An Experimental and Computational Analysis of the Flow Field in a Four-Valve Spark Ignition Engine-Focus on Cycle-Resolved Turbulence 900056

The purpose of this paper is to report on the measurement of the internal air motion in a motored four-valve S.I. engine. Two windows located in the head and a quartz top piston allow the use of Laser Doppler Velocimetry, using either a forward-scatter or a basck-scatter optical arrangement. The latter allows a large spatial exploration of the combustion chamber.

Three intake configurations, which generate various flow fields and turbulence levels, are investigated. The obtained flow patterns exhibit rather different swirl and tumble levels. Experimental data provide the mean and turbulent velocities. Despite the large exploration, the complex structure of the flow cannot be completely understood. In order to improve the description of the whole flow fields, a 3-D model is implemented. Agreement between the model and measurements is relatively good. Thus, the model is very helpful especially for the basic four-valve configuration where two inclined counter-rotating tumbling motions are revealed. For the configurations with swirl, only one inclined tumbling motion is generated. However, in all cases, squeezing of the flow during compression yields a destruction of the tumble and a significant increase of turbulence before TDC.

For the forward-scatter measurements, high data rates have been obtained. A cycle-to-cycle analysis based on a filtering technique in the frequency domain is applied. The main result is that low frequency fluctuations are highly connected to high frequency fluctuations. The ensemble fluctuations are however dominated by the low frequency contribution. The influence of the cut-off frequency is relatively weak. A spectral analysis shows that for all three configurations turbulence is roughly in an equilibrium state, such that its spectrum is almost self-similar.