A/C compressor (auto)

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frhrwa
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A/C compressor (auto)

Post by frhrwa » Fri Feb 25, 2011 12:00 pm

has anyone ever tried to run the compressor for AC in a car with an electric 12vdc motor and gear drive.. ? instead of taxing your poor motor to death, use an electric with it geared to turn at the right speed constant? was thinking on my 2.5L 4cyl, if that would work, ?... of course a second battery and larger alternator..
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Re: A/C compressor (auto)

Post by gerty » Fri Feb 25, 2011 12:08 pm

I don't see why it wouldn't work, however, I think the added strain for driving the larger alternator would cancel out your power gain. You can listen to your motor idle, turn on the headlights, and you should hear a change in your idle speed, that'll show that just a small load
(10 amps) will have an effect. Don't know how many hp it'll take to run an A/C compressor, make a good experiment though.. :grin:

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Re: A/C compressor (auto)

Post by frhrwa » Fri Feb 25, 2011 1:10 pm

seems you should be able to gear it to where it wouldn't tax the motor to heavily.. I'll have to find out the rpm range of the compressor motor and see what kind of 12vdc motor I can find .. try it and see what it does.. interesting concept, guess if it worked the auto manufacturers would be doing it?
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Re: A/C compressor (auto)

Post by hlreed » Fri Feb 25, 2011 1:26 pm

They are doing it. In fact most cars now come with electric air conditioners.
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Re: A/C compressor (auto)

Post by MrAl » Fri Feb 25, 2011 2:45 pm

Hello there,


Long time back i set up a old Cadillac AC compressor with a 3/4 hp motor to make an air compressor. Worked pretty well for years.
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frhrwa
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Re: A/C compressor (auto)

Post by frhrwa » Fri Feb 25, 2011 3:52 pm

did you have to gear the 3/4 hp or was it the correct rpm? I didn't know they were using electrics in the new cars.. but looking under the hood of new cars is a zoo now adays anyway..
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Re: A/C compressor (auto)

Post by Robert Reed » Fri Feb 25, 2011 5:47 pm

I don't know if this still holds true, but about 25 years ago I remember an article by the SAE (Society of Automotive Engineers). They stated it took 6 HP at full blast to operate the compressor. For comparison they also stated it took 12 HP to keep a full size sedan rolling down the highway at 60 MPH.

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Re: A/C compressor (auto)

Post by Janitor Tzap » Fri Feb 25, 2011 6:12 pm

frhrwa wrote:has anyone ever tried to run the compressor for AC in a car with an electric 12vdc motor and gear drive.. ? instead of taxing your poor motor to death, use an electric with it geared to turn at the right speed constant? was thinking on my 2.5L 4cyl, if that would work, ?... of course a second battery and larger alternator..
My... do we have the same vehicle?
I have a 95 Acclaim 2.5L 4 cylinder with air conditioning.

I agree with gerty that putting it on a separate electric motor,
and installing a larger alternator would cancel out any benefits.

One of the problems I noticed with my vehicle.
Was that if you didn't make sure your freon level was good.
This caused the compressor to start to bind or freeze up.
Thus, it will tax the engine when you try to use it.
I had to replace the compressor, and some hoses, and a drying canister.
Plus, the freon.
But with the system fully charged, and working correctly.
I only had a minimal difference in the amount of gas I used with or without the air conditioning on.


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Re: A/C compressor (auto)

Post by dyarker » Fri Feb 25, 2011 10:54 pm

instead of taxing your poor motor to death, use an electric with it geared to turn at the right speed constant?
Whether by belt drive drive to compressor, or by more electricity to run electric motor to drive compressor, your poor engine does the work of turning the compressor. (the alternator is harder to turn with increased load)

Converting existing compressor to electric drive would LESS efficient, engine would have to work harder than before, and more gas burned. As is now both compressor and alternator are belt driven, so call power transfer efficiency of belts even. Now convert, an alternator is not 100% efficient at converting rotation to electricity, and an electric motor is not 100% efficient at converting electric power to rotation. You've added two losses that weren't there before. Plus, you car is heavier by the weight of the electric motor and bigger alternator, further increasing load on the engine. With EXISTING compressor best bet is maintenance mentioned by J. Tzap.

