Pin valve air engine (inspired by Chuck Fellows)

[BillTodd]

Following on from my design suggestions in Chuck's  New Compressed Air Valve Idea  http://www.modelenginemaker.com/index.php/topic,4928.msg93993.html#msg93993.

This is a piston-pin operated valve that opens to a chamber that is sealed  (at least partially) from the cylinder by the piston-pin.

The pin is tapered and pushes the valve to one side thus keeping the force on the pin low

Just before TDC the pin enters a hole and operates the valve, filling the small chamber with pressurised gas. The pressure is only bearing on the area of the pin until the pin exits the hole after tdc when it escapes to press on the piston.

I've had to rethink the Fellow's exhaust valve to make it work with the above but it is still Chuck's automatic exhaust valve in essence.



I thought it was about time I went a stage further with my designs, and this time make it from metal rather than pixels.

So, based on an unfinished  engine, tonight I knocked up the necessary valves and modified the head and .... well

it works 

Pictures and video later ( the camera battery is, of course, always flat on these occasions)

Sorry about the quality of the picture (tablet!)

The chamber has to be larger than I expected  - several cm3 and a subtantial part of the cylinder volume (the chamber is missing from the picture below ,it flew off across the bench and has yet to be retrieved) .

The motor idles quite slowly <100rpm but the timing obviously late (too late for  good running) . I think the pin is reaching too far into the head (I may have to re-think the head design.

BUT IT RUNS  not bad for a first ever engine

[BillTodd]

Pics and video as promised ....


<a href="https://www.youtube.com/watch?v=o0smYDP-TuA" target="_blank">http://www.youtube.com/watch?v=o0smYDP-TuA</a>

[Jo]

 

Jo

[SHOPGUY]

Simple , but effective.
 

[b.lindsey]

That's a nice runner Bill. What does the plug in the end do as you screw it in and out. Its a little hard to tell from the video? I assume it varies the volume of the chamber but I don't detect a lot of difference in running speed.

Bill

[BillTodd]

That's a nice runner Bill. What does the plug in the end do as you screw it in and out. Its a little hard to tell from the video? I assume it varies the volume of the chamber but I don't detect a lot of difference in running speed.

Bill

I've just been laying in the bath pondering that very point :-)

I think what's happening is that the mass of air released by the valve is not being limited by the chamber volume (my initial assumption) but is limited by the valve opening time and air pressure.

My first attempt had a long pin that kept the valve open too long , but did allow for a goodly chamber full (the efficiency was ruined by the late opening to the cylinder)

I can't increase pressure ATM because of my self-ejecting exhaust port . if i can find time I'll screw the thing inplace and wind up the pressure and see if theory hold true


I also suspect that my exhaust spring is too weak, and that speed is being limited by its opening time .

[Brian Rupnow]

Good one, Bill. A few years ago  I was experimenting with air impact cylinders. A chamber was charged with air at any given pressure, with a "bleed port" into a cylinder. There was a ring of ports from the chamber into the cylinder, but when the cylinder was at top dead center the skirt of the piston covered the ring of ports. As the small bleed port allowed a small volume of air into the cylinder, the piston began to move slowly.---Once the piston had moved down far enough to uncover the ring of ports, the piston "took off" like a rifle bullet as all the air in the pressurized chamber  released at once into the cylinder. Other than making some very loud noises and scaring myself half to death, nothing came of the idea. I have wondered ever since if this technology could be applied to some form of hit and miss air engine. I don't build "air engines" now, but I wish somebody would take this idea and run with it.---Brian

[cfellows]

I've been trying for months to figure out how a palm nailer works. 

<a href="https://www.youtube.com/watch?v=kSwrj4yB9e8" target="_blank">http://www.youtube.com/watch?v=kSwrj4yB9e8</a>

It seems to me that the princple would work really well for a compressed air engine.  When the air hose is hooked up, the piston reaches equilibrium with equal pressure on either side.  When light pressure is applied to the plunger, somehow holes are uncovered that suddenly release the pressure on the underside of the piston allowing the pressure on the other side to slam the piston down.  Here is a drawing I made of my Bostitch nailer.



For some reason, I just can't wrap my brain around how this thing works.

Chuck

[BillTodd]

I feel another animation coming on ;-)

[Brian Rupnow]

Chuck--a pneumatic nail gun was the inspiration for my work  on the "impact cylinder". What I did find out was that it is almost impossible to use any normal kind of o-ring for a seal, because it gets stuck in the ports that it has to pass by. Probably a ringless piston lapped into the bore would work best for what I was doing.

[BillTodd]

Chuck,

Your drawing has to be wrong :-(

For it to work , there has to be a way for the pressure on top of the piston to escape to the outside world (and in a hurry, judging by the sound of those nailers)

Are you sure the inside diameter of the ring valve is a close fit to the  plunger (the grey bit) ?

