Steam Chest/Valve face Dimensions

[Hrcoleman66]

Hi All,

I am cureently designing and building my own Mill Engine - Loosley based on Elmers little engines and tipping a nod to Muncaster.

I was wondering whether it was absolutely necessary for the exhaust port in the valve face to be larger than the inlet ports. I have noticed that many designs seem to have the exhaust port about twice the size of the inlet.

Understand fully that in a full sized engine (or even scaled down) running in steam, there will be expansion of the steam during the stroke and that it may be a requirement to have larger exhaust porting in order to maintain the best efficiency. But I just wondered if would prejudice the smooth running of the engine to have the ports the same size.

Thanks in advance.

Cheers,

Hugh

 

[MachineTom]

Remember that the air will greatly expand so it needs more room to exit. Larger is better, even just freewheeling the engine.

 

[Hrcoleman66]

Just thought about it a bit more and answered my own question.

The Exhaust steam is going through the same chanel on the way out as when it went in!

So as far as I can see, the dimensions of the exhaust port in relation to the inlet port can only be to a timing adavantage...

I may be wrong... (Often am if you ask my wife).

Cheers,

Hugh

 

[Jasonb]

You are pushing the air into the cylinder with the power of the compressor but if you also have the same size hole for it to be pushed out of you will end up using most of the power from the piston to push it out.



Generally make the port twice the width that way exhaust pipe is twice the diameter which gives 4 times the area.

 

[Charles Lamont]

In a model purely for demonstration, not expected to produce any power, you
would probably get away with it.

 

[Hrcoleman66]

You are pushing the air into the cylinder with the power of the compressor but if you also have the same size hole for it to be pushed out of you will end up using most of the power from the piston to push it out.



Generally make the port twice the width that way exhaust pipe is twice the diameter which gives 4 times the area.

Ah, but in a double acting engine, the steam/air is also pushing on the other side of the piston on the "exhaust" stroke.

Hugh

 

[Jasonb]

Exactly and some of the energy from that push will be used up getting the air on the other side of the piston out through a tight passage when it should be going to making the engine revolve.

Assuming you have some other engines, just try putting your finger over the exhaust when running one on air and see how it slows down as more of the exhaust is covered.

 

[Hrcoleman66]

Exactly and some of the energy from that push will be used up getting the air on the other side of the piston out through a tight passage when it should be going to making the engine revolve.

Assuming you have some other engines, just try putting your finger over the exhaust when running one on air and see how it slows down as more of the exhaust is covered.

Which is an inescapeable situation, as the chanels from the Steam Chest to the Cylinder do not change in size between the steam/air entering the cylinder (Inlet) and then escaping the cylinder (Axhaust).

So, given this, why is the exhaust port on the valve face generally larger than the intet port? The Steam/Air is still having to exit through the inlet port, through the valve and then into the exhaust port.

Cheers,

Hugh

 

[Jasonb]

Simple gas and fluid dynamics, the longer the length of a restriction the more losses it will create, so get the bore upto a larger one as soon as possible.

Also worth looking at the area of the incomming steam pipe in relation to the area of the passages, passages are often more so the size has already been stepped up a bit before it gets to the passages

Although not on the scale of steam there will also be some expansion of the air, incomming air at say 30psi will take up less volume than air when it exits at atmospheric pressure

 

[jschoenly]

I would say it has more to do with timing configuration that for flow. Depending on the valve gear layout, cutoff, etc - The stroke of the valve to get full port opening on the inlet/power stroke for one end may actually cover a portion of the exhaust port from the stroke. Lets say for instance the full port opening on the intake has the inner wall of the valve centered over the exhaust port, a port "twice" as large as the cylinder port is now the same size. If you make it the same as the cylinder ports for this same configuration, you now have a port half the size.

You need to sketch out the valve and it's full travel to know for sure. Always better to have more flow for the exhaust rather than a restriction. Is there any advantage to making it smaller??

 

[Hrcoleman66]

OK... Thanks... All constructive.
Let's talk in terms or numbers then.

