Freewheel on alternator

[nicknorman]

Having been rather confused by the slowly decaying whirly whirly noises coming from our Beta 43 engine when it's taken from cruising to neutral etc, I have discovered that both large domestic alternator and the Travelpower alternator have freewheels (or sprag clutches or whatever you want to call them) built into the pulley, that allows the alternator to turn faster than the engine if it wants to. My question is why?

 

OK maybe something about sudden stoppage etc with the gearing-up, but surely the belt gives some shock absorbance and the dRPM/dt is likely to be pretty high on sudden starts (ie operating the starter motor and combustion taking the engine up to idle) as it is when a block of wood stops the prop (and the gearbox clutch slips a bit as the heavy engine is reluctant to stop instantly). Any thoughts?

 

Nick

[Nickhlx]

Having been rather confused by the slowly decaying whirly whirly noises coming from our Beta 43 engine when it's taken from cruising to neutral etc, I have discovered that both large domestic alternator and the Travelpower alternator have freewheels (or sprag clutches or whatever you want to call them) built into the pulley, that allows the alternator to turn faster than the engine if it wants to. My question is why?

 

OK maybe something about sudden stoppage etc with the gearing-up, but surely the belt gives some shock absorbance and the dRPM/dt is likely to be pretty high on sudden starts (ie operating the starter motor and combustion taking the engine up to idle) as it is when a block of wood stops the prop (and the gearbox clutch slips a bit as the heavy engine is reluctant to stop instantly). Any thoughts?

 

Nick

 

I was told it is because it saves on belt wear - the Iskra 175 pulls quite a bit of power when working hard and the clutch helps reduce some of the change in speed loads, so saving belt wear..

 

Nick

[nb Innisfree]

As I understand it the freewheel pulley allows a heavy alternator rotor to carry on at a high speed for a short time and carry on charging when the engine is temporarily slowed down. Not really very relevant I would think on a canal boat as engine revs stay more or less constant

[Sir Nibble]

As I understand it the freewheel pulley allows a heavy alternator rotor to carry on at a high speed for a short time and carry on charging when the engine is temporarily slowed down. Not really very relevant I would think on a canal boat as engine revs stay more or less constant

That's it. The energy stored in the spinning mass of the rotor would be lost and so a freewheel is fitted to allow it to retain that energy. Intuitively, it seems that you would get more of an effect by the driver having a crap before moving off but these freewheels have become widespread so it must be worthwhile. The problem occurs when one needs replacing but you can fit a solid pulley instead though it takes special tooling to shift the freewheel pulley.

[Arnot]

That's it. The energy stored in the spinning mass of the rotor would be lost and so a freewheel is fitted to allow it to retain that energy. Intuitively, it seems that you would get more of an effect by the driver having a crap before moving off but these freewheels have become widespread so it must be worthwhile. The problem occurs when one needs replacing but you can fit a solid pulley instead though it takes special tooling to shift the freewheel pulley.

I may be wrong here but I had the freewheeling pulley fail on my sprinter van and what I first noticed was a lot of vibration at low engine speeds. What I think was happening was that there was a mechanical resonance in the belt and a mechanical resonance in the alternator torque requirement and the two were beating against each other. It didn't stop charging but the engine did idle roughly and seemed harsh whilst pulling off. Replacing the alternator (which had done nearly 200,000 miles and was past it's best) immediately resolved the problem.

 

So; my take is that as well as the possible flywheel effect, with larger alternators the freewheeling pulleys may be necessary to prevent belt/alternator resonance at specific speeds. Possibly the ECU controlled alternators that shut down at lower speeds do a similar thing.

 

Regards

 

Arnot

[Nickhlx]

That's it. The energy stored in the spinning mass of the rotor would be lost and so a freewheel is fitted to allow it to retain that energy. Intuitively, it seems that you would get more of an effect by the driver having a crap before moving off but these freewheels have become widespread so it must be worthwhile. The problem occurs when one needs replacing but you can fit a solid pulley instead though it takes special tooling to shift the freewheel pulley.

 

 

Why a problem ? Beta spares dept sells them separately - mine was an upgrade as recommended by Beta ...

 

Nick

[paulcatchpole]

....did idle roughly and seemed harsh whilst pulling off.

