Flywheel question

When building an engine from scratch, especially a hit and miss, how is the flywheel weight figured out? Seems to me like that is an important part of how the finished engine performs. Too little and it would be erratic, too much and it would struggle and chug to build rpm. I don’t know if it’s something that just looks about right or if there’s a way to get at least close. I’ve got one in the mix and I am questioning if I don’t have enough mass. Any help would be appreciated

A cast iron flywheel seems to help both IC and steam engines run smoothly, as opposed to an aluminum one, due to the higher mass of iron.

I am sure there must be a formula out there somewhere, but a crude method could be to look at online videos of the same approximate size you are building, and observe how slow and smooth they will idle, and for hit-and-miss, how many revolutions between ignition.

Not scientific, but just observing what works for others.

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In the years I have been reading these forums, I have seen many approaches: As, Green Twin says above, aluminum does not have enough mass but cast iron, steel, or brass works better.

I have seen advice that the important part of the flywheel mass is in the outer rim, so some builders have put brass outer rings on aluminum hubs to make flywheels. Others have drilled an aluminum outer rim and inserted plugs of brass to increase mass.

If I am not mistaken, Brian Rupnow has done several builds where he has changed or modified flywheels to get the performance he wanted.

--ShopShoe

I think an aluminum flywheel would work, but the mass may have to be adjusted.

I have seen others add steel rims, weights and such to aluminum flywheels, but I don't really thing that is necessary, but I have no science or experience to base that on, so just talking off the top of my head.

I recall looking at flywheels before I started casting gray iron ones, and it seems like I just made the rim thicker and wider, to add mass, making sure the spokes and hubs were strong enough for the added mass.

I think aluminium is about 1/3 the mass of iron.
I wish I had some IC engine experience, and then I could talk from experience, and not just speculate.

I do recall seeing several full sized Witte diesel engines with rather massive flywheels on them, I guess to carry the piston over top dead center with the extreme compression.

Find Hansen's diesel model engines do not appear to have that massive of flywheels, although their diameter is fairly large relative to the engine.

I unfortunately can't answer the question asked though, which was "How do you figure out the mass required for a flywheel".
You definitely don't want a flywheel that will come apart at excessive rpm, and so I guess the operating rpm comes into play.

Edit:
I found this document.
No time to study it, but it looks interesting.

http://eng.pdn.ac.lk/old/mechanical/menu/class/downloads/notes/flywheel_ppt.pdf
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Flywheel weight is important to the smooth operation of an I.C. engine. There are formulas for calculating centrifugal weight and someone had even got into posting it many years ago. Some of what I'm about to say is already understood but here's some things that I have found. An all aluminum flywheel on a hit and miss engine won't have enough rim weight to overcome the compression cycle. Sure if it was big enough but then it would be way out of scale. The flywheel in total doesn't need to be heavy but rather just the weight at the rim. Most flywheels (hit and miss type) are spoked. This is to reduce the overall weight while still providing the force needed to overcome the compression. If the flywheel is too light the engine will still run but it won't run slowly. If the flywheel is too heavy the engine will run but won't respond to throttle openings properly. When I started building I.C. engines (hit and miss type) they were from casing kits. I used the dimensions from those drawings to calculate flywheel sizes for the engines that I scratch built. When I built multi-cylinder engines like the automotive type I made the flywheels proportional to the engine size. Like you took the full sized flywheel and scaled it down. The problem with this is they didn't have enough weight (centrifugal) to overcome the compression so although the engines would run fine, and accelerate quickly, their idle speeds were higher than normal. For a hit and miss engine I would just do as one responder suggested and scale the flywheel to the engine you are building using pictures of other engines. This will get you in the ball park.
gbritnell

Page 4 has some equations for flywheels.

http://eng.pdn.ac.lk/old/mechanical/menu/class/downloads/notes/flywheel_ppt.pdf
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Older "Machinery's Handbook"s have section on flywheel design. Way beyond my skill level!

Not available for download, but it is readable if you use the zoom.
Flywheel design starts on page 173.

https://issuu.com/unitape/docs/part2_mechanics.

You could make an aluminium flywheel, turn an outer facial groove and fill the groove with lead. Lead is more than four times heavier than aluminium, and melts at about 330 degrees Celsius, way lower than aluminium which melts at about 660 degrees Celsius.

Like Clockworkcheval suggests I have increased the mass of an aluminium flywheel with lead.
In my case I drilled a series of 10mm holes concentric to the axis as close to the wheel rim as feasible then melted some lead to fill the holes with a light skim to tidy up. Worked for me and created interest.

John B

See post #90:

https://www.homemodelenginemachinist.com/threads/another-knucklehead-build.27584/page-5#post-310641
Terry

With model engineering, scaling issues come home to haunt you - especially flywheels. If you make a half scale engine model - the cubic capacity will be one eighth of the original but the flywheel's rotary inertia will be a sixteenth - see attached notes.
The inertia is also a function of the square of its speed - so if you want to run it at half the design rpms (at idle) of the original motor then the problem is four times worse again.
So using Aluminium for a flywheel is not a good idea.
The maths isn't too tricky but the assumptions (guesses) are.
What speed is your model going to run at ? how much energy does the compression stroke require ? how much rotary speed are you prepared to lose during the compression stroke (5-10% ?)
I'd like to hear comments from experienced engine builders like GBritnell and Terry on such rules of thumb - as these are more significant than the maths.
I like the Lead idea as long as its cast into a hollow enclosure (or drilled holes as suggested by JohnB) - simply shrink fitting a steel hoop around a lead flywheel should be sufficient to prevent it distorting or flying apart (do the hoop stress calculations).
From the scaling issue problems you obviously want to err upwards on everthing to do with a scale flywheel i,e, thicker, larger diameters and denser materials where possible.
All my models have brass or cast iron flywheels - It wouldn't cross my mind to use Aluminium.
I do have a Chinese flame licker that has two aluminium flywheels - It point blank will not run with only the one.

Those are anodised Aluminium not brass - One flywheel's grubscrew came undone and it simply stopped mid run.

Regards, Ken

There is no easy answer to this question of flywheel design. In the real world (full size engines!) the design is not just about mass but the mass moment of inertia this being a function of mass and dimensions. Optimum moments of inertia are governed by the engine type/configuration i.e. petrol, gas, diesel, in-line, V-form, single acting, double acting, opposed piston or radial etc. Also there has to be a consideration of the operating cycle be it Otto, pure diesel or even Rankine in some large LNG marine diesels. It is then a matter of designing a flywheel to match the torque/crank position of the particular engine.

Occasionally a flywheel on its own is not sufficient to balance the irregularities of the combustion sequences and a separate balance shaft is fitted in the crank space driven, for example, by the camshaft gear train. The balancer shafts normally run at 2 x crank RPM.

Of course, the above only relates to internal combustion engines and steam recip engines present separate approaches.

Therefore for models I suggest using the time honoured principles of suck it and see!!

Graham