[aaggoswami]

Originally Posted by .anshuman

I really doubt if the compressor really is switched off with Automatic Climate control system, even in the winters when AC button is on and no cooling is required i have observed the compressor is always switched on, even the ACC uses mixing to achieve the set temperature. This comes with my experience with Climate control systems of Elantra, Punto and Fortuner.

In Swift, I have observed that the compressor is switched off and On, similarly even on Beat I have observed this. And I had read earlier in ACI that for Polo when they were working on 1.2 petrol unit ( which was then offered in Fabia ), they had added compressor clutch so compressor can be switched off. Not sure about Fortuner, but even in Innova I have observed that Compressor is switched on and off.

[IndigoXLGrandDi]

@dhanushs..
Sorry I did not post it correctly.

I have corrected the lines as below-
1)Case 1- AC Temp Knob kept on Blue Zone (Coldest Position) and Manually Turning the Compressor On and Off--
Lets assume that the AC Compressor Cools the cabin to 18 Deg C when the Knob is kept at Coldest Position-
When the AC is Turned ON, it will Cool the Cabin to 18 Deg C. Now if AC is Manually switched Off and again Turned On when feeling hot (temp. may have risen to @ 25 Deg C), the Compressor will have to cool the entire cabin over again to 18 Deg which is 7 less than that of Current (read Hot) Temp. and this will make Compressor to work more which wastes more fuel since the Compressor will SAP Power from Engine and Engine will consume more fuel to power the Compressor for longer time.

2) Case 2- AC Temp Knob kept at Near Middle Position and let the Compressor do its work automatically--
Lets assume that the AC Compressor Cools the cabin to 22 Deg C at this Position (In reality the Near-Middle position may not correspond to 22 Deg C. I am just assuming it for example purpose)--
When the AC is Turned ON, it will Cool the Cabin to 22 Deg C and the Compressor Cuts Off automatically. Now the Compressor automatically Turns ON depending on the time setting of thermostat (will vary between 30-60 sec depending on the car and Mfr). Since the Compressor does not have to cool the cabin all over again unlike in Case 1, it will Not SAP more power from Engine which results in saving fuel (and also the trouble to turn the compressor manually on and off)
Also this procedure will help you Concentrating on driving instead of Concentrating on Turning the Compressor On-Off.

Originally Posted by Suhaas307

I've noticed that my Santro gives me about 10.5 in the city with the AC switched on occasionally. And it will give me 11.5-12 if driven without the AC turned-on and if I drive with a really light foot.

You mis-understood my statement.
I said "Keeping the Temp Knob Near-Middle Position" gives more FE than "Keeping the Knob at Coldest Position and Manually Turning the Compressor ON and Off" and not like you posted.
I know that Turning On the AC will affect the FE as against driving without Turning On the AC.

[pareshraheja]

Guys,

You all are going off track with the concept of compressor functioning. Please note that the compressors in any car across the globe is a scroll compressor. NONE of these have the ability to run on variable speed. Infact no compressor whether in your house, your fridge etc is capable of running on the concept of variation in rpm.

This is achieved by external device called as the VFD or VSD i.e. Variable Speed Drive or Variable Frequency Drive. Variation in compressor RPM is achieved by varying the frequency to the motor connected to it.

No car manufacture gives you a VFD/VSD because it is very expensive. ALL compressors run on CONSTANT SPEED.

If you have the normal AC in the car and switch-on the AC, irrespective of the hot/cold knob position, the compressor runs.

If you have a ACC in your car, the compressor runs till the time the set temperature is achieved and cuts off. This is exactly how the split ac in your house also works. The circulation of air never stops. The difference b/w the two is that in your car, there are multiple thermostats and sensors which check the inside & outside air temperature, the humidity in the cabin and outside. Whereas, the split AC in the house checks only the inside air temperature and internal humidity. This is because the room is not in motion and the car AC has to work in an environment where external parameters are constantly changing.

Cheers

PS : I have worked in the air-conditioning industry and hence able to give this gyaan.

[suhaas307]

Originally Posted by IndigoXLGrandDi

You mis-understood my statement.
I said "Keeping the Temp Knob Near-Middle Position" gives more FE than "Keeping the Knob at Coldest Position and Manually Turning the Compressor ON and Off" and not like you posted.
I know that Turning On the AC will affect the FE as against driving without Turning On the AC.

Oh no, I completely understood your statement. But my question was completely different. How much of a saving are we looking at here, with the AC temp-controller kept in the middle. I know, that you're trying to say that it would not only keep the car cool, but the FE in check too. And switching on the compressor and then switching it off repeatedly affects FE. But how much of an affect is it, really?

