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Alternator Speed on Slow Revving engine


steve.sharratt

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22 minutes ago, MtB said:

 

Excellent idea, other than I don't have a boat on the K&A at the moment. Might have to put that right soon....

 

And yes that was something I loved watching on the K1, the flywheel virtually stopping and only just about passing over TDC.  The K2 doesn't really do it, surprisingly. 

Do the K1 and K2 (and even K3) share a flywheel, or do the lower cylinder count engines get a bigger flywheel?   Your remarks about cyclic speed fluctuations and cylinder count do sort of assume fixed flywheel size, if engines with lower cylinder counts get bigger flywheels then things will balance out. 

 

The torque impulse produced by a cylinder firing event is actually quite short so an engine needs a lot of cylinders before these start to merge to give a more continuous torque, I did work this out once a long time ago, I think the answer was about 20, so a smooth output torque is really all down to flywheel inertia.

 

A well known alternator man did tell me that he does not like the freewheel pulleys as he suspects that the sudden impulse as they engage is just as destructive as the fluctuations they are trying to remove, not sure I agree but its worth thinking about.

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44 minutes ago, dmr said:

Do the K1 and K2 (and even K3) share a flywheel, or do the lower cylinder count engines get a bigger flywheel?   Your remarks about cyclic speed fluctuations and cylinder count do sort of assume fixed flywheel size, if engines with lower cylinder counts get bigger flywheels then things will balance out. 

 

The torque impulse produced by a cylinder firing event is actually quite short so an engine needs a lot of cylinders before these start to merge to give a more continuous torque, I did work this out once a long time ago, I think the answer was about 20, so a smooth output torque is really all down to flywheel inertia.

 

A well known alternator man did tell me that he does not like the freewheel pulleys as he suspects that the sudden impulse as they engage is just as destructive as the fluctuations they are trying to remove, not sure I agree but its worth thinking about.

 

Yes all the K engines share the same flywheel AFAIK and yes I was assuming the same flywheel with my comments.

 

Not the case with the OP's JP. JPs have a variety of flywheels fitted IIRC, with the marine versions getting much heavier flywheels than the industrials.

 

also not sure engine torque is relevant here, oiher than how it translates into angular velocity at the various points of rotation. It's the velocity that the alternator sees. Fit a bigger/heavier flywheel to an engine and the torque remains the same but the changes in velocity over the cycle get smaller, Shirley. 

 

 

 

 

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1 hour ago, dmr said:

Do the K1 and K2 (and even K3) share a flywheel, or do the lower cylinder count engines get a bigger flywheel?   Your remarks about cyclic speed fluctuations and cylinder count do sort of assume fixed flywheel size, if engines with lower cylinder counts get bigger flywheels then things will balance out. 

 

The torque impulse produced by a cylinder firing event is actually quite short so an engine needs a lot of cylinders before these start to merge to give a more continuous torque, I did work this out once a long time ago, I think the answer was about 20, so a smooth output torque is really all down to flywheel inertia.

 

A well known alternator man did tell me that he does not like the freewheel pulleys as he suspects that the sudden impulse as they engage is just as destructive as the fluctuations they are trying to remove, not sure I agree but its worth thinking about.

 

Beta strongly recommended using freewheeling pulleys with big (high-inertia) or multiple alternators, even on a 4-cylinder engine like the Beta 43. They said it reduced belt wear, especially by avoiding tension reversals in the belt, which is particularly a problem if you have an idler wheel on the "slack" side.

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53 minutes ago, IanD said:

 

Beta strongly recommended using freewheeling pulleys with big (high-inertia) or multiple alternators, even on a 4-cylinder engine like the Beta 43. They said it reduced belt wear, especially by avoiding tension reversals in the belt, which is particularly a problem if you have an idler wheel on the "slack" side.

There may be an element of covering their liability for the well documented crank front end failures in that recommendation.

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13 minutes ago, Tracy D'arth said:

There may be an element of covering their liability for the well documented crank front end failures in that recommendation.

Nope, that was before they switched over to the splined pulley drive which has had no such problems reported. The reasons I gave for the freewheeling pulley recommendation were for the newer/current engines when I was talking to them last year.

