# Compressed Air V8



## dvbydt (May 19, 2011)

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.com/index.php?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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## cfellows (May 19, 2011)

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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## lordedmond (May 20, 2011)

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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## T70MkIII (May 22, 2011)

Looking forward to watching your build, Ian.


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## dvbydt (May 22, 2011)

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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## maverick (May 22, 2011)

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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## cfellows (May 22, 2011)

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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## dvbydt (May 23, 2011)

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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## SBWHART (May 23, 2011)

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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## dvbydt (May 30, 2011)

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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## dvbydt (Jun 1, 2011)

Answering my own question, yes they do "burble". On this very impressive site he has a video of his supercharged V8 running.

http://www.weberprecision.com/

But I think for my simple engine I will stick with even pulses on the exhaust, may sound like ripping cloth rather than a f***ing frog!

Ian


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## dvbydt (Aug 10, 2011)

I'm getting a bit of time to play with this engine. The trial conrod above was made from brass but I decided it would be better to use leaded bronze for the the real thing. So I cut off some disks and after a lot of milling and fly cutting, I now have these blanks :-











This is my drawing :-






And since I want them to be all the same ;D this is the drilling fixture :-






So, hopefully, the next post should be photos of the blanks being machined.

Ian


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## dvbydt (Aug 12, 2011)

Next job is to drill four holes in each blank using the fixture. Just used my 4mm spotting drill. The 

fixture is 0.25mm thinner than the blank and is clamped in the vice, then the blank is held in 

position by tightening the vice.

http://i757.photobucket.com/albums/xx213/dvbydt/Air%20Motor/Compressed%20Air%20V8

/Drilling.jpg

Then drill and ream the small end.

http://i757.photobucket.com/albums/xx213/dvbydt/Air%20Motor/Compressed%20Air%20V8

/Reaming.jpg

All done!

http://i757.photobucket.com/albums/xx213/dvbydt/Air%20Motor/Compressed%20Air%20V8

/40Holes.jpg

Now for the M2 tapped holes using the same fixture vertically this time. This is the edge finder I 

am using for these set-ups. 

http://i757.photobucket.com/albums/xx213/dvbydt/Air%20Motor/Compressed%20Air%20V8

/EdgeFinding.jpg

Drilled the hole and hand tightened the tap in the chuck and tapped under power. I only need to 
get a few threads down so that the thread is straight in the hole, it will be hand tapped through 

later.






Ian


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## dvbydt (Aug 14, 2011)

Fixture in use again to drill the caps.






Then the caps were milled for the screw heads.






Used this to make a wedge for the flank angle.






All done.






Ian


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## dvbydt (Aug 14, 2011)

Milling 45 Degree on the big end, if thats what it's called on this size of conrod!






Finish tapping the M2 threads. Do you like my tap wrench?






Removed the excess from the little end. I have been using this diamond lap for deburring, it is much kinder on the edges than a fine file. As an aside it also good for plastics.






Ian


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## Jeremy_BP (Aug 14, 2011)

Fantastic work, I can't wait to see how this goes!

Yes, I do like your tap wrench.


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## dvbydt (Aug 16, 2011)

Jeremy, this a photo of my 3 small tap wrenches for M2, 3 and 4 and an extension. The idea is that it restricts the torque I can apply. If they are difficult it is because the tap is blunt or it is not perpendicular to the job.






Back to the conrods. My rotary table was used to round the small ends. The head of the M4 screw was turned concentic with the screw and used to centre the rotary table.






Then the radius was milled.






The caps were screwed on.






And the 10mm hole spot drilled, drilled and reamed using the fixture upside down this time.






Next I made a 10mm mandril for the lathe and the sides of the conrods were turned.






To be continued............. 

Ian


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## dvbydt (Aug 18, 2011)

After turning both sides the flutes need to be milled.






Next job is numbering, then deburring and polishing.






Another view.