It will be very negligible, IMO, and not really worth racking your brains over!

[trumpet]

Guys,

I hope I am not adding to the confusion here. But my point is simply this. I was reading up the earlier posts where it was said that when we turn the selector towards the HOT section, it simply opens up a flap and allows the hot air from the engine bay inside.

Having said the above, why would the AC compressor need to run at all during this process ?, because during this phase we don't need cold air, we need hot air right ?

Am I making sense here ? or am I way off ?

[pareshraheja]

Originally Posted by trumpet

Guys,

I hope I am not adding to the confusion here. But my point is simply this. I was reading up the earlier posts where it was said that when we turn the selector towards the HOT section, it simply opens up a flap and allows the hot air from the engine bay inside.

Having said the above, why would the AC compressor need to run at all during this process ?, because during this phase we don't need cold air, we need hot air right ?

Am I making sense here ? or am I way off ?

What you say makes absolute sense because as a human being this is the logical outcome or this is how when we think logically the outcome should be.

But, the ac in your does not think. Hence, that's why it does know whether cold air output should be stopped if hot air intake is opened. There is no mechanism for it to understand (read no hardware chip with software).

Further, in ACC, there is a software on a chip which takes inputs like set temperature, humidity etc, similar to your 16/32 bit chip to MPFI petrol cars or an ECU to run Direct Injection Diesels.

Take for ex: if you set an temperature of 24 Deg C in Delhi summer, the compressor will keep running till its achieves says 23.5 Deg C before cutting off. Similarly, if you set the same temperature in Delhi Winter, it knows that the compressor need not be switched on and rather it will open the hot air blower and continue to do so till the temperature in the cabin reaches say 24.5 Deg C.

Hope this answers your query and also clarifies the working of normal car ac vs ACC.

Cheers

[alpha1]

Originally Posted by trumpet

Guys,

I hope I am not adding to the confusion here. But my point is simply this. I was reading up the earlier posts where it was said that when we turn the selector towards the HOT section, it simply opens up a flap and allows the hot air from the engine bay inside.

Having said the above, why would the AC compressor need to run at all during this process ?, because during this phase we don't need cold air, we need hot air right ?

Am I making sense here ? or am I way off ?

Because you have pressed the AC (on) button!
I think I will try this today (though I had long suspected the same) - drive with temp knob turned to red - all the way.
And then checking the effect of switching the AC on and off.

[vigsom]

as far as I know, the best way to run a manual HVAC is to keep the temp. knob all the way to the left(blue),switch on the compressor and adjust the blower speed as per comfort requirements.

When you need hot air,keep the compressor switch off and just turn the knob all the way to the right(red ) ; again use the blower to adjust flwo of hot air.

Mixing is an energy "inefficient" way of controlling temperature.

The only ac I know which had the thermostat was the Superking in Premiers ; there was a temperature control ring around the blower speed switch which could be used to alter the grille temperature,and consequently the cabin temperature.In the Superking set up there was no heater coil.

[Jaggu]

Why so much of confusion, the old system where there is only one fixed cut off in the circuit, using the temperature control is putting additional load on the compressor. Why? Because the compressor is cooling the system and the temperature control has opened the hot air flap = chances of grill temperature reaching the cut off is almost nil.

In the newer variable thermostat system which is electronically controlled, temperature cut off is done according to user settings by cutting off the compressor at the set temperature. This is the right method.

Easiest way to make the overcooling AC bearable in the old type system is a) to switch off the compressor occasionally b) fix an aftermarket variable thermostat (will require a small rewiring/refitting). Its easily available in the aftermarket and you can manually control the AC cut off temperature.

Of all the system worst is my Swift Vdi system, where the compressor does not have a cut off! It keeps on running only thing is the temperature is set somewhere around 21 deg cabin temperature, though it gets too cold during long highway trips.

[anandpadhye]

Yes, I agree.
For regular Airconditioners (no climate control), roating the temperature control knob is like fooling one self - if mixes hot air into the cold...It's better to keep this knob on the coolest setting and switch the A/C on/off periodically to reduce cooling when needed.

[Sutripta]

In the environment of a car (small enclosed volume compared to both number of people and blower cfm), cycling the compressor, even with a hysteresis of 1 deg, is not very comfortable. And the primary focus of a HVAC system should be comfort.

Aftermarket fittings had to have a thermostat. There is no other alternative.

The modern (though costlier) system is to use a variable displacement compressor.

The older Marutis with the Sanden system had a variable thermostat.
(It is best not use Maruti AC as an example. eg most 800s did not have either a variable thermostat, or a heater. The red/ blue knob was purely ornamental).

