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Can anyone explain the theory to me...


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...about alternator belt "sprague clutch" pulleys? What I've read is that they're there to provide an even continuous load to the belt and to remove belt chatter. 

But...

I've been messing about with my oscilloscope (monitoring alternator output) and strobe light (on the belt) to see if I can see any of this wierd phenomenon. But even just after starting up in the cold weather- glow plugs pulling current hard, headlights on and interior blower on, at idle the belt is pulling all smooth. Rev it up a bit; same. Same experiment with no load, same result. Then test again with hot engine just after coming home again - no electrical load or full load, and I still don't see any alternator clutch movement.

To me, the rotating inertia of the engine is way higher (slower) than the alternator, but so long as there's a load on the alternator, there's no scenario I can think of where it will rev faster than the crank. I can force it to happen by revving up the engine then switching off at high revs, where it will overrun, but that is the only case. The crank comes to a standstill a split second before the alternator. And even then it's uneccessary, as the alternator doesn't load the belt much in reverse.

Someone please educate me! @RAB? @Rich? @LR5V? Anyone else...? :) It's been bugging me for a while now 🙄

 

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I don't think I can teach you anything fella!

Only thing I can think of is sudden throttle off situation, the sprague allows the alternator to free spin down rather than an abrupt stop/reduction in speed.

And that could be engineered to allow for:

  • smooth out the voltage rather than spikes
  • stop the bearings getting hammered, rising revs in gear are slower than throttle off
  • reduce impact on drive belt & components

This may also allow a lower (cheaper) spec bearing to be used within the alternator, but that may be a bit of a reach.

Is this on you diesel Lupo? interesting that Lupo Gti has fixed pulley, my Mk4 Gti was also fixed and Mk5 gti has sprague

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Overrunning alternator pulley is very important only for cars with powerfull alternators. I experienced how broken pulley feels (locked) with 140A althernator and 3.3l V6 engine, engine was instantly shaking with high electricity consumption on idle, however there were no difference while driving on highway. All possible electric equpment shall be turned off to prevent shaking on idle.  Because of the small diameter pulley a powerfull alternator can make huge impact on engine crankshaft. The weaker is the alternator, the less impact it can possibly make.

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They want more money from me for things that break that I don't need.

Glow plugs are over rated also.

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I'm also starting to think that @Rich... But it must be there for some reason. The TDI also has a sprague clutch pulley on the alternator. Same as the SDI.

@ObjectiveAway the SDI has only 70A alternator and a big flywheel. Makes no sense to me. So much engine torque, it doesn't affect performance at idle or revving. Fully loaded up or no electrical load...

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Rotational kinetic energy: E rotational=1/2 I ω^2   ω is the angular velocity and I is the moment of inertia around the axis of rotation. The mechanical work applied during rotation is the torque times the rotation angle: W=τθ. The instantaneous power of an angularly accelerating body is the torque times the angular velocity. 

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ok, so we know the engine has a lot more rotating metal compared to an alternator, say 20x as much? Say the alternator spins at 5x the crankshaft RPM? So there is no way the alternator could have an effect on the lump, "instantaneously". Sure the belt might slip, but since the sprague clutch works like a bicycle freewheel- only in one direction- it would only freewheel if the engine stopped faster than the alternator. And that's pretty unlikely, unless you can somehow stall a spinning engine 'dead' from running RPM (2000ish). Then the gearbox or clutch would break. What am I missing?

 

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80-95 Bosch alternator, two screws on the regulator, you know the one.

Best alternator ever. Everyone used them and they never went wrong.

Clearly they had to evolve to make more money.

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11 hours ago, mk2 said:

@ObjectiveAway the SDI has only 70A alternator and a big flywheel. Makes no sense to me. So much engine torque, it doesn't affect performance at idle or revving. Fully loaded up or no electrical load...

I dont see that necessary on lupo as well, you can try to add an extra power consumer (like an empty battery or something) and check again, but Lupo itself has so few electric items that I hardly doubt overruning pulley will do any job.  However VW engineers didnt think so.

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It works in exactly the same way as a freewheel on a bicycle, i.e. no sudden load on the belt or anything else. When engine speed reduces, the alternator slows down more slowly until it matches the engine speed.

The pulley also absorbs crankshaft pulsations.

RAB

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