And mounted on the crankshaft.







I've still got to decide where to drill the oil holes, but that can wait.
Well, that's the first milestone reached. The order for the rest of the engine is:-

Crankcase and valve gear.
Sump.
Cylinder blocks.
Exhausts.

All these still have to have detail drawings done, so a bit of work to get through before I start machining again.

It may be a while before I get back to it, a friend has a 5 1/2 ins gauge electric garden railway and wants me to replace the 12 bogie bearings on the engine. Should proove interesting.

Ian


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## cfellows (Aug 19, 2011)

Looks really nice. The con rods and crankshaft are very impressive.

Chuck


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## kustomkb (Aug 19, 2011)

Very nice work!

That last pic looks like a piece of jewelery.


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## dvbydt (Aug 22, 2011)

Chuck, yes I'm pleased so far. Based on your earlier comments re. air leakage and my results with the Scrap Bin Motor, I am experimenting with a new idea for the inlet valve system and will have to build a single cylinder mockup to see if it will work.

KustomKB. my wife wanted one of the conrods for a pendant. She lost interest when I told her they weren't gold.
Ocasionally she gets a metalworking present :-







The pendant is 316 stainless and the rings are titanium. And yes the box is made from billet wood on my vertical mill!

Ian


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## dvbydt (Sep 17, 2011)

[The bearings in the axle boxes were replaced and the 6 wheel bogies modified so they can now go round a 20 ft. radius track]

Right, back to the V8. After a lot of time spent on experimenting, I came up with this system.






The inlet air is cam controlled by a push rod operating on a steel ball, when this shuts off, the small spring closes the shuttle valve and opens the exhaust port.

Here is the vdeo :-

[ame]http://www.youtube.com/watch?v=ZqrsJDzrOCs&feature=player_detailpage[/ame]

There is still air leakage with this valve system and I would like to get lower with the rpm, so still more experiments to come. More blocks of brass with more holes than Emmental!


Ian


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## cfellows (Sep 18, 2011)

Nice running little engine! I can hear the signature sound of the slave valve operation.

You can probably lower the attainable idle speed by using a weaker spring. However, that will tend to hurt high speed operation because it can't return the slave valve as fast. The ultimate solution would be a pneumatic return valve, but that complicates things by quite a bit.

Chuck


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## dvbydt (Sep 19, 2011)

Chuck, yes it does run quite well, but is very sensitive to adjustments. It seems to like a low volume, high pressure
supply best. It would probably be better to use a speed controll on the exhaust, like my double wobbler. But it does consume a lot of air. Bearing in mind this is a V8, I will have to get it much more efficient at low revs or get a bigger compressor ;D

The rotary valves seem impossible to seal well, so I tried a disc valve.











The problem was that if there was enough pressure from the wavy washer to seal it, the friction was too great for the engine to run. I tried a small powerful magnet, with the idea that it would seal without pressue but the friction was again too great, so it had to be a cam, push rod and ball for air supply.

Because the crank has 4 throws and I am running in "two stroke" mode, 2 cylinders will "fire" at the same time. This means that whatever system I use will have 4 air control valves.

Work is going to get in the way of playtime but I have an idea that should work for a positive, low loss valve system.

Ian


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## dvbydt (Jan 22, 2012)

I had to re-read my thread to see where I had got to.

All the different steel ball sizes, porting and springs could be made
to work over a narrow speed range but Whilst experimenting with the 
exhaust ball valve, I noticed that if the ball was held off it's seat
the engine speeded up! So I drilled a 1.0 mm hole 10 mm long and
used this instead of the ball valve and spring.

This is the video :-

[ame]http://www.youtube.com/watch?v=04RBvolzaiA&context=C308de14ADOEgsToPDskIfPb_rKcS3JNddw-wrCNkr[/ame]

The engine runs twice as long before the compressor kicks in, even though
there is a continual leak of air through the exhaust. It will run quite
happily from 300 to 3,000 rpm by restricting the input and has much more
torque than any previous experiments.

