Question about air conditioning power consumption

AC's have an efficiency band. Usually they are most efficient nearing full heat transfer. But for something as small as a car, it is not really measurable.

Now if you've got a shopping mall you're trying to cool, small efficiencies add up quickly.

 

In theory not running your a/c full blast but at a much lower setting saves energy... But in practice maybe running your A/C at a low setting constantly is not as efficient as my preference, which is to run the A/C full blast very briefly at times when the car is going downhill, flat ground, not climbing a hill or starting from a stop. This tends to annoy my passengers, but keeps me entertained on long drives.

I suspect the Hybrid Asst app has the data charts available to study this via their Hybrid reporter app. If you dig into it and research it more, please post here and let us know your findings.

 

Awesome question!

I went digging around to see if I could find anything on it-

Toyota has this, which establishes that there are at least 3 compressors used over the years. The original belt-driven model, the ES18 electric compressor used on 2nd generation cars and the ESB20 model introduced on 4th generation cars.

This thread from another forum suggests that the compressor has an integrated variable frequency inverter so it can run the scroll at different speeds for different loads, which would change the electrical power consumption.

What I didn't find:

• Does the c use the ES18 or a different compressor?
• confirmation (anything more authoritative than a forum post) that the ES18 has a variable frequency inverter

 

i don't use auto, but on manual, the higher the fan speed, the harder the compressor works. this has been documented by people with tech stream.

on 'auto', it would fluctuate as the system works easier and harder as needed.

 

I need to play with techstream one of these days. I've already got a laptop dedicated to garage pursuits and a Tactrix for my Subaru, so in theory I only need the software. Any pointers?

 

sorry, no. i don't have it. just what i've read from the experts here. there are plenty of threads with good advice.

 

The DC style inverters (like used in mini-split units) are common now. They still are their *most* efficient at high load with good temperature deltas. But are still efficient at light loads. You could argue even if the efficiency graph was perfectly linear throughout the load, that the overall efficiency would still be in favour of a full load because of the offset in power usage for the control electronics. If you use 1% duty or 100% the control electronics are still 100%. This is why most SMPS's have that close to nothing % efficiency for light loads. You're still burning power to control not burning power. Once you start regulating that overhead goes to noise and you get 98% to 99% efficiency. But at 0.5% load, you're 10% efficient lets say.

If you look up the Energy Star qualifications sheet for various ductless units you'll see the same efficiency graph basically. Starts low then rises sharply and levels out but still goes up. Most efficient at high load but still super efficient at all loads you care about.

 

For the Gen 3 (liftback, anyway), there is a spreadsheet going around that has a bunch of reverse-engineered parameter-ID data that you can load one way or another into your favorite OBD-II scanner software and read lots of operating data from the car. One of the entries I programmed into my ScanGauge II was for the watts being drawn by the A/C compressor.

From watching it while driving in different conditions, it has about a 10 to 1 power range ... it can loaf along at about 250 watts in really mild conditions, or about a third of a horsepower, and you scarcely hear it running. In the hottest conditions it'll use close to ten times that, or around 3 HP, and in those conditions it sounds a little like you clipped somebody's motor scooter with your right corner without noticing, and you're dragging it down the road.

For conditions too mild for even 250 watts, of course it'll just cycle on and off like every other air conditioner always did.

I suspect the power dissipated in the drive electronics is still only a small fraction of the total, even at the lowest power.

The c is a smaller car than the liftback, and it might have a smaller-displacement compressor, and the wattage upper and lower limits might be different. If you get an OBD reader and the right PID definition, it's easy to observe it.

-Chap

 

Please correct me if I'm wrong, but I understand that the traction battery in the c is cooled against cabin air. I understand the liftback vents overboard.

I just wondered if they were taking advantage of the relative overage in capacity to avoid another hole in the body.

I've got a scangauge lying around, I'll be interested to learn what our c tells us. Thanks!

 

The gozouta flaps on the Gen 3 liftback are behind the rear wheel wells, concealed by the bumper cover. That space is all beneath the hatch floor deck. The way the nice comfy cabin air gets into that space, so it can eventually exit through those flaps, is through the grille next to the rear seat, the ductwork, and the battery, and out beneath the floor deck.

-Chap

 

I set an XGauge on my ScanGuage, for AC Watts, and would occasionally watch it on our Prius (3rd gen hatchback, hopefully similar). the value would alternate between zero (when the compressor was not running) and various wattages, anywhere between 50 and maybe 300, in increments of 50.

 

Interesting ... I've rarely seen less than 200 or 250 on mine (it just cycles off for demand below that). But full power is well above that, by around a factor of ten. I'm also using a ScanGauge XGAUGE that I got from a spreadsheet here. I wonder if the scaling factor got changed in that XGAUGE at some point.

The one thing about relying on reverse-engineered PID definitions....

-Chap

 

Good point. Simple math says a 20k BTU/hr system (completely typical for a small/medium car) should be drawing a lot more than 250W at full output.

 

It tended to run lower values in particular, in the following conditions:

AC button off, and vent mode set to heat/defog.

With the above settings, AC would often "stealth run", even though it was not officially on, no light on the AC button. At low, near-freezing temps it wouldn't. Also, there's a sensor that detects cabin temp, and IIRC it's also detecting humidity? Or there is another sensor for humidity? Repair Manual excerpt (3rd gen):

 

I think setting the AC to "auto" mode will be the most economical. As it will manage the room temperature hence energy consumed.
As per my research the JDM "AQUA" uses ES14C electric inverter compressor .

Check this interesting document

http://www.globaldenso.com/en/newsreleases/events/otherexhibitions/2012/dae12/files/DAE12_ev_hv.pdf

 

Still depends on the temperature. If you have it set to Low, the "auto" feature runs full non stop.

 

Agreed