[Jeroen]

Originally Posted by dajay0

All,

I have been troubled by this question. Though I have an opinion, I wanted yours.

When running a simple AC (not climate control) in a car if you find that it turns out to be too cold what is the more efficient thing to do?

1. While keeping the temperature knob at the coldest, periodically switch the AC on and off to manually control at a comfortable level.
2. Increase the temperature knob to a comfortable level and keep the AC running all the time.

Can the experts please state their opinions?

Ajay

When you have a car with essentially AC with a simple on/off mode, this is what happens:

When the compressor runs it puts an extra load on the engine, and thus it uses extra fuel. An AC compressor will likely put up a load between 3-6 HP, depending on size. The actual increase in fuel consumption depends a bit at what load the engine is already running. Most car engine run at peak efficiency at around 75% load (this is not the same as fuel efficiency).

In practical terms, when you switch the AC the engine will use a bit more fuel. At idle it’s a little bit more then say at 75% loading.

If you start controlling the temperature by adding more warm air into the mix, the compressor is always running 100% of the time and thus the engine is consuming extra fuel all the time.
When you switch the AC on and off to control the temperature, the extra fuel follows the same pattern, obviously. There is a fact that starting the AC takes extra load compared to the normal run load. System needs cooling down etc. But still.

So very simplistically, on/off is more efficient. In practice there are quite a few variables so I don’t think you would be able to measure the difference easily or in a consistent way during normal use of your car. Maybe a good instant fuel flow indication would show it?

Jeroen

[ksameer1234]

Originally Posted by Jeroen

If you start controlling the temperature by adding more warm air into the mix, the compressor is always running 100% of the time and thus the engine is consuming extra fuel all the time.

I don't think happens in Indian cars. One can clearly feel AC compressor cutting on and off when lower temperature is reached. I looked up Zest's owners' manual and it mentions compressor shuts off when desired temperature is reached.

There was a discussion on this some time ago here.

[Jeroen]

Originally Posted by ksameer1234

I don't think happens in Indian cars. One can clearly feel AC compressor cutting on and off when lower temperature is reached. I looked up Zest's owners' manual and it mentions compressor shuts off when desired temperature is reached.
.

Actually, it happens on all ACs, it’s not about the air temperature but about the temperature of the cooling coil.

But I don’t think it makes a material difference in the comparison as such. Warm air is added to cooled air, so the cooling coil cut off is the same for both

Jeroen

[ksameer1234]

Originally Posted by Jeroen

But I don’t think it makes a material difference in the comparison as such. Warm air is added to cooled air, so the cooling coil cut off is the same for both

My understanding is that car AC works similar to home AC. Say if set temperature to 25C, AC will turn off when temperature reaches 25C and turns back on when temperature reaches 26C. If car AC does not work like, it is terribly inefficient.

On my budget hatchback, I would be more than happy if temperature is not controlled using warm air if it helps me achieve little bit more fuel economy.

[libranof1987]

Originally Posted by dajay0

All,

I have been troubled by this question. Though I have an opinion, I wanted yours.

I generally prefer a combination of changing temp. and blower speed.

Interestingly, at times, I feel even Auto a/c can be not optimum. A friend of mine whose car has auto a/c fixes the temp. (22-24 typically) and then manually adjusts the blower depending on how many people there are in the car and outside temp.

I use 24 on Auto mode and have found the cabin to be not comfortable after a while when there are passengers vs. when it is just me.

Originally Posted by Jeroen

So very simplistically, on/off is more efficient.

Interesting perspective. In our only current car that has a manual a/c (City), we have found that changing the temp. and/or blower works better (in terms of comfort) than switching off the a/c. I did that (turn off a/c when it got too cold) earlier and my parents complained it was uncomfortable at the rear.

[swissknife]

Originally Posted by dajay0

All,

I have been troubled by this question. Though I have an opinion, I wanted yours.

When running a simple AC (not climate control) in a car if you find that it turns out to be too cold what is the more efficient thing to do?

1. While keeping the temperature knob at the coldest, periodically switch the AC on and off to manually control at a comfortable level.
2. Increase the temperature knob to a comfortable level and keep the AC running all the time.

Can the experts please state their opinions?

Ajay

I keep the temperature set to what is comfortable for me.. 23-25 def depending on the day with it air flow on auto. The blow adjusts ok its own.

The airflow creates a wind chill. Hence, even at a slightly warmer temperature (relatively) with a larger airflow, one may feel colder. That's why, in many countries, the temperature is given as, say, 60 deg F along with "feels like" 40 deg F, due to the wind chill.

[Jeroen]

Originally Posted by ksameer1234

My understanding is that car AC works similar to home AC. Say if set temperature to 25C, AC will turn off when temperature reaches 25C and turns back on when temperature reaches 26C. If car AC does not work like, it is terribly inefficient.

I really depends on your car and or AC. There are two systems:

1) on/off
2) auto

You are referring to what in general is known as an automatic system. You set the temperature and the system will ensure that temperature is reached and maintained.

