The cycle that consists of two isothermal and two isochoric processes is the Stirling cycle.
In the ideal Stirling cycle, the working gas is compressed and expanded isothermally at two different temperatures. These steps are connected by two constant-volume, or isochoric, regeneration processes. A regenerator stores heat during one constant-volume step and returns it during the other, improving efficiency.
Robert Stirling patented the hot-air engine in 1816 with help from his brother James. Unlike the Carnot cycle, which uses two adiabatic or isentropic processes, the Stirling cycle replaces them with constant-volume regeneration. Both cycles include isothermal heat addition and rejection, but their intermediate processes differ.
A common mix-up is confusing Stirling with the Otto cycle: Otto has two isochoric heat-transfer processes, but its compression and expansion are adiabatic rather than isothermal. Stirling engines are external-combustion machines and can use many heat sources, including solar, nuclear, and waste heat.