Engineering Thermodynamics
Otto cycle consists of following four processes

Two isentropic and two constant volumes
Two isentropic and two constant pressures
Two isothermal and two isentropic
Two isentropic, one constant volume and one constant pressure

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Engineering Thermodynamics
A closed system is one in which

Neither mass nor energy crosses the boundaries of the system
Mass crosses the boundary but not the energy
Both energy and mass cross the boundaries of the system
Mass does not cross boundaries of the system, though energy may do so

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