Keeping revs up when idling with AC on?

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Ive always felt that when keeping the motor idling with the AC on, its better to keep the motor running a little higher than idle, say 1000-1200 rpm instead of 600 or so. I feel that the oil and coolant circulates better.

Waiting for my kids bus on a hot day I'll have the AC on, radio and ventillated seats which use a ton of power. Am I right or is it just my OCD?
 
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Only thing that does for sure is waste gas..

I'm sure most cars have two speed fans so cooling is not an issue.
 
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I would guess that most modern vehicles have the computer do the thinking.
 
Yep, computer can crank the RPM's if necessary on any drive by wire vehicle. Never had a overheating issue or an ac issue with the vehicle at idle.
 
Originally Posted by Rochev
Am I right or is it just my OCD?

It's your OCD.
If it needed to run at 1000-1200 rpm, it would do it on it's own.
It doesn't need you to manually intervene.
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Originally Posted by meadows
I do it in my vehicles were the compressor is run by the engine. It helps the cooling by pumping more freon.

This would only be true on vehicles without variable displacement compressors. A vehicle with a properly sized, properly charged, variable displacement compressor will pump more than enough refrigerant at idle. Any additional RPM will simply reduce the displacement of the compressor until the targeted low-side pressure. increasing rpm in this case will benefit you zero.
 
I don't raise the idle of my car, but the thought crosses my mind about how hard the alternator is working at low engine speeds and high load which is what sitting at idle blasting the AC is. No amount of computer control is going to overcome the physics of cooling and heat in an alternator spinning at slow speeds.
 
Do you drive an older car? Every car I've had just does this on its own. A/C systems consume a decent bit of power, so the engine automatically increases revs by a couple 100 RPM or so to compensate.
 
"the thought crosses my mind about how hard the alternator is working at low engine speeds and high load which is what sitting at idle blasting the AC is"

The alternator doesn't run the A/C.
 
Originally Posted by bobdoo
"the thought crosses my mind about how hard the alternator is working at low engine speeds and high load which is what sitting at idle blasting the AC is"

The alternator doesn't run the A/C.


No kidding? However, it runs the fan blowing into the interior as well as usually 2 radiator condenser fans outside as well as the AC clutch, which is usually around 5 amps. All high load items.

Also, if you're running all that, its hot outside and your under-hood temps will be elevated. Slow engine speed, high load, and high temps. All things that are hard on an alternator.
 
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I forgot to mention the power load too. Seems like a lot at low engine speed. Its a newer car. Maybe I'm a bit old fashioned with the revving lol
 
Originally Posted by Rochev
Ive always felt that when keeping the motor idling with the AC on, its better to keep the motor running a little higher than idle, say 1000-1200 rpm instead of 600 or so. I feel that the oil and coolant circulates better.

Waiting for my kids bus on a hot day I'll have the AC on, radio and ventillated seats which use a ton of power. Am I right or is it just my OCD?

Feelings are not facts. If the engine isn't overheating, and the A/C is blowing cold, and the battery stays charged... everything is working fine without bumping the RPM. I say let it run as designed. Or do whatever makes you feel good
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1. Every car I've known since fuel injection hit had idle compensation to provide additional throttle to offset the AC loads at idle.

2. I can't attest to all models, but I know that Chrysler has had "high idle" logic in their ECUs since at least 2001 to raise idle when needed if the electrical demands exceed alternator output for that RPM. If Chrysler has done this for nearly 20 years, I'd think the rest of the market has a strategy they use too.

3. as far as heat, the alternator is going to perform the same amount of WORK, producing the same amount of POWER (Watts, or V*A) regardless of rpm. And with electric fans, the air volume is not going to change with rpm.

Let 'er be.
 
Originally Posted by meep


3. as far as heat, the alternator is going to perform the same amount of WORK, producing the same amount of POWER (Watts, or V*A) regardless of rpm. And with electric fans, the air volume is not going to change with rpm.




Err, no they don't. They have a very definite output curve to them at varying speeds. You can send all the field current in the world to an alternator turning too slow but all you're going to do is make heat.

At low engine speeds (Idle and in gear) and heavy load the charging circuit in most any car, fuel injected or not is going to droop.

I'm not saying its something that needs addressing by the meat sack behind the wheel as the car will take care of it, but alternators definitely don't work the way you describe.
 
^^^ point taken - my wording was not clear, my bad. I was meaning that for a given load *within that output curve* that it can support. As long as it is within the output curve, 100 watts is 100 watts, regardless of RPM.

Once the load exceeds output curve (capability) for the rpm, then yes, it can not produce more power/work without additional RPM.

(thinking aloud) The OP was talking about stress on equipment due to idle speeds, and I read it as he was saying that dumping max volts into the alt causes it to work harder- and that's not the case - yes the field voltage is high but the rpm is low so the power output, which is what generates heat in the alternator, is no greater than a high rpm with low FC voltage. It's the stator that's affected by all this, since that's where magnetism becomes watts. I may have gone outside what he was thinking, but as long as the wattage output is constant, ie, within the output curve, it's actually the same load on the device. Since we're being precise, I'm considering FC temps as negligible since it pales in comparison to stator heat.
 
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If you look at almost any fuel injected vehicle made in the last couple of decades, you'll notice a pressure sensor in the high pressure power steering hose, as well as the obvious necessary sensors in the AC system.

The engine knows what to do when the power steering or AC makes it's demands.

It's what the IAC does.

Where the alternator is concerned, it's been pullied to make sure it can generate enough power to keep everything going. I don't think I've seen a new car that can't keep full voltage at idle with everything on.
 
If you're concerned about the engine overheating, and you're going to be idling for a long period of time, pop the hood. This is what we do with K9's in the car, never had a single issue with the AC or the vehicle's cooling system and this is in 130+ F heat.
 
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