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Question About Series-Hybrid Drivetrain Alternator ?

  1. Feb 25, 2011 #1
    My friend said he read someone claiming on his blog that in a series hybrid drivetrain consisting of a small engine and an alternator connected by a shaft & belt, the alternator acts as a motor at all times except the power stroke ? I wanted to know if anyone has experimented with this and observed any such phenomena.

    The point was raised after our drivetrain was found to be quite inefficient and wasting energy. We run a small 35cc engine to run an alternator (which is basically a modified dc brushless hub motor) that charges the supercapacitor and (reportedly) runs the motors in the wheels too. We see a lot of energy is going into waste and one of the points listed above might be the reason. Comments please !
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  3. Feb 25, 2011 #2

    Ranger Mike

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    my gasoline two cycle golf cart uses a generator that double s as a starter motor then works as a generator to charge the battery.
    dont think an alternator could do this job
  4. Feb 25, 2011 #3


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    You haven't really posed a question worth answering... A "true" series hybrid would have 3 distinct system components- an IC engine, an alternator/generator, and a motor driving the wheels. An alternator cannot act as a motor in the context you've described.
  5. Feb 26, 2011 #4
    What I precisely mean to say is that is it possible that alternator might be eating off the energy from supercapacitor ?
  6. Feb 27, 2011 #5
    I suppose it could in principle, if there wasn't a big enough flywheel on the engine. But it would require the direction of current through the alternator to reverse, and I doubt that would be possible. Maybe check if it draws any current while it's not turning - that would tell you an extreme worst case power loss.
  7. Feb 27, 2011 #6
    OK, that makes more sense of it. You did say small engine series hybrid. This automatically implies there is an energy storage medium somewhere, such as a battery or capacitor.

    The sequence of devices is engine, alternator, rectifier to motor to wheels. The battery or supercap is across the motor and can be switched in or out of the circuit.

    There's really no reason to drive power back through the alternator using an active bride so that it acts as a motor. It would only go to keep the engine spinning unless it includes a flywheel? No car currently manufactured uses one of these that I know of. Can you post a link to the blog?
  8. Feb 27, 2011 #7
    Can you tell us what kind of alternator/generator is being used as that can make all the difference. I believe it could happen given there is not enough rotational inertia like mentioned above.
  9. Feb 28, 2011 #8
    I should add that even if it does occur, the power loss should be tiny because it would come from cycling a small amount of power back and forth between the supercap and the shaft. That would just skim the top off the supercap, which I guess is the most efficiently available energy from it.

    If it wasn't for the inefficiencies of the supercap and alternator it would be harmless and equivalent to a flywheel in a normal engine - cycling power back and forth between the flywheel and shaft, which is, of course very efficient.

    Also, if it's belt driven, and this is happening, then the tension in the belt will be reversing on every power stroke! That could be bad news for any spring-loaded tensioner.
  10. Mar 2, 2011 #9
    Using a divining rod I keep for special occasions, I see that when a generator is putting out, say 94 volts into a load of 94 volts, it is moving no net energy. However, every time an armature pole in the generator passes a stator pole, currents are moving in and out of the windings. So the generator is alternately pushing forward then pushed back by the load, each pass. It's no big deal, but could be a source of amusement to someone trying to impress you with their superior understanding. This would have a tiny effect, increasing then decreasing the angular velocity of the engine insignificantly, many times a minute.

    An alternator followed by a rectifier wouldn't do this.

    However, what if you tried to throttle down the engine? A generator would act as a motor to try to keep the engine at the same angular speed. A blocking diode would stop this action. Dropping the voltage on the field windings would do this as well. I'm sure Toyota and the rest are already aware of this (understatement).
    Last edited: Mar 2, 2011
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