Steam Port / Valve Layout

[JorgensenSteam]

I am pulling this item out as a separate topic so as not to distract from Brian's great posts for his overcrank single cylinder, but as Tel mentions, maybe the port/valve/eccentric design needs a second look.

I generally enjoy the diversity of design and materials, and like to see where people go with their free-lance methods, but in this case, perhaps some additional discussion of the port/valve/eccentric will yeild some useful information, especially if a number of people decide to build the same engine.

Here is my slant on the port/valve/eccentric design.
I am not sure if this is correct, but certainly a beginning point for the start of a discussion.

Pat J

Edit: I took a try at layout out some ports, passages, and a valve for Brian's engine.
Not sure if this is correct, but maybe someone can check me and see if I am in the ballpark.

For the passages, some blank rods could be inserted into the three holes, and then holes drilled between the three holes, to get a larger and more complete passage.
These dimensions are approximate, so don't build off of any of this.

 

[JorgensenSteam]

More screen shots.

I opened up the exhaust passage into a slot where it exits into the exhaust pipe, instead of a hole.

The cuts in the valve face for the steam ports can be made a little more shallow than what I show.

Pat J

 

[Brian Rupnow]

Nice stuff Pat---But now you have to build one and prove your theory!!! ;D ;D---Tel may have something to add.----Brian

 

[JorgensenSteam]

Thanks Brian, it is just a quick attempt at it, don't know if it is anywhere near right or not.

Maybe Tel or some of the others can give it a try and see if they come up with something similar.

Pat J

 

[JorgensenSteam]

I generally design for a larger bore than Brian's engine, so the above dimensions are probably splitting hairs, but the above dimensions are designed to give the following:

(Note: All angles are measured from TDC unless otherwise noted.)

admission: 0 degrees

cutoff: 110 degrees (the typical 55 degrees typically seen on steam engines when measured from the point where the valve changed direction to the cutoff point)

release: 145 degrees

compression: 320 degrees

The lap on the valve combined with the exact valve travel is what gives the typical steam engine design values seen above.
Model engine valves often have no lap on the valve, and have a longer travel than shown above, to give 100% cutoff. For a small engine, a 100% cutooff is fine, but as the engine gets larger, a 100% cutoff would waste a lot of steam.

In a 3D program, you can actually model the valve gear, and animate it, and see the exact points for the above values. You don't really have to build an engine to test the valve gear.

Pat J

 

[tel]

Never fear blokes - when time permits I will put up a version of this engine, with modified ports and see just how hard it is. Unlike you CAD gurus tho, I have to build and then retro-sketch.

 

[Maryak]

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.

A 0.196349541
v 41.66666667
V 4000
a 0.002045308 0.065449847 wide 0.03125 high

Bore 0.5 in
Stroke 0.5 in
RPM 500

Whilst expansion ratios are generally less than 2:1
It is common practice that the Exhaust Port is 2xArea of steam port

 

[JorgensenSteam]

I started by running a calculation for steam port sizes on this cylinder, but them realized that the steam chest area had already been fixed, so I had to fit as large a valve and port I could into the existing steam chest dimensions, while still maintaining the valve travel I wanted.

The method I would use when designing an engine from scratch is to use a port length which is 75-80% of the bore, which was typical for steam engines, and then lay out the ends of the steam chest so that the steam chest end walls are up against the insides of the cylinder head flanges.

I then scale down the drawing shown below (which shows the dimensions from a Stanley 20 hp steam engine valve and port layout), and use proportionally scaled dimensions for the valve, valve travel, and port width. (Typically, I have seen the port "length" measured perpendicular to the piston travel, and the port "width" measured parallel to the piston travel).

I will try this for a cylinder designed from scratch, and maybe it will make more sense what I am doing.

Pat J

 

[JorgensenSteam]

My thoughts in general are that there are no "right" and "wrong" ways to build a model engine, since to build it "right" assumes that the builder wants to build it like some other engine, which may not be the case at all. You can build an engine any way you want, and often the engine works very well, so what is left is building an engine that adheres as much as possible to how the old engines were designed and the materials that were used. Adhering to the old standards is a matter of personal preference for me, and is an area that interests me and is fun, so I generally give it some thought, and perhaps more thought than many, but still I consider it a matter of personal preference as to how far you want to try and adhere to the old designs.

That being said, here is my REV.02 on Brian's port/valve/valve travel for his single engine.

