International regulations continuously restrict the standards for the exhaust emissions from automotive engines. In order to comply with these requirements, innovative control and diagnosis systems are needed. In this scenario the application of methodologies based on the in-cylinder pressure measurement finds widespread applications. Indeed, almost all engine thermodynamic variables useful for either control or diagnosis can be derived from the in-cylinder pressure. Apart for improving the control accuracy, the availability of the in-cylinder pressure signal might also allow reducing the number of existing sensors on-board, thus lowering the equipment costs and the engine wiring complexity.The paper focuses on the detection of the engine thermal state, which is fundamental to achieve suitable control of engine combustion and after-treatment devices. Starting from previous works on SI engines, a new technique has been developed and experimentally validated for light-duty Diesel engines. The proposed methodology relies on the estimation of the adiabatic condition between cylinder wall and engine gas mixture during compression stroke. It is based on the main assumption that the heat transfer between gas and cylinder wall undergoes to a steady-state process. Therefore, the crank angle position where the detected polytropic coefficient approaches the value corresponding to the local adiabatic one is assumed as the inversion point for the net heat flux between cylinder wall and gas mixture. In that occurrence the average gas mixture temperature is assumed as representative of the actual engine thermal state. The mean gas mixture temperature is computed from a thermodynamic relationship as function of pressure, volume and polytropic coefficient.The technique has been applied by processing the experimental data collected at the engine test stands on two Diesel engines, naturally aspired and turbocharged, respectively. Several analyses have been performed to evaluate the influence of the engine operating conditions. Particularly, the impact of turbocharging and exhaust gas recirculation has been deeply investigated. The results achieved are satisfactory and the low computational burden makes the proposed methodology suitable for on-board application.