  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.
