Speed and Efficiency Limits of Multilevel Incoherent Heat Engines
  We present a comprehensive theory of heat engines (HE) based on a quantum-mechanical "working fluid" (WF) with periodically-modulated energy levels. The theory is valid for any periodicity of driving Hamiltonians that commute with themselves at all times and do not induce coherence in the WF. Continuous and stroke cycles arise in opposite limits of this theory, which encompasses hitherto unfamiliar cycle forms, dubbed here hybrid cycles. The theory allows us to discover the speed, power and efficiency limits attainable by incoherently-operating multilevel HE depending on the cycle form and the dynamical regimes. 
Victor Mukherjee, Wolfgang Niedenzu, Abraham G. Kofman, Gershon   Kurizki
quant-ph cond-mat.stat-mech