The indicator diagram of the Diesel engine is a valuable source of information, delivering many important data. In order to obtain from it the indicated horse power, the exact position of the top dead centre is needed. Using a measuring system containing a piezoelectric sensor, a charge amplifier and a fast data aquisition system, it is possible to store, analyze and process many successive thermodynamic cycles for each cylinder in such a way as to obtain the exact position of the top dead centre without using any additional signal from, or any mark on the flywheel. For that purpose, the experimental compression part of many successive cycles is compaired continuously to a simulation compression model containing varying parameter values determined by the measured cycle itself. After the experimental and the simulation compression curves fit each other, the top dead centre can be determined. Different known heat transfer models of the compression have been used and tested, and a new method, using stepwise chaning parameters along the compression is proposed. The method has been tested for different Diesel engines running at speeds between 100 and 2500 r.p.m. with a very good success. The results given concern three types of Diesel Engines i.e. one MAN W4V 17,5/22 A DG, 140 BHP, 1000 r.p.m. coupled to a Zoellner hydraulic brake, a ship's Diesel SULZER 5RND68, 7500 BHP, 137 r.p.m. measured on sea trials and two stationary FIAT GMT A420.8, 55000 BHP, 500 r. p.m., coupled to a synchronous electric generator.