I ran some calcs. and came up with similar numbers from what Maryak came up with.
The problem I have had with using calcs on small engines is that you can end up with more of a thin line for a port instead of a slot, especially if you adhere to a port that is about 75% of the width of the bore.
A port that is too thin in width will "wire draw" (restrict the steam flow), and this will be made worse if the tolerences of the engine are slightly off.

My solution is to draw the steam chest first, with the steam chest being about 80% as wide as the cylinder outside dimensions, and then draw the ends of the steam chest against the upper and lower cylinder head flanges. The inside dimensions of the steam chest can be determined at this point.

Using a port length of 75% of the 0.625" bore gives a port length of 0.46875", which I rounded to 0.48".

I then measured the inside length of the steam chest and came up with a length of 1.125".
Since I am scaling down a full size Stanley valve and port layout, and I know the full size Stanley has a total valve travel of 2.625", then the formula for the scale factor is:

(X * 2.625") + (1)X = 1.125"

so X = 0.3103"

So I scale down the Stanley ports and valve by a multiplier of 0.3103, and then use that width (width of the port being measured parallel to the piston travel) for the steam ports.
I use twice the width of the steam port for the exhaust port, and make the bridges between the steam and exhaust ports equal to the width of the steam port.

I still maintain my port length of 75% of the bore, which I rounded to 0.48".

Reversing the calcs for the area of the steam ports, this layout yields a generous area, but for a small steam engine, you want to be on the large side with the steam ports and passages, otherwise they get too tiny to practically deal with. Efficiency is not an issue with an engine that has a 0.625" bore, so use a generous port and passage area.

Anyway, this is my slant on port/passage/valve design for a steam engine.
All of the values from the full sized Stanley engine get scaled down by 0.3103, including the valve travel, etc.

As the engine gets larger, the actual calculated port sizes can be closely adhered to, but this design should give a small engine all the power it could potentially make without restrictions in the ports, passages and valve, and also give good values for cutoff (about 75%), release, and compression. It is assumed that no lead will be used due to the small bore.

Pat J

 

[JorgensenSteam]

Here are the approximate revised dimensions for the steam chest, as well as the revised port dimensions.

Pat J

 

[JorgensenSteam]

And this diagram shows the valve in the two extremes of its travel, and shows where the edges of the valve are in relation to the ports at these two extreme positions.

The valve is designed to fit into the steam chest and be able to transverse its full movement back and forth without striking the ends of the steam chest.

Pat J

Edit: Note that often in the old designs, the valve did not fully open the steam ports when the valve was at full travel, and this can be seen in the drawing below. This is the way the Stanley engine was set up also (ports never get fully opened by the valve). The exhaust ports are about 1/2 open when the valve is at full travel.

 

[Brian Rupnow]

Thank you Pat, for digging into this so deeply. Of course, as I explained to Tel, the proof of the pudding is in the eating. Now I expect you to build one of these and run it on steam to prove your theory. I will be posting a download to all of the drawings sometime within the next two weeks.----Brian

 

[JorgensenSteam]

Brian-

I am not much of a machinist, so someone with more machining skills would probably have to build it.
I build large engines mainly because it is easier to machine something that I can get a hand around, and a slight mistake on a large piston/cylinder is not nearly as critical as a slight mistake on a small piston/cylinder.

From reading several of the old model steam engine books such as Greenlee, it is often mentioned that the ports for small model engines generally get wider and shorter than a full size engine design, and also mention that often the lap is often omitted on small model steam engines to give steam for the full duration of the stroke, again since efficiency is not a concern. Passages are often drilled in model engines since casting ports can be difficult, especially in small engines.

I saw one test setup which had a computer which did the work of the old indicator, ie: measured the pressure drop on both ends of the cylinder, and plotted that against rotation of the crankshaft.

An indicator (which would probably not be accurate at a small scale) would show whether the ports and/or passages were too small, since the pressure plot would start out near boiler pressure, but drop severely prior to cutoff if either the port/valve started wire drawing, or if the passage was too small.

The pressure plot should be as horizontal as possible between admission and cutoff, and then drop slowly as the steam expands after cutoff and before release.
After release the pressure should drop quickly up to the compression point, where it should return to near boiler pressure, but slightly less so as not to lift the d-valve off the seat.

Below is a page from Sotherns which contains a lot of information about valve gear and indicator diagrams, and links those with the piston placement in the cylinder.

In order to really get a feel for full size ports compared with smaller ports, I guess a tiny brake would have to be built and tested on two different port/valve configurations. The old books do seem to follow along with what I show above, but generally show a port that is more like 50% in length of the bore.

Pat J