Author Topic: Reversing a Piston Valve Engine  (Read 1191 times)

Offline Charles Lamont

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Re: Reversing a Piston Valve Engine
« Reply #15 on: March 10, 2021, 03:03:59 PM »
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.

Offline catceefer

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Re: Reversing a Piston Valve Engine
« Reply #16 on: March 10, 2021, 04:38:03 PM »
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.

Online crueby

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Re: Reversing a Piston Valve Engine
« Reply #17 on: March 10, 2021, 05:11:26 PM »
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.

Offline catceefer

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Re: Reversing a Piston Valve Engine
« Reply #18 on: March 10, 2021, 10:28:54 PM »
Thank you fo rthe explanation, Crueby. That makes sense to me now.

Regards,

James.

Offline steamer

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Re: Reversing a Piston Valve Engine
« Reply #19 on: March 10, 2021, 11:18:27 PM »
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
"Mister M'Andrew, don't you think steam spoils romance at sea?"
Damned ijjit!

Offline steamer

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Re: Reversing a Piston Valve Engine
« Reply #20 on: March 11, 2021, 12:34:44 AM »
"....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
"Mister M'Andrew, don't you think steam spoils romance at sea?"
Damned ijjit!

Offline catceefer

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Re: Reversing a Piston Valve Engine
« Reply #21 on: March 11, 2021, 05:13:49 PM »
Thank you, Dave.

I hope to make a start on this fairly soon.

Kind regards,

James.