Designing steam engines-oscillators specifically

[Chuck in E. TN]

I am studying and experimenting with designing my own steam engine. I’’m starting with Oscillators. Specifically a double cylinder, double acting.
A member on another forum posted a link to an Ocillator simulator program, available here, that simulates simple single and double acting oscillator steam engines. It’s a neat program, and I learned a lot from it.
One thing that I haven’t been able to figure out is whether the piston should cover, not cover, or cover half the port openings at the top or bottom of it’s stroke. I assume(and you know what that does) it has something to do with timing and ‘cutoff’ in engines run on steam, but I don’t yet have a boiler, and will be running mine on compressed air.
The above mentioned simulator shows  the piston covering half the port in one engine and not covering the ports I another. Can someone explain the difference or wherewithal of the piston covering, half covering, or not covering the ports?

[chipenter]

I don't know about the piston covering half the port , but the port is cracked just before top dead centre called lead , to get more steam or air into the cylinder , you will also get more power if the sides of the ports are flat and lined up to the pivot point .

[velocette]

Hi
As I see it this is not important and will attempt to explain.
The ports on an oscillator engine are closed when the engine is on TDC and will not open until the crank turns.
The angle before the ports are open depends on the length of the connecting rod. Short con rod less angle etc.
Do you need to reverse the crank rotation then the porting will need to be symetrical.
If not then the timing can be offset to admit Steam or Air at TDC and the exhaust to open earlier.
For maximum flow through the ports round ports are not the ideal shape. By careful marking out and shaping with the centre of the cylinder pivot used to align the flat sides of the ports. 

   

[vtsteam]

Chuck, can you give a link to that simulator so we can see what you're describing?

[Chuck in E. TN]

Here's the link to the simulator. Note: it's for oscillator types only...

http://www.labbookpages.co.uk/metalwork/singleOsc.html

[vtsteam]

Chuck, I'm not sure what the three engine types that you mention are.... are these existing designs someone else is showing. Or are these engines you designed through this program?

I would think that ideally the ports should fully line up at TDC and BDC, otherwise port area is wasted. It's the nature of the beast that a simple oscillating engine will always have symmetrical opening around TDC, whether the ports line up perfectly or not.

Whatever lead the inlet has will be mirrored by the time it takes to close after TDC. So timing is always symmetrical around TDC. No point in wasting valve opening size in order to try to change lead, just reposition the opening. This of course changes the period it's open for, as well.

On the other hand, it doesn't absolutely hurt anything to offset the opening further than the cylinder opening can reach (ie partial opening) , as long as the openings are large enough to transfer enough steam at the RPM desired to do the work you expect. But I don't see any advantage in  it.

[Chuck in E. TN]

The 4 engines in the simulator aren't my design. As far as I know they were designed by the programs author.  I have created a couple designs with the simulator by printing out a design, calculating a scale increase for each dimension and entering the new dimensions and adjusting everything as needed to make it 'look' right.
I asked the question on the piston covering the ports because the models included in the program show half the port covered in 3  and not covered in 'Steve's Workshop' model.
So, either of those port options are good? I have built a couple of simplified wobblers, but now can't remember how much, if at all, the piston covert the port at TDC. I wasn't concerned then, I just built the models to plan. I have questions now because I'm not following a plan, I'm designing my own.

[fcheslop]

When drilling the steam ports from personnel experience and lacking the ability to mark out and drill the holes where I want them I use a simple jig that canbe seen to the left in the pic.This engine is to power a gauge1 loco single acting
when the piston cover the port a tdc or bdc it shouldnt be a problem if it is then drill the passage ways as per the Duval I gave the link for on another channel .
cheers

[Chuck in E. TN]

I'm working away at my design. I'm 3d printing some parts and machining others. Have one cylinder(3d printed), both cylinder covers(3d ptd), gland nut(brass), piston(3d ptd)Flywheel(3d ptd 2 pc pattern for casting), Cylinder pivot(steel), and a partly completed piston rod(steel). Having a ball, and learning lots... Thanks to you all.

[vtsteam]