Steam engine, horizontal stationary cross compound, steel / cast iron, Marshall Sons & Co Ltd, Gainsborough, England, 1909.
In a compound steam engine, steam works first in a high pressure cylinder and then in one or more lower pressure cylinders, to achieve higher efficiency than a simple engine. There are three types of two-stage compound steam engines: tandem, Woolf type and receiver type. This engine is of the receiver type.
In the tandem engine both the high pressure and the low pressure pistons are mounted on the same piston rod (see Belliss & Morcom engine, object B1603). In the Woolf and receiver types the cylinders are adjacent to one another with separate piston and connecting rods and cranks. In the Woolf type the cranks are 180o apart, whereas in the receiver type the cranks are 90o apart, with the high pressure crank leading the low pressure crank.
In the Woolf type, exhaust steam from the high pressure cylinder is fed directly to the low pressure cylinder; the driving torque (for a double acting engine) reaches two peaks per revolution and thus varies greatly throughout each stroke. In the receiver type, exhaust steam from the high pressure cylinder is first led to a storage chamber and thence to the low pressure cylinder; the driving torque reaches four peaks per revolution and is therefore less variable.
In this engine the casting containing the high and low pressure cylinders is bulky since it also contains, on top, the receiver chamber, whose volume is normally about 50% greater than the swept volume of the high pressure cylinder.
Steam admission and exhaust, for both cylinders, is controlled by slide valves. On the high pressure side there is also an expansion valve. The slide valves, which are controlled by eccentrics on the crankshaft, regulate the supply of steam to the cylinders, whereas the expansion valve, also driven from the crankshaft but controlled by the governor, acts to cut off the steam supply to the high pressure cylinder in order to regulate the engine speed.
The governor is a Hartnell Automatic Expansion spring loaded type, which comprises two spherical weights connected to bell cranks so that, as the engine begins to run too fast, the resulting outward motion of the weights raises a spring-resisted sleeve on the governor shaft. The main slide valve is driven by an eccentric, and an additional eccentric is connected to the cut-off valve via a slotted 'expansion link'. As the engine speed increases, the sleeve raises the expansion link and the steam cut-off occurs earlier, thus reducing steam flow and slowing the engine. The governor weights are large in order to provide the force necessary to move the mechanism.
This L class engine is numbered 52700. It developed 80 horsepower at 135 rpm. Its stroke is 400 mm and cylinder diameters are 240 mm (high pressure) and 350 mm (low pressure).
On maker's plate: MARSHALL SONS & CO. LD. / ENGINEERS / GAINSBOROUGH, ENGLAND. / NO. 52700. The engine number appears on the plate twice, on each side of the word 'engineers'.