How many HP does A/C take?

[Tom Greenleaf]

An all time query. What does it take to make cool air in HP? You can't answer that and be precise. It would and must vary like this science would be split second data only that moment.

Hard trade and costly. Folks think it's a rip off so off your azz and just try to do it. Start with a guess at the question. Now you know...............

[Mikel]

It probably varies wildly based on vehicle, but my guess is about 5HP, on and off, as the compressor cycles.

I wonder how that compares power-wise with a POA system with the compressor running at all times...

I would love to see data comparing an old A6 compressor with a modern one.

[Cornbinder89]

My Text book had a chart, I would have to dig thru to find it, but it depended on compressor speed, IIRC it went well over 8 hp at higher speed.
That being said there are a lot of variables, type of system, heat load etc.
Early systems never shut off the compressor and all excess refrigerant was stored as high pressure liquid in the receiver, requiring energy input the whole time the system is running.
CCOT systems don't store excess, they cycle the compressor, to vary the flow. Now we have variable displacement compressors and "clutchless" compressors.

[Cornbinder89]

It probably varies wildly based on vehicle, but my guess is about 5HP, on and off, as the compressor cycles.

I wonder how that compares power-wise with a POA system with the compressor running at all times...

I would love to see data comparing an old A6 compressor with a modern one.

The A-6 is fairly good, the design has been copied (swash plate, moving multiple pistons), with the exception of the scroll compressors, most piston compressors in use today are based off the A-6. It can handle high speed without shaking itself apart, and move close to 40,000 BTU.

[Tom Greenleaf]

I believe the high BTU as vehicles are constantly adding outside air just less if "recirc" is chosen.
It just has to changeable with temps, a lot.

Question was just to provoke thought. It's a lot I can feel the load some systems shut it off WOT to give you power like passing another or battle with a long hill and a heavy load even more passengers add to demand of power,

[Cornbinder89]

If I have time, I'll try and dig through that old text book and see what it says. WOT clutch switches have been a thing since the 70's
I can feel it in my cars, they are both 4 cyl, and remember a "hang on unit" in my neighbors "Checker Airporter" station wagon with a 250 or 292 Chevy motor would almost stall when you turned it on at idle.
Trucks you could hear it kick on, and the rpm would drop a little, but they are governor controlled at idle, so hard to tell.
I believe, but have not seen it written anywhere, that the more modern variable displacement and even the CCOT systems use less power for the same BTU, or at least use less power when less BTU is required from the system, than an old Tx system. I think it is also why we are seeing the new stuff with a lot lower refrigerant capacity, they want "just enough" with no room for extra that has to be stored at lower heat loads.
In all cases the faster the compressor runs the more Hp it takes. That is why we are seeing variable displacement, it can reduce the Hp drag at speed but still move enough at idle to keep the car cool.
The old refrigerated rail cars used a diesel generator that had two speeds, one turned at 1800 and produced 60 cycle power to drive the refrigeration compressor, and when less cooling was required it dropped to something like 1200 and the frequency dropped with RPM and turned the 60 Hz compressor at 1/2 speed or there abouts.

[Cornbinder89]

Ok, found my 1996 textbook on the subject. It is a little out of date but does talk about scroll and variable displacement automotive compressors.
It shows 2 hp at 1 ton (12,000 BTU) and aprox 4 hp at 24,000.
The BTU capacity rises with speed but tends to flatten off at speeds over fifty, still rising but slower than the gains at lower speed.
The graph tops at 48,000 btu and 8HP Hp required rises at an almost 45 deg angle, near linear rise to aprox 90 MPH, while the BTU rises at a sharper rate but changes to a less steep angle (lower rise than hp) at higher speeds.
The book uses the figure of 2hp per ton of refrigeration for engine driven compressors and compares that to 1hp per ton for electric (constant speed) compressors.
While system may have improved in the past 2 decades, with inverter based electric compressors and multiple styles of variable displacement engine driven compressors, I think those figures will not be too far off the mark.
A truck with a front and sleeper compartment evaporator units each with 2 ton Tx is still likely to pull close to 8 hp
A van with front and rear A/C with 1.5 ton Tx on each will likely pull close to 6 hp
A regular car close to 4 hp.

[Tom Greenleaf]

Good find! It's ancient history now recall a few cars with mega HP engines still lowered idle speed a little so for lacking the right name of a thing would hold idle up but you had to touch the "gas pedal" it would cover the load one setting those didn't keep changing.

I don't think we notice much at higher road speeds strong fan clutches didn't grab so hard. If waiting like at a traffic light (stopped) would roar for a while and quit that.

Been icky hot here and lots of places full humidity the evaps pour out condensate at least if pushed to limits the dryer air really helps.

IDK, grew up without A/C cars and homes/buildings had much better air flow thru cabins (cars) and homes assorted fans as insane heat didn't last that long as a % of days per season.

Smile, dress accordingly too,

[Cusser]

My 1988 Mazda B2200 truck has only about 82hp, and on long uphill freeway stretches I sometimes switch off the AC (and NOT because the engine starts to overheat).

Of course, EVERY day in July this year in Phoenix AZ the temperature has been over 110F, and a couple of times the low was 96F, so AC is a necessity here.

My 1970 VW Beetle had the dealer-installed AC with the old Tecumseh compressor, and its 1600cc single port engine did struggle with the AC on.

