Optimization of the compressed natural gas direct injection in a small research spark ignition engine

The permanent aim of the automotive industry is the further improvement of the engine efficiency and the simultaneous pollutant emissions reduction. In order to optimize the small internal combustion engines, 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 techniques permits a deep insight into the fundamental processes such as flow development, fuel injection, and combustion process. The aim of this study was the optimization of the compressed natural gas direct injection by means of the analysis of the injection phase and combustion process. This analysis allowed the improvement of the engine efficiency in lean-burn operation condition too. The investigation was carried out in an optically accessible small direct injection spark ignition single-cylinder engine. Two different injectors were tested. The first one was the injector designed according to the results of model simulation, and the second one was a modified prototype obtained using the findings of the optical analysis carried out in the combustion chamber. The characteristic parameters of the gaseous fuel jet 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. The gas jet was analyzed using a white light source coupled with a high spatial and temporal resolution fast camera. The combustion process development was investigated for same engine operative condition performing a cycle-resolved visualization of the flame. Moreover, this methodology permitted the evaluation of motion field and turbulence during the injection process directly into the combustion chamber. The results of investigation evidenced that the modification of the injector, designed according to the optical analysis made in the optically accessible engine, allowed an optimization of compressed natural gas direct injection resulting in improved combustion and emissions reduction