Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Can you make the drive shaft the alternator?

  1. Aug 26, 2015 #1
    I recently read a post on "How much energy does an alternator use?". So I read some more online articles and found that some people have thought of running their alternator on their drive shaft. This doesn't seem more economical to me, since you will still loose engine power .
    If the alternator creates engine drag and ultimately makes the car less efficient, how can you go around this without doing away with your alternator?
    My thoughts:
    Instead of running your alternator off the drive shaft with a belt, why don't you make the drive shaft the alternator? You could assemble the rotor around the drive shaft and fix the stator to the chassis.
    Does this idea sound plausible? Or would it make the drive shaft too heavy and ultimately make the engine even lesser efficient?
     
  2. jcsd
  3. Aug 26, 2015 #2

    Ranger Mike

    User Avatar
    Science Advisor
    Gold Member

    we run a straight battery on the race car to add a few horsepower. dyno tests say you can save 7 to 15 hp. we used to run a by pass switch on the drag car to gain a few hp. the drive shaft thing is as long as you are NEVER stuck in bumper to bumper trafic for long periods of time. if you are , you are toast. todays cars use 12 volt electric fans to cool the radiator in stop and go traffic..once you are crusing at 45 mph and up the radiator can cool off by itself..but ifin you are idling i n traffic for a long time and the alternator is not charging becusae the drive shaft is not rotating...you run down the battery..not good.
     
  4. Aug 26, 2015 #3

    Merlin3189

    User Avatar
    Gold Member

    Can I just ask for clarification: are we talking about the drive shaft(s) to the wheels, or the crankshaft?

    Because I have come across a dynamo built directly onto the end of a crankshaft, I just assumed the question referred to the crankshaft in the engine, but Mike's point obviously refers to what is usually called a drive shaft, either between the gearbox and the differential or between the diff and the wheels (usually called half shafts.)

    On my car (and I think most small modern cars) it would be difficult to use the drive shaft due to the integration of clutch, gearbox and differential. The first bit you would see as a drive shaft is the floating half shafts. Because these are floating, it would be difficult to add an alternator here.

    Obviously Mike's point is valid for these final drive shafts, but it would not be a problem for an alternator directly linked to the engine on the end of the crankshaft. Though if you are like me (and many European drivers) and stop the engine whenever I'm stationary for more than 10 seconds, it doesn't matter where you put the alternator!
     
  5. Aug 26, 2015 #4

    billy_joule

    User Avatar
    Science Advisor

    I'm not sure what problems that solves? it certainly creates some.
    In addition to the problems already mentioned, the crank has a smaller rpm range so the alternator spends more of its time closer to it's peak efficiency rpm. Engine speed (crankshaft) is less variable than car speed (driveshaft).
     
  6. Aug 26, 2015 #5
    https://en.wikipedia.org/wiki/Kinetic_energy_recovery_system, check this out
     
  7. Aug 26, 2015 #6

    OldEngr63

    User Avatar
    Gold Member

    The problems associated with driving the alternator off the drive line (between transmission and wheels) have been pointed out above.

    If you drive the alternator off the crank shaft -- guess what -- that is exactly what is being done with the belt drive used in most cases now. You could replace the belt drive with a gear train, or you could even design the alternator to be mounted on the front pulley (in line with the fan drive) and thus be direct coupled to the crank shaft. That would only be a more expensive, more complicated way to do what is now done with the belt drive.
     
  8. Aug 26, 2015 #7

    Baluncore

    User Avatar
    Science Advisor

    Rubber drive belts are inherently inefficient. That is especially true of V-belts.

    The advantage of a belt is that the alternator can then spin faster than the engine. That reduces the weight of the alternator. The clearance between poles in the alternator are smaller than the vibration encountered by the harmonic balancer on the front of the crankshaft. The rubber belt isolates that crankshaft vibration from the alternator.
     
  9. Aug 26, 2015 #8

    OldEngr63

    User Avatar
    Gold Member

    The speed increase could be easily accomplished by a reverted gear train coming directly off the front of the crank shaft (if we really wanted to so something like this).

    Baluncore, you speak of vibration at the harmonic balancer. Are you thinking about torsional or lateral vibration as being a problem here? (They are both there, but which one do you think would be the bigger problem with this?)
     
  10. Aug 26, 2015 #9

    Baluncore

    User Avatar
    Science Advisor

    I am considering only direct drive as there are an infinite number of work-arounds.

    I am considering the vector sum of the axial mis-alignment and vibration. Torsional problems might require the alternator armature be part of the harmonic balancer mass.
     
  11. Aug 27, 2015 #10
    The half-drive shafts.

    Honda has an "alternator" built into the front of some engines.
    https://en.wikipedia.org/wiki/Integrated_Motor_Assist

    The only reason I thought of this idea was for Electric Vehicles. I know that they use regenerative braking to recharge the batteries, but for most people the charging time is still a problem. EVs don't use alternators because the energy gained is less than that used to turn the alternator.
    By making the drive shaft (half shaft in the Tesla Model S) the magnetic rotor and putting a stator around it, I thought you could get some extra power back to the batteries. And I am aware that this would only work when the car is moving, but that's fine with me.
    However, if the power gained is less than the power used, this idea is not feasible.
    Does this sound like it could work?
     
