The Titanic had two triple expansion reciprocating engines. The engines were located in the Reciprocating Engine Room on the Orlop Deck, aft of Boiler Room 1. The reciprocating engines drove the port and starboard propellers. Aft of the reciprocating engines was the turbine engines which drove the central propeller.
The reciprocating engines were four stories tall, running on steam from the boiler rooms. The Reciprocating Room was vented through a slight shaft going up to the Boat Deck. The leftover steam went aft to drive the turbine engine.
The selection of these engines was the result of careful investigation and trials in other ships. The system of two huge triple-expansion steam engines, powering the port and starboard wing propellers, together with a small turbine driving the center propeller, was tested on the Laurentic, which was build in 1909. Her sister ship, the Megantic (1908), had been bestowed with a pair of traditional reciprocating engines operating twin screws. The hybrid powerplant installed on the Laurentic was then adopted on the Olympic Class liners.
Engines like Titanic´s work by re-using steam that would otherwise just exhaust or returned to the condenser to be turned back into water to be used again. Working that way, it extracts the maximum amount of energy out of steam, and also provides a better level of economy, using less coal, less firemen and less boilers.
