Here's a dumb question

[Raduque]

Would it be possible to drive a car's AC compressor using a 12v electric motor? I've been thinking about ways to save gas while running the AC in my truck as the summer months are coming up and it's already starting to hit 90f here in S Texas.

One of the biggest wastes of gas is using HP to drive teh AC compressor, so if I can take that load off the engine, my gas mileage would be roughly the same as the winter months.

How much of a strain would a 12v electric motor capable of driving the AC compressor place on my truck's electric system? I have a heavy-duty battery (something like 600 amps/700+CCA) and the higher amp alternator (towing/4wd/HD packages, I think it's 150A?), but I'm not adverse to upgrading the alt to a 200+ amp unit. I would most likely wire the electric motor so that it's running whenever the engine is.

Anybody think this is a dumb idea?


I'm also considering changing to an e-fan, because using the engine to chop up air also can't be good for gas mileage.

 

[cprince]

It's not going to help with the gas mileage. There is a lost when you convert mechanical energy(the engine) to electricity(using the alternator). Then there is the lost when electrical energy is converted back to mechanical energy(using the motor) to turn the AC compressor. You would be better off driving the AC compressor directly with the truck's engine. And this does not take into account the additional weight of the electric motor. Using electric motor to turn the AC compressor only makes sense on a hybrid car in which you want the AC to be running for a little while when the engine is off(like sitting at a red light).

 

[LTC8K6]

The A/C compressor doesn't use enough extra fuel to worry about, imo.

Neither does the rad fan, imo.

 

[Pacfanweb]

Yes, it's a dumb idea. Sorry, but you asked.

There's always a payoff. The extra strain on the charging system would in turn take power away from the engine...just like the compressor does now. And at least the A/C system is designed to cut out on sudden or hard acceleration and other critical times you need the power.

 

[Raduque]

Originally posted by: Pacfanweb
Yes, it's a dumb idea. Sorry, but you asked.

That I did.

There's always a payoff. The extra strain on the charging system would in turn take power away from the engine...just like the compressor does now. And at least the A/C system is designed to cut out on sudden or hard acceleration and other critical times you need the power.

I'd actually think an alt, even a high output one, would be easier to drive than a compressor. After all, compression is harder than just spinning something. Besides, like cprince said, it would be useful for running the AC when the engine is off - something I was also thinking it could be useful for.

Originally posted by: LTC8K6
The A/C compressor doesn't use enough extra fuel to worry about, imo.

Neither does the rad fan, imo.

Actually, I have seen a noticeable decrease in gas mileage when running the AC. A tank of gas normally lasts me about 250-260 miles (about 12-12.5 mpg). When running the AC, the tank will only last about 215 miles.

And about the rad fan? Think about it for a second - when the engine is hot (which happens fast, and stays that high during a South Texas summer) and the fan clutch is engaged, it's driving a large, heavy plastic fan at between 800rpm to 3000 rpm (if the fan runs the same speed as the engine. It might run faster, it might run slower). That fan is pulling air through a tightly packed radiator and then chopping it up. There's no way that's has a minor effect on gas mileage.

 

[DivideBYZero]

Originally posted by: Raduque
And about the rad fan? Think about it for a second - when the engine is hot (which happens fast, and stays that high during a South Texas summer) and the fan clutch is engaged, it's driving a large, heavy plastic fan at between 800rpm to 3000 rpm (if the fan runs the same speed as the engine. It might run faster, it might run slower). That fan is pulling air through a tightly packed radiator and then chopping it up. There's no way that's has a minor effect on gas mileage.

You have the same issue as the A/C compressor. Energy is not free just because it comes from the battery or Alternator rather than driven by a belt.

Many cars non-electric fans have a viscous coupling to drive them. When cool it's not driving the fan, but when it's hot it's driving 100%, but if you use an electric fan system then the thermostat will start the e-fan up at the same time (theoretically) as the viscous coupling. You will still need the same amount of cooling effort and therefore use roughly the same amount of engine-provided power and, ergo, gas.

 

[mwmorph]

Originally posted by: Raduque

There's always a payoff. The extra strain on the charging system would in turn take power away from the engine...just like the compressor does now. And at least the A/C system is designed to cut out on sudden or hard acceleration and other critical times you need the power.

I'd actually think an alt, even a high output one, would be easier to drive than a compressor. After all, compression is harder than just spinning something. Besides, like cprince said, it would be useful for running the AC when the engine is off - something I was also thinking it could be useful for.

You do understand you're STILL driving the compressor, using the same amount of energy, jsut using a different form of energy to do so?