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5 minutes ago, IanD said:

Nope, that was before they switched over to the splined pulley drive which has had no such problems reported. The reasons I gave for the freewheeling pulley recommendation were for the newer/current engines when I was talking to them last year.

They still fail with the splined drive, I have seen one this month.

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6 minutes ago, IanD said:

Interesting, I thought that particular problem was fixed. What failed?

Stripped all the splines off and destroyed the center of the pulley and the crank. Due to supply problems no new 43 engines available, no short engines either but the parts to rebuild are on the way I was told. Hellish expensive.

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2 minutes ago, Tracy D'arth said:

Stripped all the splines off and destroyed the center of the pulley and the crank. Due to supply problems no new 43 engines available, no short engines either but the parts to rebuild are on the way I was told. Hellish expensive.

What on earth went wrong to strip the splines off? Overloading at low rpm, allowing bolts/pulleys to come loose, or maybe some other user error? I thought the splined interface was pretty robust by all accounts, it's easy to blame Beta when the fault may not actually be theirs at all... 😉

Edited by IanD
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28 minutes ago, IanD said:

What on earth went wrong to strip the splines off? Overloading at low rpm, allowing bolts/pulleys to come loose, or maybe some other user error? I thought the splined interface was pretty robust by all accounts, it's easy to blame Beta when the fault may not actually be theirs at all... 😉

The usual, Travelpower and large alternators. The low speed running doesn't help but the liquorice crank shafts do fail still.

 

 

Splines are fine but once there is a thou of backlash its a one way ticket to destruction. There is considerable intermittent loading on the pulley with the Travelpower.

The average user will not understand the need to keep revs up and to periodically torque up the nuts and bolts.

Edited by Tracy D'arth
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5 minutes ago, Tracy D'arth said:

The usual, Travelpower and large alternators. The low speed running doesn't help but the liquorice crank shafts do fail still.

Did the user follow Beta advice about only charging at 1200rpm or above? IIRC this is because there is a torsional resonance below this which can cause damage under heavy alternator/travelpower loads...

 

Was their engine a standard Beta setup or a customised high-power "special"?

 

I'm not trying to make excuses for Beta if there is still a genuine design problem with their engines, but the case you're describing seems unusual for a newer engine with a splined crank...

Edited by IanD
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Who knows?  The users will never admit to misuse.  All standard Beta gear, a good few years old I think. 

The habit of using the Travelpower to run the battery chargers to try to reduce recharging times is widespread with these owners I have found.

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15 minutes ago, Tracy D'arth said:

Who knows?  The users will never admit to misuse.  All standard Beta gear, a good few years old I think. 

The habit of using the Travelpower to run the battery chargers to try to reduce recharging times is widespread with these owners I have found.

 

Which could well be the cause of the problem...

 

Running the travelpower as well as big alternators means owners can get plenty of charging current even below 1200rpm, which is presumably why Beta now advise against this since it caused some failures as you described -- it's in the engine manual, and IIRC there's now even a sticker on the engine (or next to it) to remind users of this.

 

If that's why the failure happened then it doesn't seem fair to blame Beta and their "licorice crankshafts". There are lots of ways to kill a properly designed engine by misuse... 😉

Edited by IanD
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6 hours ago, Tony Brooks said:

 

Open circuiting the rotor is done many times a second by most regulators so will not produce a voltage surge, and it has the effect of dropping the output to zero.

 


Well, to be accurate, even though the regulator operates in a pulsed way, it doesn’t open circuit the rotor. The rotor has quite a lot of inductance and if one attempted to open circuit it, there would be a big back emf and arcing of contacts. V = L.dI/dt. The regulator includes a freewheel diode that allows the rotor current to decay gracefully. This is why alternators can’t tolerate a sudden load disconnection. Even if the regulator instantly stops supplying rotor current, it takes a while (few 100 milliseconds) for the current to decay, and in the mean time the alternator is still producing power that has nowhere to go.

 

So if one is going to eg install a switch to open circuit the field current, it is better to do this between the field diodes (9diode machine) or  battery supply (6 diode machine) and the regulator, rather than between the regulator and the rotor. That way the freewheel diode within the regulator is still in circuit, allowing the current to decay gracefully.

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