A steam engine is a reciprocating heat engine that performs mechanical work by using steam as its working fluid. Steam engines are external combustion engines based on modified Rankine cycle, where the working fluid is separate from the combustion products. Water is heated in a boiler until it reaches a high pressure and temperature (superheated steam) and then expanded through piston in steam engine to do some mechanical work. The reduced-pressure steam is then released into the atmosphere or condensed and pumped back into the boiler. The working of steam engine is shown in Figure 5.1.
Figure 5.1 Working of Steam Engine
In a steam engine, the movement of the valve ensures that steam is admitted to and exhausted from the cylinder at the right moment. For a typical cylinder that has two ports, i.e., double acting reciprocating steam engine, the function of the valve is to admit superheated steam at one end while allowing the exhaust steam to escape at the other as shown in Figure 5.1. As a result of covering and uncovering these ports in sequence, the piston is pushed forward and backward by the high pressure steam from the boiler. To regulate the movement of the valve, a mechanical valve gear system is used.
Lap refers to the amount of overlap between the valve and the port. In slow moving locomotives, the long lap on the exhaust port gives time for the steam trapped in the cylinder to expand fully to push the piston. On the other hand, on higher speed locomotives the exhaust port is made to open early (short lap) when the valve is in mid-position thus allowing the steam to escape faster. Higher speed locomotives also have long lead which means that the admission port is already open when the piston is at the end of its movement so there is a sufficient steam pressure that will immediately push the piston back to begin its next movement.
The indicator diagram, as shown in Figure 5.2, was used by steam locomotive engineers during the steam era to estimate the locomotive’s efficiency in converting the steam’s energy into useful power at various speeds and cut-offs. The horizontal line at the top of the indicator diagram shows the pressure as the steam enters the cylinder. At cut-off, the pressure drops as the steam expands and does work to push against the piston. Cut-off denotes the position of the piston, at the moment the valve is closing the admission port. When the engine is working hard and slowly, long cut-off admits steam for most of the stroke of the piston. On fast running locomotives this will cause back pressure to the boiler. To avoid unnecessary back pressure, cut-off is reduced so that steam is admitted for only 20% of the piston stroke and the remainder of the stroke is due to the expansion of the high pressure steam.
After the exhaust port opens, the horizontal line at the bottom of the indicator diagram indicates the return stroke of the piston. It shows the low pressure as the steam is exhausted. The curve at the end of the return stroke shows a pressure rise due to the compression of the remaining steam after the exhaust port has closed. As fresh steam is admitted into the cylinder, the pressure rises and the cycle repeats.
Figure 5.2 Modified Rankine Cycle
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