Test Bed

[Roger B]

Still following along    It's always good when someone tries to investigate the fundamentals rather than just accepting what has gone before 

[Captain Jerry]

Still following along  It's always good when someone tries to investigate the fundamentals rather than just accepting what has gone before 

I have always liked wandering in the woods.  Sometimes you bump into a tree and sometimes you find a Morel.

Today I checked out the gauges.  With the tubing clamped off past the test gauge, it reads about 7PSI below the gauge at the compressor.  I'll chalk that up to losses at the couplers.   With the clamp off and the ball valve on, the flow of air around the shaft drops the pressure another 12PSI.  Needs a packing gland.

Quick fix.  Drilled the outer end cap and tapped for #10-32.  Turned a short length of brass to about .225" and threaded with a #10-32 die. Then drilled 1/8" for the shaft.  Parted off leaving a head for adjustment.  With the end cap back in position on the valve, I took a couple of wraps around the shaft with fake graphite packing and seated the brass compression screw.  It looks like this.



It looks a little crude but is very effective a sealing the valve.  The adjustment is a bit touchy. If screwed in too tight, the valve shaft gets hard to turn.  More about that later.  Using the improved valve, I took a short video of the gauge as I oscillated the valve shaft to open and close the valve.  I haven't posted a video for the past year so it may take a while to figure it out again.  It seems to work and maybe work well enough but I am going to fiddle with it a for a while to be sure of its reliability.

While thinking about the consequences of incomplete sealing at the valve face, I had a revelation (commonly known as a self imposed head slap)    If it is the steam or air inlet,  the consequences are minimal.  If it is the exhaust valve, the consequences may prevent the engine from running at all.   In a four valve engine like the Corliss,  I think the importance of the exhaust valve has been overshadowed by the  concentration of thought and design of the steam (inlet) valve. 

RADICAL THOUGHT    I think it is possible to set up a four valve engine so that both steam (inlet) valves remain always open and still produce enough force to reciprocate the piston and rotate the  flywheel.  I am confident enough of this to offer a Romney Style $10,000 bet on the outcome.  How about it Mr. Governor?  Any takers?

Jerry

[Captain Jerry]

A word of explanation, particularly for non-US members who don't have to listen to endless US political commentary.  In the 2012 campaign to be the Republican candidate for president, one of the contenders (Gov. Mitt Romney) offered another contender (Gov Rick Perry) a ridiculous $10,000 bet on the validity of some obscure statement.  By definition, a silly, non-serious bet.

Maybe I should have offered a bucket of swarf as stakes for the sake of conversation.

Jerry

[Maryak]

Hi CJ,

Thanks for the bet explanation. I for one was totally confused. In any event no taker here for the bet. I look forward to the results of your experiments and theories.

Best Regards
Bob

[ths]

One bucket of spark in the post. Hugh.

[Captain Jerry]

I continue to work towards a better valve design and by that I mean one that will provide early cutoff at higher pressure.  My first thoughts were that the valve body needed to be disconnected from the shaft so that the face of the valve body  could seat itself against the bore face of the valve cylinder, much the way that the body of a D-valve is free to seat itself against the valve face.  My first efforts to do this used a brass valve body and a valve cylinder lined with brass tubing.  This worked only marginally better than a one piece body and shaft. 
 
I have experimented with cast iron valve cylinders with both brass and cast iron valve bodies with slightly better results with the cast iron body.  I think that the cast iron body was better because it was a better fit in the cylinder bore and because I was able to get a better finish on the cast iron.  Although better, the results were inconsistent, occasionally failing to seat well and close the valve.  My thinking was that the notch in the valve body that was rotated by the flat on the valve rod would hang up on the flat of the rod and not seat.  I think that it is due to the "window sash"  ratio and so I began to think about reducing the length of the valve body.  There are other advantages to reducing the length of the valve body.

I finally became convinced that the valve body with a notch at both ends, driven by a narrow flat on the valve shaft was at the heart of the problem.  My final design effort uses a slot on only one end of the valve body, driven by a tongue  on the end of the valve shaft.   The picture below shows several of the stages of the development of the valve body. 



