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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.

Earlier work has shown that the in-cylinder flow in internal combustion engines can be modelled, with reasonable accuracy, as the sum of an ensemble averaged mean component, a non-stationary ‘turbulence’ component and a ‘cycle-to-cycle’ variation component, the latter being phase-locked to the engine cycles. The development of the LDA technique has enabled direct measurements of the in-cylinder velocity field to be taken, either at a single position in space over the engine cycle, or over a range of spatial positions, at effectively one point in the engine cycle (scanning LDA). Previously, different approaches have been developed for separating the various flow components in the model described above, dependent on the type of data acquired. In this paper a single ‘unified’ method is presented, based on the computation of autocorrelation functions and a completely parametric representation of the various components in the flow model.

A scanning Laser Doppler Anemometer system has been developed for the measurement of spatial velocity profiles in motored internal combustion engines. Tests were carried out on a single cylinder engine with a disc shaped combustion chamber, at an engine speed of 1200 rev/min. Ensemble averaged mean and RMS velocity estimates show good agreement with conventional LDA measurements. Longitudinal and transverse autocorrelation functions have been calculated, and estimates have been made of turbulence length scales. These have been found to be comparable with length scales measured in engines by other techniques. This investigation has demonstrated that scanning LDA is a powerful technique for characterising in-cylinder air motion in engines.