1986-03-01

Threedimensional Computations for Flowfields in DI Piston Bowls 860463

The performance of Direct Injection (DI) engines is strongly dependent upon the design of the piston bowl combustion chamber. Many different shapes have been used in the course of engine development during the past years, however, the interactions between the complex flow structure within the bowl, the fuel spray and combustion are not yet well understood. The recent advances in multidimensional mathematical models can help to analyse and interpret experimental results gained by engine tests. In this study, a 3-dimensional model for the simulation of flow and turbulence in engine cylinders is applied to two reentrant bowls of different bowl shapes, which contain the same bowl volume.
The current version of the program does not allow the bowl and valve to be simultaneously in their true positions, and the intake stroke is therefore computed with a flat piston. A variety of assumed boundary conditions at the valve gap are used to estimate the flowfield at the start of the compression stroke. The computed fiowfields at the time of fuel injection are then compared for different initial swirl ratios at the beginning of the compression stroke. These computations start at inlet valve closing and are continued until TDC. The mesh contains approximately 20000 cell volumes within the solution domain.
Both bowls have been tested experimentally with the same injection system and significant differences in fuel consumption and power output have been observed. The computations can offer an explanation as to why the flowfield of one bowl is more suitable in connection with a four hole injection system than the other.
In order to facilitate the representation of the flowfield, a color graphics system has been adopted which displays velocity vectors and scalar quantities in 3D perspective views.

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