Details

Autor(en) / Beteiligte

• Nhumaio, Geraldo da Conceicao Sibia

Titel

Application of Electrostatically Charged Sprays in Gasoline Direct Injection Engines Using Plain Orifice Atomizers

Ort / Verlag

ProQuest Dissertations & Theses

Erscheinungsjahr

2000

Quelle

ProQuest Dissertations & Theses A&I

Beschreibungen/Notizen

• This study is part of ongoing research work developed at UMIST (University of Manchester Institute of Science and Technology) on the electrostatic atomization of hydrocarbon sprays and their applications. A charged spray is simulated in the context of in-cylinder flows with evaluation of a half cycle (i.e. up to the spark time) of a motored Gasoline Direct Injection (GDI) engine. An in-house engine code, named spray-EPISO (Engine Pressure Implicit with Splitting of Operators), is extended to accommodate, firstly, the three operation modes - namely early, transition and late - of a GDI engine and, secondly, the electric field effects on the spray development.A simple engine geometry is considered and this comprises flat piston surface and cylinder head. The inlet drop velocity magnitude is calculated from the nozzle discharge formula, using the typical injection pressure of 5 MPa employed in GDI engines; the decision for this injection pressure value is supported by flowrates approaching 300 mL/min, the maximum values reported in electrostatic atomization of hydrocarbon fuels for burners (Yule et al., 1995).As far as the numerical validation is concerned the real engine processes needed to be simplified to static combustion chamber with, at least, one open boundary corresponding to the cylinder roof. A time step of order 10 7 s was found reasonable to describe the motion of 10 pm drop diameters which are believed to be achievable with improved electrostatic nozzle designs, i.e. different designs from the present case of plain orifice type. The use of plain orifice electrostatic atomizers, for which experimental data are available, has led the study to mostly discuss a spray that develops in full cone fashion: this is also employed by Toyota in its D-4 GDI engine (Tomada et al., 1997) and has proved to provide the ability to vaporize ordinary sprays without risk of plug carbon fouling.It is shown through parametric studies that the use of conducting in-cylinder surfaces degrades the characteristics of the spray development; increasing the charging voltage well in excess of the "nominal/ideal" -12 kV results in the need of excessive boundary voltages which in turn seem sensible to be limited to a maximum of-325 V on the cylinder liner; and a backward stretching of the early injection window, which typically is started at 120 ° CA ATDC of intake for the case of ordinary sprays, may be made as early as 80 ° CA ATDC of intake with relaxed risks of in-cylinder surfaces impingement. The latter observation is seen as a path for improvement of volumetric efficiency of the GDI engine.The study employs the standard K-£ turbulence model of Launder and Spalding (1972). The droplet break-up, collision, heat and mass transfer mechanisms are not accounted for; this allows constant drop sizes and hence constant drop charges throughout the drop lifetime. Given the need for some tolerance of drop impaction on in-cylinder surfaces the wall impaction model of Park and Watkins (1994) for ordinary sprays is implemented, particularly at the piston surface. This tolerance is essential firstly because a limited number of droplets impinge on the in-cylinder surfaces irrespective of the boundary electric fields and secondly, due to the fact that when the GDI engine runs at full load (early injection mode) the gas pressure is near atmospheric and dilute drops impacting on in-cylinder surfaces have an enhanced evaporation rate over those freely flying in the air.