A system and method of providing real-time calculation of heat flow in an engine. A piston is disposed in a cylinder of an engine block and movable relative to the cylinder in response to combustion inside the cylinder. A temperature of the combustion inside the cylinder, an average temperature of the wall of the cylinder, and a surface area of the wall of the cylinder based on timing of combustion are determined. An estimated temperature of the piston is derived from calculating a heat fraction to the piston in real-time, via a controller, based on the determined temperature of the combustion, the determined average temperature of the wall of the cylinder, and the determined surface area of the wall of the cylinder. A state of the engine is controlled based on the estimated temperature of the piston as derived from the real-time calculation of the heat fraction to the piston.