[redcruiser]

Quick question- Will the load on the engine be more at a lower temperature say 18 degrees than at a higher temperature say 21 degrees?

[vikram_d]

Originally Posted by redcruiser

Quick question- Will the load on the engine be more at a lower temperature say 18 degrees than at a higher temperature say 21 degrees?

Quick answer - Yes.

[King_pin09]

Originally Posted by redcruiser

Quick question- Will the load on the engine be more at a lower temperature say 18 degrees than at a higher temperature say 21 degrees?

Nope. The load will be the same. The duration up to which the load remains on the engine will be longer.

[vikred]

Originally Posted by redcruiser

Quick question- Will the load on the engine be more at a lower temperature say 18 degrees than at a higher temperature say 21 degrees?

No, nothing like that. AC compressor acts as a parasitic load on the engine. If it's switched on, it IS switched on. Irrespective of what temperature your cabin is set to. If it's set to anything more than its minimum (for instance, 18˚C here), the amount of hot air getting mixed from the heater core varies.

If you remember the red-blue knob on a manual AC car, you will note that this knob has a distinct mechanical feel to it. This knob controls the amount hot air from heater core. Auto AC's just do this mixing automatically.

Car AC's work completely different from home AC's. In a split AC, when you set the temperature to 24˚C, your AC brings down the temperature from 40˚C on a summer day to 22˚C and switches off the compressor, keeping the blower running. After some time, as the temperature reaches 26˚C approximately, the compressor starts running.

[ksameer1234]

Originally Posted by vikred

If it's set to anything more than its minimum (for instance, 18˚C here), the amount of hot air getting mixed from the heater core varies.

Car AC's work completely different from home AC's.

I have had totally contradictory experience with all the small hatchback I have driven. Small ones because it is very easy to notice the drop in power with tiny engines. When I turn on the AC, I can clearly feel the loss in power. Now when the cabin has cooled enough depending on the temperature knob setting, the AC is turned off and blower gradually starts blowing air of increasing temperature till when the thermostat trips again and AC is turned on. Is this not how it happens?

[AkMar]

Originally Posted by ksameer1234

When I turn on the AC, I can clearly feel the loss in power. Now when the cabin has cooled enough depending on the temperature knob setting, the AC is turned off and blower gradually starts blowing air of increasing temperature till when the thermostat trips again and AC is turned on. Is this not how it happens?

Yes, this is how it used to work in out M-800. But with newer cars I doubt whether it is the same. Experiences with Swift as well as Xcent suggests that it doesn't work in the same way. Hot air mix is used to control the temperature as posted by vikred. That is why when I am alone, I keep temperature knob at lowest setting and manually switch on & off the AC to control temperature. This way I am sure that the compressor is definitely switched off after sufficient cooling is achieved, instead of it consuming fuel continuously.

Corrections welcome

[ksameer1234]

Originally Posted by AkMar

Yes, this is how it used to work in out M-800. But with newer cars I doubt whether it is the same.

After losing sleep thinking about how inefficient it would be to first cool air and then mix with hot air, I thought of going through the Owner's Manual for Tata Zest. Here's what it says:

Manual AC:
When AC is switched ON, engine idlingRPM increases marginally to adjust the AC compressor load.

When desired temperature is achieved AC trips OFF automatically.

For Auto Climate Control:
During ECON AC operation, the system automatically cuts off compressor at a higher temperature than normal AC.

I believe the compressor trips, at least in Zest just like home air conditioners. I suppose the mixing of air would happen in luxury cars where fuel economy is less important than passenger comfort.

Experiences with Swift as well as Xcent suggests that it doesn't work in the same way.

I had a Swift owning colleague who would switch off AC manually while keeping knob at coldest setting. I told him that he can just increase the knob to warmer setting and it would have same effect. If things don't work as expected in Swift, I can only imagine him cursing me once he find out I made him lose out on fuel efficiency!!

[vikred]

Originally Posted by ksameer1234

After losing sleep thinking about how inefficient it would be to first cool air and then mix with hot air, I thought of going through the owner's manual for Tata Zest. Here's what it says:

I believe the compressor trips, at least in Zest just like home air conditioners. I suppose the mixing of air would happen in luxury cars where fuel economy is less important than passenger comfort.

Inefficient it is, but I believe the fuel economy benefit that we get from having an on-off compressor (like home ACs) can easily be outweighed by an always-on compressor for the following reasons:

• on-off AC compressors (i) make the driving experience unpredictable, jerky and erratic, (ii) wears out the AC clutch faster, and (iii) add more complexity (more complexity = high cost and more likely to break)
• in order to have the heating functionality, a heater core and a mixer (mix cold and hot air appropriately to avoid too hot an air blowing on your face) are anyways needed.
• AC on vs off FE difference would be around 1-2 km/l for a 1200cc car. Having an on-off compressor might be able to shrink that difference by only 0.5 km/l.

However, I remember reading somewhere a few years back that Hyundai Eon's AC compressor shuts off under hard acceleration, to provide maximum power to the drivetrain. Zest, as you mentioned, might be an exception too.