About the Model:
A Free-lance, Non-release Horizontal Cross
Compound Steam Engine
Designed and Built By Mike Dennis June/July
Member - H.T.M.C.,R.M.G.
Approximate scale 1/10
This engine is similar to some small industrial
Compound engines and was built to demonstrate
the use of drop valves on the L.P. and Corliss
valves on the H.P. cylinders. The drop valve
layshaft on the L.P. cylinder is utilized
to drive an oil feed pump for valve lubrication
to both cylinders which would have been
fairly common practice. The steam valves
(Inlet) are placed at the top and the exhaust
valves, below, on both cylinders. Engines
such as this were built for medium to low
constant speed running (50 - 100 RPM) and
some had no trip gear (hence the term non-release).
A governor was also optional as steam pressure
and ingress could be controlled manually
via a gate valve which therefore had a governing
effect. The piston rods are extended by
'Tail rods' to minimize wear on the bore
and glands and covers would have been fitted
for safety as they are on the model.
The toothed flywheel would have allowed
geared, high speed, over-head counter shafts
enabling machinery to run often for 24 hours
daily over a 6 day week. Maintenance work
was carried out on the seventh! I normally
run the model at about 40 - 60 RPM to allow
viewing of the valve gear operation which
is 'timed' as near correct to 'live-steam'
conditions as I can ascertain.
The main reasons for compounding were the
advantages of overall efficiency and less
space than was required to operate a beam
engine of similar cylinder capacity. A small
compound would take up less space and produce
more power at less steam pressure on the
LP cylinder from a relatively smaller boiler.
Valve Systems - Drop Valves - LP (Large
The drop valve design used on this engine
is similar to a piston valve. It is loosely
based on the types featured on Sultzer Brothers
and Lentz engines. They are also driven
by spur and bevel gearing at 1:1 from the
crankshaft via operating rods fitted to
eccentrics on a layshaft. Drop valves were
in common use mainly on horizontal engines
from the early Victorian period. Later,
they were fitted on Uniflow and other engines
in constant productive use in the latter
half of the 20th century, some as late as
1970 in the USA.
Corliss Valves - HP (Small Cylinder)
Corliss valves are semi rotary valves controlled
from a wrist plate (center from the cylinder)
via an eccentric. Invented and patented
by the American engineer, George H. Corliss
in the 1840's; engines fitted with Corliss
valves were, like drop valves, also renowned
for efficiency and fuel economy. They were
more widely used except for the simpler
and less expensive slide valve.