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Originally Posted by Moneypit SEL
One question. How will your idea actually save gas? Running the accessories with a DC motor is all well and good, but where does the electricity for the motor come from? Regardless of your power source, the accessories will still require the same energy to turn, whether this comes from the engine directly, or from the engine via electricity generated in the alternator.
I guess that was two questions.
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Exactly, there is no free lunch here. The shaft power required to operate the larger alternator will likely be greater than the power you are saving by removing the other loads. There will be additional energy losses in each device that is added. Converting shaft power to electrical power (alternator) then back to shaft power (DC motor) is going to be less efficient than driving the device directly. There may be other good reasons to do this, but I would not expect any improvement in average mileage.
These types of modifications are useful if you are trying to increase peak horsepower. For example, air cooled Porsches sometimes convert the belt driven cooling fan to electric because it uses significant power at high rpm. This does not help the average mileage, but allows you to get maximum power for short periods of time.