To put it in simplest terms, say your AC compressor uses 5hp
Say your alternator is 90% efficient and the motor you're using to spin the compressor is 80% (there are always losses when transferring stored to kenetic and different forms)

Now lets do some math
Amps from hp (hp * constant value) / (volts * efficiency)
(5 * 746) / (12 * .8) = 388.541667
The motor would use ~390amps@12volts
Now hp from amps (amps * volts * efficiency for alternator) / (constant value)
(390 * 12 * .9) / 746 = 5.6461126
So in total, your alternator would have to draw more power to power the AC compressor compared to it being directly hooked up to the motor.

I actually have no idea where you think suddenly it would take less energy ot add more inefficient phases and that suddenly, magically the engine wouldn't have to make energy so your truck could sue energy.

It all comes form somewhere, you don't ever get a free lunch in physics. It would take more energy.

 

[nakedfrog]

I don't know about you, but I only see about a 1mpg drop when running the AC full time.

 

[Raduque]

What is this "Constant Value"? This is why I hate math, things just seem to come out of nowhere.

Also, things don't always happen the way theoretical maths say they should.

 

[exdeath]

Higher current strain on the alternator = stronger magnetic fields and high inductive loads = resistance to turning on par with a compressor.

Think "magnetic friction" or "electromagnetic drag" where the more charging current being drawn, the harder it is to spin the alternator.

Not only is it harder to spin, the ECU will raise idle speed to meet accessory current demands.

Energy isn't free. Thermodynamics guarantees you can't even break even.

Roll down your windows and don't be a poosy if you don't want to use the A/C. It's much hotter in Iraq right now... Cruise with the windows down and enjoy the fresh air.

 

[exdeath]

Actually if you hook another motor up to the alternator, you relieve the engine of drag, then you can just connect a cable from the alternator to the motor...

Or use the Zohar modifier pseudo-perpetual infinite energy engine

"During the connection tests of Zohar with the newly completed Deus, the engine started to examine infinite potential phenomena... Requiring energy, the engine connected this dimension to the higher dimensional space. As a result, that reactor 'merged' or 'synchronized' with the wave existence in that higher dimension..."

 

[LTC8K6]

If it were more efficient to have an electric A/C compressor, we'd all have them already.

If your compressor is really killing your FE, then something is wrong. Perhaps it needs more lubricant? Perhaps it is running all the time or cycling too much due to low refrigerant level?

It's not like the compressor runs that much anyway. Once the cabin is cooled down, the compressor is off much of the time when you use MAX or recirc.

 

[mwmorph]

Originally posted by: Raduque
What is this "Constant Value"? This is why I hate math, things just seem to come out of nowhere.

Also, things don't always happen the way theoretical maths say they should.

You're kidding right? I don't want to be insulting but do you understand how chemistry works?

You don't get free energy, the enrgy needed to do something only increases as additional phases are added since the conversion cannot be 100% efficent, much less 110%.

What you're thinking of is that essentially, the Alternator can function as a perpetual motion machine, creating more energy than it uses.

 

[Raduque]

I just think it'll be easier for the engine to make electrical energy through the alt then it is for it to use mechanical energy to drive the compressor directly.

You still didn't explain that "Constant Value" thing. And no, i don't care how chemistry works.

 

[mwmorph]

Originally posted by: Raduque
I just think it'll be easier for the engine to make electrical energy through the alt then it is for it to use mechanical energy to drive the compressor directly.

You still didn't explain that "Constant Value" thing. And no, i don't care how chemistry works.

If you don't care then you can't even pose the question. You asked for answers, the answer involves very basic chemistry and physics.

Energy is the same whether it's mechanical or electrical, you're still making X watts/jouses/amps/hp/btu/etc

It's all the same thing and every unit of energy can be converted into another unit of energy.

The constant value is the conversion number to make the equation work when converting between amps and hp just like in Ideal gas laws.
Ex. PV=nRT
P=pressure
V=volume
n=number of moles
R= constant value
T= temperature

Try to remember back to high school, remember when you had to calcualte all these things in chem and physics? Same thing.

Your engine needs to convert mechanical energy from a chemical reaction energy
That mechanical energy needs to be converted into electrical energy
Your energy needs to be converted into mechanical energy
the mechanical energy still has to turn the compressor

Every step loses efficiency and in the end all you get is more fuel consumption.

Let me put it this way:

If your engine was better at converting to electrical energy over mechanical energy, you wouldn't use mechanical energy to drive the wheels, you'd use electrical motors to drive the wheels with the engine charging the battery.

BTW, you never make energy, energy and matter cannot be either created or destroyed, just converted into different forms.

 

[Raduque]

Originally posted by: mwmorph

Try to remember back to high school, remember when you had to calcualte all these things in chem and physics? Same thing.

Thanks for explaining it in simple terms. I never paid attention in chem or physics because it was all boring as shit. I was busy working up lists of parts to restore my Oldsmobile with instead.

So, this was a stupid idea. On to the next: E-fans!