The one on the right has proven to be the most effective.  Using it in a test setup like the one above, it is able to hold a pressure up to about 50 PSI without noticable bypass.  I have been able to adjust the packing gland so that it does not bleed air pressure and does not add to the friction of the rotating shaft.

One problem remains.  At high pressure, the end of the valve body is forced against the end cap and the resulting friction makes it difficult to close the valve for early cutoff.  The valve is opened by the linkage through the wrist plate to the eccentric, but when the Reynolds release trips, the only force available to close the valve is the weight of the dashpot.  It takes a weight of more than 2 ounces or about 60 grams to close the valve sharply at 45PSI operating pressure.  I have to work on that. 

since the details of the valve are hidden, I will post a 3D view of the design shortly.

Jerry

[Captain Jerry]

As promised, here is a drawing of the valve body with shaft and driver.



Jerry

[Maryak]

One problem remains.  At high pressure, the end of the valve body is forced against the end cap and the resulting friction makes it difficult to close the valve for early cutoff.  The valve is opened by the linkage through the wrist plate to the eccentric, but when the Reynolds release trips, the only force available to close the valve is the weight of the dashpot.  It takes a weight of more than 2 ounces or about 60 grams to close the valve sharply at 45PSI operating pressure.  I have to work on that. 

since the details of the valve are hidden, I will post a 3D view of the design shortly.

Jerry

Hi CJ,

I wonder if you drill a small hole axially through each cylindrical end, this will allow a small amount of steam to get between the valve ends and the cover plates providing a balancing effect. Even if your valves are so tight as to prevent any loss from the ends the ensuing condensate will still hydraulically centre the valve body and this should reduce the friction of metal to metal contact.

Food for thought maybe.

Best Regards
Bob

[Captain Jerry]

Bob

Thanks for the food for thought.  I nibbled at it all day.  My first effort was to drill the rod end of the valve, thinking that there was plenty of force driving the valve in that direction so I would try to drive it the other way for balance.  No difference.  I then drilled the other end as you suggested, thinking that balance was the key.  No difference.

To be sure that the shaft was being forced against the rod end cap, I filed the end of the shaft to a polished dome shape and pushed it back toward the valve while the valve was under pressure, using a hardened steel block for minimal friction.  Major improvement.  Friction is clearly the problem.  I will work on that tomorrow by polishing the faces that bear against each other which are the inner end of the end cap and the shoulder of the valve driver which are both cast iron.  I used greased cotton thread in the shaft packing but I don't know if I should try grease inside the valve.

It appears that any effort to balance the thrust will fail due to the area of the valve shaft which can not be balanced.  Simple math indicates that at 45 PSI, there will always be more that 1/2 pound of force trying to push the rod out of the valve cylinder.  Polish and lube are the answer.  (I think)

Jerry

[dieselpilot]

I see you beat me to it. I typed this earlier in the day and hadn't posted it yet.

Interesting work and such small parts. I know nothing about steam engine design. I may have missed the 3D of the valve itself, but am under the impression it's a lost motion device? Why not use a spring to close it?

Pressure in the valve body will exert force on the shaft regardless of what you do. Two options; reduce friction between the shaft and endcap or reduce the shaft size to reduce the area the pressure acts on. Maybe a PTFE or other washer?

As far as testing the engine wouldn't that be done the same as any other engine by measuring power input and output? Pressure and flow in, torque and RPM out? Flow would easily be measured with a manometer.

[Captain Jerry]

Thanks for the post, Greg.  It is always good to get confirmation.  I don't think I can reduce the shaft size (.125") so I will work on fit and finish and cleanliness.  Working with cast iron has its challenges.

Historically, this type of valve was closed with a weight in the US, and coil spring in the UK.  Both types were partially assisted by air/vacuum spring.  The final construction here will be with springs, hidden to simulate weighted dashpot.   For testing, it was quicker to hang some chunks of metal on the valve arm than to fit a spring.

As to the purpose of this test, I have built two engines of this type and was disappointed with the power that they developed.  I suspect that the poor performance was due to poor valve fit and function so this test bed is intended to refine the valve design and construction.  The engine has four of theses valves so I don't want to build four at a time for testing. 

Jerry                                                                                                                                                                          3852