I'm not sure if this belongs here or in the fabrication forum, but I got a question for all you out there that are smarter than me.

You see people at the track (generally not with Hondas) running electronically powered oil pumps, and water pumps, and ignitions all the time.

Some people got the engine running JUST the crankshaft and everything else is powered off batteries. No alternator, and the oil pump (dry sump or not), water pump, and coil are all electronically charged.

Why can't you run the AC compressor off DC/AC power? Just the pure torque it needs to spin at a comparable speed? The resistance? What?

I mean OBVIOUSLY this has been thought of before or else if it was such a fantastic idea they woulda been using it on cars for years now to retain torque, especially low torque, low displacement motors.

So anybody wanna throw the answer to the "why" at me here??

 

I don't know the actual numbers but it must take 5 or twenty hp the drive the ac.That's a pretty big electric motor.It would take a larger alt. to keep up with it.Also a larger battery to take the surge.It might work in a new hybrid but I don't see fitting it in a conventional car.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by NJIN BUILDR &raquo;</TD></TR><TR><TD CLASS="quote">I don't know the actual numbers but it must take 5 or twenty hp the drive the ac.That's a pretty big electric motor.It would take a larger alt. to keep up with it.Also a larger battery to take the surge.It might work in a new hybrid but I don't see fitting it in a conventional car.</TD></TR></TABLE>

Yeah, that's the only reason I could think of - does anybody know if that's the reason for sure?

 

A/C units in a house use quite a bit of power, but an A/C unit for a car is much smaller. Sooner or later I can see all of this turning into electric drive, as power steering racks now use electric motors to provide assist, and traditional hydraulic brake boosters are now phased out by the electric motor as well.

In all reality, this is the best idea, because its only on when you need it, and produces no drag while its not, except for weight, and if you've all noticed, starters get smaller everyday.

Just for reference, the size of a power steering assist motor is about half of a starter motor, and can provide up to 1,500PSI of pressure (typical max output of a steering pump) on demand.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by slowcivic2k &raquo;</TD></TR><TR><TD CLASS="quote">A/C units in a house use quite a bit of power, but an A/C unit for a car is much smaller. Sooner or later I can see all of this turning into electric drive, as power steering racks now use electric motors to provide assist, and traditional hydraulic brake boosters are now phased out by the electric motor as well.

In all reality, this is the best idea, because its only on when you need it, and produces no drag while its not, except for weight, and if you've all noticed, starters get smaller everyday.

Just for reference, the size of a power steering assist motor is about half of a starter motor, and can provide up to 1,500PSI of pressure (typical max output of a steering pump) on demand.</TD></TR></TABLE>

Ah, thank you for clearing that up.

Yeah, that was pretty much what I thought, the sheer torque needed to turn the pulley was too much for an electrical motor.

Though I can def. see it coming eventually - somebody'll come up with it.

 

By the time you drive an alternator then a motor, the combined efficiency is only about 50% or 60% (but getting better all the time). So if a small motor takes about 1hp then the engine uses mabe 2hp to drive it. That's not a very bit hit.

Comparing the size of a car vs. a house isn't fair. A house is insulated far better than a car, and the % of windows (solar heat) is far greater. People expect a house to take an hour or more to cool down, but they don't tolerate that in a car. So the cooling capacity of a car AC is bigger than you think.

The AC compressor takes more power, maybe 5 hp. So that extra load on the alternator would take maybe 10 hp to drive it. Besides that's a big increase in the size/weight of the alternator.

When a belt-driven AC isn't running, the compressor clutch is disengaged. Take off the belt & spin it by hand. There really is very little drag when it's frewheeling like that.

 

Thew cost is much less, because the entire AC system can be integrated into the car itself, resulting in less parts, no belts, and so forth. Electric is the best way to go, because even if there is little drag when it is not on, its still drag. Little things like making master cylinders out of magnesium instead of cast iron are to save weight, and there are many more examples than this.

Alternators produce a load all the time, and most of the time it is never used, which is why the regulators in most new cars scale to 13.0 volts even with the headlights on, to keep load down. Less cost and better efficiency go hand in had with auto makers.

If what you say is true regarding efficiency, then why are power steering, brake boosters, water pumps and even some oil pumps going electric?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by slowcivic2k &raquo;</TD></TR><TR><TD CLASS="quote">Alternators produce a load all the time, and most of the time it is never used, which is why the regulators in most new cars scale to 13.0 volts even with the headlights on, to keep load down.</TD></TR></TABLE>Voltage isn't load. Alternators produce power. The load is drawn by all the electric devices driven by the alternator. But an alternator with the extra load-carrying capacity to drive an electric AC would have more drag. I don't know how that extra drag compares to a disengaged clutch of a belt-driven compressor.

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by slowcivic2k &raquo;</TD></TR><TR><TD CLASS="quote">If what you say is true regarding efficiency, then why are power steering, brake boosters, water pumps and even some oil pumps going electric?
</TD></TR></TABLE>
It may be because of flexibility in locating the components. Belt-driven components have to be located around the crank pulley. The extra drag (from low efficiency) is acceptable up to a certain level. Bigger loads are less acceptable.

It's also because of speed. PS is needed during parking-lot maneuvers when the engine is barely above idle. You don't hardly need any steering assist at 20mph or higher. Water pumps can be designed with substantially better pumping efficiency if they would only run at one constant rpm. Like radiator fans...

The 48v electric systems that are coming, someday, (maybe) will have advantages in efficiency, and electric components may be showing up in anticipation of that day.

I think you're right, that we'll see electric AC. But I don't think it's because of better drive efficiency. It may be done for other reasons, and it'll be done in spite of lower efficiency. Maybe it'll be done to allow a compressor that only runs at one speed.