triple expansion engine

triple expansion engine, a further development of the marine reciprocating engine. It was introduced in ships between 1870 and 1880 by adding a third cylinder to the two-cylinder compound engine. The third cylinder was introduced between the compound engine's high- and low-pressure cylinders, and its effect was to use the available steam three times instead of twice as in the compound engine. The steam was first led to a high-pressure cylinder, the exhaust steam from that cylinder being led into an intermediate pressure cylinder, and then into a low-pressure cylinder before being converted by a condenser back into the boiler feed water. It drove three pistons connected to the same crankshaft to add to the power transmitted to the propeller shaft, and was made possible by improved boiler design which produced higher steam pressures. In the 1890s quadruple expansion engines were introduced by adding a fourth stage in the expansion of the steam, and were fitted in some ships, notably the four big German ocean liners built between 1897 and 1902. And even after the introduction of the steam turbine some ships had engines on the quadruple expansion principle, using three cylinders for the first three stages and a low-pressure steam turbine for the fourth.