So I Have This Crazy Idea...

[CFLBob]

I've built two engines so far, a compressed air-powered wobbler engine and a Flame Eater. I had lots of trouble getting the Flame Eater running, but it eventually did.

While getting ready to start gathering metal for a Webster internal combustion engine, I started thinking that it's on the small side. Where I live, a 49CC internal combustion engine on a bike is street legal. I think these are called mopeds and while they're not powerful by any standards, I think they're faster than a bicyclist. Maybe not a world class cyclist, but still. They produce a useful amount of power and I guess I just like the idea of building an engine that could produce a useful amount of power.

I started looking around for plans but don't find any. I can buy a complete engine for $99, but it goes without saying the goal isn't to buy one to have, it's to build one. Preferably from bar stock.

Would it be too crazy to jump right to the 49cc engine, assuming I could find plans? It seems there are a lot of things I could learn from the Webster that would be needed to make the bigger one work so maybe it's smart to build that first no matter what.

 

[Brian Rupnow]

Go straight to the larger version. You will learn the same things, and may end up with a practical use for it. And--In general, the larger an engine is, the easier it will be to get it to run.---Brian

 

[CFLBob]

Go straight to the larger version. You will learn the same things, and may end up with a practical use for it. And--In general, the larger an engine is, the easier it will be to get it to run.---Brian

Thanks, Brian. I really value your opinion and was thinking of private messaging you, but didn't know if that was OK.

 

[lohring]

You're going to have some problems making a 50 cc engine on Sherline size machines. You could build a twin. There are lots of parts available for very high performance, two stroke, engines in this size range. Assembling engines from these parts with some custom made parts is another path. Quickdraw makes engines with this method. Here is a series of pictures of the various parts to give you an idea of the design of a similar engine. It will be tough to build a high performance cylinder without a casting, but simpler cylinders can be CNC machined. I would especially consider buying the crankshaft and connecting rod assembly from an existing engine. Other parts to buy are the piston, ring, and wrist pin. Below are pictures of transfers in an industrial engine. They could easily be machined and used with a steel sleeve.

Lohring Miller

 

[CFLBob]

Just to be clear, I'm not limited to Sherline machines. I have a 1HP Grizzly G0704 mill (roughly 16 x 6 x 8" envelope) and an 8-1/2 x 20 lathe.

That said, without a set of plans for a 49cc (3.0 cubic inch) engine, something like you suggest of buying some parts and making others might be the only path. If all I could do is buy parts I'm not likely to build one. I could try to get plans for a larger engine somewhere between the 3/4 CI Webster and the 3 CI "49er".

 

[lohring]

Those will be perfect for your size engine. Have you considered designing an engine? If you are interested, I could help. The design can be as simple as this engine in a larger size.

Lohring Miller

 

[XD351]

You could also pick up a broken unit off a line trimmer ( weed whacker ) or similar and copy it .
For 49 cc you are looking at 40 mm bore ( pistons and rings are dime a dozen ) with 39mm stroke or 1-9/16 bore and stroke .
There probably are plans out there for this size engine possibly for larger model aircraft .

 

[CFLBob]

Those will be perfect for your size engine. Have you considered designing an engine? If you are interested, I could help. The design can be as simple as this engine in a larger size.

Lohring Miller

I've never done anything like that, so don't know where to start. There's a useful relationship for how big the bore and stroke need to be, but they make a big area to trade off bigger bore with short stroke or smaller piston with a longer stroke. I have no experience or intuition of how to do these things.

Thar reduces to (2*pi)*(bore^2*stroke)

For a 3.0ci displacement, a 1.5" piston takes a 1.7 in stroke, while a 1" piston needs 3.82 stroke. 3.82 doesn't sound very practical, but that's all I can say.

 

[CFLBob]

You could also pick up a broken unit off a line trimmer ( weed whacker ) or similar and copy it .
For 49 cc you are looking at 40 mm bore ( pistons and rings are dime a dozen ) with 39mm stroke or 1-9/16 bore and stroke .
There probably are plans out there for this size engine possibly for larger model aircraft .

Great ideas. Thanks.

