In a message dated 97-08-26 15:05:25 EDT, CREICHLE@nsc.msmail.miami.edu
(REICHLE, CHRISTOPHER) writes:
> The idea is that the drain on the engine is relatively constant. The
> electric compressor pulls a lot of current but only for a short period.
The
> battery acts like a buffer and the alternator drains less HP at any one
> time. In other words, with a small engine the belt driven compressor
turning
> on can momentarily make the car dangerously underpowered where an electric
> compressor would just drain the battery which would be recharged by the
> alternator over a longer period of time. Electric units can be very
> efficient. I assume that you get your "twice the drain of a mechanical
unit"
> from the loss of energy in producing electricity at the alternator which
is
> always spinning anyway though not with a load. Hondas are usually
considered
> to be very efficient cars. If you ever purchased a car with electric ac
you
> would see the benefit of driveability with a small hp engine, the better
gas
> mileage compared to a belt driven ac unit and unfortunately the higher
cost
> of the system which is why you don't see it on cheap low hp "American"
cars.
> I quote American because all of the small low hp American company
econoboxes
> that come to mind are made by foreign companies. Electric compressors have
a
> more stable power source that does not vary depending on driving
> conditions. This allows a more efficient design in the electric unit than
> the belt driven so I would argue your claim of twice the power needed.
Chris:
In 1968, I bought a Chevy Nova with a 325 HP 327 V8. When the A/C kicked in,
it was quite noticable. In 1978, I bought a 90 HP Toyota. I had to really pay
attention to notice when the A/C kicked in, under most driving conditions (it
had a low vacuum switch to disengage the compressor clutch when under load,
where the power drain would be most noticable). I would suggest that the
major portion of the increased drivability you experience with the Honda is
due to improved A/C design in general, more than to a difference in drive
power.
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.
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.
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).
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).
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 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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