Freewheeling Alternator Pulley
12-15-2008, 08:07 AM
#1
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Freewheeling Alternator Pulley
Can anyone explain to me why my 1.9tdi has an alternator with such a complicated pulley on it . What is the benefit of having a pulley that drives one way and freewheels the other ?
12-15-2008, 09:17 AM
#3
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Well, I was wrong. It's an inertia thing with the alternator on the 1.9 engine.
Here's what I found:
On the early days of their serpentine belt implementation on the AAZ engine (MK3 chassis 1993+) VW used a standard (solid) alternator pulley, meaning that the alternator is directly connected to the crankshaft drive pulley via a serpentine belt.
This engine (and its gasser cousins the ABA and VR6) unfortunately became a bit infamous for wearing out the crankshaft sprocket, key, and keyway. It's generally accepted that a large contributing factor to this problem was that the alternator has a small pulley and a lot of angular momentum; during deceleration it tends to want to overdrive the engine, putting large stresses on the crankshaft drive pulley (and the keyway) therein in a direction opposite to normal. Over time acceleration and deceleration results in the crankshaft sprocket rocking loose... on a diesel the result can be catastrophic. The small pulley on the alternator exacerbates the problem (over previous designs) by increasing the alternator RPM and giving it a mechanical advantage when overdriving the larger crankshaft pulley.
At some point just before they moved to the TDI engine VW switched to a 1-way ratcheting alternator pulley (which they continued with on the TDI engine). This pulley allows the alternator to automatically de-clutch from the drivebelt and freewheel on its own... preventing the alternator from applying reverse torque to the crankshaft pulley when the engine decelerates.
One guy on the vwdiesel net even retrofitted his vanagon(sp?) alternator to relieve the wear and tear on the crank key and serpentine belt:
http://www.vwdiesel.net/phpBB/viewtopic.php?t=15010&highlight=aaz+alternator +pulley<ul><li><a href="http://www.vwdiesel.net/phpBB/viewtopic.php?t=15010&highlight=aaz+alternator +pulley">http://www.vwdiesel.net/phpBB/viewtopic.php?t=15010&highlight=aaz+alternator +pulley</a</li></ul>
On the early days of their serpentine belt implementation on the AAZ engine (MK3 chassis 1993+) VW used a standard (solid) alternator pulley, meaning that the alternator is directly connected to the crankshaft drive pulley via a serpentine belt.
This engine (and its gasser cousins the ABA and VR6) unfortunately became a bit infamous for wearing out the crankshaft sprocket, key, and keyway. It's generally accepted that a large contributing factor to this problem was that the alternator has a small pulley and a lot of angular momentum; during deceleration it tends to want to overdrive the engine, putting large stresses on the crankshaft drive pulley (and the keyway) therein in a direction opposite to normal. Over time acceleration and deceleration results in the crankshaft sprocket rocking loose... on a diesel the result can be catastrophic. The small pulley on the alternator exacerbates the problem (over previous designs) by increasing the alternator RPM and giving it a mechanical advantage when overdriving the larger crankshaft pulley.
At some point just before they moved to the TDI engine VW switched to a 1-way ratcheting alternator pulley (which they continued with on the TDI engine). This pulley allows the alternator to automatically de-clutch from the drivebelt and freewheel on its own... preventing the alternator from applying reverse torque to the crankshaft pulley when the engine decelerates.
One guy on the vwdiesel net even retrofitted his vanagon(sp?) alternator to relieve the wear and tear on the crank key and serpentine belt:
http://www.vwdiesel.net/phpBB/viewtopic.php?t=15010&highlight=aaz+alternator +pulley<ul><li><a href="http://www.vwdiesel.net/phpBB/viewtopic.php?t=15010&highlight=aaz+alternator +pulley">http://www.vwdiesel.net/phpBB/viewtopic.php?t=15010&highlight=aaz+alternator +pulley</a</li></ul>
12-15-2008, 12:48 PM
#5
no idea. Maybe the engine drops revs faster than the internals of the alternator, and ..
they want it to be able to stay spinning? That's the only thing I can think of.. Keeps the alternator running at a higher speed, especially if they overdrive it vs. a petrol engine.
12-15-2008, 02:40 PM
#6
AudiWorld Super User
'Ding!! FTM! LETS alternator overrun to avoid damage to the serp belt and/or crank pulley key
12-16-2008, 07:13 AM
#7
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Thanks for digging up the arcticle .... It has
answered my next question as to what the disadvantages of running a solid pulley instead of the freewheeling type would be ..... possibly disasterous ! Cheers
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19tdi, a6, aaz, advantage, alternator, audi, clutch, free, freewheeling, mechanical, overrunning, pulley, pulleys, vw, wheeling
