# Making an Engine Out of Scrap

Discussion in 'All Things Boats & Boating' started by confused, Mar 17, 2009.

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### confusedNew Member

thanks,

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### marshmatSenior Member

Hi Mike,

Is it possible? Maybe. (Not with a lead piston though.)

Do I think it is a good idea? That would depend on your design calculations. Can you prove, mathematically, that this engine will work? Can you give the temperature, pressure and volume at each key point of the cycle? How about the residual gas fraction, or the hoop stress in the cylinder wall? The piston speed curve at operating speed? The peak moment on the crankshaft?

These are all things that need to be calculated very early in the design of an engine, long before you can start thinking about how to actually put it together. Every successful IC engine ever built has a detailed set of design calculations, prepared and checked long before lathe touched steel. And there is a long, long list of engines built without such calculations- some did nothing, some fell apart, some killed their builders.

I know it sounds like a fascinating project. But do keep in mind that engine design and testing is an expensive, dangerous task at the best of times; it is nearly impossible without the calculations to guide it.

If you're still interested, check out the following books (or similar ones, just ask any university librarian or Mech.Eng. prof), learn the math in them, learn the concepts, and then you'll be in a better position to evaluate the idea:
Fundamentals of Engineering Thermodynamics, M.J. Moran and H.N. Shapiro, Wiley, 2004.
Internal Combustion Engines, Applied Thermosciences, C.R. Ferguson and A.T. Kirkpatrick, Wiley, 2001.

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### confusedNew Member

Can you give me some formulas for calculating "Temprature, Pressure, and residual gas fraction, or the hoop stress on the cylinder wall," I haven't really started on that stuff yet.
thanks,

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### confusedNew Member

thank you. you have helped me alot. I thought the lead piston Idea was not such a good idea. I might just buy some brass round off ebay and cut it to size to fit my engine. Of course I will read the books first!
thanks,

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### marshmatSenior Member

Hi Mike,

Unfortunately, it's not so simple as to be explainable in a few sentences on a forum. I wish it were. But if you want to understand enough about thermodynamics and machine design to be able to safely do what you're talking about here, there's no way around it- you're looking at several hundred pages of reading in each of half a dozen textbooks, and doing the associated practice problems yourself until you really understand where the numbers are coming from.

You could, of course, just slap something together out of scrap and pray that it starts without killing you. Keep in mind that an engine is basically a bomb inside a really big gun, with a crankshaft on the bullet. But that's not likely to work very well, and you won't learn anything useful from it.

Don't get me wrong, I'm not trying to discourage you. I love it when folks like you come up with neat ideas and really want to give them a try. (It wasn't that long ago that I was in your position!) But I do think you ought to be aware of just how difficult a problem you're up against, and I do think you ought to be aware of the resources that have already been prepared- at great expense, by world-leading experts- to guide budding engineers like yourself towards solutions that will do what you want them to do.

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### confusedNew Member

I've read alot more than you would think . I'm about to start reading a 562 page book.

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### LandlubberSenior Member

Great, read the book then come back and see if we answer more for you.

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### PARYacht Designer/Builder

The easiest way to build a motor from scrap is to use compatible scraps. An example would be the cylinder jug from a Harley Davidson , with a Chevy piston and connecting rod,, maybe married to a home made crank. You could home brew a "L" head arrangement for the valves, which eliminates the need for push rods, build the cam into the end of the crank, etc. but you do need to have a pretty fair understanding of how things work, basic engineering principles (like why and where to place a thrust washer, or why a lead piston can't work, etc.).

Without these skills, you're shooting in the dark.

Small gas engines are a dime a dozen, literally you could have a handful of 3.5 to 5 HP gas engines for a 100 dollar bill. Okay, they'd need to be rebuilt or at least resealed, but you'd have the parts to build at least one good one from this harem of engines.

I know a guy that has built a 5 cubic inch, flat head (L head) V8. It's air cooled, can sit in the palm of your hand and sounds really cool when running. It runs on alcohol and is the result of several years of machining by a fellow who's got 30+ years as a master machinist. It can be done, but it does demand more then just a casual interest in the skills necessary to make these types of adventures work.

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### confusedNew Member

Yeah, I know alot about engines. more than you'd expect. speaking of small engines, I need to find a small piston, like 19.4 mm. I don't know of any small engines that have that exact measurement of the piston.
It would be like trying to find a needle in a haystack. I need someone that custom makes pistons. Heck, custom makes engines...
thanks,

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### LandlubberSenior Member

Mike,

If the piston you need is just a bit too big, just turn it down in the lathe to suit, you can shave a small amount off with no problems, the gudgeon pin will still most likely fit anyhow without modification, otherwise just make one from billet. If you think that you are going to play engineer, you WILL need some machinery, doing things by hand is a pain, I know because I made a steam engine by hand, never again.

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### confusedNew Member

I have figured out the cylinder and piston problem! (Actually the whole ENGINE problem) I just remembered that my dad gave me a weedeater engine with a leaky gas tank, but I can't find it. well, I haven't been able to find it ever since my dad gave it to me. So, that is what I'm going to do on my spring break, is find that engine. What would happen if I put a 5 1/4 inch 6 lb flywheel on that engine, would it increase torque?
thank you very much,

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### LandlubberSenior Member

yes, it would......but at the risk of breaking the crank

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### PARYacht Designer/Builder

It's unlikely the 2 stroke would start, let alone spin up a heavy crank weight. To fling a heavy crank weight, you need a sizable amount of reciprocating mass, which a weed eater engine just doesn't have.

Placing a crank weight on an engine is just a burden and completely unnecessary, except for a few engine designs. Modern, light weight, relatively high revving engines just need enough weight to balance reciprocating mass, no more or you're just reducing available power. In fact, every effort is used to reduce the reciprocating and rotating mass on these little gas engines, just because they can't afford the power loses associated with it.

On a "make and break" engine or some, very inefficient slow turning engines of considerable internal mass, they have used crank weights, but these are the exception to the weight rule.

You should look into make and break engines as a possible option, as they're a lot easier to make then a modern engine. Of course they don't develop the power, but they're a lot easier to figure out and build.

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### LandlubberSenior Member

I have one in the shed at home, it was used as the genset in Darwin during the war, only ever started every few years or so, it is guaranteed to run, quite amazing really, I think it is a Mc Pherson from memory, but I am not in Brisbane at present so vcannot check.

It is not particularly portable though of course PAR......not unless you own a truck.

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