Thermal efficiency of reciprocating engines

[nate9519]

In a perfect, ideal world with constant specific heats the thermal efficiency of an otto cycle is dependent on its pressure ratio and a diesel cycle is dependent on its pressure ratio and cut off ratio. But since in the real world the equations with these terms do not hold since specific heats vary. Does that mean in the real world that these cycles do not depend on these ratios but on the more general form of net work / heat in

 

[Andrew Mason]

Net work/heat in - is the definition of efficiency, of course. Net work produced for a given amount of heat flow into the system in an Otto cycle or Diesel cycle engine will improve with a higher compression ratio. The analysis of these cycles is based on ideal conditions and certain assumptions about the states of the system during the process. In an irreversible, non-equilibrium process such as a real Otto or Diesel cycle, these assumptions will not be completely accurate. However, by applying the laws of thermodynamics of equilibrium states one can design the engines to optimize efficiency (e.g. by making the compression ratio as high as practicable).

AM