1. those that operate within the manufactures recommendations, probably best for longevity.
This brings up an interesting point......
I have NEVER seen a manufacturer recommend ANY engine speed.
What I do see is a power output figure at a specific RPM. Sorry to anyone that thinks otherwise but that is NOT a recommendation to operate at that speed, it's a RATED power figure at what that engine will do, normally a MAXIMUM output figure, still below max RPM, NOT what you should run the engine at.
This is why industrial engines often come with power curve charts to show what they will do at specific speeds. I am amazed people are not aware of those.
If the Max HP/ RPM figure was a recommendation, why isn't everyone, especially those so concerned with critical speed problems not driving their vehicles round at the Figure given for it's maximum power output which is what these Numbers are, NOT an engine operation speed recommendation.
The numbers merely tell you what the engine will do at that speed. That is all.
Perhaps I have missed something and if so I'd sure like to be educated if someone could show me where any manufacturer says something akin to "This is the recommended speed to operate this engine".
That is nothing like a MAX power figure at a certain RPM manufactures specify.
Further more, there is also a max Torque figure specified which is regarded as the sweet spot and where vehicle manufacturers, especially Trucks and things like locomotives try to gear their equipment to. Cruising speed in a vehicle in top gear is always closer to the much slower max torque figure than it is the max power figure which is far higher up. And again, if Operating the engine slower was going to grenade it, how do city cars especially manage to put around for a couple of decades without snapping cranks or falling apart.
Here is a power output Chart of a Cummins engine.
Engine Model Max. Power HP (kW) Peak Torque lb-ft (Nm) @ RPM Governed Speed (RPM)
X12 500 500 (373) 1700 (2305) @ 1000 2000
X12 455 455 (339) 1700 (2305) @ 1000 2000
We see here that the peak torque is at half the RPM of the peak power.
Would someone like to ring Cummins ( or Cat, Kubota, JBC, Yanmar, Detroit, Volvo, Mercedes, Ford, Deutz, MTW, Perkins, Isuzu, Honda, GM or any other manufacturer) and ask if operating their engine at 1000 RPM ( or whatever is) in the peak torque range, going to cause issues with critical speed failure or damage the engine operating at that speed?
I think we all know what the answer is going to be and that for mine shoots down any of this idea that an engine operated below max power rating will destroy it. It's simply maligned fantasy to suggest that as is the idea people would operate an engine at " extremely low speed".
I also looked at the Changfa site being a recognised small engine Builder and while they don't specify Torque Figures I could find, They do only give RATED HP/ speed figures. No where could I find anything that even alluded to as much as " Engine recommended operating speed. " If anyone can, please link to it because i'd like to see where I missed it.
As such I can dismiss the position of " Manufacturers recommendation" for engine speed as a fallacy and contortion of the truth.
It's not even a Recommended RPM speed, that where specified, is usually listed separately and is higher again.
Calling a manufacturers power output rating as the recommended operating speed is simply wrong and ignorant of what is actually being specified by those whom don't understand what they are reading.
you should be aware of the engine's critical speed, that is a valid point. no engine should be operated at critical speed that is if you want it to last and not have some form of premature failure, some of which can be quite catastrophic like broken crankshafts or engine cases, or mounts.
I agree.
I wonder how many people know that or manufacturers that could even be found from?
Again, it is a Single speed or very narrow range, it's not any speed under the max power rating of an engine.
all you have to do is ride a ferry boat or be anywhere around a diesel locomotive
you can feel the boat vibrate like hell as the throttle opens and the rpm passes through critical speed... and you will note the man with the throttle in his hand does not allow the engine to operate at that speed any longer than is necessary.
A great example. I would say though that a lot of this is in the drive train however that is just as important to avoid.
small engine's may not demonstrate this effect so plainly, but the thing also has a critical speed that should be avoided.
I'd suggest with a little observation it is discernable and it is at very low speed where anyone with any mechanical sympathy or common sense would not operate an engine and on a small engine would be at a point where the power output would be near useless anyway.
It's also dependent on load.
Just for clarity, please note I am not trying to attack you Bob or what you said personally, your comment just sparked a thought of a good point that seems to have been missed/ maligned till now and which I wanted to address and clarify.