Regards
Sutripta

[Mpower]

Originally Posted by pareshraheja

This is achieved by external device called as the VFD or VSD i.e. Variable Speed Drive or Variable Frequency Drive. Variation in compressor RPM is achieved by varying the frequency to the motor connected to it.

what is achieved? Just now you said no compressor works on variable speed?

Secondly most of the car compressors are piston and not scroll. Scroll is something new and is catching on quickly.

Originally Posted by pareshraheja

This is exactly how the split ac in your house also works. The circulation of air never stops.

Split means the compressor and condenser are housed in two separate units that are remotely mounted from one another. Its has nothing to do with the presence or type of thermostat.

[pareshraheja]

Originally Posted by Sutripta

The modern (though costlier) system is to use a variable displacement compressor.

As far as my knowledge goes there is nothing like a variable displacement compressor. The different types are scroll, stepped, screw and centrifugal compressors. ALL are Constant Speed.

Scroll are the most compact types and hence used in automobiles.

Only a VFD/VSD can be used to make the compressor function in varying RPM's.(via the motor).

Also note that the compressor in a car is not driven my an electric motor but it is connected to the engine. So, higher the rpm faster will be the cooling.

VFD's are used in central air-conditioning of large buildings/hospitals/malls. Again, the logic is simple. As a building is mostly having people coming in and leaving and temperature pattern changing over the day, VFD/VSD gives you the flexibility of varying the load on a single large compressor instead of having many small multiple compressor.

The higher cost of the VFD/VSD gets balanced by the fact that you are spending far less on a single large compressor vs many small compressors.

Therefore, having VFD/VSD on a compressor in the car does not make economic sense even if you were to run it through an electric motor.

Cheers

Originally Posted by Mpower

what is achieved? Just now you said no compressor works on variable speed?

Secondly most of the car compressors are piston and not scroll. Scroll is something new and is catching on quickly.

Split means the compressor and condenser are housed in two separate units that are remotely mounted from one another. Its has nothing to do with the presence or type of thermostat.

What I meant to say was that a compressor by itself cannot run on variable speed or for a matter of fact even on constant speed. It is the driver that governs the speed. What I probably missed out was the fact that the compressors in the car don't have any electric motor as they are connected to the engine and hence the compression of the refrigerant is based on the engine rpm.

If you need to accurately control the compression i.e. speed of movement of the compressor, you will need a VFD/VSD. This is not possible in the given scenario and even if you were to fit a VFD/VSD compatible motor instead of connecting the compressor to the engine, it will be cumbersome and an expensive affair.

The analogy given to the split AC was more to showcase the digital temperature indicator one sees on split ac's. It has nothing to do with a split or a window ac. The reason why I chose the split ac and not the window was because not many may have seen a window ac with digital display. They do exist but, very few. In fact, if you notice any new generation split ac, there will also be a light that indicates whether the compressor is running or not. What I mean is that even the ac in your home does not come with VFD/VSD else, you will not have a light for compressor on/off

Cheers

P.S. : I was not very sure about the piston compressors in car. In the air-conditioning industry, those are really obsolete and hence my ignorance.

Note from the Team-BHP Support Team : Please use "Multi Quote" option for quoting Multiple posts, instead of creating another back-to-back post.

[Sutripta]

Strange when a professional

Originally Posted by pareshraheja

PS : I have worked in the air-conditioning industry and hence able to give this gyaan.

first authoritatively says

Originally Posted by pareshraheja

Please note that the compressors in any car across the globe is a scroll compressor.

and then says

Originally Posted by pareshraheja

P.S. : I was not very sure about the piston compressors in car. In the air-conditioning industry, those are really obsolete and hence my ignorance.

and then again says

Originally Posted by pareshraheja

As far as my knowledge goes there is nothing like a variable displacement compressor.

Hi,
Could it be that there is some difference in the requirements for cars and buildings, leading to some differences in the solutions arrived at?

Just typed "variable displacement ac compressor" into Google. 86,000 results. Going through some of them might be interesting.
Or going through the product portfolio of any of the major automotive AC system vendors. Like say
Sanden International - Singapore

Incidentally, VFDs are making their way into domestic appliances like washing machines and airconditioners. No longer the preserve of industrial systems.

Regards
Sutripta

[alpha1]

Originally Posted by alpha1

Because you have pressed the AC (on) button!
I think I will try this today (though I had long suspected the same) - drive with temp knob turned to red - all the way.
And then checking the effect of switching the AC on and off.

So I did the experiment yesterday and was (not) surprised to see that the AC DOES SWITCH ON, EVEN WHEN YOU KEEP THE TEMP KNOB TO RED.



OHC Vtec 2001 model.