Quote:
Originally Posted by drmiller100
how does an old school steam engine work in a locomotive????
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Well, the steam engine is an example of the thermodynamic process known as the Rankine cycle. Basically, you have a coal fueled fire that heats up a pressure vessel that contains water. The water boils to become steam, which is then further heated so as to put the steam into superheat. This is necessary, because the steam has to be above atmospheric pressure to effectively push the locomotive forward.
The superheated steam is then admitted to one side of the piston via a reciprocating valve mechanism. As the steam enters the piston, the steam pressure causes the steam to expand and push against the piston. Since the reciprocating valve is still open, we can consider the steam pressure to be relatively constant, so we can model the steam expansion as an isobaric process.
The reciprocating valve then closes, while the steam further pushes against the piston. At this point, the steam pressure inside the cylinder drops as the piston travels. Due to Boyle's law, the steam temperature also drops. This can be modeled as an adiabatic process. The piston travels to its distant end.
The reciprocating valve then opens up in the opposing direction, and steam is admitted to the other end of the piston, beginning the process described above to push the piston back to its starting point.
It is important to note that the Rankine cycle is substantially different than the Otto cycle, in terms of execution.