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How Does It Work

There have been many books, articles and web pages covering all aspects of steam and steam engines etc. Most of these assume that the reader knows and understands the basic principles and terminology used.
Since, as far as I know, these principles are no longer taught there are probably lots of people out there (especially the younger generation) who would like to know, in simple terms, how steam engines and there asssociated components work. It is with this in mind that I am producing these pages. I therefore make no apologies for some of the basic content.

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The Corliss Valve

An American inventor, by the name of George Henry Corliss, made several improvements to stationary steam engines. One of which was the semi-rotary Corliss Valve, which he developed in 1849. In later years it proved to be very popular with large steam engine manufacturers because of its reliability and efficient use of steam.

Section of a Corliss valve in the cylinder

Corliss valves oscillate, within the cylinder casting, to open and close steam ports. They are driven by the action of an eccentric, on the crankshaft, via a wrist plate and various rods, as shown below.

The following illustrations show the position of the Corliss valves, within the cylinder, on one forward and backward stroke. This would complete one revolution of the flywheel.

Steam enters the steam chest through pipe S .
Inlet valve V1 is rotated to uncover the steam port to the cylinder and the steam forces the piston along the cylinder from left to right.
At the same time the exhaust valve E1 and the inlet valve V2 are closed.
As the piston continues to move along the cylinder the exhaust steam passes through the open port at E2

Corliss valve positions

When the piston reaches the end of the stroke, it starts to move from right to left.
The inlet valve V2 is rotated to uncover the steam port to the cylinder and the steam forces the piston along the cylinder, from right to left.
At the same time the exhaust valve E2 and the inlet valve V1 are closed.
As the piston continues to move along the cylinder the exhaust steam passes through the open port at E1.

Although not shown here, the inlet valves were usually operated by some form of trip gear, which would control the length of time the valve was open and then quickly close it. When linked to a governor, the trip gear is a very effective method of controlling the engine speed within fine limits. The exhaust valves are not usually automatically contolled by trip gear but are fully open and closed, although in some cases the control can be made by hand.

Another advantage of the Corliss Valve layout as shown, is the fact that the hot inlet steam is kept well away from the cooler exhaust outlet. This leads to a larger temperature difference between inlet and exhaust steam which tends to increase efficiency.

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