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The torque generated by a steam engine is a combination of piston area, steam pressure and crank length which is half the stroke of the engine.
A real advantage of the steam engine over some other engines is the fact that t generates maximum torque at zero rpm. This torque would remain constant all the way through the speed range except for the fact that you have to control the steam flow into the engne and get the exhaust steam out of the cylinder.
Towards the end of the stroke the the crank pin moves behind the axle and so the distance reduces and with that the torque. Therefore, rather than waste steam it is better to close the inlet valve early.
As the engine speed increases the number of strokes per minute will increase and so with it the quantity of steam required from the boiler. At some point the engine speed will be limited by the amount of steam the that boiler can produce.
This is the cylinder and connecting pipe volume at the beginning of a piston stroke just before the steam valve is opened. It is important to minimise this volume as this reduces the steam pressure that acts on the piston. Typically this volume is around 10% of the swept volume of the piston.
The pressure in the cylinder is plotted versus the cylinder volume. This was developed by James Watt and John Southern to improve the efficiency of the steam engine. The work done is the area within the red curve. more...
Steam taken directly from a boiler retains the same temperature and pressure as when it was produced and is said to be "saturated". By taking such steam and heating it further, a much greater volume of gas can be produced for a small amount of extra heat. Tests have shown that superheating can result in a 25% economy in coal and 30% in water. Introduced in the 1900's and used mainly on the larger main-line engines.
The increased efficiency from using superheated steam in a reciprocating steam engine is greater than thermodynamic reasoning would imply. The increased economy is largely due to the reduction in the initial condensation and also very probably due to a reduction in valve leakage.