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Old 19th February 2007, 17:03   #1
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Default A/C Blower Speed to Fuel Consumption

I have a small doubt. Will the AC usage on different speed levels effect the mileage proportionately? I mean :-

AC Blower Speed on 4 will consume more fuel than AC Blower say on 3rd,2nd or 1st

OR

AC switched on will consume the same amount of fuel irrespective of which blower speed its running on?
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Old 19th February 2007, 17:13   #2
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As far as my limited knowledge goes, the blower settings would not affect / improve the FE in any way. However, the Hot / Cool (the red/blue) settings do play a role wherein the MAX settings on cool will have your compressor working more often than it would on a slight chill position.
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Old 19th February 2007, 17:34   #3
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Somewhere I read that if the AC is not on max cool position it actually mixes hot air with the cold air and that way it is better to switch off AC when it is cold enough. Is it true?
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Old 19th February 2007, 17:56   #4
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Suren's reasoning is quite right. Although technically, increased fan speed might consume more power and fractional changes in FE.
Heat transfer occurs over period of time (delta T). I have argued (unsucessfully) over the point (with some one) that there is no significant more cooling happening when the fan speed is on max. It just increases the airflow. Also it significanlty reduces the time from the air to actually cool down. So there is a trade off. I find medium fans speeds to be optimal.
Thats my point anyway. Is there any difference you noticed lately??
Suren:- Mass is eqivalent to energy (just a difference of C2). So if energy is conserved, mass is also conserved! :-)
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Old 19th February 2007, 18:33   #5
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Originally Posted by kutlee View Post
Suren's reasoning is quite right. Although technically, increased fan speed might consume more power and fractional changes in FE.
Heat transfer occurs over period of time (delta T). I have argued (unsucessfully) over the point (with some one) that there is no significant more cooling happening when the fan speed is on max. It just increases the airflow. Also it significanlty reduces the time from the air to actually cool down. So there is a trade off. I find medium fans speeds to be optimal.
Thats my point anyway. Is there any difference you noticed lately??
Suren:- Mass is eqivalent to energy (just a difference of C2). So if energy is conserved, mass is also conserved! :-)
I don't know in what context you invoked the formula E=mc^2, but what it implies is that (mass+energy) is conserved, not either mass or energy. But this equation will only come into play when the velocities involved are comparable to speed of light, so we can safely ignore it in the present context.

I think increased blower speed will not affect FE if the air is set for 100% recirculation. In this case increased blower speed can only be beneficial since it will improve mixing of the recirculating air and maybe improve the efficiency of heat transfer. Of course the higher blower speed will require greater power as noted.

With fresh air intake, however, I think increased blower speed will reduce FE since fresh air at atmospheric temperature is drawn in at a greater rate and so the load on the AC increases. If the external (atmospheric) temperature is much higher than the temperature setting inside the car and if the fresh air intake is set at maximum, there may even be a noticeable difference in mileage due to the increased load on the AC.

Last edited by rks : 19th February 2007 at 18:35.
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Old 19th February 2007, 19:44   #6
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For vehicles with HVAC systems, the level of cooling selected on the knob would change the mix of cold + hot air to give desired cooling level. So, the level of cooling selected should not affect the FE.

Like kutlee said, increasing the blower speed may cause a marginal change in FE.


But, my observations:
Assumption is that I am using the re-circulate mode. No fresh air intake.

when I have only the blower on, the air-flow seems to show just a marginal increase with an increase in fan speed settings. With AC on, this increase in air-flow seems more.

RKS, if increased blower speed only affects air-circulation, then why does it feel cooler at higher speeds than lower speeds. I ask this based on the logic that - at higher speeds, if the air is passing through faster [than at lower blower speeds], the air would have lesser time to get cooled than at lower blower speeds.

To add an element of confusion, If the last point above is true, is it possible that cooling at higher fan speeds would acutally be lesser than the cooling achieved at lower fan speeds.

Last edited by condor : 19th February 2007 at 19:45.
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Old 19th February 2007, 20:01   #7
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My observation is fan speeds above 2 affect the performance. Observed it in Baleno, Accent and Petra. Even at highway speeds I felt the pull/ pickup are down with blower at 3 or 4. This observation was on a full load f 5 adults and a child. If it is affecting performance I think it affects FE too.
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Old 19th February 2007, 20:09   #8
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Quote:
Originally Posted by condor View Post
RKS, if increased blower speed only affects air-circulation, then why does it feel cooler at higher speeds than lower speeds. I ask this based on the logic that - at higher speeds, if the air is passing through faster [than at lower blower speeds], the air would have lesser time to get cooled than at lower blower speeds.

To add an element of confusion, If the last point above is true, is it possible that cooling at higher fan speeds would acutally be lesser than the cooling achieved at lower fan speeds.
To answer your first question, I think there is something called the wind chill factor, which will make you feel colder in stronger wind conditions, even though the air is at the same temperature.

Regarding your second doubt, it is true that the faster circulation means less residence time of the air inside the heat transfer section of the AC. So a fixed amount of air at a higher velocity will get cooled to a lesser extent per pass through the AC. But at the same time the said quantity of air passes more number of times per minute through the AC because of the higher velocity (here we are assuming 100% recirculation). Overall I feel that the cooling effect should increase slightly at higher recirculation rates because of better mixing of the air.
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Old 19th February 2007, 21:01   #9
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RKS, wonder if wind-chill factor makes that much of a difference in a closed environment like that inside a car.