However, I still don't like the intentional exhaust leak, so I have
decided to use a cam operated ball valve like the inlet.
This means a central cam shaft in the "V". So, I might just as well
make the "V" 45 degrees rather than 90. This will allow me to get eight
sequential pulses per rev, rather than 4 double pulses.

The cam will be high because of lack of room and this engine is small,
so I am going to try the NSU Ultramax drive system but first I have to
understand it - see :-

http://www.homemodelenginemachinist.com/index.php?topic=17315.msg177899#msg177899

The plot thickens.

Ian


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## cfellows (Jan 22, 2012)

Okay, I've got to try that. Time to drag out my prototyping engine and make yet another head for it. Or maybe I can adapt one I already have...

Chuck


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## doc1955 (Jan 22, 2012)

You are doing some really nice work there Ian That crank assembly looks impressive to say the least.
Nice!!!


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## cfellows (Jan 22, 2012)

That's really a nice looking engine. Has kind of an industrial look about it. The piece sticking out over the top of the flywheel looks nice. Does it serve any function?

Chuck


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## dvbydt (Jan 23, 2012)

Doc, thanks for the kind words - it is a long way yet 'til it runs 

Chuck - as you know the engine was a quick test bed to try out ideas. The brass lump was so that I could vary the spring pressure on the exhaust ball valve, no longer needed but the exhaust sounds better!
I hope you do give this system a try, it is very simple and your take on it would be interesting. I am just amazed how well it worked.

Ian


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## willburrrr2003 (Jan 23, 2012)

My friend, you are an artist as well as a machinist! :bow: Simply beautiful work on this project!!

Regards,

  Will R.  Everett, WA.


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## dvbydt (Feb 10, 2012)

Thanks Will, I hope I can find enough time to actually finish it!

Since I have been able to scale down the NSU cam drive system, I'd better transfer it over to where I want to use it.

Side View






The bits






The video

[ame]http://youtu.be/Z4vHcDu1XfU[/ame]

 The centre distance on this video is 40 mm and it is here driven by my battery drill, maximum speed shown is about 900 rpm. Using my mains drill, it will run up to 3,000 rpm easily. So it now needs some design work to get it to fit on the end of the crankshaft. In this application it does not need a gear reduction. 

No real attempt at precision here, so, it looks like the assembly is tolerant of a bit of missalignment.

Ian


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## dvbydt (Oct 27, 2012)

This project has not been deserted, just not been able to get as much time on it as I would like.

I have drawn up the design of the 45 Degree Block.






To recap, this will give a power pulse 8 times during one rotation.
So onto the Blocks, made from 45 mm dia brass bar.






The bores were then drilled and reamed 14 mm Dia.






Drilling and reaming for the valve ports :-






More to come.

Ian


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## dvbydt (Oct 29, 2012)

Next job was the two end plates, they are "Y" shaped but by first making them out of rectangles I could hold them in the vise and use my DRO scales to get all the hole centres accurate.






Then I could rough cut them on my bandsaw and finish to size on the vertical mill.






Using two M4 cap head screws, the blocks were bolted together and the ends milled to suit the end plates and the end plate fixing holes drilled and tapped M3.






Piston material is PEEK, a high temperature (260 C) , stable, easy to machine plastic but it is expensive, about 15 X the cost of acetal, but you can pick it up on Ebay. 16mm bar was pilot drilled for the slot and parted to 1mm up on length, then faced to size.  The clearance slot for con rod small end was then milled.

The jig for drilling and turning was made of brass and a tight fit in the slot.






Drilling and reaming  the gudgeon (wrist) pin hole.






The same jig (turned down a bit) was mounted in the 7X12 lathe.






 Using a loose centre and the rotating centre in the tailstock, the piston OD was turned to be a sliding fit in the block bores.






