Since the cycle proposed by Carnot (consisting of two isothermal and two adiabatic processes) is the most efficient one on which to run a heat engine, why do we use other, less efficient ones (like the Otto or the Diesel cycle) for automobiles?
[Physics] Why can’t the Carnot cycle be used to operate automobiles, if its the most efficient
carnot-cycleheat-enginethermodynamics
Best Answer
On the P-V diagram for the Carnot cycle the upper and lower curves are very close together. As a result, this cycle has very little area enclosed within the cycle diagram and very little work is done per cycle. Therefore, to produce a reasonably useful quantity of work the engine must be very massive, heavy, and expensive relative to other engines based on other cycles like the Otto cycle and the Diesel cycle which enclose more area on the P-V diagram and do much more work per cycle.
A larger engine with a relatively larger amount of surface area will sustain greater losses to the environment through that surface area reducing its efficiency and partially or completely offsetting any other advantages it may have. This larger engine will also have to move more mass of working fluid through it to produce the same work output as some other engine cycle with greater work output per cycle increasing the internal flow losses and again reducing the real world efficiency of the Carnot enginen relative to others. A heavier engine will also penalize a vehicle with greater per mile energy consumption once again offsetting any other advantages the Carnot cycle may present.
Bottom line, real Carnot engines can not be designed to compete with real Otto or Diesel engines on an overall economic basis.