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Old 3rd May 2006, 15:35   #1
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Default A/C and fuel economy & How to use aircon optimally.

If ones drives at low rpms with a/c on, will he get better fuel economy? like for example, we really can't drive at high speeds within city limits, so if we drive a bit slowly ensuring that the rpm does not exceed say 2000 even in top gear, will we get a higher mileage as compared to driving with a/c on and compensating for any slight loss of power by revving harder?
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Old 3rd May 2006, 16:12   #2
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At same RPM, you'll get less FE with AC on compared with AC off.

Also, best FE is usually available in RPM 2000-3000 range for most cars.

Now exactly high rev will compare with low rev + AC, that will be a complex calculation. You may have do a trial & error to figure it out...
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Old 3rd May 2006, 16:25   #3
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Quote:
Originally Posted by Lukeskywalker
If ones drives at low rpms with a/c on, will he get better fuel economy? like for example, we really can't drive at high speeds within city limits, so if we drive a bit slowly ensuring that the rpm does not exceed say 2000 even in top gear, will we get a higher mileage as compared to driving with a/c on and compensating for any slight loss of power by revving harder?
Fuel consumed = drive the car + AC power
RPM = fuel consumed

Regardless of lower or higher rpm, certain fixed amount of power is drawn out for running AC.

for e.g in my baleno

in idle gear.
If AC is ON rpm = 800
If AC is OFF rpm = 650

so the moment i switch ON AC, the in-car computer will increase fuel supply, which inturn increases RPM which incrases the power which can be used to drive the AC.
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Old 3rd May 2006, 16:43   #4
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When you "ON" the AC .The engine looses power to run the AC compressor.So whatever RPM you drive your car,the AC steals power off your engine to keep you cool once you put on the AC the engine loses some horses to run the AC.so your effective engine power goes down and the engine has to work harder to bring around the same performance .NOW you do the math !!!
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Old 3rd May 2006, 16:48   #5
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Quote:
Originally Posted by venkatrx
Regardless of lower or higher rpm, certain fixed amount of power is drawn out for running AC.
Seems like a solid argument, since my tacho readings confirm this too...

But what if the AC isn't being run at full power? Wouldn't lesser fuel be consumed them, as compared to fuel consumed when AC is running at full blast?
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Old 3rd May 2006, 16:52   #6
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AC on or off....sticking to a 2000 rpm limit gives me the best FE in my car ....
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Old 3rd May 2006, 16:55   #7
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Quote:
Originally Posted by ivor
Seems like a solid argument, since my tacho readings confirm this too...

But what if the AC isn't being run at full power? Wouldn't lesser fuel be consumed them, as compared to fuel consumed when AC is running at full blast?

yes, that one too, would the fan speed affect mileage? Low fan speed lesser power consumed ?
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Old 3rd May 2006, 17:00   #8
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Quote:
Originally Posted by ivor
Seems like a solid argument, since my tacho readings confirm this too...

But what if the AC isn't being run at full power? Wouldn't lesser fuel be consumed them, as compared to fuel consumed when AC is running at full blast?
In this case wouldnt it take longer to cool down the cabin and for that dont you think the fuel consumed is still the same?
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Old 3rd May 2006, 17:15   #9
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Well, the moment the AC is on the the RPM goes up. You can check this if you have a tacho in your car. Even otherwise, when the car is standing still hear the engine noise the moment you switch on the AC. It goes up. This is because the engine now needs to generate more power to run the engine and at the same time also power the AC. So fuel consumption does go up.

Now does the blower speed also affect the FE? Interesting question... when I think about it, it probably does in a round about way. The blower speed actually does not connect with the compressor directly. However if you have noticed, at times when the blower is running at a lower speed (say 2) and the on-board computer realizes that the compressor can take a short break the RPM does fall a bit for a while... Of course it again goes up as the blower keeps running and the gases again need to start flowing to maintain the temperature. If you increase the blower speed obviously the compressor will keep working, pumping the gas through the pipes and yet the fast running blower will never let it take that break. I think that ways your FE may be affected even if marginally.
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Old 3rd May 2006, 17:37   #10
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I have a slightly dumb question:

How much power does the compressor consume? If it is inline with let's say, a regular home a/c - let's say, 1000W or 1.5 hp, then, why is it that the power drop we notice in some of the smaller cars so significant the minute A/C is turned on?
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Old 3rd May 2006, 18:03   #11
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Quote:
Originally Posted by venkatrx

in idle gear.
If AC is ON rpm = 800
If AC is OFF rpm = 650

so the moment i switch ON AC, the in-car computer will increase fuel supply, which inturn increases RPM which incrases the power which can be used to drive the AC.
Hey, not sure on this, may be I'm wrong..

AC on = 800 and AC off = 650, that means the in-car computer increases the fuel supply so that there is no loss in performance.. (meaning the speed of the vehicle remains same)..

If this is the logic, then most of the cars slow down or performance comes down when AC is switched on.. shouldn't the in-car computer increase the fuel supply here and keep the speed constant?? or does it work only during idling??
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Old 3rd May 2006, 20:06   #12
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Quote:
Originally Posted by Laxminarayan
Hey, not sure on this, may be I'm wrong..

AC on = 800 and AC off = 650, that means the in-car computer increases the fuel supply so that there is no loss in performance.. (meaning the speed of the vehicle remains same)..

If this is the logic, then most of the cars slow down or performance comes down when AC is switched on.. shouldn't the in-car computer increase the fuel supply here and keep the speed constant?? or does it work only during idling??
You are right, when the AC is switched ON 'car slows down'....if u r in a maruti-800

I think in the newer cars, the moment AC is turned on in-car computer must increase the fuel supply.
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Old 3rd May 2006, 20:10   #13
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Quote:
Originally Posted by Lukeskywalker
yes, that one too, would the fan speed affect mileage? Low fan speed lesser power consumed ?
At home for a single room.....

AC power = 2 kw
Fan power = 0.07 kw

You see Fan requires lot less power than an AC

Even cars Ac and fan must have similar power requirements(the ratios i mean)

Car fans run with the battery, AC cannot. Fans draw very little power and that too from your battery. It cannot reduce your FE.

But if you run ur fan too long when your engine is off, your battery can go kaput
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Old 3rd May 2006, 20:41   #14
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Quote:
Originally Posted by ivor
But what if the AC isn't being run at full power? Wouldn't lesser fuel be consumed them, as compared to fuel consumed when AC is running at full blast?
Quote:
Originally Posted by Lukeskywalker
would the fan speed affect mileage? Low fan speed lesser power consumed ?
Ivor and luke,
You guys are right about that. Running the blower (fan) at the lower speeds would give you more FE than running it full blast. I had this doubt myself and wrote to MUL. I am quoting below what their engineer replied to me :
"The fuel consumption in the final speed of blower will be marginally higher than when the speed is at the lowest. This is due to the fact that the compressor cut off at higher blower speed is minimum or almost nil and hence the AC compressor load on the engine is relatively constant. Whereas in lowest blower speed the cut-off pf compressor is frequent and hence the load on engine is lesser intermittently."
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Old 3rd May 2006, 21:23   #15
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my car has a tacho (optra 1.6), agreed there is a load on the engine once the a/c is switched on, proof is increased rpm when idling, but my doubt is if i consciously make sure that i drive at a constant rpm say 2.5k, with or without a/c, my fuel consumed should be the same as the engine is revving at the same speed in either case. Pls enlighten my dumb soul
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