Dependence of Fuel-Air Mixing Characteristics on Injection Timing in an Early-Injection Diesel Engine 2002-01-0944
In recent years, there has been an interest in early-injection Diesel engines as it has the potential of achieving a more homogeneous and leaner mixture close to top-dead-center (TDC) compared to standard Diesel engines. The more homogeneous mixture may result in reduced NOx and soot emissions and higher efficiency. Diesel engines in which a homogeneous mixture is achieved close to TDC are known as Homogenous Charge Compression Ignition (HCCI) engines. PREmixed lean DIesel Combustion (PREDIC) engines in which the start of fuel injection is considerably advanced in comparison with that of the standard Diesel engine is an attempt to achieve a mode of operation close to HCCI. Earlier studies have shown that in a PREDIC engine, the fuel injection timing affects the mixture formation and hence influences combustion and pollutant formation. In order to clarify quantitatively the dependence of the mixture formation on the injection timing, in this work a computational study is performed using a multidimensional model. A measure for the fuel-air mixing characteristics termed as the degree of heterogeneity, that is shown to yield insight into the physical processes under study, is employed. Computational results indicate that there exists an optimum fuel injection timing to obtain maximum degree of homogeneity close to TDC. This timing represents a compromise between the time available for mixing and the rate of mixing that is dependent on ambient density. Additional parameters such as duration of injection, spray orientation and multiple injection rates are also explored.