Two main properties distinguish the structure of a turbulent flame from that of a laminar flame: ionization current of high intensity and high burning velocity. The first of these two quantities was directly measured in the combustion chamber of a typical European spark ignition engine by means of a Langmuir's probe, while the second was calculated starting from the pressure-time history measured with a piezoelectric transducer. The experimental results show the strong influence of the initial flame propagation on the type of the subsequent combustion during the rest of the cycle. The mathematical elaboration of the experimental data showed that the flame propagation mechanism is consistent with the hypothesis that turbulent combustion is caused by successive self-ignitions, as suggested by Russian authors. Useful information can be obtained from the measurement method used in this work on the quality and quantity of the ions that are the precursors of some of the most interesting pollutants found in the exhaust stream of a spark ignition engine.