###### Abstract  
Combustion stability, engine efficiency and emissions in a multi-cylinder spark-ignition internal combustion engines can be improved through the advanced control and optimization of individual cylinder operation. In this work, experimental and numerical analyses were carried out on a twin-cylinder turbocharged port fuel injection (PFI) spark-ignition engine to evaluate the influence of cylinder-by-cylinder variation on performance and pollutant emissions. In a first stage, experimental tests are performed on the engine at different speed/load points and exhaust gas recirculation (EGR) rates, covering operating conditions typical of Worldwide harmonized Light-duty vehicles Test Cycle (WLTC). Measurements highlighted relevant differences in combustion evolution between cylinders, mainly due to non-uniform effective in-cylinder air/fuel ratio. Experimental data are utilized to validate a one-dimensional (1D) engine model, enhanced with user-defined sub-models of turbulence, combustion, heat transfer and noxious emissions. The model shows a satisfactory accuracy in reproducing the combustion evolution in each cylinder and the temperature of exhaust gases at turbine inlet. The pollutant species (HC, CO and NOx) predicted by the model show a good agreement with the ones measured at engine exhaust. Furthermore, the impact of cylinder-by-cylinder variation on gaseous emissions is also satisfactorily reproduced. The novel contribution of present work mainly consists in the extended numerical/experimental analysis on the effects of cylinder-by-cylinder variation on performance and emissions of spark-ignition engines. The proposed numerical methodology represents a valuable tool to support the engine design and calibration, with the aim to improve both performance and emissions. limit the in-cylinder production of pollutant emissions for SI engines, such as the adoption of innovative combustion modes moving towards the Low-Temperature Combustion (LTC) concept: the homogeneous charge compression ignition (HCCI), the spark-assisted compression ignition (SACI) and the turbulent jet injection [2]. Turbulent jet injection (TJI) demonstrates to be a promising technique to reduce the exhaust emissions of SI engines [3], especially in the case of an active pre-chamber thanks to the ultra-lean combustion; on the other side, HCCI and SACI combustion modes allow significant improvements in NOx emissions, while some penalties on the HC and CO are obtained [4]. In addition, well-established technologies, such as the employment of cooled EGR [5], direct or port water injections [6] and alternative fuels, including ethanol/gasoline blends [7] or methanol/gasoline blends [8], allow certain benefits on the main exhaust emissions. In addition to the above-discussed techniques to improve pollutant species, a particular attention has to be devoted to the control of cylinder-by-cylinder variation, since it could lead to a combustion deterioration with consequent increase in the exhaust emissions, especially of HC and CO species. Different factors may play a role on the onset of cylinder-by-cylinder variation in SI engines. Indeed, the increasing complexity of engine subsystems, the high number of mechanical components, the manufacturing tolerance, the components aging can be considered as examples inducing the cylinder-by-cylinder variation and leading to a worsening in both efficiency and exhaust emissions. Of course, this aspect cannot be overlooked and should be taken into account during the engine development phase. An individual control and optimization of combustion for cylinders in a multi-cylinder engine can further contribute to suppress the cylinder-by-cylinder variation and to improve emissions. To this aim, both experimental and modeling approaches are employed. Experimental and numerical methods are increasingly combined to merge their relative advantages and to offer the availability of validated numerical tools capable to reproduce the behavior of both engine and single cylinders under various operating conditions.