Valve bounce

Re: pure v applied engineering

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not starting the engines until all the lines have been let off and you're ready to put them straight into gear

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well, you could take it too far of course.
 
Sounds like you need a second opinion - I'd recommend Col on this forum, I expect hearing it rev/stutter down your mobile phone will let him put your mind at rest.

Also bear in mind your rev counter may be inaccurate, if set up properly when new then you won't exceed the engines rated speed unless airborne, in neutral your actual revs could be higher than the counter indicates, hence if only 200rpm out (if you get 4600rpm wot then it sounds good or slightly low), 5000rpm on the gauge will certainly be into limiter territory at approx 5200ish....

You need to exclude the limiter possbility, call Col for an opinion....he's in your area I think....
 
I don't believe for one minute you have valve bounce, as 5000 rpm is pretty low for this to occur on any motor.
But more to the point this engine is only supposed to achieve 4600 max, so your so called 'surveyor' is already exceeding the manufacturers specs with his ridiculous obsession of trying to maintain 5000 with no load.
No self respecting engineer I know would do this, perhaps only briefly blip the throttle but never hold it up there, and you always make sure that you know what the max rpm is for a motor before such a test.
He should go back to surveying lawnmowers as he sure doesn't know anything about boat engines.
If the boat is pulling 4500 and making 30-34 knts then there isn't much wrong with it. But as others have mentioned cast an eye over the manifolds and risers, they shouldn't be too hot to touch if the engine is just seawater cooled, if they are its a sign they are getting blocked with rust. Also when was the impellor last changed if its an Alpha leg.
I recently tested the same boat for a guy locally who had just imported one from the US and was concerned that the engine didn't reach 4500 and the boat only achieved 26kts, he was amazed when I took over the helm and magically the boat sat up and accelerated up to 33kts and the revs wound round to 4500, he was about to have the engine pulled out.
How do you tactfully tell the owner that he was using the trim flaps incorrectly, he was driving with them full down all the time thinking this was how you trimmed the boat, and never used the trim on the leg.
Needless to say he was delighted that the boat performed as it should.
If you have little confidence in your surveyor it could be worth a 1/2 hrs labour with your local Mercruiser dealer to have a mechanic take the boat for a spin.
 
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Frexample the max acceleration of the piston caused by the little end pushing it upwards occurs at max rpm and 1/2 way between TDC and BDC. (And the accelerative force is of course zero at TDC and BDC,

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Totally wrong! (apart from max rpm)
Maximum acceleration is at TDC and BDC and maximum velocity half way between.
Half way between TDC and BDC acceleration is zero.
To demonstrate, I suggest that you check your own geometry and calculus and perform first and second differentials with respect to time on a sine wave.
 
Re: pure v applied engineering

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not starting the engines until all the lines have been let off and you're ready to put them straight into gear

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Absolutely correct Moose 1. We had a thread about that a while ago. All this starting of engines, THEN faffing about with shorepower cable, lines, cups of tea, is just pants. I start engines after lines let go.
 
90deg out on calcs

Quite correct fatlady, sorry, was late. I was (roughly) 90deg out on my positions for max Accel (as opposed to Vmax). Thanks for correcting!

The actual position (with respect to TDC) of max Accel isn't the main point (though, eek I'm the one who introduced it). The point is that max accel of engine internal components occurs at max rpm, and isn't a function of how fast you blip the throttle and how fast the engine rpm rises from 500 to 5000 or whatever. You concur with that?

Thanks :-)
 
Re: pure v applied engineering

Dunno about the m'bike engines, mebbe different types of engines in the zing zing bikes. Lots of variables. Also a wizz wizz m/bike is often 2 stroke so no oil pump? And you do get materially different piston accel forces with compact engines where the conrod length is short compared to stroke. The theoretical answer (where Vmax is 90deg +/- TDC, and max accel is at TDC/BDC) only occurs if the conrod is infinitely long and thus remains vertical. In real life it aint, and the accel forces (as well as positions of the max numbers wrt TDC) are materially different

Agree that thrashing when cold is bad. We're saying the same thing. But the point is, and praps I'm being pedantic/ purist, that it's thrashing a cold engine that is bad, not running an engine at high rpm under no load,. The latter, per se, isn't at all bad.

And I agree the Mole Sud neighbours thing. It's nuts, engineering wise, to "winter run" engines for 40mins no load till they reach full temp. If you want to run them (to exercise impellers and seals and sloosh a bit of oil around internally) it's much better to run them 2 minutes ish, and shut down before they reach full temp. We had a thread on precisely this a couple months ago and many people (not me) felt it was better to lettem run till full temp reached
 
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Hmmm, but it's not really a "common fact", rahter it's a commonly made claim that's actually wrong and has no engineering basis at all :-)

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No Im sorry you are wrong. re petrol engines.
I have been in the motor repair business for 30 years & still am. I am talking from experience. One of the favourite car thief tricks used to be sticking a brick on the accelerator & seeing how long it took the engine to self destruct. Ive lost count of the amount of dropped valve. or melted/holed piston jobs we have had where Billy Bigwheels has blown his engine reving the nuts of it in front of his mates. Admittedly now many have rev limiters so this doesnt happen. but on the older stuff it was very common & still is.
 
