COMPOUND HORIZONTAL ENGINE
Clayton and Shuttleworth exhibited a very beautifully made compound Engine with a wrought iron frame being extremely simple to operate. The cylinders are 6 ¼ in. and 10 ½ in. by 12 in. The engine is fitted with Tyrrell and Deed’s governor with a peculiarity that the steadying dashpot is put into the dead weight. This is a very elegant device and work very well. Messrs. Clayton and Shuttleworth make the same type of engine in various sizes.
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Compound engine, a development from the single cylinder reciprocating marine engine in which the steam, after leaving the first cylinder, was passed through a second low-pressure cylinder of larger diameter before being drawn off to a condenser to be changed back to boiler feed water. This second use of the steam added to the thrust produced by the engine results in a higher engine efficiency for the same amount of steam. Although the principle of compounding an engine was patented as early as 1781 by Jonathan Hornblower, a contemporary of James Watt, it was not until the 1850s, when higher boiler pressures were introduced in marine boilers, that the compound engine, designed by John Elder and Charles Randolph, became practicable at sea. As steam expands in a cylinder the temperature falls, and the greater the expansion the lower the temperature of the cylinder wall. When steam is admitted to the other side of the piston for the next stroke this steam encounters the cool cylinder wall and some of the steam condenses. This results in less steam being available to do useful work. To avoid this, steam was expanded in two states so that the temperature drop in each stage (cylinder) was less, resulting in reduced condensation and higher efficiency.
As steam expands in a high pressure engine its temperature drops; because no heat is released from the system, this is known as adiabatic expansion and results in steam entering the cylinder at high temperature and leaving at low temperature. This causes a cycle of heating and cooling of the cylinder with every stroke which is a source of inefficiency.
A method to lessen the magnitude of this heating and cooling was invented in 1804 by British engineer Arthur Woolf, who patented his Woolf high pressure compound engine in 1805. In the compound engine, high pressure steam from the boiler expands in a high pressure (HP) cylinder and then enters one or more subsequent lower pressure (LP) cylinders. The complete expansion of the steam now occurs across multiple cylinders and as less expansion now occurs in each cylinder so less heat is lost by the steam in each. This reduces the magnitude of cylinder heating and cooling, increasing the efficiency of the engine. To derive equal work from lower pressure steam requires a larger cylinder volume as this steam occupies a greater volume. Therefore the bore, and often the stroke, are increased in low pressure cylinders resulting in larger cylinders.
Double expansion (usually known as compound) engines expanded the steam in two stages. The pairs may be duplicated or the work of the large LP cylinder can be split with one HP cylinder exhausting into one or the other, giving a 3-cylinder layout where cylinder and piston diameter are about the same making the reciprocating masses easier to balance.
Two-cylinder compounds can be arranged as:
• Cross compounds - The cylinders are side by side.
• Tandem compounds - The cylinders are end to end, driving a common connecting rod
• Angle compounds - The cylinders are arranged in a vee (usually at a 90° angle) and drive a common crank.
With two-cylinder compounds used in railway work, the pistons are connected to the cranks as with a two-cylinder simple at 90° out of phase with each other (quartered). When the double expansion group is duplicated, producing a 4-cylinder compound, the individual pistons within the group are usually balanced at 180°, the groups being set at 90° to each other. In one case (the first type of Vauclain compound), the pistons worked in the same phase driving a common crosshead and crank, again set at 90° as for a two-cylinder engine. With the 3-cylinder compound arrangement, the LP cranks were either set at 90° with the HP one at 135° to the other two, or in some cases all three cranks were set at 120°.
The adoption of compounding was common for industrial units, for road engines and almost universal for marine engines after 1880; it was not universally popular in railway locomotives where it was often perceived as complicated. This is partly due to the harsh railway operating environment and limited space afforded by the loading gauge (particularly in Britain, where compounding was never common and not employed after 1930). However although never in the majority it was popular in many other countries.