[Tom Greenleaf]

You also have HOT weather, real air temp is thinner, so even less engine HP.
Those "Beetles" I think were like 40 HP? IDK even I was real young once a bus version mid 1960s I think had like 18-20 HP OMG wind out low gear still not keep up speed was full of passengers too really stressed the thing out. No A/C or probably choice of moving or cooled air :-)

Hot here too all local few hills to speak of not high altitude so don't notice not far real mountains in NH and VT or nowhere ME isn't very populated where mountains are.

It all matters!

[Cornbinder89]

The 80's Saab's use a Seiko vane compressor, "ear" mount. I don't know if it took more or less energy, only that mine has been incredibly "tight" only "boosting once in 13 years (and I don't know when it was last charged before I bought it, so could be 20 years or more), and less than a can at that.
They are NLA however. I think you can still get seal kits but no internals. They had a HPCO on the compressor body, I suspect the vanes would crack if deadheaded, so they put the cut-out on the compressor itself.

[Mikel]

On the "WOT compressor shut off" - It may have more to do with overspeeding the compressor than with squeezing every bit of acceleration out of the car.

I can't imagine an A6 likes 7000 RPM.

[Tom Greenleaf]

Just thinking about that? Sure it would. It would shut down and just pulley/clutch would make such low pressure would cut out. Look at diameter of the balancer is 2-3 times what clutch is would only take 3-4,000 engine RPMS most would do that TMK same bearing as many alternators are smaller and take it.

Idea was power TMK the clutches power was routed thru a vacuum switch on manifolds when an engine works full power vacuum is close to zero from a stop if you just floor it to take off hard,

[Cornbinder89]

On the "WOT compressor shut off" - It may have more to do with overspeeding the compressor than with squeezing every bit of acceleration out of the car.

I can't imagine an A6 likes 7000 RPM.

somewhere I had the spec's, the A-6 can spin fairly fast. I'd have to search for the exact limit. They move close to 40,000 BTU, there is no connecting rod, or crank throw to cause a problem. balanced so will not shake at high speeds.
ripped from the net:
"The A6 is rated at 6,500 max continuous rpm and 7,500 intermittent (downshift) rpm"
another place states 42,000 BTU.
Haven't looked further.

[Cornbinder89]

I found this years ago, then lost it. They make (re man?) A-6's in lower displacements as well as stock. Found it interesting.

http://www.almaproducts.com/Transtar.ALMA/media/ALMA-Media/Products/ac%20compressors/A6-Flier.pdf

[Tom Greenleaf]

Somehow I knew Cornbinder would find some stuff out there on these. Look at first pic of the link. Top right you see the drilled spots on that one is to balance it for wild speed?

Actual BTU ability with things missing off the web? Factor it was first made for R-12 is 20% more capable than 134a. There's a reason,

[Cornbinder89]

What I found interesting is they are making them in both rotations and are offering two optional smaller displacements. Gm only made one displacement, although I have seen "backwards" rotation listed at one time.
I've seen many "GM era" pulleys balanced with drilled spots. so I think that was always done.

[Tom Greenleaf]

Backward rotation: It's nuts I guess but "trucks" with same appearing engines are backwards! Why? Not sure for just routine trucks GM, Ford, Chrysler's Dodge but those 3 perhaps IH (International Harvester) did sell engines for marine use!
Never saw an engine mounted compressor however twin engines (yachts) had some serious time with have to really the wash propellers make would either interrupt the other or at least make them go in a slow circle!

That's big biz out there pleasure or work boats.

Hate to waste $$ owned smaller ones chose "single" engine just because I only wanted one headache not 2.

Propellers are also backwards so if keeping a spare need 2.

GM kept up Ford got out of that and IDK for the rest.

Outboards also followed suit.

It's trivia but marine singles are right handed (starboard side) you add the other if two.

Scratch head most larger had power steering as they pull at assorted speeds is/would be annoying it's a lot of force depending.

I think alternators don't care are not automotive those and starter motors CAN'T MAKE SPARKS and have shields to limit that.

Just think especially for gasoline a bilge full of vapor is or would be the end of them in short order - BANG > you wouldn't notice just gone!

[Cornbinder89]

Reverse rotation: where it came up for me, was a aux power unit I built for my truck. The A/C compressor was driven off the flywheel end and was mounted "backwards" so needed a compressor that could run "backwards". The other place it shows up is in waterpumps and fans that are driven off the back side of serpentine belts. Compressor draw too much power to be driven off the back side. I suppose counter rotating engines, you might need counter rotation.
Some engines have accessory drives which don't "have" to turn in crankshaft rotation, so a belt drive off one of those might need a "backwards" rotation compressor.
All in all, it would seam that normal rotation but the compressor facing the opposite way (clutch toward the rear of the vehicle) with the engine facing forward, would be the most common place you would see them. This was the reason I needed a reverse rotation compressor.
All this is why we don't see many compressors that are directional made in both directions.
Many compressors are splash lubed, so don't care (if piston type), The vane compressor in my Saab would care if turned backwards, and scroll types would also.
The A-6 only cares because the lube pump in them is directional, the rear cover that houses the oil pump would have to be different for counter rotation. The pistons and valving will not care and will work if turned backwards, but the parts would not receive the needed oil.

[Cornbinder89]

Found something interesting for comparison.
An air brake truck compressor of the size normally fitted to road tractors (12CFM) take 2.4HP at rated engine rpm, or about what a 1 ton a/c comp would take.
a 7 1/2 CFM (used on older trucks with fewer axles) take 1.6 hp
so a car with between 1 and 1.5 ton system uses the same or more HP to drive the compressor than an air braked truck uses for brakes.
I found this while researching air compressors for trucks.