Author Topic: engine size formula  (Read 3421 times)

Offline jct842

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engine size formula
« on: July 23, 2012, 02:41:19 AM »
I have often wondered if there is a formula for flywheel size verses engine size.  What I am getting at is a small engine with a large flywheel does run smoother and slower,  I eventually would like to build one with a over sized flywheel,  way out of proportion.  I have thought of possibly gearing down the flywheel too.  Some where around a 1" bore and a 2" stroke.  Possibly using a auto brake disc for the flywheel since they are easy to come by and a matching bearing and mount can come from a junker.    john

Offline Maryak

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Re: engine size formula
« Reply #1 on: July 23, 2012, 03:07:53 AM »
A very basic starting point is 15 lb per 1 HP.

Now you have to work that back to the engine BHP the size of the rim and the diameter of the wheel to give the equivalent energy in ftlbs.

Safety 1st...............Flwheels if oversped, (peripheral speed) and under designed, (factor of safety less than 10) can burst and even a small one on a model could be very nasty if it's chasing you around your shop.

Machinerys Handbook has a pretty comprehensive section on flywheel design and it would pay to use this or something similar before setting things in motion.

Hope this helps
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Offline jct842

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Re: engine size formula
« Reply #2 on: July 23, 2012, 03:35:08 AM »
thanks, I do have the handbook.  I will do some research with it.   john

(I do want as slow a speed as earthly possible though)

Offline Captain Jerry

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Re: engine size formula
« Reply #3 on: July 23, 2012, 06:13:13 PM »
I have often wondered if there is a formula for flywheel size verses engine size.  What I am getting at is a small engine with a large flywheel does run smoother and slower,  I eventually would like to build one with a over sized flywheel,  way out of proportion.  I have thought of possibly gearing down the flywheel too.  Some where around a 1" bore and a 2" stroke.  Possibly using a auto brake disc for the flywheel since they are easy to come by and a matching bearing and mount can come from a junker.    john

Gearing down a flywheel can work well to simulate a load without distorting the proportions of the model.  Here is a video of an engine that I built last year with a small flywheel that can be started and run on lung power.



This is the same engine but it is driving an 8" bar type flywheel through a 5.25:1 planetary reduction gear.  It is only a temporary jury rigged setup to provide some starting load and slow speed operation to the engine.  The "chuff-chuff" sound was an added benefit.



Jerry
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There are things that you can do and some things you can't do. Don't worry about it. try it anyway.

Offline jct842

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Re: engine size formula
« Reply #4 on: July 23, 2012, 09:06:29 PM »
I like slow.  For me to see each of the engines parts working is a joy.  Just about any one can have an engine go so fast you miss all the different parts working together.  I would love to have a copy of yours on the end of my table to run at will.   john

Offline Dean W

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Re: engine size formula
« Reply #5 on: July 23, 2012, 09:42:47 PM »
I had never see that second iteration of your engine, Jerry.  That's a lot of fun to watch!  Does make quite a good chuff, too.
Dean
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Offline Captain Jerry

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Re: engine size formula
« Reply #6 on: July 23, 2012, 10:29:38 PM »
Thanks John and Dean

I think I might have misspoken about the gearing. In fact it is geared UP so that the flywheel (bar) can be seen whiling around in the background 5 times faster than the crankshaft.  Is that anti-reduction?  Another advantage of this method is that it does not overload the engine bearings with the weight of a huge flywheel.

Jerry
NOTARY SOJAK

There are things that you can do and some things you can't do. Don't worry about it. try it anyway.