RMS Titanic - Engines

One of Titanic's two reciprocating engines.

Titanic had two reciprocating engines, one on each side: The term "reciprocating" comes from the mechanical motion of the pistons inside the engine's cylinders, which move up and down in a reciprocating motion. The motion of each piston, by means of a large piston rod, helped turn the large crankshaft below. In fact, these massive engines were constructed on similar lines to an automobile's internal combustion engine, the key difference being that instead of the expansion of burning gasses being used to move the piston in each cylinder, the expansion of steam was used for the same purpose. Another significant difference is that, unlike the engine in a car, where the piston is only driven in one direction, steam reciprocating engines are “double-acting”. This means that steam is admitted first to one side of the piston and then the other, driving the piston both down and then back up. Additionally, there are no “cycles” as found in internal combustion engine; every stroke of the piston, up or down, is a “power stroke”, near the end of which the expanded steam is released from one side of the piston just shortly before the steam is admitted on the opposite side, driving the piston again in the opposite direction.

One of Titanic's engines.

The steam supply, coming straight off the boilers, was initially piped to the throttle valves located at the receiver of high-pressure cylinder on each engine, where it arrived at a pressure of 210 PSI. After this high-pressure steam expanded in this cylinder, thereby driving the piston up or down, and losing some pressure in the process - it was exhausted out of that cylinder at a pressure of 78 PSI, and directed into the next, or intermediate-pressure cylinder. Here the steam was used again, to drive the piston up or down in the cylinder. From here, having been expanded again, the steam was exhausted at a pressure of 24 PSI and was piped into two low-pressure cylinders for use again. Because the same supply of steam was expanded in three stages, Titanic's reciprocating engines were known as triple-expansion type. Each was designed to produce 15,000 horsepower at 75 revolutions per minute - far slower than an automobile, which runs at thousands of rpms, but vastly more powerful.

As with an automobile engine, the engine was mechanically arranged so that each cylinder "fired" at slightly different times, thereby transmitting energy to the shaft in a coordinated and continuous manner.

When the steam left the low-pressure cylinders, it was exhausted at a pressure of 9 PSIA (pounds per square inch, absolute). The difference between “PSI” - also known as PSIG, or “pounds per square inch, gauge” - and “PSIA” here is very important, as “PSI” is a measurement of pressure above that of the atmosphere at sea level, which is approximately 14 PSIA; so, it can be seen here that the low pressure cylinders were actually exhausting steam at a pressure below atmospheric pressure. This was possible due to the terrific vacuum created at the exhaust side of LP cylinders by the cooling of the steam being exhausted from the engines into the condensers. (More will be said about the condensers a bit further along.) Even though it would seem that every last bit of available energy had been gotten from the steam at this stage, there was still considerable latent energy that could be extracted from this steam, though not by any reciprocating engine: this is where the low pressure turbine came in.

Unlike a reciprocating steam engine, a turbine can be designed to produce power from the continued expansion of steam all the way down into the high vacuum range that exists within the condensers. The turbine driving the center screw of the Titanic was designed to utilize the steam exhausted from the reciprocating engines at a pressure of 9 PSIA, expanding this steam all the way down to a terminal pressure of 1 PSIA, at which point, the steam – now virtually a warm fog - was exhausted into the main condensers.

A steam turbine is designed to operate something like a windmill. Using this analogy and modifying it somewhat will help to understand how a turbine works -- imagine a windmill with a hundreds of very small blades, rather than 6 or 8 large blades. Then, assemble many rings of such blades - like connecting multiple windmills front-to-back on the same shaft – and place it inside a large cylinder, or housing, and you have the basic concept of a turbine engine.

Although it used steam at a pressure of only 9 PSIA, the turbine engine was designed to produce a considerable amount of power - 16,000 horsepower at 165 RPMs. Unlike the reciprocating engines, the turbine engine could not be run in reverse and so a means for the steam to bypass the turbine engines had to be incorporated into the system. When the reciprocating engines were running in reverse, a pair of large changeover valves routed the steam from the reciprocating engines directly to the condensers, bypassing the turbine entirely.

1. Steam from the boilers, at 215 p.s.i., enters the small high pressure (HP) cylinder, moving the piston in the cylinder.
2. The steam exits the HP cylinder, and is routed to the next cylinder along, the slightly larger intermediate pressure (IP) cylinder, moving the piston in the cylinder.
3. The steam exits the IP cylinder, and is routed to the next cylinder along, the much larger low pressure (LP) cylinder, moving the piston in the cylinder. (On Titanic, each engine actually had TWO LP cylinders.)
4. The steam, at 9 p.s.i., was then passed to the turbine that powered the center propeller.
5. The steam is then passed to the condensers, where it is cooled, turning it back to water, ready to be passed to the boilers, where the whole process starts all over again.

All of the above process might seem long-winded, but in reality it only took a couple of seconds from the time the steam entered the first cylinder to the time it left the turbine.

N.B. The centre turbine was non-reversible.