 

[Brian Rupnow]

CFL BOB--This has been a very popular engine, hundreds of people have built it. It is an engine I designed and built and there is a build thread on it on this forum. It is a 1" bore.
http://www.mediafire.com/file/fh7wnz2d4cgt0s4/VERTICAL_CYLINDER-RUPNOW_ENGINE.zip/file
https://www.homemodelenginemachinist.com/threads/vertical-i-c-rupnow-engine.25727/

 

[Brian Rupnow]

I also have plans for an opposed cylinder air cooled engine with 1" diameter pistons.--this would be more suited to powering a bicycle. I built and designed this engine a few years ago. I sell the plans for this one for $25 Canadian funds paid to my paypal account under [email protected] This engine is a very nice running engine, but it's not for sissies. I think you would want to have built two or three i.c. engines before you tackled this one.---Brian
https://www.homemodelenginemachinist.com/threads/opposed-twin-i-c.24899/

 

[CFLBob]

This engine is a very nice running engine, but it's not for sissies. I think you would want to have built two or three i.c. engines before you tackled this one.

It is a nice looking engine as it's running. I will respect your advice and restrict myself to single cylinder engines - for now.

The engine that XD351 describes, 40mm diameter pistons and 40mm stroke, or about 1-9/16" bore and stroke, is very close to the 49CC or 3.00ci numbers I'm leaning toward. I downloaded the zip file of your vertical engine. I had to be out a few times today, so I didn't search much for engine plans. I'll do more looking tomorrow.

 

[popnrattle]

I've never done anything like that, so don't know where to start. There's a useful relationship for how big the bore and stroke need to be, but they make a big area to trade off bigger bore with short stroke or smaller piston with a longer stroke. I have no experience or intuition of how to do these things.
View attachment 107722
Thar reduces to (2*pi)*(bore^2*stroke)

For a 3.0ci displacement, a 1.5" piston takes a 1.7 in stroke, while a 1" piston needs 3.82 stroke. 3.82 doesn't sound very practical, but that's all I can say.

hello, i keep plugging the bore and stroke values into above equation with much higher than 3 c.i.. ima getting 24c.i. please check formula. thanks. later, rt.

 

[Jennifer Edwards]

Hi Bob,

Just my two cents:

whilst a two stroke is pretty simple to build compared to a four cycle engine, you may want to consider the latter as they generally have more Torque at the low end than a two cycle of the same displacement. If you are looking to use the engine to power a vehicle it is a consideration you should not ignore.

With adequate low end power you could save yourself the complexity of a gearbox/clutch by using a simple centrifugal (slipper). clutch like those on a chain saw.

Whatever you do remember no idea for a project in a hobby you love is a “Crazy Idea”.

Jenny

 

[CFLBob]

hello, i keep plugging the bore and stroke values into above equation with much higher than 3 c.i.. ima getting 24c.i. please check formula. thanks. later, rt.

The formula is one of those geometry formulas you can look up anywhere, the volume of a cylinder. They just say bore and stroke, where the geometry formula says diameter and height.

The only thing I found that gives odd results is that it needs the same units you're looking for, so if you want CC you have to enter the numbers in cm not mm.

pi/4 is 0.7854, so using inches, the first one I mentioned turns into displacement is 0.7854* 1.5*1.5*1.7 = 3.004

The example XD351 used of 40mm bore and stroke is odd because the answer comes out in cubic millimeters unless you convert to 4.0 cm first

0.7854 *(4*4)*4 = 50.27 cc. A 40mm piston needs a 39 mm stroke if you want to get 49cc.

Hope that helps.

 

[goldstar31]

49cc is the classic moped size and quite a capable machine was the NSU Quickly. 25mph isn't bad.

What my sit on lawnmower is capable of is anyone's guess. Quite fast, 4 stroke and a lovely crash gear box beloved of us oldies who could 'double de-clutch'.

Again, the French, the Italians and the Spanish had any number of these mopeds with more noise than speed.

Even the late 'Ned' Westbury serialised a 49cc two stroke 'Busy Bee' in Model Engineer when my remaining few strands of hair were not grey.

