Warren.Allen@infores.com (Warren Allen) Writes
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Fellow Scions,
The recent cooling fan/generator/alternator thread brought
back a question that I have wondered about for years: Does
increasing electrical load really result in increased load
on a generator/alternator, and therefore in increased gas
mileage? Here goes: A generator consists of an
armature (a metal shaft covered with wires) rotating between
two magnets. There's no physical contact (other than
brushes). How, therefore, can increased load make the
armature harder to rotate?
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Warren,
What you are describing is a shunt regulator and although it would
work it would waste energy and cost more than the system in use.
An anology would be filling buckets with a hose. when a bucket
needs filling you hold the hose over the bucket. When the bucket
doesn't need filling you hold the hose over the ground. With this
technique you use the same amount of water no matter how much water
you really need. Controlling the output of an alternator this way
would use up noticably more gasoline than the method used which
controls the internal parameters of the alternator to control the
output to provide the current neede and no more.
Have you ever made an electric magnet when you were a kid? Surely
you are familiar with the principal. Current flows in a coil
creating a magnetic force with which you can pick up paper clips.
And the more current you put through the coil the stronger the
magnetic field. Well the current that comes out of the alternator
creates a magnetic field which repels the armature (field) and
requires force (torque) from the engine to overcome. Higher values
of current will create higher values of magnetism which require
higher values of torque to overcome. Hence when there is a greater
load on the alternator the alternator requires more horsepower to turn.
Please note that the voltage regulator acts to maintain a constant
voltage out of the alternator over varying load and speed conditions.
The net result is that the alternator/regulator system is a constant
voltage device that delivers the current required by the electrical
system.
No there is no excess bled off to ground.
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Related question: When I have my air conditioning on in the
car, and I turn the temperature dial towards warmer, does
the A/C compressor therefore start working less hard, and my
gas mileage goes up, or does the compressor work just as
hard, but the cool air get mixed with warm uncooled air in
order to warm the air coming out of the vent?
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Ok, here there are two possibilities. On some older cars there is
a thermostat that will cycle the compressor to maintain the desired
temperature. If you turn up the setting on this type of system the
compressor will cycle more and you will save gas.
However, most modern cars use this knob to control reheat. It is the
same control used to control the heat in the heating mode. The air
first passes through the cooling coil (evaporator) and then it passes
through (or is bypassed around) the heater core. So the air is cooled
down first and then reheated to achieve the desired temperature.
Cooling systems on most modern cars cool the air to 40 to 50 degrees F
(5 to 10 degrees C) and if that is too cold you can warm the air back
up to taste. Although it sounds like a waste (it is) there is one
side benefit which probably explains why most manufacturers do it this
way: You get dehumidification with this process. When you cool the
air to 50 degrees you also lower the dew point to 50. And when you
reheat the air the dew point remains at 50 so when you are driving
around in southern Louisiana at night it feels more like Reno.
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Warren Allen
1960 TR3A (And no, it doesn't have A/C. That question
concerns a BAC (Big American Car.)
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I hope this was simple enough. I have been immersed in this stuff
for so long that some of these principals start to seem like they
are intuitively obvious. But they aren't.
One parting thought: The more air you blow through the cooling coil
(on your AC) the more the load on the compressor - because you are
cooling more air after all. So if you lower the fan speed instead of
turning up the heat control you will use less gas (you might save 1-2
miles per gallon)
Dave Massey, on the Digest
(Don't ask me why halogen lights are so much brighter than regular, I
will harangue about that, too)
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