If the cooling effect increases slightly at higher recirculation rates ..- and the higher recirculation [by higher blower speeds] causes only a slight decrease in FE. So using the higher blower speeds doesnt really decrease FE significantly.

OT: Is there any way I can change the method of cooling level in a car with HVAC - something other than varying the cold-air, hot-air mix?
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Old 19th February 2007, 21:05   #10
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Quote:
Originally Posted by sreenivass View Post
My observation is fan speeds above 2 affect the performance. Observed it in Baleno, Accent and Petra. Even at highway speeds I felt the pull/ pickup are down with blower at 3 or 4. This observation was on a full load f 5 adults and a child. If it is affecting performance I think it affects FE too.
Agreed. When you increase your blower speed beyond level 2 it affects FE and performance. This was also clarified by a Maruti service advisor for my Esteem. What I was told is that if you set the blower level to 2 the AC cools the front compartment alone housing the driver and the co-passenger seated on the left. Once the required temperature is attained, the AC is cut off and it is just the blower that is working. This reduces the strain on the engine and results in better FE and performance. On the contrary, if the blower level is set beyond 2, it goes ahead cooling the entire car and thereby the AC compressor is on for most of the time. This service advisor also gave me a pamphlet on fuel saving tips and the topic under discussion was one of them.

Last edited by rr_zen : 19th February 2007 at 21:06.
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Old 19th February 2007, 21:15   #11
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Quote:
Originally Posted by rks View Post
To answer your first question, I think there is something called the wind chill factor, which will make you feel colder in stronger wind conditions, even though the air is at the same temperature.

i totally agree with that/the cool air on a higher settiing comes out really fast and it appears to be cooler but as mentioned above on a higher sttting the air has less time to be colled hence on say 1/2 it would be much cooler having more time to be cooled.


the best option for optimum cooling and performance would be to start the blower at maximum till the car is cool and then reduce the setting to 1/2 whatever is preferd by you and the temp outside the car.


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Old 19th February 2007, 21:15   #12
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Quote:
Originally Posted by condor View Post
RKS, wonder if wind-chill factor makes that much of a difference in a closed environment like that inside a car.
I think it will make a difference in the sense that faster-circulating air at the same temperature will take away more body heat and make you feel colder. Exactly what speeds are needed for this effect to be significant is something I am not sure of.
Quote:
If the cooling effect increases slightly at higher recirculation rates ..- and the higher recirculation [by higher blower speeds] causes only a slight decrease in FE. So using the higher blower speeds doesnt really decrease FE significantly.
That is what I feel, provided the air is set for 100% recirculation. Although if the car is loaded with people and if the external temperature and humidity are already very low, the AC may function more efficiently with some fresh air intake -- because the humans are a source of heat and humidity for the air. So it is a complex calculation.

Last edited by rks : 19th February 2007 at 21:18.
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Old 19th February 2007, 21:38   #13
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Quote:
Originally Posted by rr_zen View Post
Agreed. When you increase your blower speed beyond level 2 it affects FE and performance. This was also clarified by a Maruti service advisor for my Esteem. What I was told is that if you set the blower level to 2 the AC cools the front compartment alone housing the driver and the co-passenger seated on the left. Once the required temperature is attained, the AC is cut off and it is just the blower that is working. This reduces the strain on the engine and results in better FE and performance. On the contrary, if the blower level is set beyond 2, it goes ahead cooling the entire car and thereby the AC compressor is on for most of the time.
So what the service advisor is saying is that with the blower set for low speeds, the entire car doesn't get cooled to the same temperature; i.e., there is a temperature gradient, with the front seat area being cooler. I think this may only be a transient state; eventually the whole car ought to cool to the same temperature. Even though the air is blown from the front, there will surely be vents at the back side of the car also, through which the uncooled air will flow back into the AC inlet (at least with a proper AC design). What may be true is that the front seat passengers feel colder due to the wind chill factor if the cold air is blown directly at them.
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Old 19th February 2007, 21:46   #14
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Quote:
Originally Posted by rks View Post
So what the service advisor is saying is that with the blower set for low speeds, the entire car doesn't get cooled to the same temperature; i.e., there is a temperature gradient, with the front seat area being cooler. I think this may only be a transient state; eventually the whole car ought to cool to the same temperature. Even though the air is blown from the front, there will surely be vents at the back side of the car also, through which the uncooled air will flow back into the AC inlet (at least with a proper AC design). What may be true is that the front seat passengers feel colder due to the wind chill factor if the cold air is blown directly at them.
What you say is true regarding the cold air moving to the second compartment eventually but as far as I know there are'nt very many cars that have AC vents at the back except for the premium cars like Toyota, Lancer etc.
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Old 19th February 2007, 22:12   #15
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Quote:
Originally Posted by rr_zen View Post
What you say is true regarding the cold air moving to the second compartment eventually but as far as I know there are'nt very many cars that have AC vents at the back except for the premium cars like Toyota, Lancer etc.
The cold air is *blown* from the front in most cars. I am assuming that there are air vents located at the back side of the car also, through which this cold air recirculates back to the AC inlet or else is partially vented out with fresh air intake. If there are no such vents on the back side of the car, then there will be no air circulation there and the AC will not be effective.
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