How much HP do auto A/Cs use?

[gwalchmai]

Wide topic, I know. Is there a database anywhere? Mainly curious. I drove about 250 interstate miles yesterday in my Tacoma and got about 32 mpg, which is about what I usually get without the A/C. The A/C or defog was on all the time. I'm guessing auto A/Cs are pretty efficient nowdays. 

[railfancwb]

Can you feel a change in engine RPM when stopped if you switch the AC off, wait a few moments, then switch it back on?

[Huaco Kid]

For a long time now,  the compressor runs all the time.  If you shut the AC "off",  it just redirects the "cold".

[gwalchmai]

6 minutes ago, railfancwb said:

Can you feel a change in engine RPM when stopped if you switch the AC off, wait a few moments, then switch it back on?

Will check...

[Duluth]

I read somewhere recently that keeping your windows closed and running your A/C will get you better mileage at speeds over 40 mph. Having the windows  at higher speeds creates drag.

[crockett]

Most car ACs use a clutch that only engages the compressor when there is demand.

Car ACs produce 40,000 to 60,000 BTU (3.3 to 5 Tons). That's more than many home ACs.

The compressor may steal between 3 and 10 HP. More when wear increases.

A big fat V8 with 5 plus liters won't be hassled, unlike a small 1.6 liter 4 banger.

[Cougar_ml]

Some modern AC pumps also use variable vane type pumps, so they are constant on, but only using just as much power as they need, and not more, unlike many older systems that were either all or nothing.

[gwalchmai]

7 hours ago, crockett said:

Most car ACs use a clutch that only engages the compressor when there is demand.

Car ACs produce 40,000 to 60,000 BTU (3.3 to 5 Tons). That's more than many home ACs.

The compressor may steal between 3 and 10 HP. More when wear increases.

A big fat V8 with 5 plus liters won't be hassled, unlike a small 1.6 liter 4 banger.

7HP is the last number that I remember, and I think that was on my 90 Dakota V6. 

[GT4494]

Don't forget the alternator load (interior fan) and the resulting HP loss.

[jmohme]

15 hours ago, crockett said:

Most car ACs use a clutch that only engages the compressor when there is demand.

Car ACs produce 40,000 to 60,000 BTU (3.3 to 5 Tons). That's more than many home ACs.

The compressor may steal between 3 and 10 HP. More when wear increases.

A big fat V8 with 5 plus liters won't be hassled, unlike a small 1.6 liter 4 banger.

I think few, if any modern cars use clutches on the compressors anymore. They run full time and just divert the flow of refrigerant when not needed.

[crockett]

11 minutes ago, jmohme said:

I think few, if any modern cars use clutches on the compressors anymore. They run full time and just divert the flow of refrigerant when not needed.

Negative.

I just checked my 3 vehicles from 3 brands (2011, 2018 and 2019). All use a clutch.

[crockett]

39 minutes ago, GT4494 said:

Don't forget the alternator load (interior fan) and the resulting HP loss.

Checked on my Minivan, pulls 4 amps on highest setting. 45 watt is 0.06 HP, that's nothing.

[jmohme]

1 hour ago, crockett said:

Negative.

I just checked my 3 vehicles from 3 brands (2011, 2018 and 2019). All use a clutch.

My Dodge, my wifes Ford, and my Daughters VW are all clutchless. So I guess it's just all mixed up.

[railfancwb]

On a kindred subject, increasing numbers of vehicles have removed the engine cooling fan from the water pump, made it electric, controlled it thermostatically, and clamped it to the radiator frame. 

[jmohme]

3 hours ago, railfancwb said:

On a kindred subject, increasing numbers of vehicles have removed the engine cooling fan from the water pump, made it electric, controlled it thermostatically, and clamped it to the radiator frame. 

I did that on my Ford pickup back in the 70s. Those old fans robbed a lot of power.

[G19Tony]

On my 78 Plymouth Arrow, I could either use the AC, or go up a hill. But not both. 

[gwalchmai]

8 hours ago, G19Tony said:

On my 78 Plymouth Arrow, I could either use the AC, or go up a hill. But not both. 

My buddy had a 49 Chevy with vacuum wipers. He had to make similar choices.

[railfancwb]

On 7/22/2021 at 9:37 PM, Duluth said:

I read somewhere recently that keeping your windows closed and running your A/C will get you better mileage at speeds over 40 mph. Having the windows  at higher speeds creates drag.

