In the paper a new method of T.D.C. determination based on the net heat release calculated for measured non-fired cylinder pressure has been presented. Heat flux during non-fired engine operation changes its direction shortly after T.D.C. i.e. heat flows from gas to the walls on the compression side, and from the walls to gas - on the expansion side. It has been proved that the function Vdp being a part of the net heat release has always inflexion at the maximum pressure point. Therefore the second derivative of the Vdp is equal to zero, and gives a condition which is met for the one and only pressure versus volume (crank angle) position.However, the second derivative of the net heat release means that the first three derivatives of the measured pressure are necessary, thus a method to calculate those derivatives has been discussed in the paper.The presented method for T.D.C. determination is universally applicable for the engines with non-divided combustion chambers as it has no assumptions, and results from the First Law of Thermodynamics. Impact of heat flux is taken into consideration without need to know its character. The method has a form of a numerical program.