When you hear someone give a talk on Titanic, or watch something on TV about Titanic, they usually give a description of Titanic that will include “… Titanic had two, four-cylinder, triple expansion, direct acting, inverted type, reciprocating steam engines … .”
So what does all of that mean? Just what IS a “… four-cylinder, triple expansion, direct acting, inverted type, reciprocating steam engine … .” What it says is that Titanic had the best state-of-the-art steam engines of its day, for those who know marine steam engines, and it means absolutely nothing to the rest of the world. Part 1 of this article will give a simple explanation so you can impress your friends, family, and co-workers, with your knowledge of marine steam engines.
Here goes (like I said this is easy).
“… Titanic had two …” Titanic had two steam engines side-by-side on the lowest deck in the engine room. Each engine was attached to a propeller shaft that turned one of Titanic’s two outboard, three-bladed, propellers (the center, four-bladed, propeller was operated by a steam turbine, we will discuss later). The Canadian stamps below show the two outboard, three-bladed, propellers and the center, four-bladed, propeller.
The two steam engines could be operated independently of each other. When Titanic wanted to turn very quickly or shorten the amount of space needed to turn (like when they were leaving Southampton and when Titanic was passing the SS New York) they could move one engine in the ahead direction and one engine in the reverse, or back, direction. This is a handy ability when you are turning in a narrow river. Unfortunately it takes quite a bit of time to reverse an engine, so it doesn’t do much good if you are trying to avoid something, like an iceberg suddenly spotted in your path.
“… four cylinder …” Each engine had four cylinders that the steam expanded into to move the pistons up and down. This moved the piston rod up and down, which moved the crankshaft in a circular or rotating direction, turning the propeller shaft and the propeller outside of the hull. The steam exhausted from the first cylinder went into the second cylinder, steam from the second into the third, steam from the third into the fourth. The steam from the fourth cylinder normally went to a condenser to become water again. Titanic used this steam, exhausted from the fourth cylinder (of both engines) to operate the steam turbine that operated the center four-bladed propeller (but we’ll come to that later). Below is a photograph of a four-cylinder, triple-expansion, marine, steam engine. The first (to the right) and last (to the left) cylinders are easy too see in the photograph. This engine was smaller than Titanic’s, but was built about the same time.
Below is an animation of a double expansion, triple-cylinder, steam engine operating.
“… triple expansion …” When steam expands in the cylinder it cools, so steam enters at a high temperature, high pressure and leaves at a cooler temperature, lower pressure. This causes a loss of power. So of course someone figured a way not to lose this power, and it is called a compound engine and that was improved upon to create the triple expansion engine (which is a compound engine, but a two-cylinder engine is the only engine called a compound engine, all others are called triple or multiple expansion steam engines). To get the same work from the cooler, lower pressure steam they made each cylinder larger than the last. So the smallest cylinder is the high pressure first cylinder and the largest cylinder is the lowest pressure fourth cylinder. It is all about getting more power from the same steam.
“… direct acting …” All this means is that the piston rod, which is connected to the piston inside the cylinder (cylinder is the case that the piston is in, and the piston is moved up and down in the cylinder by steam), is attached to the crankshaft. The crankshaft is connected to the propeller shaft. The up and down motion of the piston rod, becomes the circular (or rotating) motion of the crankshaft. If you look at the animation above you will see that the crankshaft is just a cylindrical rod, but at each place where a piston rod is attached to the crankshaft, that place is off to the side of the crankshaft. This is how the up and down motion of the piston rod becomes the rotating motion of the crankshaft. Older steam engines did not connect to the crankshaft directly. The piston rod operated a lever and the lever operated the crankshaft.
“… inverted type …” A vertical steam engine has the cylinder at the bottom of the engine, with the piston rod above the engine. A vertical inverted steam engine (also called an inverted type steam engine) has the cylinder head at the top of the engine and the piston rod under the cylinder. The photograph and animation above both show an inverted steam engine. There are also horizontal and inclined steam engines, where the cylinder is on its side and the piston rod moves out from the cylinder instead of up and down.
“… reciprocating …” This is what a piston engine is. It just means that the piston moves up and down, which moves the piston rod up and down, which moves the crankshaft in a circular or rotating direction. There are also rotary and turbine engines as well, they do not have cylinders, pistons, and crankshafts.
And there you have it, you are now a marine steam engine expert. So, the next time you are watching a Titanic show and it cuts to commercial, you can show your significant other they aren’t the only one who knows a thing or two about engines.
Part 2 will cover Titanic’s steam turbine, and part 3 will cover Titanic’s boilers which were used to make the steam.
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