HOWEVER, "mechanical POWER engineering" would have to be considered separately. I don't know the answer to your question personally but I just wanted to clarify so you didn't get confused between the two. Mechanical power engineering will be much much more specific to a certain industry. It will probably pertain to the examples given by Rohit above.
The various disciplines of mechanical engineering come into play within any of these scenarios: thermodynamics, heat transfer, fluid mechanics, dynamics, stress analysis and even system control. So, you can get involved in "power" engineering in any number of ways.
As long as humans continue to rely on machines and vehicles to do their daily activities, we will have secure jobs as either "mechanical" or "power" engineers. Good luck to you!
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