The Rankine cycle describes the ideal thermodynamic cycle for a heat engine to produce the best performance and convert heat into mechanical work while it undergoes phase change. Is it possible for us to build an ideal heat engine that conforms as closely as possible to the performance of the ideal Rankin cycle?

  • 3
    $\begingroup$ I thought it was already used for steam engines. Perhaps I missed something. $\endgroup$
    – NofP
    Jan 22, 2018 at 15:48
  • $\begingroup$ I do not understand the question either. $\endgroup$ Jan 22, 2018 at 15:49
  • $\begingroup$ "The Rankine cycle is a model used to predict the performance of steam turbine systems." (Wikipedia) Not only we can make thermal engines which use the Rankine cycle, we do make such engines; steam turbines produce the vast majority of electric power used in the world and power countless ships. In practice, the efficiency of a large steam turbine reaches about 50%, similar to the efficiency of a large marine diesel engine. (Small steam turbines and small diesel engines have worse efficiencies.) $\endgroup$
    – AlexP
    Jan 22, 2018 at 16:46
  • $\begingroup$ Are you asking if it is possible to build a heat engine that meets the theoretical ideal performance statistics as calculated by the Rankin cycle? In other words, an engine that is as perfect as possible to the performance of the ideal Rankin cycle? $\endgroup$ Jan 22, 2018 at 17:57
  • $\begingroup$ Yes, exactly as described by @JustinThyme. $\endgroup$
    – Prometheus
    Jan 22, 2018 at 19:40

1 Answer 1



By definition, no engine is ever truly ideal (assuming 'ideal' means 'loss-less') as per the law of conservation of energy. No engine will ever operate at 100% efficiency, and further no Carnot engine can operate above ~63.4% efficiency; this is what the Rankine Cycle tells us.

However a 63.4% efficiency is assuming that only the entropy generated by the heating/cooling process itself is taken into account. Once you introduce practical inefficiencies of mechanical systems (imperfect fluids, friction, structural defects) this number decreases rapidly.

Modern gasoline-driven internal combustion engines operate around 30%, diesel engines around 45%, and coal power plants around 40%.

So, assuming by 'ideal' you mean the theoretical maximum of 63.4% then the answer is still no, because no engine will ever be completely devoid of practical inefficiencies.

To put it a bit more scientifically...

As the practical inefficiency of an engine approaches zero, the absolute efficiency of a Carnot engine approaches 63.4%.

And thus...

Because a real Carnot engine will never have a practical inefficiency equal to zero, the absolute efficiency will never be equal to 63.4%

  • $\begingroup$ Where does the 63.4% number come from? It's been exceeded. The formula itself has no such limit, as long as working temperature spread is increased. $\endgroup$
    – Therac
    Sep 3, 2023 at 6:03

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