Reversing a Piston Valve Engine

[catceefer]

Good evening.

I wonder whether anyone can help with a question about reversing a piston valve engine?

I have sketched out the attached plan, taking the dimensions and general layout from information on the internet. Whether they are good or not is another matter.

Have I understood it correctly that if I simply use a reversing valve to put steam in the pipes currently labelled "Ex", using the current "In" as the exhaust, the engine will run in reverse? The plan is to make this a twin cylinder, so it should be self-starting. I should like to avoid valve linkages to reduce complexity.

Regards,

James
piston valve engine.jpg

[kvom]

That's correct.  Reverse inlet and exhaust for reverse.

[catceefer]

Thank you. Does that work if it is double acting? As far as I can work it out, if both of the outer ports on the valve are used as inlets, the piston locks up as they are both trying to let in steam at the same time.

Have I misunderstood the operation of the valve timing?

Regards,

James.

[crueby]

The valve should only connect the center port labeled In and just one of the outer ports labeled Ex to the cylinder at a time. The yellow squares show where the valve blocks passage, they slide side to side in the diagram together. In the diagram you posted, the valve is in the transition state from moving the piston one way or the other, so no pressure is going to the cylinder at all at that instant.

[catceefer]

Thank you. So, if I understand correctly, if steam enters the centre port the steam is acting on on both sides of the piston in turn as the engine rotates making it double acting in one direction, but when the reversing valve changes to flow so that the steam goes in to one of the end ports, it'll become single acting. Have I interpreted that properly?

Regards,

James.

[Jasonb]

As you have two passages into the cylinder that is what makes it double acting - steam pushes on one side of the piston and then the other.

It would be a single acting engine if you only had a connection at one end eg steam pushes the piston in a single direction and then the momentum of the flywheel exhausts the steam through the same single connection.

[crueby]

Right - it will still be double acting, the steam will just take an alternating path from the ends of the valve.

There were locomotives that were set up this way, they refered to them as either inside admission or outside admission piston valves, depending which way the steam/exhaust was hooked up. The original diagram you showed was inside admission, what you are proposing would be outside admission.

The more common way to reverse a double-acting engine is through a second set of eccentrics and a reverse linkage to select one or the other.

[catceefer]

Thank you for your replies. I think that I need to do another set of drawings and try again. The sketch was done in Word. When I moved the valve in a second drawing using the same sizes, I found that both end inlet pipes were allowing steam in at the same time. I need to have a another look.

Thank you.

James

[crueby]

If you compare them, an outside admission piston valve is basically the same thing as a slide valve (D-valve) with steam on the outside and exhaust on the inside portion. The oly difference is that for the piston valve you have to route the steam to both ends seperately, and with the slide valve the steam can flow around the valve by itself.

[Charles Lamont]

To make it run double acting, you need both the outer connections to be connected to steam.

Probably an easier way to make a reversible engine, without using a normal reversing gear, is to use a 'slip' eccentric. The eccentric driving the valve is free to rotate on the crankshaft. Beside the eccentric is a disk that is fixed to the crankshaft. The disc has an arc cut away, or a crescent shaped slot. The eccentic has a peg sticking out of the side that engages with the slot, so limiting the free rotation of the eccentric relative to the crank to 180°.* The only thing is that it does not reverse automatically. To change direction you have to turn the flywheel half a turn in the desired direction to slip the eccentric round to its other stop.

* Not necessarily 180°, but I don't think you are ready for that yet.   

[steamer]

You will run into problems with Lead and Lap trying to just swap ports.    If you remove the lap, you should be successful, but a Stevenson link and another eccentric would work better...just my 2cents


Dave

[Jasonb]

I don't think lap will make much difference but to get equal running in both directions you will effectively end up with no lap. Lead would give problems as when you change direction that would become lag but as you have your eccentric shown at 90deg there is no lead to worry about.

I've just blown both ways into my single acting piston valve engine and it runs either way, Better in the intended direction as I have it timed for shorter inlet time than exhaust. Though I expect if I used a compressor it would be better running in reverse as I can't blow hard enough for long.

