Diesel engine design is a very complex challenge, which aims to find the best compromise between different hardware and calibration parameters. The guidelines that lead the diesel engine combustion system design are the need to comply with pollutant emissions regulations and to reduce fuel consumption and consequently to lower CO2 emissions levels. Moreover, to improve customer acceptance, noise, harshness and vibration have to be minimized as well, and vehicles performance has to be enhanced as much as possible. Although the current emissions standards for light-duty vehicles can be met only with an optimized aftertreatment system design, the “in-cylinder” control of pollutant emissions remains nevertheless of crucial importance.