Thermodynamic cycle

A sequence of thermodynamic processes that returns a system to its initial state. Its net heat transfer can produce work or require work input.

First law of thermodynamics: The principle that energy is conserved, with changes in a system’s internal energy balanced by heat and work transfer. It accounts for the balance between net heat and net work over a complete cycle.
Thermodynamic process: A change in a system’s state described by a path through thermodynamic variables. Each leg of a cycle is a thermodynamic process connecting two states.
Carnot cycle: An ideal reversible cycle consisting of two isothermal and two adiabatic processes. It provides the maximum efficiency possible between two fixed-temperature reservoirs.
Thermal efficiency: The fraction of heat supplied to a heat engine that is converted into net work. It measures how effectively a power cycle turns heat input into useful work.
State function: A property whose value depends only on a system’s state, not the path taken between states. Internal energy returns to its initial value when a cycle closes, regardless of its path.
Isothermal process: A thermodynamic process that occurs at constant temperature. Some ideal cycles use constant-temperature legs to exchange heat with reservoirs.
Otto cycle: An idealized spark-ignition engine cycle with constant-volume heat addition and rejection. Its process sequence approximates the operation of gasoline engines.
Coefficient of performance: A ratio of useful heating or cooling delivered to the work required by a heat pump or refrigerator. It measures cyclic devices that move heat rather than produce net work.
PV diagram: A graph of pressure against volume used to represent thermodynamic states and processes. The signed area enclosed by a pressure–volume cycle gives its net boundary work.
Adiabatic process: A process in which no heat crosses the system boundary. Adiabatic legs connect heat-exchange stages in many idealized cycles.

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