Size- and Speed-Scaling of Turbulent Premixed Combustion in a Spark-Ignition Engine

Tess, M. J. Size- and Speed-Scaling of Turbulent Premixed Combustion in a Spark-Ignition Engine. University of Wisconsin-Madison, 2014.

The size- and speed-scaling of the turbulence and combustion properties in an internal combustion engine were investigated using multiple optical diagnostic techniques in two geometrically similar, single-cylinder optical engines scaled in size by a factor of 1.69, and operated at mean piston speeds ranging from 0.5?3 m/s. The engines were homogeneously fueled and spark ignited.

The bulk mixing characteristics of the in-cylinder flow, measured using planar laser-induced fluorescence, were observed to closely scale with engine size and speed, giving similar stratification trends throughout the intake and early compression strokes. The flow became very well-mixed during compression where PDFs of the fluorescence intensity showed a nearly Gaussian distribution about a homogeneous condition.

The scalar field turbulence length (integral, Taylor, and Batchelor) scales were measured either directly or by a spectral method and compared to corresponding values from the velocity field. The scalar integral scale was independent of engine speed or valve type and scaled with the engine size at a slightly larger ratio than the size-scaling factor. The Taylor scale varied with engine size, as predicted by Reynolds number scaling, but was only weakly dependent on engine speed. In-plane and out-of-plane resolution effects on the accuracy of the Batchelor scale were parametrically investigated, resulting in methods to correct for under-resolution. The corrected Batchelor scale strongly agreed with Reynolds number scaling theory for both engine size and speed.

The turbulent flame structure was examined for a premixed, stoichiometric operating condition. Dynamically similar operation was achieved by operating the engines at similar mean piston speed and adjusting the spark timing for similar combustion phasing. The required spark timing at constant piston speed was similar in both engines, and the cylinder pressure data revealed similar rates of combustion at a range of speeds. A fractal analysis of the flame structure showed that the fractal dimension increased linearly with mean piston speed and was similar in both engines.