Air conditioner run off of alternator vs crank?

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bp944
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Air conditioner run off of alternator vs crank?

Post by bp944 » Mon Jan 14, 2013 2:24 pm

I was curious why the air conditioning system isn't run off the of the alternator and why a small car couldn't be cooled this why. I know some of the DIY electric cars run off of a low power Cool Blue compressor and fan or other similar small alternatives.

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Re: Air conditioner run off of alternator vs crank?

Post by avengerengines » Mon Jan 14, 2013 2:47 pm

It would take a huge alternator. The air compressor takes a lot of power to run. The electrical power used to turn it would probably equal the crank power used. No gain.

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Re: Air conditioner run off of alternator vs crank?

Post by Warpspeed » Tue Jan 15, 2013 4:59 pm

avengerengines wrote:It would take a huge alternator. The air compressor takes a lot of power to run. The electrical power used to turn it would probably equal the crank power used. No gain.
Yes,
And if you are stuck in bumper to bumper traffic with the engine idling, alternator output will be low, just at the time you really need max air conditioning capacity.
Cheers, Tony.

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Re: Air conditioner run off of alternator vs crank?

Post by dirtracr5 » Tue Jan 15, 2013 10:10 pm

The one example i can think of is a chevy volt. It has an electric compressor. its 360v and 3 phase. Would be very hard to do on 12 volts.

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Re: Air conditioner run off of alternator vs crank?

Post by Circlotron » Thu Feb 07, 2013 4:42 am

Automotive alternators have a lot of corners cut to the extent that when they are hot and near full load they are only about 50% efficient. I have seen figures for car a/c compressors saying they draw between 5 and 10 hp. To drive a 5hp dc compressor motor needs 3730 watts. 3730/12 volts = 311 amps. That's a lot.

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Re: Air conditioner run off of alternator vs crank?

Post by Dodge Freak » Sun Feb 10, 2013 11:57 pm

dirtracr5 wrote:The one example i can think of is a chevy volt. It has an electric compressor. its 360v and 3 phase. Would be very hard to do on 12 volts.
Yes it might be coming. Even if these new electric cars don't fully pan out, they might help to bring more electric run systems, like power steering and even A/C

That would seem to make more sense then powering the whole car off batteries, which is the main goal right now :roll:

Higher voltages would be more efficient, why 120 and 240 volts are common everywhere else. When voltage goes up, current flow goes down. Only high voltages in cars right now is in the ignition systems, that someday will change.
Last edited by Dodge Freak on Mon Feb 11, 2013 12:03 am, edited 1 time in total.

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Re: Air conditioner run off of alternator vs crank?

Post by crazyman » Mon Feb 11, 2013 12:00 am

I'm confirming nor denying the OP's statement. I'm just curious as to this technology. It's not too energy demanding.

http://reviews.northerntool.com/0394/71 ... eviews.htm

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Re: Air conditioner run off of alternator vs crank?

Post by Dodge Freak » Mon Feb 11, 2013 12:07 am

And that unit only cools off 1.7 cf of very well insulated amount of space. A car with windows allowing sunshine inside, needs a much more powerful cooling system. The technology is lacking today. Food doesn't give off heat either, people are close to 100 degrees, they even throw off heat, you want the car to be only 70 degrees or cooler. That is better then 42 degrees for refrigerators but a car interior is much larger and it heats up when the car is parked outside during the day. You need to cool off a 140 degree car in mins or folks won't be too happy with their new cars

Now if we get pint size cars like those Smart cars it help.

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Re: Air conditioner run off of alternator vs crank?

Post by Circlotron » Mon Feb 11, 2013 6:55 am

That little refrigerator would likely use a peltier effect device. They are only about 1/4 to 1/6 the efficiency of the normal vapour compression cycle refrigerators and air conditioners. #-o -> http://en.wikipedia.org/wiki/Thermoelec ... erformance

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Re: Air conditioner run off of alternator vs crank?