Removing the side vents in the late 1960s, on the grounds that side vents made easy entry for thieves, made use of the roll down windows for climate adjustment at speed very uncomfortable. Led to AC becoming standard as well…

[LostinTexas]

For some reason 3-5 HP sticks in my head.

The scenario you described is not the correct observation though. HP has little to no effect on fuel mileage. Well, unless you are under full load the entire time, but even then,HP has little to do with it.

They do rob a bit. That is why most auto makers have gone to electric fans, power steering units, brake boosters, fuel pumps and everything else they can. Some, if not all, have better performance with electric anyway.

[gwalchmai]

56 minutes ago, LostinTexas said:

For some reason 3-5 HP sticks in my head.

The scenario you described is not the correct observation though. HP has little to no effect on fuel mileage. Well, unless you are under full load the entire time, but even then,HP has little to do with it.

They do rob a bit. That is why most auto makers have gone to electric fans, power steering units, brake boosters, fuel pumps and everything else they can. Some, if not all, have better performance with electric anyway.

If you drive the same speed, won't a parasitic load require more fuel?

[LostinTexas]

5 minutes ago, gwalchmai said:

If you drive the same speed, won't a parasitic load require more fuel?

Yes, but it isn't that simple, and AC is rather minuscule in the picture. The actual HP number it takes isn't going to be a big factor. Rolling windows down can create enough drag to offset the AC use. Some auto makers advise to only roll windows down in city driving thanks to this.

Start and stop, pulling, habits, towing, and other things make more of a factor. At a steady higher revving speed the compressor won't factor so much.

If just asking for the sake of curiosity, that has been pretty covered in the general consumption. The particular vehicle will vary.

[jmohme]

21 hours ago, G19Tony said:

On my 78 Plymouth Arrow, I could either use the AC, or go up a hill. But not both. 

I don't know if you are serious or not, but this did remind me of a true story.

It was early fall, and I was working at my dads auto repair show when a young lady that I had gone to school with brought in her very first new car for its first oil change. She bought a Chevy Chevettte and was very proud of her new ride and religiously brought it to us for all scheduled maintenance.

Fall, winter, and spring passed, and summertime arrived. One day she rolls up and says her new car won't go over 52 mph, so she leaves it with us. The car was due for a tune up so we started there. Put it on the Sun scope and all checked out. I took it on a test drive and all seemed well. Got it up to the speed limit and a little past with no problem. Then I turned on the AC and that sucked the speed right out of it and down to 54mph. That pathetic little engine just did not have enough power to run the AC and the car at highway speeds.

I talked to a friend of mine that worked at the Chevy dealer and told him I was sending some warranty work his way. He confirmed that many air conditioned Chevettes had this problem.

Yet another reason that I would never own a GM product!

[G19Tony]

On 7/24/2021 at 7:18 PM, jmohme said:

I don't know if you are serious or not, but this did remind me of a true story.

It was early fall, and I was working at my dads auto repair show when a young lady that I had gone to school with brought in her very first new car for its first oil change. She bought a Chevy Chevettte and was very proud of her new ride and religiously brought it to us for all scheduled maintenance.

Fall, winter, and spring passed, and summertime arrived. One day she rolls up and says her new car won't go over 52 mph, so she leaves it with us. The car was due for a tune up so we started there. Put it on the Sun scope and all checked out. I took it on a test drive and all seemed well. Got it up to the speed limit and a little past with no problem. Then I turned on the AC and that sucked the speed right out of it and down to 54mph. That pathetic little engine just did not have enough power to run the AC and the car at highway speeds.

I talked to a friend of mine that worked at the Chevy dealer and told him I was sending some warranty work his way. He confirmed that many air conditioned Chevettes had this problem.

Yet another reason that I would never own a GM product!

I was serious. If it was a long hill, I'd have to shut off the AC. I still liked the car. 

[inthefrey]

I have a 2008 Subaru Forester with 208,000 MI. 2.5l boxer. I routinely turn off the air conditioner when climbing Western Pennsylvania hills. I also routinely turn the air conditioner on when descending, adding engine braking, giving the brakes a rest and getting free air conditioning from the downward slope. I can definitely feel it in my 2.4l boxer. No clue how many horsepower..

[aomagrat]

Turning the AC on at idle in my 2005 Mustang and 2008 Ranger definitely causes a slight rise in RPM's.