Reciprocating engine advice please

[paul]

I'm looking at building a very simple horizontal reciprocating engine, single action with piston-type valve parallel to the cylinder, from the old book 'Things to Make' which has been mentioned elsewhere on the forum.



The plans don't give any specific dimensions and aside from getting the valve travel right and the con rods lined up it doesn't seem too tricky. The overall size as you can see is about 6 inches.

I plan to make the valve from a piece of rod with the centre section turned to a smaller diameter to allow steam to exhaust at the rear of the valve tube.

I'm not sure whether the piston should be made from tube as per the diagram or to use solid rod for that as well, I suppose tube is suggested to reduce the weight while maintaining a enough length
to provide a steam-tight seal.

Does anyone have any suggestions as to materials, valve or cylinder dimensions or construction? I suppose I should calculate the cylinder diameter (to make sure it's big enough to accommodate the vertical travel of the con rod) by using the crank to end-of-cylinder distance and the throw.

Am I likely to need any form of seal around the valve and piston or will a good fit suffice?

I'd probably use brass for most parts, except the rods/crank/wheels - any thoughts?

If the piston was solid rather than tubular I could make the back end of the cylinder flat. As I don't have the equipment to make a blind bore of suitable size I'd probably make the cylinder from drilled square rod with a soldered or bolted-on cover.

If anyone has any suggestions or tips I'd very much appreciate them before I go wasting lots of time on a hiding to nothing.

TIA

Paul


EDIT: If a solid piston was used I'd probably make a piston rod/crosshead arrangement instead.

[alanstepney]

If you are making that engine as a "toy type", then almost any material will suffice, as it will only be run light, and probably not often.

If you want a degree of authenticity, then mild steel or silver steel for the crankshaft, piston rod, valve rod, etc.
Bronze or gunmetal for bearings, cylinder, eccentric straps, etc.

Brass for valve chest cover.
Brass or bronze for the valve.

Or, cast iron for cylinder/piston/valve/bearings.

In the former case, the piston-cylinder seal could be soft packing, (or an O ring), but for cast iron a proper CI ring is best, although an O ring will work there too.


However, as you said, dimensions are sparce and the design is old.

Might I suggest that, before you go ahead, you look at Theseus, by Tubal Cain. It is similar to Perseus, which is featured in Reeves etc catalogues, but is built from bar stock not castings.
It was fully described in ME, and runs well. Importantly, it is fully detailed, so no guesswork at dimensions.

Dont have the ME reference to hand, but I do have copies of the drawings & articles if you get stuck.

[paul]

Thank you Alan. I'd definitely want a degree of authenticity, so I note what you say with regard to materials.

I've been giving some thought to the valve dimensions when looking at that old drawing. I came up with this rough sketch for it but I've no idea if these dimensions are 'reasonable'.



I've looked at Perseus on the Reeves site and that is what I'm aiming for (a nice starter project) but, as you say, fabricated from stock rather than castings. If you have the articles/drawings for Theseus it would be greatly appreciated

Thanks again!

[baggo]

Hi Paul,

your valve dimensions should work ok. Using the calculation in Martin Evans' book on valve gears the dimensions would give a cut off of 75% i.e. steam would be admitted for the first 75% of the stroke, the remaining 25% would be expansion. You could increase the cut off and hence reduce steam consumption by reducing the valve travel to say 3/8" although arriving at the best figure would probably be trial and error. The eccentric would be set so that the port just opens to steam when the piston is at top dead centre, or perhaps slightly before to give a bit of lead.

John

[paul]

Thanks John

Now the question is what size and stroke for the piston? In the drawing it seems to be about an inch in diameter and is constructed from brass tube, but a shorter solid piston might be easier to make.

Is there any real relationship between the piston per se and the valve in this sort of engine?

Paul

[baggo]

Hi Paul,

Obviously the bigger the bore and the longer the stroke the more steam you are going to need so you will have to think of the size of the boiler required. Perhaps one of our stationary engine friends could help there?

As a rough guide the piston valve diameter is usually about half that of the cylinder diameter. You could make the piston from solid bar and perhaps slot the end to take the connecting rod and cross drill for a press fit gudgeon pin, something like the ram on a water pump.

John