[cyclops]

Originally Posted by viper_711

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

I doubt whether the car AC mixes hot air with cold to control the temp. because in a small car like alto, one can easily make out when the compressor is getting cut-off. And in my personal experience i've observed, that whenever I move the dial towards the red markings, the compressor cut-off's frequently. If i keep it on the coldest, the compressor hardly gets cut-off.

[viper_711]

Originally Posted by cyclops

I doubt whether the car AC mixes hot air with cold to control the temp. because in a small car like alto, one can easily make out when the compressor is getting cut-off. And in my personal experience i've observed, that whenever I move the dial towards the red markings, the compressor cut-off's frequently. If i keep it on the coldest, the compressor hardly gets cut-off.

Cyclops, check these links:
HowStuffWorks "How Car Cooling Systems Work"
Car Air Conditioner System
First one explains how the heater part of the car AC works. the second one explains how temperature is controlled.

[cyclops]

Originally Posted by viper_711

Cyclops, check these links:
HowStuffWorks "How Car Cooling Systems Work"
Car Air Conditioner System
First one explains how the heater part of the car AC works. the second one explains how temperature is controlled.

Hmm.. Good link, explained in plain english how an AC works. Well I dint get much understanding on the working of the thermostat by the links and I am still pretty much sure that the temp. control knob when set on less cold, cuts-off the compressor frequently. This is what I have felt and observed everyday. But I don't have any link or something to prove it.

EDIT: I'm not sure about the cars with climate control, as I have never experienced one. The working specified in the link might be true for the climate control systems.

[viper_711]

Originally Posted by cyclops

Hmm.. Good link, explained in plain english how an AC works. Well I dint get much understanding on the working of the thermostat by the links and I am still pretty much sure that the temp. control knob when set on less cold, cuts-off the compressor frequently. This is what I have felt and observed everyday. But I don't have any link or something to prove it.

- Please find one - I would be quite interested in your theory!

Originally Posted by cyclops

EDIT: I'm not sure about the cars with climate control, as I have never experienced one. The working specified in the link might be true for the climate control systems

. - It's true for most cars - not too sure of really high-end ones like the S class or the 7 series though!

I suggest you try this - Switch on the AC with the knob/slider at the coldest position. After it has run for about 5-10 mins bring the temperature knob/slider to somewhere in between the blue and the red - once you feel the air coming out of the blower is now less cold than before - switch off the compressor. After a few seconds - you should get hot air coming out of the blower - now move the knob/slider to the coldest position again (without switching on the compressor). The air should feel cooler - this is because the system stops mixing the hot air! This should tell you that it basically mixes hot and cold air.

[cyclops]

Originally Posted by viper_711

I suggest you try this - Switch on the AC with the knob/slider at the coldest position. After it has run for about 5-10 mins bring the temperature knob/slider to somewhere in between the blue and the red - once you feel the air coming out of the blower is now less cold than before - switch off the compressor. After a few seconds - you should get hot air coming out of the blower - now move the knob/slider to the coldest position again (without switching on the compressor). The air should feel cooler - this is because the system stops mixing the hot air! This should tell you that it basically mixes hot and cold air.

Yes, i will try this and update you with my observations..

Originally Posted by cyclops

I am still pretty much sure that the temp. control knob when set on less cold, cuts-off the compressor frequently. This is what I have felt and observed everyday. But I don't have any link or something to prove it.

I am not sure if ALTO has any inbuilt Thermostat. If there would have been any thermostat, the compressor cut out can easily be experienced on Alto (you suddenly get a boost as if you have turbocharger :P, by switching off the AC). In my 400 km long drive, i never felt anything like that (auto shutt off of the compressor)

Going by this fact, I guess your assumption might be wrong. The mixing of hot air and cold air sounds logical.

[viper_711]

Originally Posted by cyclops

Yes, i will try this and update you with my observations..

Dude! Waiting for your observations!

[cooldude1988765]

Originally Posted by nitin.dangayach

I am not sure if ALTO has any inbuilt Thermostat.

I am sure any car equipped with an ac/heater will have a thermostat. The knob with the blue and red markings is the thermostat.

[humyum]

Originally Posted by nitin.dangayach

I am not sure if ALTO has any inbuilt Thermostat. If there would have been any thermostat, the compressor cut out can easily be experienced on Alto (you suddenly get a boost as if you have turbocharger :P, by switching off the AC). In my 400 km long drive, i never felt anything like that (auto shutt off of the compressor)

Going by this fact, I guess your assumption might be wrong. The mixing of hot air and cold air sounds logical.

Even my 1998 carburated 800 has a thermostat which cuts the compressor off once the desired temperature is reached and an alto definitely definitely has one.
Get your car checked if its not cuttin off.

[abhibh]

Originally Posted by nitin.dangayach

I am not sure if ALTO has any inbuilt Thermostat. If there would have been any thermostat, the compressor cut out can easily be experienced on Alto (you suddenly get a boost as if you have turbocharger :P, by switching off the AC). In my 400 km long drive, i never felt anything like that (auto shutt off of the compressor)

Going by this fact, I guess your assumption might be wrong. The mixing of hot air and cold air sounds logical.

