Rena Crank Works

[WXYZCIENCE]

The crank is coming along well. Here is the number one journal and connecting rods. All eight journals were machined together, then reamed on a machined lathe cup chuck. This was done so that the journals run very square to the main shaft.




The pistons are stationary, so an addition linkage is required to complete the connection. The piston units are aircraft auto pilot cylinders.







Manufactured in 1945, the bore is 1.25" and the stroke is 4". The are double acting and made from aluminum with steel sleeves. I purchased 4 of them for twenty buck each. Each of them also has a small locking piston that I will use to balance the pressure and lubrication control.


Here the four cylinders are set to the crank for alignment measurements. Main crank shaft bearing will be added between each journal.




Only the cylinder on the right is completed. There is a 90 degree shift between each cylinder. The front end of each cylinder will be used for the hot side and the rear for the cold.


The Rena Stirling Engine requires four regenerative heat units and four separate cooling units. This is the main part of a pressurized air exchanger that will transfer the heat to the working fluid. It was an air cooler for a heavy truck blower. I should be able to transfer 50,000 btus through this puppy. I intend to heat the hot side working fluid to 320 degrees C.







The heated air will then be passed through a copper/carbon regenerater into the water cooled exchanger. This should complete the stirling cycle.


I will post a simplified diagram of the unit in my next post. This should help those who are new to stirling engines understand them a little better.


I like to build stuff because in my mind it works real good. Simple, take it out of the mind and build it. How close I get, tells me how sane I am.
Joseph.

[Nando]

Joseph:

I hope You are sane !!

And of course your project is a success.

Nando

[willib]

Joe , have you thought about using a small 4cyl engine crankshaft ?

you could use the connecting rods also

i would very much like to see the diagram or the rena (simplified)

as much time as you have put into it allready , i would hate to see it fly apart at high speed

most engine cranks are a bit beefier and are cast iron , for good reason.

the stresses involved , at the speeds you are contemplating ...

i'm sorry , but i cant see it lasting a few min/seconds at 500 rpm.

just my opinion , but maybe at this point in time , before you go any farther and put any more time in an unworkable crankshaft, ...

 a junkyard crankshaft is cheap enough , for sure

you do nice work ,

Homemade crank-> bad idea

storebought (or junkyard crank ) good

[pepa]

hi willib, with a support bearing on each side of each journel, he should be fine as long as the crank shaft is well balanced. i sure wish i could do machine work like that, great work joseph. pepa.

[WXYZCIENCE]

Willib, I expect to get 1/4 to 1/2 a horsepower from this engine. The challenge is making the crank run perfectly square. Also the rpm of this engine will probably not exceed 1500 rpm. The working fluid (air) can not expand and contract fast enough with the heat energy supplied ( around 300 degrees C.). I may try a different gas, if the engine runs as expected.
The weight to power ratio of a Stirling Engine makes them impractical for most vehicle use, but for production of electricity they are proving to be well suited. Besides a co-generation into heating domestic water is also a plus. Thanks for your input.
Joseph.

[richhagen]

I don't understand how it will operate, but the machining is marvelous.   I guess if those compression fit clamps where the aluminum is fastened to the shaft work on the crankshaft at up to 1500 RPM, the little ones I built for my Ameteck motor shafts should be fine.  Rich

[Stonebrain]

Hi Joe,

I think for finding a cast crankshaft with the right stroke

and spacings you must be very lucky.

I have a question:

Wouldn't it be good,for eliminating source of vibration at high

speed,if every separate crank was balanced?

I'm not very connaiseur of motor technology,but that's what

my intuition says.That's often,but not allways,wrong.

cheers,

stonebrain

[maker of toys]

a couple reasons not to use a junkyard crankshaft:  a single-plane crank, with a single-plane cylinder 'block'  denys the the possibility of self-starting. Too, you'd have to look a long time to find a 4" stroke 4 cylinder engine. . . maybe a diesel crank out of a piece of construction equipment, but that won't be cheap. (or light)

having a two-plane crank as shown here allows the engine to self-start without user intervention.  you could step up to a big-block V-8 chevy crank and get the phasing right for self-start, but you might still have to mess with regrinding to get the right stroke.  Plus that's a LOT of mass for a 1hp engine.  

on the flip side:  People who build model engines grind/build up cranks quite frequently, and rarely have problems, even at 10,000 rpm. at 1 HP, 1500 rpm, I don't forsee any show-stoppers, but short dowel pins to keep the webs from fretting could be a reliablity refinement. (would also make phasing the engine easier?)

on to the questions:

So I'm looking at this, and thinking that you're likely to have a nasty rocking couple on this thing.  Not that it can't be done; look the flogging NASCAR gives a similar crank. . . a little bit of counterbalance helps a bunch.

I'm just curious why you chose that form over one with better primary balance. . . any reason not to make adjacent pairs 180' out from each other; maybe go down to a 3-main crank with a double-length web beween the two throws;  and change planes across the center main?  true, you'll still have some primary imbalance and some secondary couples, too, but it seems like the magnetude would be smaller?

another question:  are you planning any sort of cross-head to keep from wearing the packing glands out with side thrust?

just thinking out loud. . .

(can't wait until my milling machine arrives. . .)

-Dan

[WXYZCIENCE]

Dan, I was planning the dowel pins after I get the phase correct. This stirling design does not have a compression stroke. It uses both the expansion and contraction of the working fluid on all cylinders. The movement is amazingly smooth. I will add counter balance weights to each arm later. One of the main problems in making this type of design work is the piping from the hot and cold exchangers through the regenerators. Position of the cylinders is the main reason why I chose this type of crank. A very constant expansion and contraction of the working fluids volume is not easy to obtain. The heat exchanger has to be positioned just above the cylinders with the piping as short a possible.


The cylinders are very unique in the fact that they have an additional bronze shaft support and oilers in front of the triple seal front packing.


The engine will actually stand vertically with the crank and flywheel assembly on top. This configuration will make the engine over three feet tall.
Joseph.

[maker of toys]

Yeah, I think I understand that the stirling is an (effectively) closed, relatively (generally low) constant pressure, variable displaced volume cycle, with a recuprative element to reduce heat rejected to the environment. (not having built one of my own, I'm not completely clear on all the implications of that statement, other than that leaks and long plumbing runs are really bad news. I'm following Ed's and your experiments with some relish to see how things work out. . .)

perhaps I was not clear in my wording. . . I was wondering about the form of the crank alone, not the position of the cylinders. . . . I had originally thought that an opposed cylinder arrangement would be a better fit, but the plumbing seemed a little involved when I got to assembling the thing in my mind. . .   I then realised that many of the same benefits accrue to using the same 'opposed' crank with an inline layout; the 'twist' (if you'll pardon a bad pun) was to rotate the second pair of crank-throws 90' to the plane of the first pair.  just looking at the balance of the crankshaft as you've laid it out above.

of course, I'm not the one building this engine, (yet) so I guess I'll just clam up and watch.

I should have guessed that the Sperry guys would have thought about cross-head wear when they designed those cylinders. . . . do you have any pictures of that end of the cylinder you can post? (i'm curious is all. don't put yourself to extra effort on my behalf.)

keep up the good work!

-Dan

(off to wikipedia to see what horrible oversimplifications I've just made. . .)

[powerbuoy]

The chance of finding an off the shelf crank for your beast is slim to none. Your are doing the right thing. I am looking forward to see this one in operation. (Can't wait) Are you planning on pressurizing it?

Powerbuoy