Question

Carnot cycle is called the ideal cycle.
What is the process that is impossible in reality in the Carnot cycle (among isothermal expansion, insulation expansion, isothermal compression, and insulation compression)?

Answer 1

Carnot cycle is the most efficient heat engine cycle consisting of two isothermal processes and two adiabatic(insulation) processes. The Carnot cycle is a reversible process therefore the entropy change zero. But all real engines or any real process entropy change will be positive. Therefore it is an idealisation of real engines. When the second law of thermodynamics states that not all the supplied heat in a heat engine can be used to do work, the Carnot efficiency sets the limiting value on the fraction of the heat which can be so used. In Carnot cycle heat transfer into the engine in the isothermal process is too slow to be of practical value.Think this way.Isothermal process means temperature should be constant. But how will it be possible in reality? Well we can make the process very slow or use a coolant. And adiabatic process demands insulating surrounding or make the process extremely fast.But both these processes are part of same cycle, ie we cannot have coolant and insulating cylinders at the same engine. So isothermal process should be very slow..so yes we have an car having an efficient engine but it would be better to walk than ride it!