Analysis of the Combustion Process of SI Engines Equipped with Non-Conventional Ignition System Architecture

he use of lean or ultra-lean ratios is an efficient and proven strategy to reduce fuel consumption and pollutant emissions. However, the lower fuel concentration in the cylinder hinders the mixture ignition, requiring greater energy to start the combustion. The prechamber is an efficient method to provide high energy favoring the ignition process. It presents the potential to reduce the emission levels and the fuel consumption, operating with lean burn mixtures and expressive combustion stability. In this paper the analysis of the combustion process of SI engines equipped with an innovative architecture and operating in different injection modes was described. In particular, the effect of the prechamber ignition on the engine stability and the efficiency was investigated in stoichiometric and leanburn operation conditions. The activity was carried out in two parts. In the first part the investigation was performed in a research small direct injection spark ignition (DISI) engine, running at 2000 rpm WOT, and fueled with Methane. The combustion process was studied using optical diagnostics. Methane was injected both in the prechamber and in the main chamber through the port fuel injection (PFI) mode. The ignition was obtained with a properly designed fueled prechamber prototype. It was equipped with a gas direct injector, used to inject the fuel into the prechamber, and a spark plug used to ignite the mixture. The gaseous fuel in the main chamber was ignited by the plasma jets coming from the prechamber. The combustion of the prechamber mixture generates four plasma jets that quickly ignite the mixture into the combustion chamber, and the flame speed is much faster than the traditional ignition. The optical data were correlated with the engine performance and indicated measurements that showed an increase of the Indicated Mean Effective Pressure (IMEP) and the reduction of the Coefficient of Variation (CoV). In the second part, the optimization of the gasoline combustion by means of a passive prechamber was performed. The investigation was carried out in a commercial small SI engine at 2000 rpm, fueled with gasoline in PFI mode and equipped with the same prechamber used in the first part of the activity. But in this case the prechamber works in passive mode.