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A General Question About Compressor Cycling . . .

NewbieQ on Thu May 01, 2014 9:23 PM User is offline

Greetings!

Hope I have a simple question for the forum today...

I've been reading a bit about compressor cycling as an approach to better fuel economy during moderate AC usage; and seem to have come to a bit of an impasse regarding typical baseline specs for compressor-safe cycle time/operation. Hoping to put together a nifty DIY project for the weekend if I can...

From what little I can glean, "tribal knowledge" seems to place the minimum safe compressor off time at about 5 seconds in a rapid cycling scenario to prevent the possibility of compressor damage. While the exact and specific values would vary, is this a good general "safe bet" time-off timeframe?

Just curious --

Thanks, and have a great day

Edited: Thu May 01, 2014 at 9:30 PM by NewbieQ

Dougflas on Fri May 02, 2014 6:43 AM User is offline

It is my opinion that today's compressors are so efficient that you will not notice any difference in fuel usage. Years ago, compressors did use energy but not so at this time. Not too sure what you're asking. Under high heat loads, the compressor will not cycle which is normal.

mk378 on Fri May 02, 2014 8:50 AM User is offline

Every single cycle wears a little off of the clutch surfaces.

A long time ago, an EPA (boogaa boogaa) study found that dropping out the compressor at wide open throttle could save a little gas. It was the only simple DIY "gas saving" mod that was found to actually do anything toward saving gas. But a driver interested in high mpg would rarely go to WOT in the first place.

wptski on Fri May 02, 2014 9:17 AM User is offline

I used to maintain a bunch of machines that used 6" diameter Warner electric clutches. They ran a 15 second part cycle at two different speeds which cycle two clutches. If the coils went bad, I'd change the plates as well but it was common to see an 1/8" groove in the plates and they worked just fine.

I know that Ford is using a new battery management system is some of it's vehicles where under low battery drain the alternator is shut off and even as the battery ages the charge rate varies, all to improve MPG. This also means that when a battery is replaced the info has to be updated in the PCM by a Ford Tech. Proper battery replacement DIY no more!!

Edited: Fri May 02, 2014 at 9:24 AM by wptski

NewbieQ on Fri May 02, 2014 9:56 AM User is offline

Thanks, folks, for dropping by! Great stuff...

On the note of compressor cycling for MPG, one of the things I found a while back that got me 'a thinkin' was an SAE paper out there somewhere (was about ten years ago, I reckon) which looked at this with a generally positive spin. Seemed interesting (and possibly worth a try), particularly for the old "Queen Mary"

However, not having the old paper, there didn't seem to be any "safe" guidelines out there as far as how long one should let things "rest" between run cycle portions without having pressure sensor/temp feedback. Going WAY back to before '85, there seemed to be no minimum off period at all: One could manually rapid cycle a unit by just twisting the fan knob off-and-on at will...

So, I don't know. At any rate, would something like a 5 sec rest between spinups be reasonably safe for an auto A/C compressor; or is there really any reason why a quick periodic "off blip" would be harmful to the compressor during this type of operation???

Thanks so much again.

HECAT on Fri May 02, 2014 1:25 PM User is offline

Clutch cycling = clutch wear. I would not purposely cycle more than what the system design allows for.

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MrBillPro on Sat May 03, 2014 10:35 AM User is offlineView users profile

Quote
Originally posted by: HECAT
Clutch cycling = clutch wear. I would not purposely cycle more than what the system design allows for.

I always figured it was not good on the front compressor seal either. I mean at 2000 rpm or more, you would think the cycling on and off would create more heat around the seal area.

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Don't take life seriously... Its not permanent.

wptski on Sat May 03, 2014 11:28 AM User is offline

Quote
Originally posted by: MrBillPro
Quote
Originally posted by: HECAT
Clutch cycling = clutch wear. I would not purposely cycle more than what the system design allows for.

I always figured it was not good on the front compressor seal either. I mean at 2000 rpm or more, you would think the cycling on and off would create more heat around the seal area.
If that generated excessive heat, it would cook the pulley bearing first, I'd think.

MrBillPro on Sat May 03, 2014 11:56 AM User is offlineView users profile

Quote
Originally posted by: wptski
Quote
Originally posted by: MrBillPro
Quote

Originally posted by: HECAT

Clutch cycling = clutch wear. I would not purposely cycle more than what the system design allows for.



I always figured it was not good on the front compressor seal either. I mean at 2000 rpm or more, you would think the cycling on and off would create more heat around the seal area.

If that generated excessive heat, it would cook the pulley bearing first, I'd think.

So it would overheat the bearing before the shaft seal would start leaking?

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Don't take life seriously... Its not permanent.

wptski on Sat May 03, 2014 1:36 PM User is offline

Quote
Originally posted by: MrBillPro
Quote

So it would overheat the bearing before the shaft seal would start leaking?
That's been mentioned here before as a cause of bearing failure.

On the other hand, a few posts above I mentioned that I used to service machines with electric clutches. We had a problem where the clutch coils would get so hot that the wire insulation would melt and bubble out at the wire connection terminals! That never caused a bad bearing. It started on one machine and got to the point where they were flying in the parts. It was blamed on mechanical which was my job! I thought it was the coils which were under specs for resistance which means more current draw but since it wasn't my job, I was ignored. I can remember being on a conference call with Warner who also blamed mechanical. It just stopped happening after about a half-dozen coils!

Edited: Sat May 03, 2014 at 1:38 PM by wptski

MrBillPro on Sat May 03, 2014 1:57 PM User is offlineView users profile

You would think in the 21st century, they would have already found some different method to engage the compressor, guess that will be a hard component to replace.

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Don't take life seriously... Its not permanent.

bohica2xo on Sun May 04, 2014 1:08 PM User is offline

Audi, Mercedes, Toyota & others are using a Denso compressor with no clutch at all. They simply unload the compressor internally & let it spin.

Cycling a clutch causes NVH issues for the automakers, as well as wear on parts. GM went to variable displacement compressors to eliminate cycling. Saturn & VW are variable displacement units as well.

If you are chasing a fuel economy issue, a lot more info about the exact vehicle would be necessary to offer any solid advice.

.

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"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

MrBillPro on Sun May 04, 2014 7:29 PM User is offlineView users profile

Quote
Originally posted by: bohica2xo
Audi, Mercedes, Toyota & others are using a Denso compressor with no clutch at all. They simply unload the compressor internally & let it spin.



Cycling a clutch causes NVH issues for the automakers, as well as wear on parts. GM went to variable displacement compressors to eliminate cycling. Saturn & VW are variable displacement units as well.



If you are chasing a fuel economy issue, a lot more info about the exact vehicle would be necessary to offer any solid advice.



.



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Don't take life seriously... Its not permanent.

MrBillPro on Sun May 04, 2014 7:33 PM User is offlineView users profile

Audi, Mercedes, Toyota & others are using a Denso compressor with no clutch at all. They simply unload the compressor internally & let it spin.

Very smart! They are finally catching up with residential and commercial air conditioning.

Guess I need to learn how to post here all over again.








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Don't take life seriously... Its not permanent.

Edited: Sun May 04, 2014 at 7:38 PM by MrBillPro

zzyzzx on Mon June 02, 2014 3:08 PM User is offlineView users profile

Quote
Originally posted by: Dougflas
It is my opinion that today's compressors are so efficient that you will not notice any difference in fuel usage. Years ago, compressors did use energy but not so at this time. Not too sure what you're asking. Under high heat loads, the compressor will not cycle which is normal.

I thought the not noticing part was due more to the grossly oversized engines that all cars have now.

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