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Fly wheel size

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john brown 1707/10/2019 18:21:07
114 forum posts
3 photos

Am asking this question for a friend who has not got a computor he was given a set of drawings of a kerzel type engine that is bigger than the engine that shows up on the net ,any way its looking good but the plans state that the od of the fly wheels are to be 178mm and all he has is stock with a od of 150mm do you engine experts think that difference will be ok please .


JasonB07/10/2019 18:36:16
17842 forum posts
1951 photos
1 articles

It is not just the diameter that matters but the section of the rim so a pair of smaller flywheels could have the same effect as larger ones if the rim is thicker and or wider so without knowing what the drawings call for and what your friends flywheels measure it's a bit hard to say. Any reduction will mean the engine does not coast for as long between hits.

You can always add a bit more rotating mass in the way of a take off pulley as many Hit & miss engines did not take the belt off the flywheel rim.

Andrew Johnston07/10/2019 19:03:50
5410 forum posts
627 photos

The energy stored in a flywheel is proportional to the mass of the rim and to the square of the velocity at the mean radius of the rim. So, for a given angular velocity, reducing the diameter of the flywheel has a disproportionate effect on the stored energy and is the worst thing you can do. Follow the drawings and buy the correct diameter stock.


Edited By Andrew Johnston on 07/10/2019 19:11:58

john brown 1708/10/2019 12:30:50
114 forum posts
3 photos

Well just had to prove a point,the local farm still uses an old hit an miss engine to drive a saw bench for logs ,and l know they have been getting it ready for the winter job ,now there engine has the two fly wheels and on one side a belt pully ,so asking about it if the belt pully is removed the engine run rough ,put the pully back and the old girl runs sweet ,so looks like jason you are about right.

And yes andrew l think my mate would get the right size stock ,but as he has a length of 150mm dia round ,and its not easy to find bigger ,well unless you buy a full length and take out a loan to buy it ,l think he want to use what he has.


Hopper08/10/2019 12:46:47
4403 forum posts
93 photos

Add a bit extra to the width and it should be ok.

SillyOldDuffer08/10/2019 14:51:17
5624 forum posts
1157 photos

As Andrew said except it may be possible to restore the energy by loading the outer rim with Lead or Tungsten Putty.

Could be more trouble than it's worth. The centripetal forces on the rim of a flywheel are substantial and it's not unknown for them to explode, cast iron being notorious.  The problem is making sure the added weight won't fly off, which probably requires an external rim, or drilling a large number of holes to contain the weights. Making a flywheel from full sized stock avoids such complications.

The extra width needed to compensate for a smaller diameter as suggested by Hopper could be considerable. As the energy in a flywheel is proportional to the square of the wheel's diameter, so the width has to be increased by a distinctly greater weight of metal than is lost due to reducing the diameter. Assuming that is the reduced flywheel must contain the same amount of energy.

Always good to ask 'does it matter'? Building a hot and miss engine is on my to do list but I've no practical experience yet. Flywheel size wasn't critical on the small stationary engines I've built - the engines run OK with flywheels smaller than specified in the plan. No load, so their flywheels only needed to store enough energy to carry the valves past top dead centre. Jason is far more experienced: are hit and miss engines sensitive to flywheel size, or does the builder have plenty of leeway before they refuse to run?



Edited By SillyOldDuffer on 08/10/2019 14:53:48

JasonB08/10/2019 15:51:02
17842 forum posts
1951 photos
1 articles

John, it would be helpful to know what plans your friend is working to.

Georgineer08/10/2019 16:04:06
340 forum posts
16 photos

In Parsell & Weed's 1900 book Gas Engine Construction there is a section on flywheel design:

I haven't studied it in detail, but it gives their reasoning and may help you come to a conclusion.


john brown 1708/10/2019 17:58:46
114 forum posts
3 photos

Jason l did ask him today what his plans for the engine were , and its j.a.m de waal ,the engine is 2x larger than the original kerzel engine,l had a ring round today for him about finding some 200 mm round bar but no luck ,unless he orders 3m of it ,so it looks like one way or the other his 150 mm stock will have to do,l did think that mybe you could make it from 20 mm plate joined together ?


Tricky08/10/2019 18:05:25
47 forum posts
2 photos

M-Machine do black EN3 up to 260mm and 185mm dia is £12.60 per inch

JasonB08/10/2019 18:32:00
17842 forum posts
1951 photos
1 articles

Ah one of Julius' enlarged copies that he has doubled up the sizes of the original Kerzel. Most of his drawings take no account of the scaling factors so a 150mm flywheel of say 40mm rim width by 25mm thickness would do and up the central web thickness to 12mm for good measure.

M-Machine do 180mm cast iron or 7" EN1A by the inch but cheaper and better looking to get a couple of the 7" part machined flywheels from RDG

john brown 1708/10/2019 18:46:12
114 forum posts
3 photos

Many thanks jason will pass that info on to him in the morning,that will make the old fellas day,l no he was getting into a worry about it,again thanks and to the others for there posts,but must admit them flywheels from rdg look good,may have to get a copy of his plans,then again have had a enough problems with the juliius lanz tractor drawings,but with help from this forum am under way ok now,we hope .


Paul Lousick08/10/2019 22:37:52
1376 forum posts
532 photos

If you cannot get a piece of round bar, get a piece cut from plate to the diameter that you want ?


John MC09/10/2019 08:08:26
274 forum posts
38 photos

My advice here would be to use the flywheel thats available, it may well be OK. If not then try again with the size recommended.

I was once involved in IC engine design, one engine, a diesel of about half a litre swept volume had a flywheel about one tenth of what was recommended, it ran fine, it could idle at 500rpm and, much to everyone's astonishment rev to 7000rpm while still making useful power! 5500rpm was the designed maximum rpm.


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