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Re: A/C compressor (auto)

Post by MrAl » Sat Feb 26, 2011 7:36 am

frhrwa wrote:did you have to gear the 3/4 hp or was it the correct rpm? I didn't know they were using electrics in the new cars.. but looking under the hood of new cars is a zoo now adays anyway..
Hi,

Oh yes, im sorry, i forgot to mention the gearing. Actually i used a belt and pulley because the compressor already had a pulley (from the normal setup in a 1959 Cadillac) and the motor already had one too if i rem right, and the ratio was geared down (speed reduced, torque increased). The pulley on the compressor was about 5 inch diameter (from memory now) and the one on the motor about 2 inch, so that's roughly 2 to 1 or 3 to 1 gear down. The speed of the motor would have been the typical around 1500 RPM.
In the automobile the compressor is also geared (belted) but i dont know the ratio there and didnt think to check that out way back then (sometime in the late 70's).
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Re: A/C compressor (auto)

Post by frhrwa » Sat Feb 26, 2011 9:09 am

so basically, running the electric fans to cool the radiator use more power than the metal fan blade? everyone tells me the electric fan gives them quite a boost in power over leaving the old metal blades on? I know most new cars have the electric, but like an older jeep with a 4banger, or even the 6, power is lacking .. so boost's of this sort are nice.. no engine mod's like cams, etc..
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Re: A/C compressor (auto)

Post by MrAl » Sat Feb 26, 2011 9:26 am

Hello again,

Well, there are other considerations too such as wear and tear on the bearings. Fan blades are subject to some unusual torques in directions other than what they are primarily designed for. Taking the fan blade out of the picture would help there as well as make clearance for the billion other parts under the hood these days.
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Re: A/C compressor (auto)

Post by CeaSaR » Sat Feb 26, 2011 9:35 am

No, the big difference between the 2 fans is in duty cycle. The belt driven fan is on 100%
of the time. The electric fan only runs when needed.

Example:

At a dead stop, no air is moving through the radiator except what the fan moves. The belt
driven fan at 100% duty cycle pulls some air through the radiator, using x amount of energy.
The electric fan comes on when signalled by the thermostat control using y amount of
energy for z time, producing a duty cycle of less than 100%. The electric fan may use
more power when on (at this particular point) but uses less overall due to low duty cycle.

When moving down the road, air moves through the radiator without the need for a fan. For
each vehicle and condition, there is a speed at which enough air moves through the radiator
to do the cooling without help. At this point, the belt driven fan is still running at 100% duty
cycle, producing increasing amounts of energy drag, while the electric fan is not running at all.
This means that no extra energy is being used to cool the engine, hence more power is available
for vehicular motivation.

Simply put, anything that doesn't run in order for the whole process to work frees up additional power.

CeaSaR

**This assumes a fixed fan. Clutch fans are more efficient than fixed, and electric fans are most efficient.
Hey, what do I know?

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Re: A/C compressor (auto)

Post by haklesup » Tue Mar 01, 2011 1:38 pm

All you need to do is differ to the laws of thermodynamics. The 2nd one in particular says every time you convert to another form of energy you loose something. So if all you do is convert from chemical to heat to rotational (the motor) then to compression it takes less energy than if you do all that plus convert rotation to electricity and electricity back to rotation.

While the process of converting electricity to rotation is better than chemical to rotation, you still need to make all that electricity somehow. Until electric cars evolve to have 5X the electrical storage capacity they have now, electric to A/C will be impractical. Even hybrid cars run the engine when you turn on the A/C, all those engineers haven't found a better way yet. And don't even go down the road of non mechanical refridgeration (like from a Peltier device) the process of electric to cold is immensely inefficient generating 3x to 5x more heat than it does cold.

I have never noticed a significant change in my mileage with or without A/C. One might even say A/C saves energy by making it so you don't have to open the windows which greatly increses drag (I can feel my sun roof open in the gas pedal). In all cases, idling in the sun on a still humid day with the A/C on will cause the worse case mileage

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Re: A/C compressor (auto)

Post by Janitor Tzap » Tue Mar 01, 2011 6:44 pm

Hi frhrwa,

I'm wondering now.....

What are you trying to achieve here?
Better power for climbing hills?
Or better gas mileage?

On newer vehicles they not only employ a large radiator,
with electric fans that are temperature controlled.
So the fans are only used when the coolant temperature goes above a set point.
Then turns them off when the coolant temperature goes below a set point.

The air conditioners on many of the newer cars is designed to cycle the compressor on and off.
When the temperature in the passenger compartment has reached the set temperature.
It then only comes on intermittently to maintain the set temperature in the passenger compartment.
Also, they are designed to turn off the compressor when the engine requires more power,
when you are climbing a steep hill, or just quick hard acceleration.

But even with these modifications.
You can only expect a 10 to 15% increase in power and fuel economy.

There are plenty of other things to take into account as well.

The weight of the vehicle.
The rolling resistance of the tires.
Shocks and struts.
Wheel Alignment.
Properly tuned and maintained engine.
Wind resistance.

These will all add up to how well the vehicle will perform.


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