The difference between a normal AC and ACC is that, the normal AC only senses the grille temperature, or the temperature of the air blown.

In ACC, along with the grille temperature, cabin temperature is also being sensed.

The reason normal AC's sense the temperature of the air blown/pressure of the refrigerant is to prevent icing the lines.

[revintup]

Which of these below two scenarios give a better fuel efficeincy in a manual AC?I drive an ANHC.

A. Adjust the ac temperature control to a medium level where you feel comfortable. I am assuming that the compressor would shut down more often than at its lowest temperature setting.

B. Temperature control set at the coolest. Switch off the AC switch when the climate is comfortable and switch it back on when its getting a bit hot.


I know its a bit confusing. My basic question is does the ac take just as much fuel in a medium setting (both compressor and heater working to give desired temperature) as it would in the coolest setting ( just the compressor, but at maximum load) ?

[jalsa777]

Originally Posted by revintup

I know its a bit confusing. My basic question is does the ac take just as much fuel in a medium setting (both compressor and heater working to give desired temperature) as it would in the coolest setting ( just the compressor, but at maximum load) ?

the amount of load on engine (fuel consumed, power loss, etc) will be same under all conditions when the compressor is ON. Period.

[mkpiyengar]

Originally Posted by revintup

Which of these below two scenarios give a better fuel efficeincy in a manual AC?I drive an ANHC.

A. Adjust the ac temperature control to a medium level where you feel comfortable. I am assuming that the compressor would shut down more often than at its lowest temperature setting.

B. Temperature control set at the coolest. Switch off the AC switch when the climate is comfortable and switch it back on when its getting a bit hot.


I know its a bit confusing. My basic question is does the ac take just as much fuel in a medium setting (both compressor and heater working to give desired temperature) as it would in the coolest setting ( just the compressor, but at maximum load) ?

As per my understanding, the compressor would cut off based on the thermostat setting. As earlier pointed out in the thread the difference between ACC Vs manual A/c would be the thermostat monitoring the temperature. As long as the desired temperature is achieved it would cut off. So in case your a/c is set in middle then the compressor will cut off soon which is better.

However, in case you keep it in coolest setting then the compressor would be on till the coolest temperature attained. Now, it would be relative what is the coolest temperature achieved based on the manufacturer's settings and also the rating of the A/C unit in the car.

AFAIK For manual a/c (not ACC) we cannot verify the temp achieved unless you have thermometer in car to check it.

So to sum up in my opinion the A/C with medium setting would be better than keeping it coolest and manually switching of.

Cheers

[Aroy]

Originally Posted by revintup

Which of these below two scenarios give a better fuel efficeincy in a manual AC?I drive an ANHC.

A. Adjust the ac temperature control to a medium level where you feel comfortable. I am assuming that the compressor would shut down more often than at its lowest temperature setting.

B. Temperature control set at the coolest. Switch off the AC switch when the climate is comfortable and switch it back on when its getting a bit hot.


I know its a bit confusing. My basic question is does the ac take just as much fuel in a medium setting (both compressor and heater working to give desired temperature) as it would in the coolest setting ( just the compressor, but at maximum load) ?

What the AC is doing automatically in A, you are doing manually in B. The AC does just that, keeps running till temperature is achieved and switches off.

As discussed before, the AC consumes constant power when it is on. The temperature setting is just to tell the AC when to switch it off and again on. So the answer to your question is that AC consumes same amount of fuel. The only difference is that if you are using heater to control the temperature, the AC will work longer, consuming fuel all the time.

The only method of reducing the fuel consumption, while at the same time keeping constant temperature is to use Variable Refrigerant Flow (VRF). Here the AC regulates the quantum of refrigerant either by reducing the speed of the motor (Non Automotive AC), or by varying the vane geometry to adjust the flow. In both the cases the flow is reduced after achieving the temperature and so is the power consumption. Variable vane AC are yet to make their entry in most vehicles.

