Valve gear

This is not the ideal arrangement for a steam engine, though, because greatest power is achieved by keeping the inlet valve open throughout the power stroke (thus having full boiler pressure, minus transmission losses, against the piston throughout the stroke) while peak efficiency is achieved by only having the inlet valve open for a short time and then letting the steam expand in the cylinder (expansive working).

The point at which steam stops being admitted to the cylinder is known as the cutoff, and the optimal position for this varies depending on the work being done and the tradeoff desired between power and efficiency.

A further benefit may be obtained by admitting the steam to the cylinder slightly before front or back dead centre.

This advanced admission (also known as lead steam) assists in cushioning the inertia of the motion at high speed.

Both slide and piston valves have the limitation that intake and exhaust events are fixed in relation to each other and cannot be independently optimised.

Another effect of early cutoff is that the valve is moving quite slowly at the cutoff point, and this creates a constriction point causes the steam to enter the cylinder at less than full boiler pressure (called 'wire drawing' of the steam, named after the process of making metal wire by drawing it through a hole), another wasteful thermodynamic effect visible on an indicator diagram.

Valve gear was a fertile field of invention, with probably several hundred variations devised over the years.

At high speed, a Walschaerts-type gear is said to give better steam distribution and higher efficiency.

Harold Holcroft devised a different method for conjugating valve gear by linking the middle cylinder to the combination lever assembly of an outside cylinder, creating the Holcroft valve gear derivative.

The use of separate valves and port passages for steam admission and exhaust significantly also reduced losses associated with cylinder condensation and re-evaporation.