Ian


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## dvbydt (Nov 2, 2012)

Pistons all completed.






This photo shows all the bits made so far. The gudgeon pins have small discs of PTFE  to  stop them scoring the bores.






Completed all the valve bits. The "O" ring is just to stop the stainless steel balls making a bid for freedom - not that it would do them any good, as they would probably be devoured by my shop spider ( I can never find small things that get dropped)






Ian


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## SilverSanJuan (Nov 2, 2012)

Dang!  That's some beautiful work Ian.

Todd


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## dvbydt (Nov 4, 2012)

Todd - thanks for looking in.

Now for the first BIG TEST - will they all go together without anything fouling






 and the crankshaft rotates OK.

Camshaft is the next problem to solve. I will use my single cylinder test engine to establish the timing for the inlet valves. With eight air pulses per rev, the V8 will use a lot of air,  but  if I use the steam engine idea of cut off, then maybe, at say 50%, it won't be so bad. the exhaust will be 180 degrees for each cylinder. Then  the machining of the cam can be completed using my small rotary table. 

The centre main should be fairly straightforward to make using a split leaded bronze bush. 

But guess what, I'm busy again, so it may be a while till the next installment - but it will  get finished!

Ian


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## dvbydt (Nov 18, 2012)

Dug out the little single cylinder engine, here, for those who havent seen it.


[ame="http://www.youtube.com/watch?v=04RBvolzaiA&feature=plcp"]http://www.youtube.com/watch?v=04RBvolzaiA&feature=plcp[/ame]

This is how it works.

(Photobucket is playing up, had to use imageshack .)





This is the set up for measuring the RPM.





These are the results of gradually making the cam duration smaller but keeping it opening at TDC.





Having got to a 70 Degree cam





It looks smaller than 70 degrees but the pusher lifts the ball and admits air long before the full stroke of the cam. The cam has a hex (Allen key made) broached hole to connect to the hex on the end of the crankshaft and I was able to retard the air admission by 60 degrees for the final test.

So what did I learn from all this, bearing in mind that this is an odd type of single cylinder set up? 

1. Cut-off works on air as well as steam and, in this engine configuration, can give higher RPM.

2. Torque goes down as cut-off increases - I did not have any way to measure this but in between altering the air pressure I stopped the fly wheel by hand, well, fingers, so got an impression of the torque.

3. Retarding the air admission in the last test reduced top speed but seemed to increase torque over the top dead centre air admission, presumably because the air pressure was acting at a better mechanical advantage on the piston. In this, the last of my tests, air consumption was much reduced, judged by how often the air compressor kicked in. 

Conclusions - I will be able to run the V8 at 70 degree cut-off and retarded air entry for more economic use of air and that this little engine deserves to be made into a proper project of it's own with full drawings and a bit of bling!

Final cam timing will need more experiments on the V8 engine with only one piston installed, but I now have a few guidelines from the above.

Ian


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## dvbydt (Apr 20, 2015)

It's been a while but I have found time to finish this engine.

Here are the YouTube links :-

[ame]https://www.youtube.com/watch?v=yXgaZM_b9iE[/ame]

[ame]https://www.youtube.com/watch?v=8ysh0kwC6mw[/ame]

Getting the cam right was a problem:wall:
The followers are 6mm PEEK and they are what you can hear clattering in the background. The camshaft video was taken using the 100 frames per second on my TZ40 camera.

Engine will run from about 150 up to 1600 rpm and because of the 8 pulses per rev, it is quite smooth and does not walk across the table whilst running.

Ian


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## dvbydt (Jul 21, 2015)

I decided to make a Simple Single using the techniques I had learned on the V8. The crankshaft, piston and conrod are all similar and the inlet valve and pushrod are a modification of the experimental single.

Here is the engine on YouTube.

[ame]https://www.youtube.com/watch?v=9RZL3ZBo6D4[/ame]


Ian


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