ONE TO MODEL

WILLANS HIGH-SPEED TRIPLE EXPANSION ENGINE

THE The Willans high speed two-crank triple-expansion engine is unusual in that the cylinders are located vertically rather than horizontally. This challenge to the model engineer consists of two similar engines side by side.

In the drawing, a represents the high-pressure pistons,  b the intermediate pistons, and c, the low-pressure pistons. The air buffers d are pistons working in cylinders e. On the upward stroke the buffer d closes the ports f in the sides of the cylinder e, and the air in the cylinder is compressed as the buffer rises, overcoming the inertia of the reciprocating parts and maintaining a downward thrust on the connecting rods. The air buffer d also serves as a crosshead, with the sides of the cylinder e as the guides. The buffer carries the crosshead pin g from which the pressure on the pistons is transmitted to the crank-shaft h through the two connecting-rods i.

All the valves for each line of cylinders are placed on the same spindle, which is driven by an eccentric j mounted on the crankpin. As the valves work inside the trunk or hollow piston rod k, there is a certain amount of relative motion between the valves and the trunk. A downward thrust is always maintained on the valve spindle owing to the fact that high-pressure steam acts continually on the uppermost valve while the engine is running. The speed is regulated by a centrifugal shaft governor which controls the throttle valve m.

Steam is admitted through this throttle valve to the steam chest n, and as the pistons approach the end of the upward stroke, steam enters the high pressure cylinder through the ports o and p in the trunk, as shown by the arrows. The valve then moves upwards relative to the trunk, and in so doing the upper edge q of the distribution valve cuts off the steam supply. Toward the end of the downward stroke the valve passes still farther up the trunk, re-opens the port p by its lower edge r, and allows the steam to pass through the trunk and the port s into the chamber if at the lower side of the high-pressure piston. From this chamber the steam passes, through the cycle of operations, through the intermediate and low-pressure cylinders and finally enters the chamber u, and the exhaust pipe.

As the thrust on the crank-shaft is always downwards, the main bearings require the bottom halves only, although, to prevent the shaft from lifting when starting up, narrow upper halves are also provided. The Willans engine is single-acting so that the force transmitted through the connecting-rod acts in one direction. If the direction of action of the pressure is reversed at the end of each stroke, as in the case of a double-acting engine, there is a tendency to produce pounding, especially at high  speeds.

In this engine, the steam pressure acts only on the upper surface of each piston, and so the thrust on the connecting-rod and crankpin is always downwards. During the greater part of the return, or upward, stroke, the piston, piston rod, and connecting rod are lifted by the crank pin, the result being a continuous downward pressure on the pin. On nearing the upper end of the stroke, however, there is a tendency for the piston, piston rod, and connecting rod, due to their inertia, to exert an upward pull on the crank pin.

To avoid this, an air buffer is provided by which air is compressed on the upward stroke. This not only cushions the reciprocating parts, it ensures that the pressure on the crankpin will not alter in direction.


Photos of a Willans 2500HP and other high-speed engines can be seen at:

http://www.douglas-self.com/MUSEUM/POWER/highsped/highsped.htm

Click on drawing to download