The document discusses Carnot and Otto cycles. The Carnot cycle consists of two isothermal and two adiabatic processes and is used as an ideal benchmark for comparing real engine cycles, though no real engine achieves its theoretical efficiency. The Otto cycle, which models spark-ignition engines, consists of isentropic compression, isochoric combustion at constant volume, isentropic expansion, and isochoric heat rejection, returning the system to its initial state.