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.