Newbie question on valve function and timing

[cogburn]

I am hoping someone can point me in the right direction. I am very new to the whole metal working thing but I have built one model - a Rudy Koutouph designed little horizontal engine. It is not pretty but it works fine. I want to understand the valving on these engines like horizontal mill engine models. The valving is a mystery to me. I can copy plans and I can fool around and get things to fit but I need some theory so I can scale some plans up or down or I can design my own engines. Is there an good source for explaining the timing and valving on these type engines? I have recently gotten a good explanation for the porting and placement for a wobbler but I am not clear on other types of valving and timing.
Thanks beforehand-
Scott

 

[Blogwitch]

Scott,

The attached pdf shows a basic piston valve setup, and by looking at the ports, and figuring how the rods move, you should be able to see how it works.

A basic slide valve is similar, but once you get to the reversing of a slide valve, things start to get complicated.

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View attachment Piston Valve.pdf

 

[cogburn]

Thanks Blogwitch,
That is a big help. I will study the drawings and see if I can make application to my projects.
Scott

 

[BigBore]

Hi Scott,

Type "steam engine valve" into Google and get yourself a beer. Ton's of theory in the midst.

Ed

 

[cogburn]

Ok- I am wondering if we can discuss a second the specific valve that is designed on the E-Z engine that is I think called a "spool" valve. On the sheet from Blogwitch, it says that the length of the large diameter part of this particular valve should be the same as steam port width. I think on the EZ engine, the port width is 1/2 of the diameter of the large diameter part of the valve. Am I reading the plans wrong or is there leeway in this dimension? Is there a rule about the size of the intake and exhaust ports in regards to the displacement of the cylinder bore?

Also, I am wondering about a double acting engine's effect on the placement and the size and spacing of the valve's larger diameters. I know this has to have an effect on its engineering.

 

[Maryak]

Is there a rule about the size of the intake and exhaust ports in regards to the displacement of the cylinder bore?

Cogburn,

This was provided some time ago by Sandy C.

The actual formula for port cross sectional area is: -

a = Av/V

Where A = the area of the piston in sq ins.;
v = piston speed, in feet per minute;
V = velocity of flow of steam, in feet per minute.
a = port cross sectional area, in sq. ins.

The values of V commonly used for the steam velocity are 4000 for the EXHAUST and 6000 for the INLET, however, in the case of a slide valve or piston valve, where the same port is used for both Inlet and Exhaust the lower figure of 4000 should be used.
This will determine the required (minimum) cross sectional area for the ports.

Hope it helps.

Best Regards
Bob

 

[BigBore]

I found this free Google book that is chucked full of valve info. The Table of Contents is hyper linked so it is fairly easy to get around in.

http://books.google.com/books?id=NsNCAAAAIAAJ&printsec=frontcover&dq=valves+and+valve+gears&as_brr=1#v=onepage&q=&f=false

Ed

 

[spuddevans]

I came across this website Linky ages ago. There are a number of downloadable valve-gear simulators that you can alter the parameters and slow down the rpm to see exactly how it works, how the steam expands, the effects of changing the cut-off and so on.

The great thing is that there are a number of different valve linkage types to try out. I found them fascinating and very educational, try one of them and see if it helps. ( if a picture paints a 1000 words, what does an interactive, animated picture tell ?? )


Tim

 

[cogburn]

Thanks for the info and the links. I have some stuff to chew on for a good while. I appreciate all the replies.
Scott

 

[Blogwitch]

Unfortunately Scott, I don't ever think I have seen the plans for this engine, otherwise I would have been able to explain things in a more easily understood form.

I know how difficult it can be trying to understand things from a drawing, where it is just one part that has no real corelation to another part.

All I can say is that in a piston valve (another name for a spool valve), that the width and distance apart of the control faces on the spool, should be the same width and distance apart as the port holes in the main block. See C-o-C.

The other thing to understand is that the overall throw on the eccentric should be exactly double the width of a spool or diameter of a port hole.