 

Arnot

 

The big ribbed-belt-driven one on the car seized whilst running last winter (225k), filling the cabin with clouds of belt smoke, which was bad enough, but when shut down, the alternator 'grip' on the belt was enough to stop the starter turning the engine over - incredible power transmission on these modern belt systems, I was impressed! But annoyed at the same time...

 

PC

[Nickhlx]

The big ribbed-belt-driven one on the car seized whilst running last winter (225k), filling the cabin with clouds of belt smoke, which was bad enough, but when shut down, the alternator 'grip' on the belt was enough to stop the starter turning the engine over - incredible power transmission on these modern belt systems, I was impressed! But annoyed at the same time...

 

PC

 

They fit belts on bikes these days - instead of the drive chain - but only on e.g. low powered Harley Davidsons, and a few others, up to about 75-100 bhp, I think - it's not really a friction drive though, more of a cogged belt. Not particularly practical though as a bit of gravel causes havoc apparently

 

Nick

[paulcatchpole]

They fit belts on bikes these days - instead of the drive chain - but only on e.g. low powered Harley Davidsons, and a few others, up to about 75-100 bhp, I think - it's not really a friction drive though, more of a cogged belt. Not particularly practical though as a bit of gravel causes havoc apparently

 

Nick

 

Sounds fair - I can believe it working, but as you say, wouldn't like to get stuff 'in' it! Mind you, chains are bad enough for wearing out...

 

PC

[Arnot]

They fit belts on bikes these days - instead of the drive chain - but only on e.g. low powered Harley Davidsons, and a few others, up to about 75-100 bhp, I think - it's not really a friction drive though, more of a cogged belt. Not particularly practical though as a bit of gravel causes havoc apparently

 

Nick

I used to have one, a Kawasaki 305?. A bit strange at first but once I got used to it very good. Smoother, quieter transmission and a lot cleaner. No problems in about 10,000 miles.

Edited June 1, 2011 by Arnot

[Timleech]

I used to have one, a Kawasaki 305?. A bit strange at first but once I got used to it very good. Smoother, quieter transmission and a lot cleaner. No problems in about 10,000 miles.

 

I seriously investigated using timing belt as a final reduction drive for a particular need for a boat, carrying about 60bhp @ 400rpm to the prop. It was all possible, using the biggest single belt available, I was quite keen to go through with it but then had a change of engine plan. Not a cheap solution, though.

 

Tim

[Sir Nibble]

Why a problem ? Beta spares dept sells them separately - mine was an upgrade as recommended by Beta ...

 

Nick

Yes they are readily available but you need special tooling to remove and re fit.

[Nickhlx]

Yes they are readily available but you need special tooling to remove and re fit.

 

 

Only It is on the Electrolux Travelpower systems - the standard Iskra alternators are a straight "on and off" action.

 

The over-run pulley is designed to take out the torsional vibrations, so prolonging service life of the associated items...

 

Nick

 

edited to remove the "only" - special tools may also be needed on other systems

Edited June 1, 2011 by Nickhlx

[Sir Nibble]

Only on the Electrolux Travelpower systems - the standard Iskra alternators are a straight "on and off" action.

 

The over-run pulley is designed to take out the torsional vibrations, so prolonging service life of the associated items...

 

Nick

Well I was not aware that the iskra didn't need a special tool to shift the pulley, all the alternators with freewheel pulleys I have seen do. It's certainly not "only" travelpower.

[Nickhlx]

Well I was not aware that the iskra didn't need a special tool to shift the pulley, all the alternators with freewheel pulleys I have seen do. It's certainly not "only" travelpower.

 

Sorry - I can't help with the others - I just asked the tech guy at Beta whether it was needed for the Iskra and he said "no, but on the Travelpower it was". I guess there are different mounting arrangements.

 

Nick

[markb]

It also helps reduce load on the crank nose.

 

Some older volkswagen engines with large alternators suffered from crank nose failures before they fitted the freewheel pulleys. Over time it would destroy the keyway.

[Guest]

That's it. The energy stored in the spinning mass of the rotor would be lost and so a freewheel is fitted to allow it to retain that energy. Intuitively, it seems that you would get more of an effect by the driver having a crap before moving off but these freewheels have become widespread so it must be worthwhile. The problem occurs when one needs replacing but you can fit a solid pulley instead though it takes special tooling to shift the freewheel pulley.