Um, I'm not wrong. Of course if you rev nuts off an engine by putting a brick on the pedal or revving it in front of mates it will bust. But that's not cos you are running it under no load, it's cos you are running it at 9000rpm or whatever. It busts becuase of the internal forces caused by high rpm, not becuase it is being run unloaded. The actual no-load aspect of the 9000rpm BillyBigwheels stunt isn't any contribution to the engine blow up - in fact at the margin it might give the motor a slim chance of survival.

But if you think running a motor at 5000rpm loaded is better than 5000rpm unloaded, mebbe explain why using engineering principles?
 
Re: 90deg out on calcs

Tee hee. The "roughly" was reference to the fact real-life conrods are nowhere near infinitely long, they are same order of magnitude, lengthwise, as the engine's stroke. So the positions of Vmax and Accel max can be something like +/- 20deg away from the "throretical" TDC/BDC or the midway point between TDC and BDC. But I guess you knew that. Thanks!
 
Re: 90deg out on calcs

I realised that was what you meant. However, correct me if I am wrong, but maximum acceleration still occurs at TDC/BDC. When the big end, crankshaft centreline, small end and cylinder axis are in alignment, the conrod length has no influence on the value of, or the crankshaft angular position for, maximum acceleration. On Vmax you are quite right.

However, for the purposes of this thread, approximation is sufficient. That is why I did not suggest that the second order effect of "blipping", depending on the rate of change of angular velocity, could cause a minor peak in acceleration just before maximum rpm is reached.

All fun to argue, but of no real consequence to the thrust of this thread.
 
fatlady only - not recommended reading :-)

As you say, of no consequence to the thread, only to tcm's theories (!) but here goes.

First off, engines can be built with the cylinder centre line not passing through the crankshaft centre, so your "all vertical alignment" scenario would never happen in such an engine. But let's ignore that

If we're getting precise (and I'll make mistakes - 18 years since I did proper study) then yes Accel max is always at TDC. The other peak in accel occurs at BDC (subject to the last para below) but the BDC accel is always much less than at TDC. In a car engine at 4000-5000 rpm with conrod length around 1.5xstroke the sort of values you get are 1200-1400g accel at TDC and half that at BDC. This is due to the conrod angle geometry. It's the same simple trig analysis that tells you the piston moves about 65% (not 50%) of its stroke during the first 90deg past TDC and the other 35% during the 90deg before BDC.

During the first 90deg past TDC the piston is pulled (on the induction stroke!) by two components of crank pin movement both pulling it down - it is being pulled by a crankpin that is moving downwards AND outwards. Whereas once you get past 90deg past TDC the two components of crankpin movement work AGAINST each other so the piston is still pulled down but also pushed up a bit (as it were) as the crankpin moves inwards towards the engine centreline. Hard to explain, but get my drift? As I say, same geometry as the 65%-35% thing above.

This is why peak piston accel at TDC is much higher than BDC

Now, to confuse things, i seem to remember that as you get to conrods that are short compared to infinity, say 1.5 x stroke, as found in real engines, the accel curve at BDC is very flat and even has a tiny "dent" in the peak, like a double high tide, ie piston accel at BDC is actually lower magnitude than at the positions +20 and -20 from TDC, approx. In other words the curve is W shaped. But to be honest I can't remeber why, from fundamental trig and geometry, it's been too long....
 
Re: fatlady only - not recommended reading :-)

I agree, I think the confusion is with the difference between max piston velocity and max piston acceloration, but either way the stresses on the valve gear are proportional to revs regardless of load on the engine.

However this actual case does puzzle me, software managment systems dont have a huge range for rev limiting. I would like to hear this engine; the fault (if there is one) could be anything from a faulty coil, low or high tension side to valves or fuel delivery - I am sure a competent person on site with a mark one ear would sort it out very quickly though.
 
Re: fatlady only - not recommended reading :-)

I accept that the cylinder(s) can be offset, but we are talking about a conventional engine, not a swashplate, or whatever.

Your comments about lower acceleration at BDC make sense, althought I am surprised at the difference in magnitude. Hadn't thought about that before.

I can also envisage the W effect. The crankshaft rotation will produce a piston displacement sine wave at the crankshaft rotation frequency, whereas the crankpin lateral displacement will superimpose a piston displacement cosine wave at twice the crankshaft rotation frequency.
Differentiating twice for acceleration gives the same thing with a phase shift. At TDC the crankpin effect will reinforce the crankshaft effect, whereas at BDC they will be in opposition.
If the conrod is short enough, at BDC the acceleration due to crankpin displacement could be greater than that due to crankshaft rotation, hence the W effect.

Of the top of my head, will that do?
 
unloaded

face-saving as far as i can make out. I explanied how the engine is vulnerable at cold i.e no-load and suddenly we have a load of flippin maths homework! Surely any fule can see the engine is most knackerable at extended TDC cold. heyho.
 
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