Then there was the Corgi which were dropped with the British Para's. I recall mending one -which belonged to a rather nubile young lady- and was rewarded with 'favours'-- nudge, nudge,, wink , wink.

Now my 8 year olds do 'wheelies' with grandpa's mower.

It's just a case of more time 'Googling'

Have fun

Norm

 

[CFLBob]

Hi Bob,

Just my two cents:

whilst a two stroke is pretty simple to build compared to a four cycle engine, you may want to consider the latter as they generally have more Torque at the low end than a two cycle of the same displacement. If you are looking to use the engine to power a vehicle it is a consideration you should not ignore.

With adequate low end power you could save yourself the complexity of a gearbox/clutch by using a simple centrifugal (slipper). clutch like those on a chain saw.

Whatever you do remember no idea for a project in a hobby you love is a “Crazy Idea”.

Jenny

Thanks, Jenny.

I wasn't really clear that I don't really want (or plan) to put this 49cc engine onto a bike. The reason I got interested in doing a engine like this is I'd like to stretch up from engines that move but can't do useful work. Whenever people who aren't model makers see an engine, they ask "how much power is it?" I figured the 49cc engines are street legal here in Florida, so that must be about the limit of useful work. I didn't know they get used in the big weed trimmers and chain saws. That just makes them sound even more useful.

My question about "am I crazy" is starting from the idea that in other hobbies I've played with you can do a first project that's way too big for you. Let's say I made this as my first IC engine and takes me so much longer to make it than it would have taken to make a smaller one first, and then the bigger one, put together. Maybe I could make three smaller engines in the time it takes to make the one bigger engine. Then it wouldn't make sense to build the bigger one first. Does that make sense?


Bob

 

[Jennifer Edwards]

Thanks, Jenny.

I wasn't really clear that I don't really want (or plan) to put this 49cc engine onto a bike. The reason I got interested in doing a engine like this is I'd like to stretch up from engines that move but can't do useful work. Whenever people who aren't model makers see an engine, they ask "how much power is it?" I figured the 49cc engines are street legal here in Florida, so that must be about the limit of useful work. I didn't know they get used in the big weed trimmers and chain saws. That just makes them sound even more useful.

My question about "am I crazy" is starting from the idea that in other hobbies I've played with you can do a first project that's way too big for you. Let's say I made this as my first IC engine and takes me so much longer to make it than it would have taken to make a smaller one first, and then the bigger one, put together. Maybe I could make three smaller engines in the time it takes to make the one bigger engine. Then it wouldn't make sense to build the bigger one first. Does that make sense?


Bob

Hi again Bob,

My personal take in the larger/smaller question is this:

For me anyway, I find that a larger engine is much easier to make because the dimensions of the parts are not all these teeny tiny bits that are super tedious to make.

For example it is much easier to turn a 4 or 5mm valve stem vs say a 1.5 mm one. Personally working on small bits either on my lathe or mill require much higher speeds and super accurate setups.

Also my old eyes are not what they used to be the bigger the easier for me to see.

Of course there is s limit to how large this “rule” applies to. At some point I guess things can get too big for comfort as well.

 

[XD351]

I didn’t bother with any math i just used an online calculator for engine capacity . I know from past experience that that a 40 mm bore and 40 mm stroke was around 50 cc as i have bought pistons and ring for these engines off ebay - why stuff around making rings when you can by a set for $3 ! I also picked up a reamer that is a couple of thou under 40 mm so i can ream then hone a bore to final size easily , i can either make a piston or buy one off ebay for a few dollars .

 

[CFLBob]

I didn’t bother with any math i just used an online calculator for engine capacity . I know from past experience that that a 40 mm bore and 40 mm stroke was around 50 cc as i have bought pistons and ring for these engines off ebay - why stuff around making rings when you can by a set for $3 ! I also picked up a reamer that is a couple of thou under 40 mm so i can ream then hone a bore to final size easily , i can either make a piston or buy one off ebay for a few dollars .

I'm with you on those. It's nice to say "I made every part from bar stock", but I think your way is really good. There are minimum purchases for metal, and I'm sensitive to buying large pieces to cut off an inch or less. Some scrap is always good to have around for odd projects, but too much is too much.