No heat engine operating in cycle between the two given thermal reservoir, having fixed temperature can be more efficient than reversible engine operating between same thermal reservoir.
* Thermal efficiency η th = Work out/Heat supplied
* Thermal efficiency of a reversible engine ( ηrev)
ηrev= (T1 - T2)/T1 ;
* No engine can be much efficient than reversible carnot engine that is ηrev > ηth
= (Heat added - Heat rejected) / Heat added = [mRT2lnV2/V1 - mRT4lnV3/V4]/ mRT2lnV2/V1
= 1- T1/T2