George Corliss built his first automatic engine in 1848. This horizontal engine had several interesting innovations in its valve action. Corliss used four separate rotating valves in place of the common single sliding valve. The two exhaust valves at the bottom of the cylinder were separated from the two steam admission valves at the top. (In this way Corliss was able to reduce the heat losses that result from the temperature difference between input steam and exhaust steam, and he provided a convenient path for trapped water to flow from the cylinder. Corliss was able to achieve a fine control over the instant in the cycle at which the admission valves were closed by breaking the physical link between the eccentric rods and the valve stems. The instant at which the link was broken was determined by the speed of the engine as sensed by the governor.)
In this model showing the action of Corliss valves the motion of the eccentric rod rocks a circular wrist plate about its center. The exhaust valves at the bottom of the cylinder are driven directly from the wrist plate, since there is no need to vary their instant of cut-off. The steam admission valves at the top of the cylinder are pulled open by the wrist plate, but are strongly urged toward the shut postion by the spring action of the dash pots which are mounted on the floor under the valve stems.
Rotation of the wrist plate forces each admission valve open, in turn, against the pull of the dashpot. At the proper instant a latch on the valve stem is tripped, breaking the link between the stem and wrist plate, and the valve snaps shut. The part of the cycle at which the link is broken can be set by external controls and be varied by the governor to compensate for variations in engine load or supply steam pressure. If the engine speeds up for any reason the governor causes the valves to be tripped earlier in the cycle, thereby admitting less steam to the cylinder per power stroke and slowing down the engine. Conversely, if the engine speeds up the action causes more steam to be admitted to the cylinder per power stroke.
The dashpots for closing the valves were usually evacuated cylinders mounted on the floor, or on the base of the engine. There were many variations of the original Corliss design, however, and some of these used springs or weights in place of dash pots. Also, in some designs the dash pots were mounted on the body of the cylinder rather than below it.
Corliss Mill Engine
The Corliss engine was an immediate success because of its increased efficiency and sensitive speed control. Engines of this design were used for over fifty years and were manufactured by many builders, all over the world. Of these, the engines built at the Corliss Works in Providence, Rhode Island, were noted for their craftsmanship and perfection of design detail. This is a model of a single cylinder, horizontal mill engine, built around 1890. In this engine the cylinder is insulated with a wood jacket. Steam enters the cylinder through the pipe at the top of the cylinder and exhaust is carried away through the pipe at the bottom.
The model has the four separate valves and the wrist plate action that is characteristic of all Corliss engines. The wrist plate operates, through the eccentric rod, by rotation of the eccentric which is on the crankshaft, directly behind the flywheel.
Despite its many fine characteristics the Corliss engine was almost obsolete by 1900. Superheated steam had been introduced and generally accepted by that time and the semi-rotating Corliss valves were not compatible with it.