Triple expansion engines were the backbone of the shipping industry for many years, indeed the triple expansion industry was the driving force for the widespread change from sail to steam propulsion. Triple expansion engines were more fuel efficient than earlier simple expansion or compound expansion engines thus it cost less to transport a set amount of cargo; they also used less fuel per unit of power developed so the ship could travel further on the same amount of fuel or conversely the amount of space taken by bunkers was less for a given distance and so more cargo could be carried. 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.
One of a pair of three cylinder triple expansion engines built by John Penn Co. for the battleship HMS Magnificent. These engines could develop 12,000ihp with the boilers working under forced draught. Cylinder dimensions were 40in (HP) + 59in (IP) + 88in (LP) by 51in stroke. The cylinders were supported on forged steel front columns and cast steel rear frames which also provided mounting points for the guide bars.
Triple expansion engines effectively had three stages of steam expansion, high pressure (HP), intermediate pressure (IP) and low pressure (LP) from where the steam exhausted to the condenser. Some large engines had the low pressure divided into two cylinders, making a four cylinder triple expansion engine, as this reduced the overall diameter of the LP piston and made for better engine balance of rotating parts which in turn reduced vibration. In order to keep engine length limited and also help with balance some engines had cylinders arranged in tandem manner, one above the other. In such cases pistons would be connected together by means of a rod. 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 minimise but multiple expansion is only possible when steam pressures are high enough. For triple expansion steam pressures above about 75 psi are needed otherwise compound or double expansion is satisfactory. Although quadruple expansion engines were developed they were not a success as steam pressures were never high enough to justify their adoption; for very high pressures, and temperatures, the turbine is more suitable as problems exist with reciprocating engines with respect to high pressures and temperatures. High pressures on piston rings and glands result in wear and leakage whilst cylinder lubrication difficulties exist when temperatures are very high.
When manoeuvring a triple expansion engine, or any steam reciprocating engine for that matter, the engineer at the controls would have to 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 generally 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 one of the eccentrics alone (say the ahead eccentric) would actuate the cylinder valve whilst 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 the cylinder at some intermediate point in the piston stroke allowing steam to expand in the cylinder thus achieving maximum economy of steam usage whilst 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 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.
The pair of four cylinder triple expansion engines built by Scotts of Greenock for the British cruiser Defence (1908); 40in + 65.5in + 75in(two) by 48in stroke, 13,500 ihp. The expansion linkage and eccentrics can be seen in this view of the after end of the engines; the toothed flywheels connect with the turning gear for turning the engines when making adjustments or repairs.
One of the triple expansion engines built for the battleship HMS Prince George (1896). This vessel was in the same class as HMS Magnificent and had the same type and size of triple expansion propulsion plant but her engines were built by Humphrys, Tennant & Co. The column cylinder supports at the front and cast frame supports at the rear can be clearly seen as can the manoeuvring wheels.
Maudslay, Sons & Field constructed the pair of three cylinder triple expansion engines for the Russian ironclad Admiral Oushakoff (1895) and this was one of the last orders completed by that famous concern before it went out of business.
The ship itself was built at St. Petersburg. Cylinder dimensions were 31in + 46in
+ 68in by 33in stroke. The condenser can be seen at the end of the engine.
This book tells the story of the development of the marine steam engine from its early days in the late 18th century to the present day when the last steamships are being built.