On the history of valve gears:
In 1698, the English mechanical designer Thomas Savery invented a pumping appliance that used steam to draw water directly from a well by means of a vacuum created by condensing steam.
A new design by Thomas Newcomen in 1712: the valves of his Steam Condensing "Atmospheric" mine-water pumping engine had no rotative motion for valve operation, but the valves were operated by the Engine driver. His valves were one for admission of steam to raise the heavy engine piston and rod, and lower the pump rod, and a second valve that admitted water to the engine (later a condenser) to quench the steam and create the vacuum, so the atmospheric pressure on the outside of the piston could raise the pumping rod and pump the water up and out of the mine.
Move on to James Watt: His developments to improve the Newcomen design developed a new design to be introduced commercially in 1776: When he improved the Newcomen design with a condenser, the second valve became an exhaust valve from the cylinder.
Some later engines had dogs and levers operated by the piston rod - the timing produced by the position of the piston rod.
In 1781 introduced Watt a system using a sun and planet gear to turn the linear motion of the engines into rotary motion. He could not use the crank as someone else held the patent for that. Later this became out of patent and he used cranks.
Because he developed the sun and planet gear arrangement, he had a reliable way of converting the reciprocating motion of the piston into rotative motion of the crank. But the valves were very like the original "reciprocating" lever operated valves on some engines (right up until the late 19th century?).
In his first sun and planet gear driven flywheel engine you can see the rods to operate the separate valves per beam position.
Although by 1832 Watt appeared to be using D-Valves on his beam engines.
However, Richard Trevithick, amongst others, keen to develop engines using high pressure steam, started using a 4-way valve, driven from the crankshaft. But possibly he was not the inventor of the steam carraige. It is not clear what valve arrangement was on his first steam engine powered carriage (1803), but appears to be rotating valves (possibly his 4-way valve?) operated by the engine driver - operating the long lever to the valve?
William Murdoch had developed and demonstrated a model steam carriage, initially in 1784, and demonstrated it to Trevithick at his request in 1794.
Certainly, in 1803~4 Thevithick still had "engine driver" arranged valve gear: as can be seen on his Coalbrookdale loco here: - The looped handles that were pushed and pulled to operate the valves for steam and exhaust.
The Blenkinsop engine: predecessor of George Stephenson's locos - shows rotating valves for steam inlet and exhaust: (at 2.53mins). You can see the valve drive rods at 4.44 mins of this video.
George Stephenson's early engines - e.g. Killingworth no 2 - appear to have rotating valves, operated by some linkage not shown in this engraving.
Once locomotive design took off (in a hurry?) the rotative motion from the cranks became the driver for the valve gear - using an eccentric - and reversing valve gears were developed. The Stephenson gear was probably the earliest commercial design of this? And I figure it was after 1833ish? But the earliest rail locomotives on the Stockton and Darlington railway were still required to stop before changing from forward to reverse direction. In 1831 or 32 - my direct ancestor - was recorded in the railway records as being the only engine driver who could reverse the engine's gear while still in motion. (Thus acting like a brake!). Against the usual practice of stopping the train, dismounting and changing the gear to reverse operation.
What I do not know, is where in the line of history did the development of the D valve, and similarly the piston valve, get introduced? I figured somewhere within the James Watt development?
And when did "cut-off" become a known advantage for economy? - Possibly as early as Newcomen?
I must read more history on this. e.g. the valve gear of Trevethick's "Catch me who can"
and Stevensons Rocket all seem to have had rotating valves, later driven from eccentrics (on the Rocket)? But Newcomen had single valves hand operated. And Watt, through to Stevenson had linkages driving separate rotating valves.
Also curious to note that when Otto developed his first engine (1867), it used some gear mechanism driven off the piston rod to operate the valves - rotating type.
So, with the later steam engines using Corliss gear and rotating valves it seems the design has gone full circle?