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The Engine. What it is and how it works. Non-condensing, Fixed Cut-Off, Steam Engine. Quite a mouthful even for those who know what it means. Lets take it one step at a time. Have you ever seen a steamboat, either in the movies or real life? Did you notice the large puffs of white steam shooting into the air from pipes near the back of the boat. Those graceful white puffs are the Exhaust Steam from each stroke of the engines. Those clouds of white are really Condensed Steam. And all that steam you see puffing into the sky is being wasted. Both early steamboat designs and railroad locomotives, are examples of "non-condensing" steam engines. A technical term meaning they do not re-use the steam. Once it has done it's work, the spent steam was vented directly into the atmosphere. A tremendous waste of both water and fuel for more water had to be pumped into the boilers and heated. Water which came from a rather muddy and cold river to boot. So what is the difference between a Non-condensing and Condensing engine? A "condensing" steam engine does not exhaust its spent steam. Instead the spent steam is piped into a devise which, like a car radiator, cools the steam turning it back in to liquid water (condensing the steam). That still very hot water is pumped back into the boilers where it is reheated and reused. Since the returning water is still very hot it requires less heat to bring it back up to boiling temperature. A big savings in fuel and boiler maintenance, for this reused water is free of mud and dirt like normal river water. The term "fixed cut-off" refers to the method of opening and closing the steam valves. In a fixed cut-off engine the rate at which the steam valves open and close is "fixed." Not adjustable, just open or closed at a given rate. Later designs included an additional device which allowed the valves to be controlled. That in turn allowed the engineers more control over the rate steam was used by the engines. Looking at the illustration you will quickly notice that steam has entered the cylinder by way of the Steam Supply Valve at the right end and is beginning to push the piston to the left. The piston is connected to the drive shaft which in turn is connected to the Pitman Arm Yoke. A yoke is a type of hinge shaped something like a tuning fork or the letter 'Y'. There is a large bearing or pin which runs between the two ends of the 'Y' which connects the Pitman Arm to the Yoke. The other end of the Pitman Arm is connected to a crank on the paddle-wheel and forces the wheel to turn. So, steam pushes on the piston, which moves the drive shaft, which moves the Pitman Arm yoke, which moves the Pitman Arm which moves the paddle-wheel crank causing the paddle-wheel to turn. Simple! Also connected to the pitman arm yoke is the Pendulum Rod which received its name from the way it rocks back and forth like the pendulum in a grandfather clock. As the Pitman Arm Yoke moves, the Pendulum Rod moves with it sliding through a hollow, pipe like, guide. As the Pendulum Rod moves back and forth with the yoke, it moves the Reach Rod in the opposite direction. If the pitman arm yoke is moving aft (left) the reach rod is moving forward (right). The reach rod in turn is connected to the Reach Rod Cam located on the engine. As the reach rod moves it makes the reach rod cam oscillate or rock back and forth. As the cam slowly oscillates it raises one side which lifts a Lever. The upward movement of this Lever opens a valve at the end of the cylinder allowing steam to flow. At the same time the other side of the cam moves downward lowering and closing the valve at the opposite end of the engine. If this did not happen there would be live steam pushing on both sides of the piston and it would not move. What is not obvious from the illustration is that there are four (4) levers and four (4) valves. Two steam supply valves and two exhaust valves. One of each on either end of the engine and mounted directly across from each other. A series of small cams and levers connects the reach rod cam to Exhaust Cam on the other side of the engine which, as its name implies, operates the Exhaust Steam Valves. And just like the supply valves, one of them is always closed. When the Drive shaft reaches its furthest point of travel, the pendulum rod has swung its full arch which pushed the Reach Rod causing the Reach Rod Cam to rotate as far as it can go. And that is one half (1/2) turn of the paddle-wheel. As the wheel continues to turn, the pitman arm begins to move in the opposite direction which makes the pendulum rod reverse its swing, which in turn pulls on the reach rod, which causes the Reach Rod Cam to reverse it oscillation. This closes the previously open supply valve and opens the other (previously closed) supply valve. Steam not flows into the opposite end of the cylinder forcing the piston to move backward. Pulling instead of pushing the drive shaft and pitman arm along with it. One complete rotation of the paddle wheel
Although both the steam supply and exhaust valves sit on top of a common manifold, they are in no way connected. Separate pipes deliver live steam to, and remove spent steam from, the engine. The metal case surrounding the steam pipes was an early attempt at insulation. Keeping the steam HOT was (still is) a very important factor in making a steam engine run properly. By 1889 the steam driven turbine engine was being perfected and the fate of the piston engine sealed. Smaller, less complicated, reliable and more efficient the turbine engine was soon to replace almost every piston steam engine in maritime service. For more info. visit the steam power section of HowThingsWork.com Marine Engine Info. This site has info. on marine steam engines, including an Engine Identification Section to allow users to post a photo and description of an unknown engine. 
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