{edit}  Ah ha NO sorry Chuck  I'm wrong (I'm allowing my piston to drop too far thus closing the bottom port - I have it now ... I think

[cfellows]

Actually, the top of the yellow part is open.  You can see the grey Cylinder liner through the opening.  In the equilibrium state, the grey part is pushed down as far as it will go.  When the nailer "fires" the grey cylinder liner pops up a bit and air escapes out the top of the nailer.  The ring valve is a close fit to the yellow part and the grey part.  And, in the equilibrium state, the ring valve is half way in between the top and the bottom, partially exposing the rings of holes above and below the ring valve.

Chuck

[BillTodd]

Thanks for confirming that Chuck :-)  ,I found a patent drawing that suggested the same.

I'll try to finish the animation this evening.

[BillTodd]

This is how I think it works

<a href="https://www.youtube.com/watch?v=buGztIYT4PU" target="_blank">http://www.youtube.com/watch?v=buGztIYT4PU</a>

[Brian Rupnow]

I lay in bed thinking last night what a perfect driving force one of those "impact" type cylinders would be for a hit and miss engine. After it fired and the flywheel went into "high revolution speed" mode, the lockout lever would simply interrupt the air supply to the cylinder. While the engine coasted (the miss part of the cycle), the air chamber would have time to recharge. When the flywheel slowed down enough, the governor weights would let the lockout lever return to it's normal position and let air flow resume, and the "hit" cycle would repeat. The air chamber requires a bit of time to recharge with air between shots, and the miss cycle of the engines would give it that required time.

[BillTodd]

There seems to be a couple of designs of these palm nailer, one of them is a proper impact device and has the plunger and piston shaped to take the blow. The one Chuck has drawn is only designed to thrust the nails in using air, it , might be a better choice for a hit-n-miss :-)

I had another (better)  idea for the pin valve  engine , that uses a second pin and spring in the head to maintain the open air port until cut-off by a variable spring force i.e. it'll cut-off when the air pressure in the head drops as the piston accelerates (much like Chuck has been noodling of late)

[cfellows]

I lay in bed thinking last night what a perfect driving force one of those "impact" type cylinders would be for a hit and miss engine. After it fired and the flywheel went into "high revolution speed" mode, the lockout lever would simply interrupt the air supply to the cylinder. While the engine coasted (the miss part of the cycle), the air chamber would have time to recharge. When the flywheel slowed down enough, the governor weights would let the lockout lever return to it's normal position and let air flow resume, and the "hit" cycle would repeat. The air chamber requires a bit of time to recharge with air between shots , and the miss cycle of the engines would give it that required time.

The Palm nailer would be ideal for an air engine, even one that fired continuously.  Its recharge rate is very fast and it hits with surprising force.  Unfortunately, it's kind of complex and requires pretty tight tolerances on the parts.  A multiple cylinder engine requiring this assembly for each cylinder would quickly get pretty complicated, not to mention the design challenges of keeping it looking like a real engine.

Bill, to your points on the pin valve, I would like to keep the design as simple as possible and the parts count to a minimum.  This is particularly important for multiple cylinder engines.  I like your ideas and hope you aren't discouraged by my list of evolving requirements.  The more ideas we have, the better our chances of developing a new design.

Chuck

[cfellows]

Here is an alternative to the pin valve arrangement that uses a small vertical slot in the side of the piston to uncover ports in the cylinder wall at near top dead center until slightly after TDC.  Timing should be similar to the pin valve arrangement in operation.  Thoughts?

[tvoght]

When I was a kid, we had a book around the house called The How and Why of Mechanical Movements. I believe it was published by Popular Science magazine. I looked at it for hours and hours, and took a lot of inspiration from it.

Anyway, the book described a couple of what were called "automatic engines". One was the bump or bash valve type used in CO₂ airplane engines (similar at least on the inlet side to what's being discussed here). The other I thought was simpler, and as a kid fantasized about building one.

The attached picture shows  it schematically. The piston (in red) had a deep depression that was filled by a long projecting head (blue) at top dead center, where the inlet port in the piston aligned with the inlet port in the cylinder. Near bottom dead center, the piston port aligned with the exhaust port, while the inlet was covered by the piston.

--Tim

[cfellows]

Thats an interesting design, simpler than mine.  The only issue I see is trying to incorporate the slave ball valve for the exhaust.  A slight variation would be to make the inside piston walls and the head protrusion straight, then drill a small hole down from the top of the piston to connect with the hole coming in the side of the piston.  Might even be worth adding an o-ring between the head protrusion and the inside wall of the piston to make it air tight.  Still don't quite see how a slave exhaust valve could be implemented.

Another source of inspiration is high pressure airguns.  These include pellet guns as well as paintball guns.  Due to the popularity of paint ball, there are a lot more commercial designs out there, although most of them are quite complex and include automatic reload and cocking.  Still, here is a relatively simple valve I found that looks like it might have possibilities.



It's similar in principle to the one you uploaded, Tim.