Zoldak, P., John Joseph, J., Shelley, W., Johnson, J. et al., "Characterization of Partially Stratified Direct Injection of Natural Gas for Spark-Ignited Engines," SAE Technical Paper 2015-01-0937, 2015, doi:10.4271/2015-01-0937.
The increased availability of natural gas (NG) in the United States (US) and its relatively low cost compared to diesel fuel has heightened interest in the conversion of medium duty (MD) and heavy duty (HD) diesel engines to NG fuel and combustion systems (compressed or liquefied). The intention is to realize fuel cost savings and reduce harmful emissions, while maintaining or improving overall vehicle fuel economy. This is a potential path to help the US achieve energy diversity and reduce dependence on crude oil.Traditionally, port-injected, premixed NG spark-ignited combustion systems have been used for medium and heavy duty engines with widespread use in the US and Europe. But this technology exhibits poor cycle efficiency and is load limited due to knock phenomenon. Direct Injection of NG during the compression stroke promises to deliver improved thermal efficiency by avoiding premixing and extending the lean limits which helps to extend the knock limit.In this study, an optical combustion vessel (CV) chamber was used to capture shadowgraphy images of direct injection of methane fuel under constant upstream pressure for various chamber pressures simulating injection at various injection timings during the compression stroke. The resulting transient turbulent jets were visualized and macroscopic details were determined including; penetration distance, jet tip velocity and enclosed angle for changing chamber pressures. Microscopic details were also observed using Schlieren imaging and observations were made of various downstream flow patterns including; choked flow, normal shock and barrel shock patterns.