Turbulence Characteristics of Tumbling Air Motion in Four-Valve S.I. Engines and their Correlation with Combustion Parameters

An experimental investigation has been carried out of the turbulence characteristics of tumble air motion in four-valve pent roof combustion chambers. This was conducted on an optically accessed single cylinder research engine under motored conditions at an engine speed of 1500 rev/min. Four cylinder heads with varying tumble magnitude were evaluated using conventional and scanning Laser Doppler Anemometry (LDA) measurements. Analysis algorithms developed to account for the effects of mean flow cyclic variations and system noise were used to obtain unbiased estimates of turbulence intensity and integral length scales. The cylinder heads were also evaluated for combustion performance on a Ricardo single cylinder Hydra engine. Mixture and EGR loops at 1500 rev/min and 1.5 bar BMEP were carried out and cylinder pressure data was analysed to derive combustion characteristics.

Tumble air motion in a four-valve pent roof chamber resulted in a bi-modal turbulence generation, the first peak occurring at 110° BTDC and the maximum turbulence at 20° BTDC compression. This bi-modal turbulence generation occurs with moderate or high tumble builds and limits the turbulence generation close to TDC. Chamber shrouding to increase tumble does not increase turbulence near TDC due to early mean motion breakdown.

Turbulence at the spark plug at ignition correlates well with early combustion delay angle suggesting a linear relationship. Mean flow cyclic variations do not show any correlation with combustion variability from these measurements. The use of a combustion model for further correlations is suggested.