Why the clutched alternator pulley?

[belome]

So I'm reading the thread about alternators. What makes the TDI special that it requires a clutched pulley? Does it have to do wtih the fact that TDI's turn relativley low RPMS?

 

[JB05]

I think the clutched pulley eliminates voltage spikes to a certain degree. It's like riding a bike; when you stop peddling the bike will coast along instead of coming to an abrupt stop. The rotor inside the alternator continues to spin faster than the engine rpm when you let up on the accelerator. At least that is what I observed when I tested mine with the serp. belt removed.

 

[lkchris]

Because of its high compression the diesel stops rotating in an instant when it's shut off. The clutch prevents damage to the belt/pulley/alternator by mitigating this shock.

 

[Mach1]

The alternator is not directly coupled to the engine, which lessens vibration, allows the alt to turn a diferent RPM from the engine, minimizes vibrations..

I want to see the next Gen of pulleys get on the TDI's..a dampened clutch like which is use on the Jeep Liberty by Motori Engine.

The longevity is greatly lengthened.

 

[LurkerMike]

Because of its high compression the diesel stops rotating in an instant when it's shut off. The clutch prevents damage to the belt/pulley/alternator by mitigating this shock.

Yes, as well as every time you lift the throttle on a 5-speed to change gears, the rpms drop, the serpentine belt must pull hard to slow down the big amperage heavy spinning mass TDI alternator.

I suspect it is probably as much or more a factor of how heavy the spinning mass of the alternator is than it is the diesel engine dropping rpms faster.

 

[AndyBees]

Because of its high compression the diesel stops rotating in an instant when it's shut off. The clutch prevents damage to the belt/pulley/alternator by mitigating this shock.

Right on! And, the "big" 120A Alt has far more rotating mass than the 90A and smaller alts.

And, the clutch action is only in one direction!.........slow down!

 

[Vince Waldon]

One clue is *when* VW started using them... in response to AAZ engines having their crank nose's torn off.

The AAZ was the first VW diesel engine with a serpentine belt... and if you look at the diameter of the pulley on the crank sprocket versus the alt pulley you come to realize that the rotating mass of the alternator will have a large leverage advantage... so that when the engine is decelerating and the momentum of the alternator wants to keep it rotating large counterforces are applied to the poor nose of the crank. Many many AAZ engines suffered an early death as the crank nose eventually worn out enough that the crank sprocket wobbled loose and pistons met valves.

VW responded with a beefier crank nose design *and* the clutched pulley that lets the alternator overrun the crank pulley during deceleration if it wants to. Anyone with a solid AAZ pulley that retrofits the clutched pulley immediately notices a night-and-day difference in belt vibration, and you can actually hear the alternator overspin when you blip the throttle.

 

[compu_85]

I came across this video, I think it shows nicely why the clutched pulley is important with today's larger alternators:

https://www.youtube.com/watch?v=4EXYP1CmL9Q

-J

 

[Rico567]

^^^^^^

This is worth watching. Settles the question very well. Sometimes, "-a video is worth a thousand words."

 

[Scoutx]

+1

Video certainly helps.
Further I would disagree that the speed the engine stops at isn't as important an issue. During a normal shut down it probably isn't, but when you stall it with a manual, the engine can got from run to stop almost instantly. That would be a massive force acting on the belt if the alt wasn't decoupled and allowed to overrun.

 

[vanbcguy]

The need for the alt clutch isn't so much the engine stopping as it is the pulses of power from each piston causing the crank to slow during compression and then accelerate during combustion. The crank does not turn at a continuous speed, especially at lower RPMs when the actual injection event lasts just a few milliseconds. This basically hammers on the crank pulley constantly.

Incidentally this is the same reason why single mass flywheel clutches tend to chatter. Same cause, same forces.

Sent from my HTC One XL using Tapatalk

 

[Chris]

Arguably this wouldn't be as big an issue if the engine had a heavier flywheel but then it would be more sluggish.

 

[Ski in NC]

Ever heard an older Dodge/Cummins shut down? r-r-r-r-r-r-r-r--r----r----r------SCREECH! Inertia of alt slips belt. I think the clutched alt pulley and DMF are nice features, provided they hold up!! My alt pulley pooped about 130kmile (??), no big deal, fixed it. DMF still hangin'.

 

[Abacus]

The B4 model came without a clutched alternator pulley and did just fine throughout its lifespan. I just switched mine to the Gates AOD at 440,000 miles and the original crank sprocket was fine. I switched the wagon at 325,000 miles and it was also fine. My brother's B4 with 332,000 miles is still going strong on the solid pulley and he has no plans to change since he's cheap.

I'm not saying there wasn't an improvement, just that it's not an end-all if the pulley is solid.

 

[Ol'Rattler]

Didn't the B4 Passat 2.0 TDI have the chain driven balance shaft that was problematic or am I thinking of a different TDI?

I think that any time engineers design something they get some thing really right, some things kinda OK and ocasinally they creat some engineering night mirrors.

So really, it just depend how they implement the engineering compromises that are part of anything that gets designed and manufactured.

 

[Abacus]

That was the B5/5.5. The B4 was '96/'97.

 

[94x]

The B4 model came without a clutched alternator pulley and did just fine throughout its lifespan. I just switched mine to the Gates AOD at 440,000 miles and the original crank sprocket was fine. I switched the wagon at 325,000 miles and it was also fine. My brother's B4 with 332,000 miles is still going strong on the solid pulley and he has no plans to change since he's cheap.

I'm not saying there wasn't an improvement, just that it's not an end-all if the pulley is solid.

Yes, but the b4 harmonic balancers do not last as long as the alh balancers.

If the serp belt tensioner on an alh is rattling, bouncing, and the dampener is leaking, 9/10 times the alt pulley is siezed.

 

[ta79pr]

I came across this video, I think it shows nicely why the clutched pulley is important with today's larger alternators:

https://www.youtube.com/watch?v=4EXYP1CmL9Q

-J

can we upgrade to that pulley?

 

[compu_85]

From the reading I've done the decoupler pulley is tuned to the rest of the system, so it's not as simple as throwing one on.

-J

 

[CoolAirVw]

The need for the alt clutch isn't so much the engine stopping as it is the pulses of power from each piston causing the crank to slow during compression and then accelerate during combustion. The crank does not turn at a continuous speed, especially at lower RPMs when the actual injection event lasts just a few milliseconds. This basically hammers on the crank pulley constantly.

Incidentally this is the same reason why single mass flywheel clutches tend to chatter. Same cause, same forces.

Sent from my HTC One XL using Tapatalk

I'll second that.

All engines but diesel especially don't "spin" continuously. During the compression stroke the slow radically and during the combustion stroke the speed up radially. The weight of the alternator guts slowing and speeding like that make the tensioners rattle. The clutched pulley allows it to freewheel and keep spinning during the slowdown of the compression stroke, and avoids the tensioner rattle. Sometimes the one-way clutch fails where it doesn't freewheel and it makes the tensioner rattle.

 

[compu_85]

For comparison this is my 91 Mercedes without and with a clutched alternator pulley:

http://www.youtube.com/watch?v=DGI9XLuhaYY

HUGE difference.

-J