Continuously Variable Pneumatic Control
Application to Steam Engines
by Mark J E Bellis, 14th November 2009

The original pneumatic steam engine was the second model of set 8868, a refuse truck, from 1991.
The grab and arm raising functions of the truck formed a logical loop that repeated its sequence as
long as air was supplied.

My first steam engine in the early 1990s, based on this system, used two cylinders and two
valve switches to turn a shaft.
Reversing this engine pneumatically would take quite a few valve switches because there are two
pairs of pipes to swap over.  Using two 4-switch reversers would need another 8 valves, making
it large and expensive.
A gear system could reverse the direction of rotation with no additional valves, but there is a
large dead-band in the valve positions.

The Continuously Variable Pneumatic control system may be applied in multiple as a steam engine.
Each cylinder uses 2 valves for control but no additional valves are required for reversing.
Small cylinders are used because they fit better in the loading gauge of an L-gauge train.

The output to the wheels is taken from the feedback beam (blue in the original mechanism,
now dark bley, behind the cylinder on each side)

The variable-phase valve gear drive is fed into the input beam (red in the original mechanism,
now also dark bley, behind the red valve lever extension parts on each side)
The input beam is lowered in order to be more representative of a steam locomotive.
A small (black) beam, used for the dither mount, is fixed to the input beam at the higher level,
using 2 red beams to transmit the movement.

The dither is applied to each set of valve positions.
The dithers of the two (or more) cylinders are operated by a single motor.  This helps to ensure
consistent perfomance across all cylinders, as well as saving motors!
Since the input beams of the cylinders slide at different phases, the drive has to accommodate it.
This is done by using pulleys and belts.  The motor shaft is above the set of sliding dither pulleys
so that the tension in the belts varies little throughout the sliding motion of the dither mounts.
The motor fits between the ends of the 16M beams at the top.  this would be inside the boiler
of a steam loco and should be lowered to fit the loading gauge.

The phase variability for the valve gear drive could have used Walschaert's valve gear or Stephenson
Link Motion, but the expansion links would be large and would introduce too much play in the system.
Therefore a gear drive has been used.
A valve gear shaft is geared from the driving wheel shaft.
The idler between the two shafts is mounted on a tilting bracket.
the tilt of the bracket caries the phase between the two shafts by up to about 20 degrees, which
is enough to cause the steam engine to drive the wheels in one direction or the other.
The bracket is tilted from the black lever.  The dark bley piece limits its travel to prevent
disengagement of the gears.

I have shown 6 wheels here, though it could be any number, depending on the steam locomotive.
The lowest beams under the valves could be trimmed by using a studless beam, though the slide
of the main piston beam would move vertically during operation.
Combining this trimming with the use of BBB train wheels and a front bogie would allow the system
to run on L-gauge track.
The height of the valve levers and dither mechnism should be squashed to fit under bridges!
A compressor in the tender would supply the air.
Perhaps more use could be made of the space between the valves with further development.
The pneumatic hoses might be inside too.
A working pneumatic L-gauge loco is surely one of the holy grails of LEGO trains!

Mark J E Bellis