Christos Chryssakis Homepage

CFD in IC Engines

KIVA-3V

W.E. Lay Automotive Laboratory

ERC - UW Madison

Digital Library

University of Michigan Library

Science Direct

General CFD & Fluid Dynamics

CFD-Online

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Conferences

ILASS-Americas

COMODIA 2004

SAE Spring
Fuels & Lubricants

ASME Internal Combustion Engine Division Fall 2004

General Interest

DieselNet

Fuel Spray Modeling for Internal Combustion Engines

General

My work focuses on fuel injection modeling for Direct Injection (DI) Gasoline and Diesel engines. The major challenge in both engine types is the air utilization during combustion, which can lead to the formation of excessive amounts of soot that cannot be burned up prior to exhaust. Furthermore, the high temperatures developed locally in the combustion chamber due to mixture stratification can also lead to high NOx formation rates, while the liquid wall films from the fuel spray impinging of the piston or cylinder walls can lead to unburned HC emissions. Hence, the injection and mixing processes have to be understood and optimized to achieve optimum engine performance with minimum pollutant emissions generation. Computational models can be a valuable tool to gain insight on in-cylinder mixing phenomena.

Fuel Injection Modeling

Fuel sprays used in IC engines are produced in various ways, depending on the requirements of each application. There are several basic processes associated with all methods of atomization such as the internal flow in the nozzle, the primary and the secondary atomization processes. All these characteristics of the spray depend on the internal geometry of the nozzle, the injection pressure and the ambient conditions in the combustion chamber.

The internal flow in the nozzle can include flow separation and reattachment and to the limit cavitation phenomena that strongly enhance turbulence levels and atomization. Additionally, the design of the nozzle has a major effect on the structure of the spray and its properties. A multi-hole injector nozzle, such as those used for diesel applications, results in dense solid-cone sprays favoring a stratified charge. On the other hand, swirl and fan injectors, typically used in DI Spark-Ignition (DISI) engines, lead to a more disperse spray appropriate for a homogeneous or stratified charge.

The primary atomization of the spray also depends directly on the internal geometry of the nozzle. This structure can vary from a liquid core for diesel sprays to a liquid sheet (either flat or conical) for gasoline sprays. These structures interact with the ambient gas and result in disintegration starting with long ligaments that further disintegrate into spherical droplets.

Once spherical drops are created, after the primary atomization has been completed, the secondary atomization starts and its governing mechanism is common for any type of spray. It only depends on the initial droplet sizes, velocities and physical properties of the system. These three parameters determine the breakup mechanism under which a droplet will further disintegrate. Even though in a given spray a certain mechanism may be dominant, it is most probable that more than one mechanism will be relevant and they all have to be modeled successfully. The fact that breakup mechanisms are independent of the primary atomization process, offers the flexibility of developing a breakup model that will be able to handle all possible cases if appropriate criteria can be established.

Primary Atomization

The primary breakup mechanisms vary considerably with injection pressure, nozzle design and operation. In Figure 1 (from Faeth) the primary breakup for a diesel spray (liquid core) is schematically shown. On the other hand, a gasoline spray typically emerges from a High-Pressure Swirl Injector, resulting to a rotating, conical liquid sheet. There are substantial differences on the size, shape and durations of these mechanisms. Their common feature is that they connect nozzle design properties and spray properties and they also define initial conditions for the dense sprays. In both cases rapidly growing waves on the liquid-gas interface cause instabilities that lead to primary breakup.

Secondary Atomization

The secondary breakup of drops is an important multiphase flow process and is common for all sprays, independently of their primary atomization mechanism. Numerous studies of dilute sprays, as well as of isolated droplets have been performed in order to increase our understanding of this spray region. Existing experimental observations and theoretical considerations of secondary breakup show that breakup regime transitions are functions of the initial Weber and Ohnesorge number of a drop, which are the ratio of aerodynamic and liquid viscous forces to surface tension forces respectively. This is demonstrated in Figure 2 (from Hsiang&Faeth, 1995).