Also, the operation of the steam for fwds/reverse is the same as you would use for controlling a double acting oscillator, that is by switching the pressure/exhaust lines over, if the piston valve has been designed and made properly, the engine will run in fwds/reverse.


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[bearcar1]

That appears to be a very useful website on valve gearing Spud', unfortunately I am in the 'other world' and not a Big Blue user so the files will not run for me unless I go through all of the harrangue of installing the patches to run such files Still it would seem that being able to change the dimensions of the various links given would prove invaluable to a scratch builder. I could sure use it to complete my "Smitty" build.


BC1
Jim

 

[cogburn]

I figured out how to attach part of the plans here as .pdfs. It shows the valve and cylinder plans. This might help you see what I am asking about. I added the crank throw and valve throw plans too. If I read the plan right the crank throw is 3/8 and the valve throw is 1/2.

 

[Dan Rowe]

There is one crucial missing piece if information and that is the angle of advance. This angle is the difference between the eccentric throw and the crank throw minus 90 degrees.

The Dockstader programs are very useful for checking a design but some theory is needed to know which variable to change in order to refine any design. The latest release of the programs included slip eccentric. This is really a non-reversing design with a movable eccentric for reverse. I was really happy to see it included as it reduces the problem down the only error in the valve timing is the angularity of the connecting rod and the eccentric rod.

A valve can be designed for slip eccentric first then the curves produced can be compared to one of the reversing gear curves. I like the oval diagram the best as it shows the whole design at a glance. I use the slip eccentric oval curve as a goal post. If a modification makes the curve look more like the slip eccentric diagram then an improvement has been made.

Dan

 

[bearcar1]

Hiya Cog', I am just a bit confused here but if I am reading those partial drawings correctly the valve throw is .250" which would make the stroke .500" according to the drawing callouts. Now then, if that small rectangular piece shown in the flywheel drawing plate is the crank, then the throw is .375" which would make the stroke .750". Remember now that the stroke is always going to be twice the dimension of the throw. (two distinctly separate entities to be discussed, not interchangeable as there is a major difference between the two) I'm certain that this has probably not answered your question but I wanted to point out this fact in order for clarity in asking. Could you possibly re-phrase your question for us please. ??? ;D

BC1
Jim

 

[Blogwitch]

Now I understand Scott, no wonder you were getting confused by all the bullsh being thrown at you.

Your engine is only a very simple single acting one, and in fact if you make it as is, it should work just fine.

Forget all about the complicated directions coming in from everywhere, and just make it as per plans.

Rather than a complicated and tolerance critical control system, as it would be if it was a double acting cylinder, this system is just using the piston valve as a very basic pressure on/off valve.

I hope the C-o-C explains it a little easier. As the eccentric moves the spool down, it allows pressure to the top of the piston and forces the ram out, so turning the flywheel. As the flywheel continues to turn from the push received from the piston, the eccentric moves the spool up, so allowing the air that was above the piston to be let out to atmosphere, then the flywheel continues to turn and another cycle starts. Depending where the spool is to begin with, you might have to flick the flywheel over to start the engine running.


Blogs

 

[cogburn]

OK- This last diagram helped much. I am sure my terminology lacks correctness or specificity but all of the replies were useful in that I am gaining knowledge with each one. As I said I am pretty new to all this but it is fascinating to me. Some people like to go out and watch birds and such. I could sit all day and watch these engines run. I want to build an example of each kind and then scale them up and build them again. I want to eventually design my own from scratch and build them and watch them go. I found this site a day or two ago and already it has been a great help to me. It is nice to find people who have a similar interest. My grandpa worked for the Missouri Pacific Railroad for 42 years from the 30's on to late 70's. He always had a working steam engine at his house and he used to fire it up for us kids. I guess I inherited his mechanical curiosity but probably not his intuitive skills.
Scott

 

[compressor man]

Hi Scott,

I once found this on the internet and saved it. I dearly wish I could remember where in order to give the site credit but I cannot. This really was a great help to me.


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