Optimization of the CNG Direct Injection in a Small Research SI engine

The permanent aim of the automotive industry is the further improvement of engine efficien-cy and the simultaneous emissions reduction. In order to optimize the small internal combustion en-gines it is necessary to further improve the basic knowledge of the thermo-fluid dynamic phenomena occurring during the combustion process. In this context, the application of optical diagnostic tech-niques permits a deep insight into the fundamental processes such as flow development, fuel injec-tion, and combustion process. In this paper the analysis of the injection phase and combustion pro-cess of gaseous fuel were performed. The investigation was carried out in an optically accessible small Direct Injection Spark-Ignition (DI SI) engine fueled compressed natural gas (CNG9. Two differ-ent injectors were tested. The first one was the injector designed according to the model simulation in-formation, the second one was an optimized prototype obtained using the findings of the optical analy-sis carried out into the combustion chamber. The characteristic parameters of the gaseous fuel spray were evaluated through an image processing procedure. The fuel rail was modified in order to allow the injection of seeding particles into the gaseous fuel and the spray was analyzed using a white light source coupled with a high spatial and temporal resolution fast camera. The combustion process de-velopment was investigated for same engine operative condition performing a cycle resolved visuali-zation of the flame in the combustion chamber. This approach allowed the acquisition of important in-formation about the flame propagation and the abnormal combustion events. All the optical data were correlated with the engine performance and the exhaust emissions