I've been browsing this site (<!-- m --><a class="postlink" href="http://www.railarchive.net/randomsteam/dmir1311.htm">http://www.railarchive.net/randomsteam/dmir1311.htm</a><!-- m -->) looking to get some ideas for upgrading my steam roster, and I see that cylinder dimensions are given as "27"x 32", or some such numbers. What do these numbers refer to..??

Thanks for any insights....

If you know anything about autos think of it as bore and stroke.
Charlie

Sometimes you get another number -- usually 2, 3 or 4 -- which is the number of cylinders.

Thanks for the info.
I presume the same rules apply to steam cylinders as they do to auto engines, where a stroke longer than cylinder diameter will result in a higher torque, whereas a diameter greater than the stroke will result in a higher RPM's/speed..??

Steamtrains Wrote:Thanks for the info.
I presume the same rules apply to steam cylinders as they do to auto engines, where a stroke longer than cylinder diameter will result in a higher torque, whereas a diameter greater than the stroke will result in a higher RPM's/speed..??

I think that the ratio of diameter and stroke is a more complex problem - and a greater stroke will limited a larger RPM.

A very important fact is the diameter of drivers and there you can say - the larger the wheel diameter of a steam locomotive may be the greater the speed. There are of course exceptions, such as the class J of the NW with relatively small driers while a high speed.
Another fact is the mean piston speed, which may not exceed a certain value and this value depends on the wheel rpm and also from the stroke of the piston. This value can be a critical at high speed and with very long legs! Consider the piston has moved each time in the middle of the cylinder with its maximum speed and have to stop at each end point to change the direction of its movement. And another point is that large pistons have a larger mass and thus the total mass of piston, piston road and crosshead twice must change their direction of movement per revolution of wheels.
So the best way should be a short stroke and a relatively large diameter of cylinder - but there is a more problem.
A short stroke gives only a small diameter of the crank pin circle! But that no large force can be generated on the outer diameter of the driving wheels. The ratio of the pitch circle diameter of the crank pin to wheel diameter should be as small as possible, others - So it needs a large large crank circle and thus be a great piston stroke to transfer a lot of power to the wheels. And a long piston stroke is standing again in contradiction to the large pistons.
In result - the locomotive designers need to find a happy medium to produce as large a force as possible with small cylinders (piston) and a possible short piston stroke. But in most cases you are right. The diameter of cylinders is almost smaller as their stroke. On other side at low-pressure cylinders of (real) Mallets is a good bit larger than their diameter. In this case the power with a low steam pressure must come by an enlarged cylinder, but that is associated with a poor ratio of the moving stroke and mass, because the distance is identical to that of the high-pressure cylinder, since usually identical drive wheels of the two drive groups are used. How I said already - it is a very complex process to find the right ratio of stroke and cylinder diameter including the steam pressure for getting the wished speed and maximal power.
Compare this page of steam locomotive dot com - page N&W class Y, scroll down near to end of table and compare values of "High Pressure Cylinders" and "Low Pressure Cylinders". There you can find these dates of diameter and stroke of cylinders.

I hope that I could help you.

A short addition.
Compare also the VGN 2-10-10-2 class AE with a low pressure cylinder of 48" x 32" (dia x stroke) - the largest cylinders ever used on steam locos. However these engines compensed with a max. speed of 15 mph (also because of their small drivers) the speed of the hugh mass of engine's large pistons. On other side larger drivers would give an enlarged speed without more piston speed however the engine would equally lose pulling power. All in all the class AE was trimmed for heavy power and speed was a second-rated factor.

Bernhard,
Thanks for that information - very enlightening. Us train fans (nuts) rarely appreciate the complexity of these machines.
Now I like them even more..!!

One thing to remember (although it makes no difference to modelling) is that steam cylinders are double acting, i.e. steam is introduced to the cylinder on both sides of the piston alternately.

Talltim,
yes, yes you are right. And there is more yet! The movement and behavior of the forces is influenced by many more factors than previously described.
Yes, the steam engines of a steam loco are double acting. To make the forces at changing the direction of movement of the piston and cross head as 'gently' as possible, some precautions are taken.
First, the supply of fresh steam at the movement of the piston in each direction, the so-called 'cut-off' is blocked well before the dead point of the movement, which can even set the engineer. However, the cut-off is set by the valve gear to max. 70 percent of the distance of movement of the cylinder piston. More or later (in the time scale) is not possible, but the engineer can reduce the cut-off point near to zero by using the valve gear. After the cut-off no more steam flows into the cylinder and the cylinder 'caged' steam can only produce force by which the steam pressure reduces in the cylinder - at each direction of piston movement. As a result, the force that the piston moves at the end of the movement is just before the dead point at its lowest.
On the other is controlled by the valve gear that opens the exhaust ports already nearing the end of the movement, before the dead center and thus before the change of direction of the piston. Thus one could say that the piston changes its direction of movement without force only with the inertia of its own mass about the dead center.
However, in order to reduce this kinetic energy before the end of the movement of the piston in one direction the valve gear opens the valve inlet channels of the opposite direction in order to give fresh steam to the back side of the piston. This steam in the opposite direction the piston is braked at the last part of the way and at the same time from the first point of the movement of the piston in the opposite direction, the full force of fresh steam for the piston movement in return direction is available.
All in all, this is a very complex procedure in the development of the forces in the cylinders. And so far is only the horizontal movement of pistons, piston rod and crosshead were written and how they arise from the forces due to the pressure of steam. But a second procedure must be considered. The horizontal back and forth motion and the high forces to move thousands of tons, must be converted into a rotary motion in a circle and evenly as possible, with little vibrations and swingings and often by high speed. But the balancing of linear moving masses by rotating masses and at end of steam era also by the cross balancing of both sides of driving system is an additional complex theme - but there I do not know enough about this.
I find that the steam engines, their gearing and and the entire system of the drivers are a high development of engineering. I am extremely excited them!

I hope for understanding of my post, here. I must use the Google translater for some parts of this text and I hope that it will help. In other case I would write until late night yet.

WoW..!! Now THAT's some complexity....It's mind-boggling how simple they started and evolved to the complexity of latter day Mountains, articulateds, etc...

It reminds me of a quote by Kurt Vonnegut in one of his novels..."How God must have laughed when He saw how man came up with the steam engine" - or something along this line.

Gus.

The steam engine could have been developed quite a bit earlier but they had to wait for metallurgy to catch up -- we needed steel boilers that would hold a bit of pressure.

For fun, look up some of the early mine engines. In the first designs they actually filled the cylinder with steam and then sprayed in cold water to condense the steam and create a vacuum which sucked the piston down.
The temperature changes were very hard on the cylinders.
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