Single Cylinder Oscilator - Page 2

**Bunkerbarge** · 08-02-2007

One of the biggest problems with oscillators is maintaining a seal between the cylinder and the mainframe to allow the passage of steam to the cylinder between two sliding faces. Very early marine design actually used oscillators in ships and got away with it as the boiler pressures used at the time were relatively low.

As power requirements increased however and boiler design improved the steam pressures started to rise and the problem with the sealing face became more of an issue. The answer was to incorporate a valve in a chest alongside the cylinder which directed the steam to the cylinder at the appropriate time. The valve was driven by an additional throw on the crankshaft and the rod that operated the valve was relatively easily sealed by a stuffing box. The cylinder no longer oscillates, so those associated losses are removed, and the steam pressure can now be raised giving more power for a given volume. Most slide valve engines are double acting as the valve is used to direct the steam to both sides of the piston so, unless the engine stops at top dead centre or bottom dead centre, it should self start. Another major advantage with this engine is that the steam admission can be timed and adjusted so steam can now be controlled in a way that an oscillator could never achieve.

The first picture is probably one of the most popular of model vertical slide valve engines and that is the Stuart 10V. This was also an Ebay buy but part of a complete installation that incorporated two vertical engines, one with an upside down crank, a horizontal engine, boiler, feed pump, dynamo and a lamp. This is obviously very old and from the time when well to do people bought toys to demonstrate a few of the principles of physics to kids. Anyway I purchased it with the intention of restoring the whole set up one day but there is certainly a lot of work required for that. All the items are Stuart though so there is a ready supply of spare parts available for these things so, hopefully one day, it will all become a working model again. Probably one for the retirement fund!!

dipstick · 08-02-2007

i don't mean to be rude but i think that boiler has seen some better days. but they are some nice little stuart models with it. and richard i really am quite fasinated by that triple cylinder engine i really would be nice to see some pics of the set up.

**Bunkerbarge** · 08-02-2007

I'll add a few more pictures of the triple in a few weeks time when I am back home.

It is a very interesting design and a fascinating model, however it is not my favourite. I'm saving that till last!!

I agree the boiler looks a bit tired though!

Kiwi · 09-02-2007

I've made a couple of single oscillators (one on a traction engine similar to a Mamod) and I have all the castings for a Stuart Double 10 (the box has a price of £2 9/9d) on it which may tell you how long I have had them!)The intention was to build a "Wide-a-wake" an Edwardian steam launch but model aircraft have got in the way and with only a Unimat SL it wouldn't be an easy machining job doing something of that size and the thought of machining that double throw crankshaft terrifies me.

**Bunkerbarge** · 09-02-2007

I agree it would be a bit of a challenge, especially with something like a Unimat.

If the kit is that old though it may well be worth keeping as an interesting item as it is.

**Bunkerbarge** · 18-02-2007

If we look at the next example we see it as not quite so much another forward step in the evolution of the steam engine but more of a step sideways.

This engine is basically the same configuration as the last one, being a single cylinder, double acting slide valve engine but, instead of being stood up vertically it has been laid down horizontally. Thus we have an engine that is basically of the same configuration that powered much of Britain’s industry for many years during the 19th century.

The configuration lent itself to being fitted into buildings as it could be fitted between floors easily and it could be worked on easily. The engine was usually used to power a main drive that invariably ran up the side of a building and from there a shaft was driven that ran the full length of each one of the floors. From this shaft belts were driven that then, in turn, operated various machinery in the building such as weaving machines in the case of mills or lathes, shapers, drills etc. in the case of a machine tool manufacturer. This very engine played a major part in the whole industrial revolution of the United Kingdom and in conjunction with it’s low pressure boiler with it’s inefficient combustion led to vast areas of the country becoming black and dirty with mill chimneys pumping out smoke into the atmosphere. There are still many areas in the north of the country today living with the legacy of such industry in such ways as blackened buildings and lack of trees used to power the boilers.

There are still a number of museums around, such as the one in the Piece Hall in Halifax, that have operational horizontal steam engines that are powered up occasionally and which actually run a working textile mill. When you see the drive belts all flapping around it is easy to see why they were considered as such dangerous places.

The model is, as usual, an Ebay purchase and is a nice example of a relatively simple single cylinder horizontal model. This one hasn’t yet run in anger as it appears to be a bit sticky to turn and it hasn’t yet been fitted with a flywheel. With such a relatively slow turning engine a flywheel is essential to carry the momentum of the power stroke through the exhaust stroke and so the engine will not run anyway until it has been fitted with a flywheel. Again though you can see the two rods of the main piston and the slide valve in just the same way as the vertical engine was laid out. I have actually purchased a flywheel this time away from a supplier in the United States and I hope to get it machined up and fitted during my next time home.

I also have a Stuart 10H that came with the same layout as in the last posting and is in pretty much the same condition as the 10V I Have. One day it will get restored, one day!

bogstandard · 18-02-2007

Here is a link to a slightly different slide valve design, this one has a rotary valve driven directly from the crankshaft. I built from castings and plans a few years ago.
http://www.floridaame.org/GalleryPages/g1Ex0124.htm

**Bunkerbarge** · 20-02-2007

Now moving one rung further up the evolutionary tree the obvious next step is tom stick two slide valve cylinders together and get a vertical twin slide valve engine.

Stuart make a popular but somewhat expensive model in their twin 10V and you will find a number of similar engines around from other manufacturers. In reality the twin slide valve engine was used in very early marine applications but was very soon superseded by the advent of the compound engine. The basic vertical twin supplies steam to both cylinders evenly and both exhaust to a common manifold. In a compound engine the exhaust from the first stage is then used to drive the second stage piston. This has the effect of getting considerably more of the energy from the steam and makes the engine more efficient. This was then progressed to a triple expansion compound engine which provided marine propulsion for many years during the first half of the 19th century.

A compound engine is very easy to spot as the cylinders increase in size as the steam is expanded from one cylinder to the next. The tops of a triple expansion are quite distinctive in their size progression,

The model is manufactured by a German company by the name of Hansen who make a very nice range of brass engines. The standard twin is self starting but not reversible and with this type of engine it is not simply a case of putting steam in the other way as the slide valve has now allowed us to admit steam at just the right time of the cycle and shut it off just as we want as well. To reverse a slide valve engine you have to use a complicated reversing mechanism which does not lend itself very well to smooth model operation. For a start it requires two servo’s, one for the reversing lever and one for the speed control so it is not the smoothest of operations and the reversing mechanisms seem prone to sticking.

The solution, as with this example, is to reverse the propeller externally so this engine is destined for an open launch model coupled to a controllable pitch propeller. This means that the engine can be turning at the same speed, although I am probably going to control that as well so as not to waste steam, and the reversing process will be controlled by reversing the propeller pitch. It took me a while but I eventually came up with an example of a Controllable pitch prop which will suit this job perfectly. Hopefully the engine will tick away nice and smoothly and the boat will go backwards and forwards without the engine changing speed or direction. Should prove interesting.