With the right drilling of the valve block it is very easy to have the two outer "exhausts" connected to one common point and the "inlet" to the other. Then you just want a simple reversing valve as found on a lot of oscillators so not too hard to do if you don't want to go the slip eccentric or other valve gear route.

James, is this for a small Loco application and maybe with Solenoid valves?

[catceefer]

Thank you for all of your replies. I have have studied them, made some adjustments to my diagrams and carried out some simple animations. Everything now makes sense. I had the two inlet pipes at the ends of the valve too close together and it was causing confusion for me. I have moved them towards the ends of the valve which has made the operation more obvious.

Jason: Initially, I should like to make a small engine to go into a small boat which I have. This will be radio control. If successful, I wish to put a similar design engine into a steam locomotive with possible radio control as well.

 regards ,

James.

[Jasonb]

If you want the boat and indeed the loco to be reversible by RC then you will need two cylinders to make them self starting with the cranks at 90deg to each other.

[catceefer]

A useful reminder, but I do plan to build twin cylinder engines. The boat currently has a single cylinder, single acting oscillating engine. The boiler is vertical, unstable and lacks a safety valve. It relies on forcing off the steam pipe if the pressure is too high. It is a nice looking, but impractical set up.

Regards,

James.

[Charles Lamont]

The simplest engine for your requirements is probably a V-twin double-acting oscillator.
You could do worse than the Stuart kit:
 https://www.stuartmodels.com/item/89/stuart-twin-oscillator
or you can just buy the drawing (look under spares).
The primary advantages of a fixed cylinder engine would be that it looks more authentic, and it can be more economical (longer run, or faster, for same amount of fuel/water). To achieve better economy it needs to use the steam expansively, and you can't really do that with an arrangement that swaps the steam and exhaust.

[catceefer]

Thank you for the suggestion of the Stuart kit. Being happy to show my ignorance: why does swapping the inlets and outlets limit the ability to use steam expansively?

Regards,

James.

[crueby]

With a normal reverse gear linkage, it lets you set up the valves so that the steam inlet is cut off partway through the stroke - the steam will keep expanding through the stroke, but you are not wasting any more by keeping the inlet open. With an engine under light load (or none) you can save a lot of steam that way - under a heavy load they would have to open the timing up to keep it open longer. The Stephenson type linkage lets the operator change that setting on the fly. For the case where you are reversing the inlet/exhaust, you would have to keep the timing set more to the open-longer case.

[catceefer]

Thank you fo rthe explanation, Crueby. That makes sense to me now.

Regards,

James.

[steamer]

With a normal reverse gear linkage, it lets you set up the valves so that the steam inlet is cut off partway through the stroke - the steam will keep expanding through the stroke, but you are not wasting any more by keeping the inlet open. With an engine under light load (or none) you can save a lot of steam that way - under a heavy load they would have to open the timing up to keep it open longer. The Stephenson type linkage lets the operator change that setting on the fly. For the case where you are reversing the inlet/exhaust, you would have to keep the timing set more to the open-longer case.

Yes....otherwise known as "linking up"....

Dave

[steamer]

"....I don't think lap will make much difference but to get equal running in both directions you will effectively end up with no lap. Lead would give problems as when you change direction that would become lag but as you have your eccentric shown at 90deg there is no lead to worry about....."

Without steam lap, you will have cut off at the end of stroke.   Not good for steam economy....most models are set between 0.75 and 0.88 of stroke.   Lead, the amount the valve is open at dead center,  is usually set to "just opening"   Linking up will shorten the cut off, depending on the valve gear geometry but up to 30 percent or perhaps more!....it also adds some compression.   Exhaust lap is usually not added on model engines as the condensation losses are so great, you end up with "water lock"...which isnt good

My first steam engine was a small single cylinder 3/4 bore x 1" stroke engine for a model tugboat.   It was a single cylinder, and to get reverse, it had a tumbler crash gear box....the engine just kept running


Good luck with your project

Dave

[catceefer]

Thank you, Dave.

I hope to make a start on this fairly soon.

Kind regards,

James.