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Old 05-19-2011, 10:59 PM   #1
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Default Compressed Air V8

This engine was started a while ago and I just had to experiment with the crankshaft as it seemed that everyone said they were difficult. I posted my attempt here :-

http://www.homemodelenginemachinist....?topic=11472.0

It turned out so well that it was good enough for this compressed air V8. The air porting was to be in and out like my Scrap Bin Motor (Thanks for all the interest) but Chuck Fellows showed how he had made his porting with a ball and spring. This is so elegantly simple that I had to copy this idea for the V8, so that meant I had to rethink my ideas. The Twin D/A Oscillator took up some of my spare time so the V8 has been on hold.

However I am back to it now. It will be 12mm bore 12mm stroke with the "camshaft" in the middle of the V driven at the same speed as the crankshaft by nylon gears at the front. On an air engine of this size, three bearings should be enough, ball races front and rear and a split plain bearing in the middle. The experimental pistons will be made from PEEK which is a high temperature plastic which turns easily is harder than nylon and does not have the same water absorption problems. It is expensive but I managed to strike lucky on Ebay.

Here is the "Proof of Concept" drawing :-



It will be made from brass bar stock. The finning is just bling. Lots of detailing to do yet, exhaust should be 4 into one both sides, so I am hoping that it will sound interesting!

Ian



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Old 05-20-2011, 01:21 AM   #2
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Default Re: Compressed Air V8

Hey Ian, this looks like a great project. Glad to see someone trying a V8. Have you considered using a hollow tube cam like I used on the opposed 4? I think air leaks might be less of an issue...

Chuck



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Old 05-20-2011, 10:52 AM   #3
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Default Re: Compressed Air V8

peek is good stuff I use in for the piston valves on my 5 inch gauge loco's I just make the polo mint like bits from peek the rest is Stainless. make it cheaper to replace /adjust them

they just work fine but the price for 25 mm dia is ouch



Stuart

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Old 05-22-2011, 12:13 PM   #4
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Default Re: Compressed Air V8

Looking forward to watching your build, Ian.

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Old 05-22-2011, 03:23 PM   #5
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Default Re: Compressed Air V8


Chuck,
Thanks for looking in, I will need some hand holding on this one.
Air leakage.
The top half of the crankcase will have the valve shaft running in a reamed hole.
No problems on the ends. The air connection will be on a manifold bolted to the back end of the crankcase and the front will be sealed by the face of the nylon gear with a wavy washer for tension. You might check my logic on the next bit. The engine will be running as a "two stroke", this will mean that at any "TDC" two cylinders will be "firing " together in opposite banks for 180 degrees. This means that the air supply is always flowing and the only back pressure will be from the springs pushing the steel balls into the closed position. So, hopefully there will be little air leakage along the length of the valve shaft.

Stuart,
I have built up a small hoard of PEEK from looking on Ebay there are sometimes bar ends for sale, it does not seem to attract much attention. It is harder than PTFE, a bit like acrylic and machines nicely. I will experiment with it for the pistons, using PTFE as plugs at each end of the gudgeon pin (wrist pin).

I made up a conrod, just to see if I could! The screws are stainless M2 panheads. Pictures of my method will follow when I make the rest.




Ian
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Old 05-22-2011, 03:54 PM   #6
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Default Re: Compressed Air V8

Ian- If that con rod is any indication, this will be a very nice looking engine. A two stroke with 4 into 1 exhausts
should sound great.

Mike

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Old 05-22-2011, 04:25 PM   #7
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Default Re: Compressed Air V8

Very nice job on that con-rod. Did you use any jigs, CNC, or

The air leaks, if any, are more annoying than problematic. The only issue you might have is air leaking across into adjacent cylinders. My air engines are usually made to look and sound like 4 stroke IC engines and the sound of hissing air definitely detracts from that vision. I've just never been able to get that kind of valve rod to seal very well. Probably just me.

Chuck

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Old 05-23-2011, 10:31 AM   #8
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Default Re: Compressed Air V8

Chuck,

No jigs or CNC on this one, just made it up to see what problems there were.
I did think of using a long spool valve fitted with "O" rings to keep it air tight, but decided that it was better to keep it simple!

Thanks all for your interest, you may need patience as this one, it may take some time.

Ian

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Old 05-23-2011, 01:04 PM   #9
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Default Re: Compressed Air V8

Ian

What a great design, I've got a design bouncing arround in my head for a V4 that was stimulated from Chucks great designs, you section drawing as helped get a few issues clear in my mind, its going to be a good few months before I start to cut metal, so I'll be following your progress with great interest.

Stew

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Old 05-30-2011, 12:42 PM   #10
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Default Re: Compressed Air V8

Need a bit of advice.

The simple valve shaft would only have four cutouts for feeding air into adjacent, oposite cylinders, giving me a firing order of 12567834 giving equally spaced pulses into the exhaust pipe. If I went to eight ports on the shaft, I could get the usual 18436572 for a full size V8. As I understand it the reason for the "burble" on a full size V8 is that this firing order give exhaust puffs that are 1357 and 8462 into the pipe, but in the gap between 1 and 3 firing, 8 and 4 fire, in the gap between 3 and 5, 6 fires but the is no gap between 5 and 7. My brain hurts

So is anyone able to advise me whether small model V8s "burble"?

Ian



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