Hereticus II-- By Mike Dennis




Special Parts


About the Model:

A Free-lance, Non-release Horizontal Cross Compound Steam Engine
Designed and Built By Mike Dennis June/July 2002
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 Cylinder)

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.


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