Links
Summer A/C Guide, How it Works, and When it Doesn't - Honda-Tech
Types of Automobile Compressors | eHow.com

[SirAlec]

Originally Posted by mayankjha1806

Thanks, a question.

Wouldn't this be applicable on a Climate control AC only and not a normal one? My understanding is in a normal AC it does not sense the temperature inside and keeps blowing cold air without cutting it off ever. Is my understanding wrong?

As per the original post the question was around normal AC and not climate control AC.

I am sure you are confused. Major difference with climate control, is that to keep the cabin temperature to the one specified on Dial, climate control also may switch on the heater.

Normal AC just works like home AC. I am sure you might have heard a cut off and switching on of compressor in windows AC unit automatically.

[dhanushs]

Originally Posted by revintup

Which of these below two scenarios give a better fuel efficeincy in a manual AC?I drive an ANHC.

A. Adjust the ac temperature control to a medium level where you feel comfortable. I am assuming that the compressor would shut down more often than at its lowest temperature setting.

B. Temperature control set at the coolest. Switch off the AC switch when the climate is comfortable and switch it back on when its getting a bit hot.

Originally Posted by mkpiyengar

As per my understanding, the compressor would cut off based on the thermostat setting. As earlier pointed out in the thread the difference between ACC Vs manual A/c would be the thermostat monitoring the temperature. As long as the desired temperature is achieved it would cut off. So in case your a/c is set in middle then the compressor will cut off soon which is better.

Originally Posted by Aroy

What the AC is doing automatically in A, you are doing manually in B. The AC does just that, keeps running till temperature is achieved and switches off.

Hi,

AFAIK, Scenario 'B' will offer better FE.

Reason: In a manual AC, there is nothing to sense the temperature in the cabin. Only the Grille temp or Air trow temp is sensed (or refrigerant pressure, i think). And, this is done only to prevent icing.

This compressor cut off temperature is independent of the hot-cold knob.

So, in scenario A, what happens is: The hot air is mixed with the cold air. Here the grille temperature is much higher than the compressor cut off temperature, and hence, the compressor is continuously ON. (This is also done to prevent misting/de-humidification)

In scenario B: When the AC is in max cold setting, whenever the grille temp (refrigerant pressure) reaches a specific min (pressure:max) value, the compressor is shut off, to prevent icing.

So, actually, the duration for which the compressor is OFF is max in scenario B, and hence lesser engine load, and more FE.

Regards,

Dhanush

[mkpiyengar]

Originally Posted by dhanushs

Hi,
So, in scenario A, what happens is: The hot air is mixed with the cold air. Here the grille temperature is much higher than the compressor cut off temperature, and hence, the compressor is continuously ON. (This is also done to prevent misting/de-humidification)

How would hot air and cold air get mixed in the A/C ? Could you kindly elaborate?
According to me once the desired temperature is achieved, the condenser would go off and only the blower would be blowing air i.e fan mode. This is how the Room A/C operates. I hope the concept is same in cars. Also, the AHNC manual says some cars have heating also, which is achieved by some heating mechanism.(Not available in Indian cars I think)

However, in case you have the coldest setting then that would take longer to achieve that temp resulting in condenser being ON for longer time. Also, in case you are driving in blazing sun then it would take more to achieve the coldest setting than say intermediate.

P.S: In case of Diesel the difference in pick-up is negligible with A/C on, whereas in petrol it is more prominent. Relatively speaking.

Cheers,

[dhanushs]

Originally Posted by mkpiyengar

How would hot air and cold air get mixed in the A/C ? Could you kindly elaborate?

Heat, in most cars this side of 10L is obtained from the engine. There is a AC diverter valve which controls the flow of the hot coolant from the engine, which is used to heat up the air.

Originally Posted by mkpiyengar

AAlso, the AHNC manual says some cars have heating also, which is achieved by some heating mechanism.(Not available in Indian cars I think)

Almost all cars with AC have the heater. Except may be the Indica.

Originally Posted by mkpiyengar

However, in case you have the coldest setting then that would take longer to achieve that temp resulting in condenser being ON for longer time. Also, in case you are driving in blazing sun then it would take more to achieve the coldest setting than say intermediate.