The simpler system are just on/off. You switch the AC on and you adjust the temperature just be feel by either switching the AC off and on, or by letting it run and adding a bit of warm air in the mix. As pointed out the fan speed is of influence as well.

We were discussing inefficiency in terms of using extra fuel and what is uses the least fuel is not necessarily the most pleasant, comfortable and or convenient.

Originally Posted by libranof1987

I generally prefer a combination of changing temp. and blower speed.

Interestingly, at times, I feel even Auto a/c can be not optimum. A friend of mine whose car has auto a/c fixes the temp. (22-24 typically) and then manually adjusts the blower depending on how many people there are in the car and outside temp.

I use 24 on Auto mode and have found the cabin to be not comfortable after a while when there are passengers vs. when it is just me.

Interesting perspective. In our only current car that has a manual a/c (City), we have found that changing the temp. and/or blower works better (in terms of comfort) than switching off the a/c. I did that (turn off a/c when it got too cold) earlier and my parents complained it was uncomfortable at the rear.

What is comfortable for one persons, might not be the same for another person. Also, even in a small compartment like a car cabin there can be differences between the different positions. Also, whether you are in sunlight or shade makes a difference.

The more advanced system also measure sun intensity and have multiple controls for different seats. In some of my cars I can set different temperature for driver and person next to the driver. Even more elaborate system might have individual controls for the rear seat passengers as well. Not just a little duct you can close or open, but a proper individually temperature set and control.

There is one important consideration of keeping the AC always on and that is it’s the best remedy against condensation!

Jeroen

[amit_purohit20]

Originally Posted by dajay0

All,

I have been troubled by this question. Though I have an opinion, I wanted yours.

When running a simple AC (not climate control) in a car if you find that it turns out to be too cold what is the more efficient thing to do?

1. While keeping the temperature knob at the coldest, periodically switch the AC on and off to manually control at a comfortable level.
2. Increase the temperature knob to a comfortable level and keep the AC running all the time.

Can the experts please state their opinions?

Ajay

The most fuel efficient thing to do when you have a manual AC and not a climate control one is to switch off and on.

But I would never recommend you to do that, especially on a long drive. Imagine doing this on a 15 hour drive. I did it, this miserly act led me into trouble. Why?

In a long drive if you repeatedly switch off and on the AC to save fuel the temperature of the car follows a saw tooth pattern (sort of) on a graph of temperature vs time. The amplitude of the temperature drop is higher than what it would be in a climate control AC. These large bumps and drops in temperature fluctuation can lead to a severe headache and even make you feel ill. Your body is not designed to withstand continuous temperature fluctuations for a long time.

I was returning from Bangalore to Pune and I followed this practice and I had a severe headache and felt little bit feverish. Luckily I took a break, took a short nap and then kept the car AC on continuously and adjusted the car temperature with the temperature setting knob and the blower speed and all was fine. I would have been in deep trouble if I fell ill because I am the only one who knows driving a car in my home.

[dajay0]

Thanks everyone for an enlightening discussion so far.

My opinion also is that switching the AC on / off is more efficient as it switches off the compressor (assuming its a simple unintelligent system). But does frequent start / stop of the compressor hurt the system in some way?

Ajay

[SS-Traveller]

Originally Posted by dajay0

When running a simple AC (not climate control) in a car if you find that it turns out to be too cold what is the more efficient thing to do?

1. While keeping the temperature knob at the coldest, periodically switch the AC on and off to manually control at a comfortable level.
2. Increase the temperature knob to a comfortable level and keep the AC running all the time.

Can the experts please state their opinions?

You missed out on the third and fourth options - turn down the blower fan to position 1, and adjust the vents to cut down / change direction of airflow.

The less air that flows over the cooling coil, the cooler (and more moist) it stays, and the compressor remains in the off position automatically. The moisture attracts dust and dirt over a period of time, whether in recirculate or fresh air modes. With a dirty cooling coil, however, icing would be more of a problem.

However, the less the compressor runs, the more humidity that gets a chance to build up inside the cabin (and fog up the windscreens & windows).

So, my choice is to run the AC on blower position 1 or 2 (and switch the vents to face-and-footwell), and turn up the temperature control to mix in some warm air - this prevents fogging inside the cabin, prevents icing on the cooling coil, keeps a constant airflow over the cooling coil that allows it not to accumulate dust on a moist surface, and bumps down the FE by 0.5 kmpl compared to switching off the AC totally.

[deep_bang]

I too switch on and off the AC and this is particularly irritating to the other passengers. So is there a way to set a higher temperature cut-off of the cooling coil? That basically is what most of us need, so its not chilling, but cool enough.

[vrprabhu]

Originally Posted by SS-Traveller

You missed out on the third and fourth options - turn down the blower fan to position 1, and adjust the vents to cut down / change direction of airflow.

So, my choice is to run the AC on blower position 1 or 2 (and switch the vents to face-and-footwell)....

+1.

It is more convenient to set the blower at an higher speed (3 or 4) and let the warm air in the car to be pushed out (by keeping the windows open). After a couple of minutes, close the windows, switch on the re-circulation mode, and bring down the blower speed to 1 or 2, and regulate the air flow by using the direction vents - to maintain the desired level cooling.