Post by tresi » Mon Apr 08, 2013 7:52 pm

Circlotron wrote:Automotive alternators have a lot of corners cut to the extent that when they are hot and near full load they are only about 50% efficient. I have seen figures for car a/c compressors saying they draw between 5 and 10 hp. To drive a 5hp dc compressor motor needs 3730 watts. 3730/12 volts = 311 amps. That's a lot.
I'll take his word because my brain hurts too much tonight to check his math but I can still confirm it. I use to work on airplane. We had this old A90 that had a york compressor in the nose driven by a 5 hp 24 volt DC motor. It had a 150 circuit breaker. 24 volt at 150 amps vs 12 volt at 311 amps
= pretty close

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Re: Air conditioner run off of alternator vs crank?

Post by peejay » Wed Apr 24, 2013 7:50 am

Dodge Freak wrote:
dirtracr5 wrote:The one example i can think of is a chevy volt. It has an electric compressor. its 360v and 3 phase. Would be very hard to do on 12 volts.
Yes it might be coming. Even if these new electric cars don't fully pan out, they might help to bring more electric run systems, like power steering and even A/C
Power steering is already electric on a large percentage of new cars. Possibly a majority. GM started converting everything to electric about five-six years ago, putting the assist motor right in the steering column. If you can get a column out of a Cobalt/HHR/whatever and a controller box out of the UK, you can put power steering in anything. I think ididit sells Saginaw style columns using the GM components and the controller as one slick little package, for less than buying new parts from the dealer and figuring it out yourself.

The "new" way of doing it is an electric motor mounted to the rack, driving the rack directly with a worm or recirculating-ball setup. Mustang/BMW/basically everyone using TRW racks. Which is why most of the new electric P/S cars have the steering feel of a video game. No feedback at all since it all gets lost in the drive setup to the motor. Most other people use an electric motor to drive a fluid pump, just like an '89 Subaru. I hear the one from the Mini is a three wire hookup, just power ground and ignition sense, so that's an easy swap too.

You'll only see electric A/C compressors on hybrids or electrics. Need the high voltage system to run the compressor. Plus, hybrids shut the engines off sometimes, which is the only reason to not use an engine-driven compressor. The electric compressors are noisy since they run at full-tilt all the time, and they require special (EXPENSIVE, like tens of dollars per ounce) non conductive A/C oil.

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Re: Air conditioner run off of alternator vs crank?

Post by eikoor » Tue Apr 30, 2013 2:13 am

How powerful of an electric motor would it take to drive all of the belt driven accessories and could it be solar powered with a separate battery?
Note; water pump and fan could be electric driven so only A/C and power steering would need a separate motor, the only thing the engine would need to drive is the alternator.
I ask this because the scan gauge on my 2001 2500HD shows a 3MPG drop when you turn on the A/C, so I would think if you could disconnect all accessories from the engine you would see a 5 to 7 MPG increase if not more.

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Re: Air conditioner run off of alternator vs crank?

Post by peejay » Tue Apr 30, 2013 8:19 am

The fuel economy would drop more because you'd be driving the alternator to convert energy to electricity so you could convert electricity to compressor work. Every conversion will lose some energy because you never get 100%.

Electric P/S is a net gain because most of the time, you don't actually need power steering, so you're just wasting power pumping fluid back and forth 99% of the time. Same for the fans - leave them off until the coolant actually needs to be cooled off. When they do come on, they generally pull more current than the alternator is capable of providing because they generally only come on at idle. If you're moving, you don't need very much power steering and probably could get away with having no fan at all...

A/C is trickier because when on, it generally STAYS on, instead of only being needed once every five minutes or so. Modern compressors are generally variable-displacement so they only use as much power as is actually needed to cool the interior, to boot. They don't cycle anymore, and a lot of cars don't even have compressor clutches nowadays. Not needed anymore.

To directly answer your question, for a 12V system you'd need roughly a 1000 amp alternator and at least a 10hp motor. How large is a constant duty cycle 10hp electric motor?

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