A Carnot's engine  is made to work between 2000C and 00C first and then between 00C and −2000C.  the ratio of efficiencies of the engine in the two cases is

η=1−T2T1=1−273273+200=200473                              η=1−T2T1=1−273−200273+0                                         =200273                 η1η2=273473=1:1.73