  12. Aug 27, 2015 #11

    billy_joule

    User Avatar
    Science Advisor

    No. What you propose is a perpetual motion machine. discussions on pmm's are banned here so tread carefully :-)
     
  13. Aug 27, 2015 #12
    Not exactly sure what you mean by inherently inefficient.
    Multigroove belts act similar to flat belts which can have efficiencies up to 98%. Also known as serpentine belts, they can bend backwards and can drive mutliple pulleys, rather than having multiple v-belts for each pully drive.
    Thus the engine has less dynamic mass to accelerate/decelerate than that associated with chains or gears or multiple pulleys and belts, and with a corresponding energy saving that goes along with less dynamic mass.

    Alignement is not as critical for belts as with other types of drives.
     
  14. Aug 27, 2015 #13

    Ranger Mike

    User Avatar
    Science Advisor
    Gold Member

    ifin these belts were no good they would not run um to drive camshafts in NASCAR vs. gears or the old roller chain rig. gilmer belt drive cuts harmonics, takes minimum horsepower maximum durability.
     
  15. Aug 27, 2015 #14

    Baluncore

    User Avatar
    Science Advisor

    Bending rubber wastes energy as the temperature cyclically falls in tension then rises in compression, which results in thermal losses. Thinner belts, with maybe Kevlar tension members, can be more efficient because there is less elastic volume change.

    Ribbed or multi-groove belts are more efficient than V-belts because they are thinner. Belts without steps can be used where some slip is acceptable. But slip always occurs in belts without steps. When slip is significant, greater friction losses results in higher belt temperatures and a shorter life.

    Light weight stepped belts can replace timing chains and gear trains. But belt lifetime is poor, so stepped timing belts need regular replacement. NASCAR can afford to do that. It is often the failure of a timing belt that destroys a poorly maintained engines due to the resulting piston–valve interference.

    But this topic is titled “Can you make the drive shaft the alternator?”.
    I fail to see how a rubber belt can be used in that direct drive application.
     
  16. Aug 27, 2015 #15
    Exactly in agreement with that. The mechanical obstacles could probably be overcome, but solutions would be not that simple.
    Driveshaft is subjected to the elements - water, grit, mud - detrimental affects to ovevercome for the alternator itself, and any power transmission from the driveshaft
    Floating of the driveshaft has already been mentioned.
     
  17. Aug 27, 2015 #16

    OCR

    User Avatar

    Well, I'm not seeing that... unless you have a better source...?
    And, for you younger guys (lol) that don't know what a Gilmer Belt is...



    Dragster_Kompressor_2005.jpg



    If you really wanted to, I suppose you could... It would basically be the opposite of this...

    http://www.frenelsa.es/en/producto-aplicaciones/vi-gama-semirremolque.asp

    http://www.frenelsa.es/en/producto-aplicaciones/vehiculo-industrial.asp
     
    Last edited: Aug 27, 2015
  18. Aug 27, 2015 #17

    Baluncore

    User Avatar
    Science Advisor

    Spur and helical gear efficiency is usually quoted as being between 98% and 99%. That is better than belts because there is rolling contact between gear faces.
    http://www.roymech.co.uk/Useful_Tables/Drive/Gear_Efficiency.html

    Roller chain on sprockets is also more efficient than belts. Roller chain friction is at the roller pins between the links where the chain direction changes at the sprocket contact. 98% efficiency is normally assumed. I have never seen an efficient bicycle with a rubber belt in place of the usual chain.

    Efficiency is important. Rubber belts are more sensitive to heat than are gears. Belts waste significantly more energy as heat than do gears, yet belts are less able to handle the temperature difference needed for cooling. A chain wastes energy only when the chain is under tension and changing direction. A belt is inefficient no matter how much energy is being transferred because the rubber still flexes.

    A belt is used because it is flexible, light-weight and cheap. It is not as efficient as other mechanical solutions available.
     
  19. Aug 28, 2015 #18
    Thank you for getting us back on track.
    I agree. Originally I asked if I could assemble the rotor around the drive shaft (so that the drive shaft is the "iron core") and put a stator around that assembly. No belt or gears would be used!!

    Thank you for a straight answer. ;-)
     
  20. Aug 28, 2015 #19

    Ranger Mike

    User Avatar
    Science Advisor
    Gold Member

    i looked into this a while back.
    http://www.experimentalaircraft.info/homebuilt-aircraft/electric-aircraft-engines.php
    i was going to fabricate a generator for the formula car on the half shaft hub where it connected to the CV joint. it was based on an ultra light aircraft power generator.
    this is the area you need to explore.

    as a final note omn the off track subject - i did a good post on chain vs belt drive in this forum ..
    Sep 13, 2013 #1
    pnorm91
    avatar_m.png 30
    mechanical design -- cam timing chain versus gear-driven
     
  21. Aug 28, 2015 #20

    OldEngr63

    User Avatar
    Gold Member

    The idea that there is rolling contact between the teeth in a gear pair is a popular misconceptions. As a tooth first begins to engage, the engagement is at the tip. The point of contact moves inward with a sliding motion; this is not rolling. Engagement ends when the tip of the other tooth breaks contact. Look up "angle of approach" and "angle of recess" for more information.

    The idea that there is rolling contact probably comes from the fact that, with conjugate gear teeth, the relative motions of the two gears is the same as would happen if they were replaced by two rolling cylinders with their respective pitch diameters.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Can you make the drive shaft the alternator?
Loading...