Surely this would make a mess in the car?

[nicknorman]

OK thanks for the replies folks. I must say I am a little surprised that an alternator that is converting a lot of mechanical energy into electicity (a few horsepower's worth) would ever want to spin faster than the crank, and/or if the torque oscillations as each pole of the rotor passes results in the freewheel disengaging, bearing in mind the number of poles per rotation and the rotational speed, I would have thought that all that mechanical movement (rollers sliding on ramps or whatever) in a modestly lubricated environment would create a fair bit of heat and lead to it wearing out very quickly. But not in a position to argue, and can't think of a better reason!

 

Nick

[nb Innisfree]

I used to have one, a Kawasaki 305?. A bit strange at first but once I got used to it very good. Smoother, quieter transmission and a lot cleaner. No problems in about 10,000 miles.

 

Many years ago a bike mag did a comparison road test between an easy rider style Kawasaki 440 LTD? with belt drive and its european chain driven version and the difference was very noticeable, in fact it transformed the engine from an intractable jerky PITA to a smooth and tractable dream. The chain version couldn't be driven comfortably below 30 mph in top gear due to chattering and juddering whereas the belted version could be driven to a standstill and the engine went to zero revs without the slightest hiccup, just the elimination of backlash was all it took to reveal an engine's true capabilities.

 

OK thanks for the replies folks. I must say I am a little surprised that an alternator that is converting a lot of mechanical energy into electicity (a few horsepower's worth) would ever want to spin faster than the crank, and/or if the torque oscillations as each pole of the rotor passes results in the freewheel disengaging, bearing in mind the number of poles per rotation and the rotational speed, I would have thought that all that mechanical movement (rollers sliding on ramps or whatever) in a modestly lubricated environment would create a fair bit of heat and lead to it wearing out very quickly. But not in a position to argue, and can't think of a better reason!

 

Nick

 

I would hazard a guess that modern high revving engines have a lightweight flywheel to allow it to rev up, and down, as quickly as possible and so is more likely to slow down faster than a loaded alternator will, so it's not a case of the alt going faster than the engine but allowing the engine to go slower, albeit momentarily.

[cereal tiller]

i came across a freewheeling alternator 3 years ago,when i made enquiries as to why it had a clutch mechanism,a london diesel injection company told me it was a fuel saving device.

 

in vehicle applications it saved energy during engine speed fluctuations when changing gear,the alternator did not require as much power to speed it up again.

 

 

another gain was increased belt life.

[by'eck]

I would hazard a guess that modern high revving engines have a lightweight flywheel to allow it to rev up, and down, as quickly as possible and so is more likely to slow down faster than a loaded alternator will, so it's not a case of the alt going faster than the engine but allowing the engine to go slower, albeit momentarily.

 

I agree & would add that in todays energy conscious climate any saving in power will be exploited, this being one such.

 

Remember that the alternator rotor will be spinning faster than the engine, being rated well over 10,000 rpm. Kinetic energy stored in such will overcome load for a finite amount of time as engine revs fall.

[Nickhlx]

I would have thought that the torsional vibration it is meant to address was at a higher frequency than "power strokes" - perhaps somewhere between the frequency of the poles passing the coils in the alternator and the "ringing" of the crank / belt / alternator stator. ( terminology ?)

 

Naturally, the sudden lowering of revs as you drop back to idle could well have the alternator running on as well, especially if it is loaded lightly when the battery is well charged. Could it be that the over-run pulley addresses both scenarios, at least partly ?

 

Nick

[WotEver]

... the "ringing" of the crank / belt / alternator stator. ( terminology ?)

Resonance

 

Tony

[Sir Nibble]

All this is very well thought out but over the 35 years I spent working on this stuff I have identified why these things are fitted. It's to annoy me!

[by'eck]

Whoops & there's me thinking car manufacturers were concerned about the environment seems its just to extend drive belt life & reduce crank pulley key wear - see here

 

Another innovation with diesel cars is the dual mass flywheel - Designed to isolate torsional crankshaft spikes created by diesel engines with high compression ratios. By separating the mass of the flywheel between the diesel engine and the transmission, torsional spikes can be isolated, eliminating potential damage to the transmission gear teeth.