On long drives you won't feel much difference as car is in motion doing consistently 80 -100 km. Alto does have a auto cut off and it also have NOS. While overtaking just turn off the AC u will experience the NOS. Maruti should rename the AC switch to NOS and write in instructions "DEPRESS TO GET POWER BOOST"

[merve_extreme]

This has been discussed many times but lets do it all over again

1)Cars With HVAC System
They do have a thermostat which is fitted right in front of the evaporator core(the internal cooling coil from where the fan blows cold air).This thermostat is mostly electronic type and is set at 2-5C depending on the manufacturer.Once the Air coming from the blower reaches this set temp the AC compressor will disengage.As the temp of the air rises the compressor will cut in again.The Knob with the blue and red markings controls either the adjuster flap for the heater or the flow of engine coolant to the heater core which sits in front of the evaporator.even at fully hot position the Ac compressor will cut in and out but it may take longer as the cabin air is at higher temp.

2)Cars with Climate Control
They have temp sensors,single or multiple depending on the manufacturer.The desired Air temp can be set by the driver through a temp knob or digital display.The Ac compressor will disengage when the air in the cabin reaches the set temp as sensed by the sensors.If the temp is lower in the cabin than what is set then the heater core is used automatically as mentioned earlier to bring the cabin temp to the set temp.In fully Auto mode the fan speed is also adjusted depending on the cooling requirements.


All Car AC systems have two other devices :
1)Expansion valve or orifice tube to control the flow of refrigerant to the evaporator so that the evaporator temp never dips below 0C as the evaporator will freeze and reduce cooling efficiency.

2)Pressure Switch to cut off the compressor if the refrigerant pressure goes too high(due to a faulty thermostat or blockage) or low(due to a refrigerant leak).In some cars this pressure switch is also used to control the raditor(condensor) fan speed depeding on the refrigerant temp which is directly proportional to its pressure.

[Path_Finder]

Originally Posted by merve_extreme

This has been discussed many times but lets do it all over again

1)Cars With HVAC System
They do have a thermostat which is fitted right in front of the evaporator core(the internal cooling coil from where the fan blows cold air).This thermostat is mostly electronic type and is set at 2-5C depending on the manufacturer.Once the Air coming from the blower reaches this set temp the AC compressor will disengage.As the temp of the air rises the compressor will cut in again.The Knob with the blue and red markings controls either the adjuster flap for the heater or the flow of engine coolant to the heater core which sits in front of the evaporator.even at fully hot position the Ac compressor will cut in and out but it may take longer as the cabin air is at higher temp.

2)Cars with Climate Control
They have temp sensors,single or multiple depending on the manufacturer.The desired Air temp can be set by the driver through a temp knob or digital display.The Ac compressor will disengage when the air in the cabin reaches the set temp as sensed by the sensors.If the temp is lower in the cabin than what is set then the heater core is used automatically as mentioned earlier to bring the cabin temp to the set temp.In fully Auto mode the fan speed is also adjusted depending on the cooling requirements.


All Car AC systems have two other devices :
1)Expansion valve or orifice tube to control the flow of refrigerant to the evaporator so that the evaporator temp never dips below 0C as the evaporator will freeze and reduce cooling efficiency.

2)Pressure Switch to cut off the compressor if the refrigerant pressure goes too high(due to a faulty thermostat or blockage) or low(due to a refrigerant leak).In some cars this pressure switch is also used to control the raditor(condensor) fan speed depeding on the refrigerant temp which is directly proportional to its pressure.

merve_extreme put it very right and precise. Additional point to note is that Alto being a small engine and less power a/c, the cooling effect is low and the built-in thermostat hardly gets a chance to cut-off. So it is very true that Nitin would have felt this. I felt the same in my long ago M800 – but in my CEDIA now, I can feel it turning off & on almost every minute (if the car is idle, the engine rpm increases marginally on the needle)

Coming to Nitin’s usage queries - I would recommend not doing the manual off/on very frequently. This is not good for compressor which is why your home a/c stabilizer usually has a 1 minute cut-off.
Also as bblost said, I also do not think this makes much difference to FE on long drives – the incremental load of the compressor is only marginal at higher speeds, engine is more at a flat performance range and windows down increases the drag of the car. But it does make some difference in city driving – but do give at least 2 minute gap in between operating the switch.

[Shashank.A]

Alto gets a major impact with the AC on. I noticed this couple of weekends back while on a long drive. Putting the AC off really makes the car fly.

[Path_Finder]

Originally Posted by Shashank.A

Alto gets a major impact with the AC on. I noticed this couple of weekends back while on a long drive. Putting the AC off really makes the car fly.

Very True - The difference is very much noticabe - with smaller cars switiching off the a/c is specificaly useful while overtaking to get that extra thrust.

[humyum]

Originally Posted by Path_Finder

merve_extreme put it very right and precise. Additional point to note is that Alto being a small engine and less power a/c, the cooling effect is low and the built-in thermostat hardly gets a chance to cut-off. So it is very true that Nitin would have felt this. I felt the same in my long ago M800 – but in my CEDIA now, I can feel it turning off & on almost every minute (if the car is idle, the engine rpm increases marginally on the needle)

Coming to Nitin’s usage queries - I would recommend not doing the manual off/on very frequently. This is not good for compressor which is why your home a/c stabilizer usually has a 1 minute cut-off.
Also as bblost said, I also do not think this makes much difference to FE on long drives – the incremental load of the compressor is only marginal at higher speeds, engine is more at a flat performance range and windows down increases the drag of the car. But it does make some difference in city driving – but do give at least 2 minute gap in between operating the switch.

Sirji,Alto has one of the best AC around with a very big 117 cc compressor against the zens 80 cc compressor.
Yes engine size is 800 cc agreed but its mated to a big compressor as compared to the 50 cc compressor that is mated to an 800.
So when in a zen the cut off is soo noticable in traffic/highway and everywhere should it not be more evident in an alto ?