[ITGuy]

The alternator spins all the time when the engine is running ... whether at 0% duty cycle or 100% duty cycle ... the charge controller determines whether charging happens or not. An alternator spinning at less 100% duty cycle is wasting fossil fuel energy. This explains why efficient HHO boosters installed on my car, two family members and an office mate noticed extra power.

Originally Posted by Baggage Handler #2

No. Or at least, "It Depends."
Link: Alternator Field Coil - MiniMopar Resources

The alternator field coil is the magnetic coil inside the alternator that controls its output current. The more current passed through the field coil, the more current the alternator produces. Since the coil is resistive, this appears as a voltage across the coil terminals. The current through the coil is regulated by the power module, which is controlled by the logic module. The logic module uses its voltage sense input to monitor the charging system voltage and adjust it accordingly. One side of the coil is connected to the ignition key switch "on" terminal (for 12V) and the other side is switched to ground by the power module's switching control circuit. The logic module cycles this end of the coil on and off at a fixed frequency and varies the amount of time the terminal is grounded, which is called the duty cycle. At 0% duty cycle, the terminal is always open and no current is produced by the alternator. At 100% duty cycle, the terminal is always grounded and the alternator produces maximum output current. The duty cycle is varied by the logic module, depending on the electrical demand placed on the charging system, and varies the target charging voltage, based on battery temperature. The greater the demand (more accessories on), the greater the duty cycle.

So, in a nutshell, if you don't need the HVAC fan on, turn it off, and you will burn slightly less fuel.
This is also true for the HHO thingy. It will take extra fuel to power that thing.

Physics is not only a harsh "mistriss," it is supremely indifferent to our wishful thoughts.