Nowadays, the growing importance of human emissions on the environment has led to decreasing emissions from internal combustion engines, and consequently reducing consumption, with ever more stringent regulation. For this region, the study of lean combustion spark-ignited engines was improved, but this type of combustion is less stable than the stoichiometric one, different researchers are studying what causes this Cycle-to-Cycle Variability (CCV) and how to solve the problem. In this thesis, the kernel formation was investigated in an optically accessible Direct-Injection Spark-Ignition (DISI) engine to correlate its formation with CCV. The work is divided into two main parts, in the first one was reported the thermodynamic analysis of the engine parameters (Indicated Mean Effective Pressure (IMEP), Covariance of the previous parameters (COV_IMEP), Peak of pressure and its covariance (P_MAX and COV_(P_MAX )), Mass Fraction Burned (MFB), Net Heat Release Rate (NHRR) etc...); two different spark plugs were used (i.e.