Abstract: The invention concerns a method to measure parameters in the combustion chamber of a piston engine that comprises an ignition system. An alternating voltage is applied across the secondary winding of the ignition coil and the value of the current that arises in a measurement circuit that co-operates with the secondary winding is measured. The value of the current depends on the resistance (R1) of the measurement circuit, on the inductance (L1) and resistance of the secondary winding, and on the impedance of the combustion chamber, i.e. on its capacitance (C1) and its resistance. For example, top dead centre, the pressure in the cylinder, analysis of the ionic current and change of the burning time can be determined by means of the method.