It is my experience that after switching on the AC, the high speed of the radiator fan kicks in and after some time when the AC cuts off, the radiator fan too goes back to normal speed. This cycle keeps repeating; so the condenser must be cutting off at preset levels. (Mine is a decade+ old Palio - so no automatic climate control; but, the AC is quite effective even after 10 years. One feature available is that, even if the blower is kept off, cold air mildly keeps blowing).

So, instead of switching on-off the AC, it appears to be more practical to control the cooling using the blower fan speed.

[Styler]

Hi

I have a Grand Vitara Petrol 2.0, 2007. The AC has been excellent and in fact at times it cools so much that I have to turn the temperature knob to warmer mode, or turn the vents away.

A few weeks ago the AC suddenly stopped cooling. The compressor was not coming on. The AC Gas pressure was fine and there was no leak. It was diagnosed that the Clutch / Magnet of the AC Compressor had failed.

The Authorized Service Center would replace the complete compressor in this situation (36K is the cost). I went to my trusted Local AC mechanic and they took out the AC compressor. The AC Clutch / Magnet was fixed permanently to the Compressor and it had to be sent to a specialized workshop to separate the magnet. Since parts for the Grand Vitara are not easily available we could not source a replacement AC Clutch Magnet. So the existing Clutch / Magnet was repaired and everything fixed back.

Gas was refilled and the AC is back to normal. Earlier the AC gas was original factory filled.

I believed that repairing of the Magnet should not effect the cooling in any way as the role of the magnet is just to turn on the compressor. Everything including the Cooling Coil, Compressor etc remain the same.

In the next few days of driving I just noticed that the Cooling was good but not as before.The AC was not cutting off after reaching the set temperature as it did earlier, specially when ACs are over efficient in the humid monsoon weather.

Another thing I noticed that if the Vitara was unused for a day or two, and then when the AC was turned on it took 30 - 60 secs to notice the cooling start. Earlier it was much faster.

I thought maybe they have filled the AC gas a little less to I went back to get it checked after a week. They did notice that cooling was a little less and topped up the AC gas a little and told me everything was fine. The AC is cooling but it is still not as efficient as before with the company filled Gas.

My questions are :

1.Are there different qualities of AC Gas available ? Could this effect the cooling ?
2. What would be the optimum pressure of the AC Gas ?
3. What could be other reasons for the Cooling not being as before ? Could it be that filling of AC gas in Factories at time of manufacture is precise with proper vacuum created before filling but the local AC workshop can not get it done as precisely as that ?

Styler

[akmehta]

Originally Posted by Styler

Hi

1.Are there different qualities of AC Gas available ? Could this effect the cooling ?
2. What would be the optimum pressure of the AC Gas ?
3. What could be other reasons for the Cooling not being as before ? Could it be that filling of AC gas in Factories at time of manufacture is precise with proper vacuum created before filling but the local AC workshop can not get it done as precisely as that ?

Styler

1. Yes there are different AC gases available, R22 being used in age old cars to the latest one i.e. R410A(inverter ac), for your case it should be R134A only which had been most widely used since long. You can double check the same with the person who topped it up.

2. Low pressure end: 45-50Psi and at high pressure end: 300+ psi are the optimum pressure readings

3. After getting checked that pressure readings are just fine i.e. within the prescribed limits, make sure following things are also working fine:
i) AC Compressor - Clutch working should be smooth and not intermittent(should work instantly as soon as AC is switched ON); the coupler connecting the compressor clutch should be tight fit.
ii) AC relay - Incase compressor working is intermittent and wires are intact, get the relay check done(Read operator manual to locate the AC relay)
iii) Evaporator coil unit - I suspect it hasn't been cleaned since long/the date u bought the car, ask the local AC mechanic to get it cleaned.

These steps should ensure proper working of car AC like new.

Do revert as soon as you get aforementioned checked

[pixantz]

Originally Posted by Styler

My questions are :

1.Are there different qualities of AC Gas available ? Could this effect the cooling ?
2. What would be the optimum pressure of the AC Gas ?
3. What could be other reasons for the Cooling not being as before ? Could it be that filling of AC gas in Factories at time of manufacture is precise with proper vacuum created before filling but the local AC workshop can not get it done as precisely as that ?

Styler

1) Yes, there are different qualities of the same gas type which may read the same gas type(ie., 134a) on the label but may be of inferior quality depending on brand. Yes, this will surely directly effect the rate of cooling if the quality is not optimal.

2) The optimum pressure readings at about 32deg ambient should be around 40-55 at low side and 150-225 at high side at idle rpm. Give or take a few depending on the overall condition of your Ac system. People tend to forget that the Ac system is also a working mechanical machine and also ages like all other components with corresponding drop in performance.

3) Taking into consideration that no other changes were made to the system, the drop in performance can be primarily due to two main reasons: one, wrong technique of evacuation and/or improper charging of the system, which I think is the main culprit here. And secondly, inferior quality of gas.