You are confusing the climate control with Manual AC. This can be achieved only if the temperature control knob is directly related to the compressor cut off temperature. Or this can happen only if the compressor cut off temperature is controlled by the temperature control knob.

But. that is NOT the case in manual AC. In most cars, what the temperature control knob does is just to control a diverter valve. This valve decides how much air is to be diverted towards the hot coolant for heating up.

Also, think of it, if what you where saying was true, wont the compressor completely shut off, when we turn it completely to the HOT position? But that doesn't happen.

[mkpiyengar]

Originally Posted by dhanushs

.......

OK, I always thought it was better to keep the a/c in medium compared to coolest setting. Will try this method.

In my Fiat Siena, the a/c chills very quickly and it gets quite cold especially during night, so I keep it at middle and the fuel efficiency drops quite a bit compared to non-ac driving.

Will try keeping it coolest and switching off manually and post the results.

BTW, read some where that the blower speed is also related. Not sure how will the blower speed matter?

Cheers.

[Aroy]

Originally Posted by dhanushs

But. that is NOT the case in manual AC. In most cars, what the temperature control knob does is just to control a diverter valve. This valve decides how much air is to be diverted towards the hot coolant for heating up.

Also, think of it, if what you where saying was true, wont the compressor completely shut off, when we turn it completely to the HOT position? But that doesn't happen.

At least in MUL products, I have heard and felt the AC switching off faster when on warmer side of the temperature control. In fact the AC in the car behaves exactly like the AC at home - the thermostat determines when the AC will cut off/on.

The temperature knob has a blue and a red area. The blue is for AC and the red for the heater, will try out the AC with thermostat set in red area.

[Sutripta]

Originally Posted by SPIKE ARRESTOR

Turning the switch to red zone means you need less cooling right (as there is no heater) ? I am asking how dehumdification process happens when there is cold air / hot air requirement.

Yes, you are right, all this happens inside the Evaporator unit. I am more concerned of how and why? Strangely, I never gave this a thought anytime.

Spike

Hi,
If you cool air below its dew point, water will condense out. If you bring it back to its original temp, it now carries less moisture - it has been dehumidified. The RH value has dropped. Note though that if you just cool it below the dew point, though you will be precipitating moisture, RH now is 100%. Uncomfortable. Cold and clammy.

So you first cool the air to remove moisture. There are some standard rules (incorporated in tables) as to when a person feels comfortable in terms of temperature, humidity, wind velocity, whether clothed or not etc. Then you heat it to the degree needed.

All this assumes that the overriding priority in a HVAC system is comfort. If cost or fuel efficiency start becoming overriding priorities, designs will change.

The ultimate is if one can setup the system for a 'cool head, warm feet' setup no matter what the ambient temp.

Regards
Sutripta

[SPIKE ARRESTOR]

Yes ^^ this is what I was looking for, trying to relate things to the Psychometric chart. I was sure it is not just the dehumidification happening there must be dehumidification + (---) process. I can recollect things now.

Spike

[dhanushs]

Originally Posted by Aroy

At least in MUL products, I have heard and felt the AC switching off faster when on warmer side of the temperature control.

That.. is a better method, but expensive. The only downside in that method I guess is the dehumidification, will not occur when the compressor is off.

In fact the AC in the car behaves exactly like the AC at home - the thermostat determines when the AC will cut off/on.

I think ACC in cars behaves exactly like home AC. You set a temperature, and the system does the rest in maintaining it.

Manual AC I believe, is different. Let me try to source more concrete info in this regard.

[anujmishra]

Is everyone sure that AC cuts off in Manual AC when certain temperature reaches? Is there any source of information for this?

If this was the case cars with smaller CC should sometimes should not feel load on engine(Atleast for some time). I have never felt this during my seven year of driving of 800. I was driving 800 with almost AC on for 80% and there was drastic power reduction when AC is switched on.

I feel people making statement based on his experience of Window AC at home.