Just a few things and then I'll stop.
----
I agree with you absolutely that any system designed to operate at one speed
can be more efficient than one operating at varying speed. Under general
driving conditions, though, how much does your engine speed vary - probably
no more than 1000 RPM max (2000 - 3000), in a car with a 5-speed. I wouldn't
think a small speed variation that small would drastically alter the
efficiency, given that the manufacturer would most likely design the unit to
be most effiecient in the middle of that range.
*Small 4 cyl cars are driven over a wider rpm range than 1000 and you are
not taking in to account the fact that the compressor must work at idle
speed when it is hardest for the ac to cool the car. So the range is really
700 rpm - about 3500. I would guess it would not work properly if it worked
best at 2000 rpm. At idle it would crap out. So I think the design would
have to be less efficeint to be able to work at all speeds. I could be worng
I also agree with you that an electric motor CAN draw power to overcome
starting torque from the battery, instead of the engine, but WILL only if
the
current drain exceeds the alternator capacity. Let's compare the two A/C
types under starting conditions. When the mechanical drive is engaged, there
is a heavy load placed on the engine till the compressor reaches full speed.
At that time, the load drops off to steady state. The same is true of the
electrical motor driving an electric A/C unit. The initial surge is drawn
from the battery, til the motor reaches steady state. What happens then,
though? The alternator now has to provide steady state current to the A/C
motor, plus it now has to replace the energy drained from the battery during
compressor spin-up, which places an extra load on the engine to drive the
alternator. So we trade a larger peak load over a short period of time with
a
smaller peak load over a longer period of time (this is just a re-phrase of
what you said). The later will be less noticible to the driver then the
former, but since the former is virtually un-noticable anyway, what
difference does it make. As for fuel economy, I can't see much difference.
*Running at "steady state", the compressor still must work hard. We are not
talking overcoming inertia here upon startup, it's compressing. The
lightening of the load that you feel is probably the compressors electric
clutch disengaging the compressor after it completes a cycle. The compressor
does not stay on when you have the ac on, it cycles off and on. How many
amps do you think the motor could pull? Compared to defrosters, seat
heaters, headlights aux lights, seat motors I don't think it's that much.
Most cars have an alternator rated at 65 or 80 do you really think an ac
motor would overload one?. I think you made a very good point which is that
the electric ac does not turn off every time you hit the gas. Having driven
my girlfriend's Hundia (sp? what a piece of crap that car was) with the ac I
can tell you it was a pig with the ac on and when you did floor it all of a
sudden the ac would disengage giving a boost of power and the air would soon
begin to blow warm.
As for the statement I made about "twice the power," I will back off from
that a little - that was just a WAG anyway. Before I decide how much I'm
willing to back off, let's do another comparison. In both systems, the
starting and ending points are the same. They both start with an engine and
end with a compressor. I have to assume that both compressors are of the
same
efficiency. If not, there would be nothing to stop the designers from using
the electric driven compressor with a belt drive (at least nothing that I
know of).
* Cost...
Between the engine and the compressor in a belt driven system is a pair of
pulleys and a belt. Assuming a 1% loss in the belt drive, due to friction,
etc, a one horsepower compressor will require 1.01 hp from the engine. In an
electric A/C system, between the engine and the compressor there is a pair
of
pulleys, a belt, an alternator, and an electric motor. Assuming the same 1%
loss in the belt, and a 5% loss each in the alternator and the motor, there
will be a total of 11% loss in this system. Therefore, for a one horsepower
compressor, it takes 1.11 hp from the engine. (I am pulling these numbers
from the A.I.R. and biasing them towards your argument. I believe the real
numbers would be much larger).
* Well the belt driven compressor will cause some drag on the engine even
when it is not engaged. And the alternator will too. It has a mass which is
always spinning and causes a drag. It will always produce electricity so why
not use it. True it will cause more drag on the engine when you pull more
current but I can tell you that in a honda, turning on the headlights causes
a noticeable momentary dip in rpm and brightness of the guage lights etc.
which turning on the ac does not produce.
In summary, I agree that an electric system would exhibit less "loading" on
starting than a mechanical system, but I doubt the difference would be that
great, particularly since loading is nearly undetectable now. After the
electrically driven compressor is up to speed, there will be more power draw
from the engine. I don't agree that there will be less gas consumption - in
fact, I believe the opposite. I also believe that there are so many other
factors affecting gas milage that the difference attributable soley to
differences between one type of system or the other would be difficult for
an
owner to detect in a modern car.
*I don't know about you but I think it's kinda funny that noone has jumped
in to correct any of our hypothetical guesswork in deriving which is more
efficient etc. I'm basing my arguement on what I do know about it and my
experience with the systems. I don't know for a fact which is more efficent
but it seems to me, for the reasons I've stated, that the electric ac is at
least as efficient if not more than the belt driven. It's been fun tossing
it back and forth though.
I think the decision to use an electric A/C unit was made based on
packaging,
rather than any derived benefit. Front wheel drive, coupled with compact
body
shells, doesn't leave a lot of room under the hood.
Of course, I am an electrical, not a mechanical, engineer, so maybe I am
missing something. This is the first I've heard of electrically driven
compressors in a car (been stuck in the past too long with LBCs!), but it
sounds like a neat idea. I can see where this could be very handy in car
design. I'm curious as to why the street rod fraternitiy hasn't jumped on
this. The hot ticket in street rods today is to hide every thing - door
handles, license plates, taillights, gearshift levers, gauges, etc, - so it
seems only a natural that an electric driven compressor, hidden out of site,
would be the next "hot button."
Dan Masters,
Alcoa, TN
'71 TR6---------3000mile/year driver, fully restored
'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
http://www.sky.net/~boballen/mg/Masters/
'74 MGBGT---3000mile/year driver, original condition
'68 MGBGT---organ donor for the '74
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