45
« on: November 10, 2021, 11:53:38 AM »
(i typed the following late last evening, but forgot to hit "post" so it is probably out of sequence)
i have been guilty in the past of getting into the weeds, and over time found it to be non productive for myself and for the members of the forum.
we can argue fine points, and believe me i can do that as well as anyone
but what is to be gained?
when i mention the oem recommendations on operating speed, i should have expanded on that a bit perhaps, and related it as recommended "operating range", rather than speed.
i think most folks understand that.
most all stationary engine manufactures, or those that build for that purpose, and for purposes of heavy loading in particular do recommend an operating range. by that it is meant that this is the range of speeds that the oem recommends the use of their engine under load. stationary engines are likely far more heavily loaded than the typical automotive engine which can get by quite nicely operating over anywhere from idle to redline (which btw is a recommended don't go over line)
lets take hwew' kubota single cylinder diesel, iirc it is rated for 2600-2800rpm at rated load, and the power curve stops at something just over 2200rpm or so, its not that the engine won't run under 2200, its just that it vibrates badly at around that speed. likely one of the critical speeds that they don't recommend running at, especially under load.
(my bet is hwew has talked to the kubota engineers about this issue.)
it is apparent to the user that this is not a good speed to operate at, because the thing shakes and vibrates so bad that it works parts loose, which generally is not a good thing, unless you want a asphalt packer or concrete screed vibrator ?
getting back to automobiles and critical speeds, these engines are generally very stiff, compact, well balanced, well engineered, with serious attention to things like harmonic balancers, and engineered mounting systems, along with the decoupling effect of the almost ubiquitous automatic transmission.
go back to the 40's and early 50's and the 6cyl stick shift transmission cars and trucks, one could certainly determine by feel what he might not understand as a critical speed but knows that "hey, i don't want to drive in this gear, this thing vibrates my teeth out"
its doubtful that the oem's had to make a recommendation on what engine speed to operate under, they left it to the user who figured it out within the first few blocks of leaving the dealership, or....
he was like some of those old farts that lugged the crap out of their car or truck and you could hear every part of the vehicle vibrating like mad, gears clatter, driveline ringing a horrible tune, and the old deaf guy oblivious to what was happening, or he was driving down the road in first gear at 55mph turning 6 grand! (that was my poor dad, he couldn't hear a thing)
as for your point on industrial engines and power curves, and folks not aware of such, i think you are sadly misinformed. while there might be some newbie that doesn't know about such things, it is doubtful that you will find but a tiny percentage of the members on this and other forums that are unaware.
(skipping down a bit to your cummins reference)
if we go back to the 60's and early 70's cummins made a taper nose ntc335 engine
it had a critical speed problem in that if you loaded the engine at a certain speed you ended up with a broken crankshaft
ford tractors, 50's with the 4cyl diesel had a similar problem, recommendation was to not load the tractor at some specific rpm, i don't recall what it was either, but broken cranks were not uncommon.
the reason that cummins is now able to get max torque down to 1000rpm is they have built the engine massively stronger than it has ever been. you can build any engine heavy enough to stand up to about any critical speed issue you like, but almost without exception, the bean counters don't want an oz. of extra iron or expense put into a design without very good reason.
car and pickup reference again
the engine in the typical 5k lb suv has what? 300hp or more these days?
what does it take to in hp to move a 5k lb suv at highway speeds, maybe 100hp?
what does it take to plug around town at 30mph? maybe 25 or 30 if the a/c is on?
the typical car engine is asked to deliver on average probably a quarter of its capacity
and likely half of that driving around town, with brief excursions. so maybe you are lugging around at some critical speed in your car, putting around town, however because the engine is built to make upward of 300hp it is over built for the need required to putt around at that speed, it is decoupled and isolated so the driver has no feel that it is even happening, and yes it is not a problem. don't feel it therefore it doesn't exist? ya right.
every engineering book ever written on internal combustion engines, beit for cars, trucks, planes, trains, boats, or anything else has a chapter on critical speeds and what is done to deal with them.. you either overbuild and isolate the engine, or if you can't you instruct the operator on where not to run the engine under load.
i mentioned the ferry boat
they are an interesting case study in critical speed
the engines are generally very rigidly mounted to the boat superstructure, and as such engine vibration is transmitted through the hull and around the boat.
when the captain leaves the dock and starts to throttle up he does so , increasing rpm slowly, sometimes remaining at a lower rpm while he clears the dock and/or other obstructions, but as he increases the rpm the engine will enter critical speed, at that precise moment you can feel the boat start to violently vibrate and you can also feel the surge as the captain shoves the throttle open so he can quickly pass through that rpm.
it is actually very interesting to witness having studied the phenomenon, to actually feel the process from before, during and after. its not something most of us would think about, but that captain sure does.
after witnessing that i went looking for other examples, and came across a railroad yard, and found the same thing taking place, they start to slowly throttle up, then at some rpm the ground starts to vibrate and the engineer cracks up the throttle and he gets through that rpm quickly. again very interesting.
if you would like i can refer you to a two volume set of engineering books from MIT on engine design, believe me critical speed is a very real thing that must be dealt with one way or another.
its been a very long day, i will part with you for now
and time willing, pick up where we left off tomorrow.
fair enough?
bob g