For the sake of argument, I'll use the engine I'm building.

Inlet port slot is 2mm wide. Then there is a 2mm gap and a 2-4mm (depending on the upshot of this discussion) wide exhaust port. Then another 2mm gap and the other 2mm wide inlet port.

The lands on the valve are 2mm wide. The total width of the valve needs to be 12mm to cover the total width of the ports and gaps. Thus, the lands cover the inlet ports both fully at mid stroke.

The travel of the slide valve needs to be 6mm total in order to allow full opening on inlet at each end.

So my valve chest needs to be the 12mm plus 2mm at each end of the stroke... plus some clearance.

The whole point of my original question was that my external length of the team chest is only 20mm.

So I'm getting seriously short of wall thickness. But I can regain some of this if I narrow the exhaust port back to 2mm.

The engine WILL work in this configuration, but will it work better with the wider exhaust port?

In my case, it's a moot point, because I just want the engine to run - Not nessecarily to be a world beater in terms of efficiency. It's my first engine after all.

Cheers,

Hugh

 

[Charles Lamont]

Right. Let's go back a step if we can. What is the intended bore & stroke?
I take it no dimension is finally fixed yet?

 

[Jasonb]

Yes as Charles says we need a bit more info, you don't say how wide the ports are a 5mm x 2mm one will flow less gas than a 10mm x 2mm one.

Whats the incomming supply area and the area of the actual passages between port and cylinder and what volume of gas needs to pass all these.

It may be possible to come down to multiples of 1.5mm rather than 2mm if the ports can be made wider or keep the same width if they don't need to flow much gas.

 

[Hrcoleman66]

Sorry guys. I should have been a bit more detailed.

The intended bore and stroke are 2.5 and 18mm respecitively.

But I am beginning to think that I need to go right back to the drawing board anyway, as the cylinder blank I turned up is only 24mm long.
And this will make it VERY tight to fit a boss on the cover plates as well as the thickness of the piston itself.

I may need to start again and go up to 26 or even 28mm. This will of course mean that I have a load more room in the steam chest!

Watch this space.

Cheers,

Hugh

 

[Charles Lamont]

2.5mm bore? You sure?

 

[Jasonb]

Who said the 2.5 was mm?

I expect its 2.5cm which is mixing units or 25mm to keep things the same

J

 

[Hrcoleman66]

Sorry.

12.5mm. Typo.

Cheers,

Hugh

 

[Charles Lamont]

To anyone wanting to design their own, I would strongly recommend Model Stationary & Marine Steam Engines by K N Harris.

He suggests port dimensions based on the bore D, with a port width about D/10, port bar the same, and exhaust port D/4, all about 3/4 to 7/8 D across.

This suggests a 1.25mm x 10 mm port in this case.

I tried drawing it out, and the biggest port I could comfortably get in was
1.5mm with a 3mm exhuast, 0.5mm lap, and 4mm stroke. The valve was
10mm over all, with a 6mm cavity.

I know 1.5mm slot drills are available, 1.25mm I am not so sure.

In trying things out bear in mind that the ports do not have to be fully
opened.

In many ways, a bit bigger would be easier.

 

[Hrcoleman66]

Thanks Charles,
I have 2 mm slot drills, so a 2mm wide port was going to be easily accomplished.
I could I guess use drills to run a line of holes for the inlet ports on the port face and then recess the back of that piece withe the slot drill prior to soldering this onto the cylinder blank.
The cylinder is going to be constructed as a turned and milled blank with the valve face as a soldered on block.

I'll see if I can track down that book.

Thanks for the info.

Cheers,

Hugh

 

[Hrcoleman66]

Well, I found a PDF copy on the internet and have had a good read of the Cylinder design section.

Now I know the answer to my original question... In no uncertain terms.

But from the sounds of it, there is certainly no harm in going bigger than the recommended dimensions... just so long as I have room in the steam chest (which apparenty needs to be the same volume as the swept volume of the piston!)...

Cheers,

Hugh