In Memoriam: RMS Titanic
The Reciprocating Engines
Triple expansion engines were the backbone of the shipping industry for many years. They were more fuel efficient than earlier simple expansion or compound expansion engines. Because of this the marine engineers who built the ships of the time could trade fuel (coal) for cargo and distance. For larger ships the reduction in fuel burned per unit of power also meant a reduction in boiler capacity and fewer firemen and coal trimmers. Triple expansion engines required steam to be at higher pressure and that also resulted in smaller boilers or a reduced number of boilers.
Triple expansion engines had three stages of steam expansion: high pressure (HP), intermediate pressure (IP) and low pressure (LP) from which the steam exhausted to the condenser, or in the case of the Olympic class ships, to the Parsons turbine. Some large engines had the low pressure section divided into two cylinders, making a four cylinder triple expansion engine. This reduced the overall diameter of the LP piston and made for better engine balance of rotating parts which in turn reduced vibration. (This was the arrangement Harland and Wolff chose for White Star.)
The reason for triple expansion of steam was the same as with double expansion or compounding, namely to limit the degree of expansion in any cylinder and so limit the temperature reduction which takes place during expansion. When steam enters the opposite side of the cylinder for the next stroke (steam engines are double acting) some of it will condense on the cold cylinder walls and so its potential effect for power generation will be lost. By limiting the degree of expansion this condensing effect is minimized.
When maneuvering a triple expansion engine, or any steam reciprocating engine for that matter, the engineer at the controls must deal with the steam supply for the power and the direction of rotation. A steam throttle valve would be opened to supply steam to the engine. The amount of opening would regulate the supply of steam and the power developed.
Cylinder valves were actuated by means of eccentrics driven by the crankshaft, there being two eccentrics, one ahead and the other astern, connected to an expansion link which operated the cylinder valve. With the expansion link full over on one side, the ahead eccentrics would actuate the cylinder valve. while, if the expansion link was fully over the other way the astern eccentric would control the cylinder valve. With the expansion linkage at the middle point both eccentrics would have equal influence and the cylinder valve would not admit steam for either astern or ahead operation even though the steam supply valve was open.
At other intermediate positions of the expansion linkage, steam would be shut off to the cylinder at some intermediate point in the piston stroke, allowing steam to expand in the cylinder, thus achieving maximum economy of steam usage while obtaining the desired power output. In order to reverse the engine the engineer at the control would move the reversing lever which would cause the expansion linkage to move fully to the ahead or astern position. For a large engine, a steam cylinder would be used to move the linkage. With the engine running either ahead or astern, adjustment of the linkage to give the desired degree of expansion in the cylinders would then take place. In practice, setting steam engine valves accurately required a great deal of skill and experience.
