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Engine torque: Can an alternator increase ICE torque?

  1. Oct 2, 2015 #1
    Hi there.
    What would happen if, while generating electricity with a fixed inlet power from the ICE, the alternator suddenly increase the out put demand by increasing the voltage?
    Does the engine would augment the torque while reducing RPMs?

    consider a self excited alternator, the voltage is for the electromagnets in the alternator.
  2. jcsd
  3. Oct 2, 2015 #2


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    Alternators are a parasitic load on the engine. They don't generate torque, they convert torque and rotational speed into electrical power.
  4. Oct 2, 2015 #3
    Due to energy balance, i guess that the output (generated) power of the alternator should remain the same, but What would happen to the engine With a fixed power if the alternator suddenly increase its magnetic field?
  5. Oct 2, 2015 #4


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    I'm not sure what you mean by "suddenly increase its magnetic field," but an alternator's drag scales with the electrical load it is supplying. If it's not supplying any power (like if its unplugged) it has essentially zero drag; conversely if it's supplying its full power capacity drag is increased.
  6. Oct 2, 2015 #5
    the alternator basically is formed by a rotor and a stator, within the rotor there is a winding that wrap around an iron core on the rotor shaft which will produce a magnetic field while current is applied, so, more current equals a bigger field.

    the magnetic field in turn induce voltage into the stator.

    when this inlet current is low, the stator must spin faster to produce the required energy (X kw) but when it augment, less revolutions are required to meet the same energy (X kw).

    if we consider a 100% conversion efficiency, the engine which we are converting energy from, must have an output of X kw, same as the alternator.

    if this engine produce 100% of its torque at 5000 rpm and the alternator meets this requirements by applying a certain current producing X kw, what would happen if the current in the alternator increase?

    the engine have a fixed power, or a constant fuel supply which would not change.

    i believe that the engine would decrease rpm´s and produce more torque in order to be capable of moving the alternator shaft
    what did you think?
  7. Oct 2, 2015 #6
    That is a very astute observation. The Alternator does in fact convert torque to electrical energy. The wild card, at least in automotive use, is the human operator. When the throttle plates are partially open the engine operates at a reduced efficiency. This is the volumetric efficiency expressed as a ratio of actual intake air compared to theoretical perfection.

    If an engine that could produce 200 ftlbs of torque was operating at 20% volumetric efficiency it would make 40 ftlbs of torque. Assuming that the vehicle involved required 40 ftlbs of torque at the input shaft to maintain speed the system would be in equilibrium. If the electrical system suddenly needed more power and the alternator reacted to the draw ( let's pretend a requirement of 5 ftlbs). The system would no longer be in equilibrium and speed would suffer (speed at the shaft is after the engine where alternator is a parasitic loss.) until equilibrium is again achieved. The driver if they are paying attention would simply and very minorly adjust the "gas pedal" to maintain speed.

    This is in reality a 2.5% change in the effective torque required. Barely perceptible to a human foot. Very achievable to a relatively skilled driver. The answer is yes and no Would you notice it? Probably not. Does it change things? most definitely so.
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