if all you change is the flywheel mass in a car, and place it on a dyno there will be no measurable difference in hp
changing to lighter wheels but using the same tires will also not increase hp to those wheels
its really simply physics, and what you are describing is measured in fractions of a percentage point, at best
and beyond the means to be able to measure by any common dyno on the planet.
as for turning a shaft by hand
a well conditioned man can produce approx 1/2 hp, that is with his legs predominately, and of this total amount of power
his ability to produce hp by "hand" might well be measured in hundredths of a hp.
properly mounted a heavy flywheel will not require measurably more hp to turn it than a light flywheel, even though you might
feel like you actually feel it, remember it is unlikely you can produce even 1/100th hp spinning a shaft by hand. this is not to say if
the difference in weights is taken to extremes that this might not be the case, but it will come down to predominately windage losses
and some brg friction "if" one were to change from perhaps a 10lb flywheel to a 500lb flywheel, but that is an exception and not the rule
generally we have a relatively narrow range to work within when it comes to flywheels, taking even a lb off of a 10lb flywheel is generally pretty hard to do without causing issues with strength. we certainly are not going to reduce the flywheel weight by half?? and even if we did
the engine is likely going to exhibit operational problems because of it, and it will not become a more efficient producer of power. it just isn't going to happen.
there is only one reason folks reduce rotating mass with cars in relation to dyno's, that is to reduce the reaction time of the engine. in other words
a lighter rotating mass will accelerate quicker than a heavy rotating mass, the hp will remain the same. drag racers and circle track boys reduce flywheel weight so that the engine can rapidly gain rpm, providing fast times and a bit quicker out of corners, they give up a bit of instantaneous torque in the process and also usually end up with an even rougher running engine at lower speeds.
as for exhaust temps and the differences between the otto and diesel cycle, that is a tangent apart from you assertion that reduction in rotating mass will increase efficiency.
the largest engine's on the planet are those used in ocean going container ships, there rotating mass is measured in tons and their efficiency is second to no other engine.
on the other end of the spectrum are small displacement single cylinder diesel engine's that do well to make upper 20's in efficiency, cutting rotating mass is not going to improve the efficiency or the manufactures would have done so.
something a bit more common around my quarters is the changfa s195 idi diesel engine, it weighs approx 350lbs, is rated at 12hp continous
at 2000rpm, and is dyno good at 32% thermal efficiency. with an appropriately sized genhead of 7.5kwatt capacity the engine will make 7kwatts continuously and return an overall efficiency of approx 25.6%
no gas engine in this class will even come close, most will be doing good to return 15% overall efficiency and none will make 7kwatts continuous
with a 12hp rating.
i don't care how much rotating mass you remove the efficiency is not going to change in a measurable way, i know this because i have a test cell
where i can actually measure the difference in bsfc when switching in a 40 watt light bulb.
the same engine driving a 12kwatt head (typically thought of as a bad match, because of higher windage losses) works out to actually be more efficient overall by returning an overall efficiency of approx 27% and will make 8kwatts continuous
same engine, same drive but one head is 50% larger physically, heavier and much higher rotating mass, but it returns a higher overall efficiency?
and makes more power? obviously the bigger head is more efficient to start with, because we know that the windage losses are higher with the larger head, as well as excitation power.
i would challenge you to go ahead and build up a small genset, and do some serious testing, then do your reduction of rotating mass, and then
come back and report your findings. you will find that documenting that supposed increase in efficiency is going to be extremely difficult if not impossible.
my basic point is there are countless other ways to increase efficiency that are easier to do, cheaper to implement, and will return measurable and significant result. all of which should be done and exhausted long before worrying about reduction of rotating mass.
i am no newbie when it comes to this sort of thing, i have a test cell, complete with all the requisite tools to measure just about everything one wants to know, i am also anal about tracking down every last percentage point in efficiency gains and am known for it and often derided for it as well.
the simple and basic gensets that are typical here are those put together often times with small gas engine's belt driving salvaged car alternators
with simple reostat control, made to charge a 12volt battery bank when the wind doesn't blow or the sun doesn't shine.
in my opinion none of them ever get to 15% overall efficiency, and none of them will make any gains by reducing rotating mass, if no other reason than there just isn't much mass to trim away at to start with.
when i mentioned replacing the cast iron flywheel from a horizontal briggs 5hp with an aluminum flywheel from a vertical briggs, it was from experience, i used to build race karts 40 years ago that ran on nitro additive using such modified engine's. there was only one reason to make the change, that was to reduce kart weight and to allow for faster engine acceleration, but
the tradeoff was a pair of engine's that were very hard to start, ran irratically at lower speeds, and were just more finicky than they were with cast iron flywheels.
the difference in weight between iron and aluminum was not much, iirc about 2 lbs tops.
one thing for sure is this, making such a modification for a small genset would be setting yourself up for a very hard to start, erratic running engine, that would be downright miserable to maintain operation of.
but by all means do as you please, don't let me stop ya
bob g
btw, when you get your alternator up close to 80% efficient, from the more typical 45-50% come talk to me about reducing rotating mass