Corliss engines have four
valves for each cylinder, with steam and exhaust valves located at each end. Corliss engines incorporate distinct refinements in both the valves themselves and in the
valve gear, that is, the system of
linkages that operate the valves. The use of separate valves for steam admission and exhaust means that neither the valves nor the steam passages between cylinders and valves need to change temperature during the power and exhaust cycle, and it means that the timing of the admission and exhaust valves can be independently controlled. In contrast, conventional steam engines have a
slide valve or
piston valve that alternately feeds and exhausts through passages to each end of the cylinder. These passages are exposed to wide temperature swings during engine operation, and there are high temperature gradients within the valve mechanism. Clark (1891) commented that the Corliss gear "is essentially a combination of elements previously known and used separately, affecting the cylinder and the valve-gear". The origins of the Corliss gear with regard to previous steam valve gear was traced by Inglis (1868).
Corliss valve gear George Corliss received for his valve gear on March 10, 1849. This patent covered the use of a
wrist-plate to convey the valve motion from a single eccentric to the four valves of the engine, and it covered the use of
trip valves with variable
cutoff under
governor control that characterize Corliss Engines. Unlike later engines, most of which were horizontal, this patent describes a vertical cylinder
beam engine, and it used individual slide valves for admission and exhaust at each end of the cylinder. The inlet valves are pulled open with an eccentric-driven
pawl; when the pawl
trips, the rapid closure is damped using a
dashpot. In many engines, the same dashpot acts as a vacuum spring to pull the valves closed, but Corliss's early engines were slow enough that it was the weight of the dashpot piston and rod that closed the valve. The speed of a Corliss engine is controlled by varying the
cutoff of steam during each power stroke, while leaving the throttle wide open at all times. To accomplish this, the
centrifugal governor is linked to a pair of cams, one for each admission valve. These cams determine the point during the piston stroke that the pawl will release, allowing that valve to close. As with all steam engines where the cutoff can be regulated, the virtue of doing so lies in the fact that most of the power stroke is powered by the expansion of steam in the cylinder after the admission valve has closed. This comes far closer to the ideal
Carnot cycle than is possible with an engine where the admission valve is open for the length of the power stroke and speed is regulated by a
throttle valve. The Corliss valve gearing allowed more uniform speed and better response to load changes, making it suitable for applications like rolling mills and spinning, and greatly expanding its use in manufacturing. In this engine, the wrist plate was moved to the center of the cylinder side, as on later Corliss engines. This was still a
beam engine, however, and the semi-rotary valve actuators operated linear slide valves inside the four valve chests of the engine. Corliss valves are in the form of a minor
circular segment, rotating inside a cylindrical valve-face. Their actuating mechanism is off along the axis of the valve, thus they have little "dead space" such as the stem of a
poppet valve and the entire port area can be used efficiently for gas flow. As the area of a Corliss valve is small compared to the port area, the effects of gas flow generate relatively little torque on the valve axle compared to some other sorts of valve. These advantages have led to the Corliss form of valve being used in other roles, apart from steam engines with Corliss gear. The
Rolls-Royce Merlin aero-engine used a rectangular
butterfly valve as a throttle. Gas-flow forces acting asymmetrically on this butterfly could lead to poor control of the power in some circumstances. Late models, from the 134, used a Corliss throttle valve instead to avoid this problem.
Barring and barring engines A common feature of large Corliss engines is one or two sets of narrow gear teeth in the rim of the
flywheel. These teeth allow the flywheel to be
barred, that is, turned with the aid of a
crowbar. This may be needed during engine maintenance, for example, to set the cutoff and admission valve timing, and it may be needed during engine starting. The need for barring the engine during starting is most obvious on single-cylinder engines, where a careless engine operator might stop the engine with the piston in or near
dead center. Once stopped in this state, the engine cannot be started under its own power, so it must be barred to a more favorable position for starting. Large Corliss engines cannot be safely started cold, so it is common to admit low-pressure steam to both sides of the cylinder to warm up the metalwork. Turning the engine slowly during this process ensures that the entire engine is uniformly warmed, and it ensures that oil is uniformly distributed through the mechanism before applying power. Again, barring may be used to do this, although operators sometimes do this by careful manual manipulation of the valves. For large engines, muscle powered barring is sufficiently difficult that
barring engines are frequently installed. These are small engines with gear teeth cut to mate with the teeth on the flywheel. Generally, the drive gears of the barring engine are designed to automatically disengage if the engine begins running under its own power while the barring gears are engaged. ==Company history==