A coupled model of thermochemical multiphase mantle convection and parameterized heat balance in the Earth's core is used to investigate the need for radioactive potassium in the core, and chemical layering above the core-mantle boundary (CMB), to obtain a successful thermal evolution of the core, i.e., one in which the magnetic field exists over geological time and the final inner core size matches that observed. The mantle convection model includes both the olivine and pyroxene phase change systems linked via the compositional field. The most successful core thermal evolution is obtained when the compositional density difference between subducted MORB and pyrolite in the deep mantle is 1.1% and the core contains 100 ppm radioactive potassium, both of which are consistent with estimates from laboratory experiments. In that scenario, the CMB heat flow at the present time is 8.5 TW, and the time-averaged ohmic dissipation is 2 TW. However, during the modeled magnetic evolution, the ohmic dissipation associated with the geodynamo occasionally becomes zero, which means that the geodynamo stops working, although these large fluctuations could be an artifact of two-dimensional geometry. Various model uncertainties still remain.