Lateral integral length scales were measured directly using a novel two—point, single probe-volume, laser Doppler velocimetry (LDV) system in a motored, ported, single—cylinder internal combustion (I.C.) engine with a pancake—shaped chamber. The engine was motored at 600 rpm with a top-dead centre (TDC) swirl ratio of about 4. The measurements were made between 43 degrees before and 20 degrees after TDC. Three compression ratios (5.7, 7.6, and 11.4) were used corresponding to TDC clearance heights of 18.1 mm, 12.8 mm, and 8.2 mm. The LDV system developed in this work was designed to make length scale measurements without and with combustion. It is capable of measuring in—cylinder length scales of the order of millimeters (from about a quarter of a millimetre to about five millimeters).

This study contains the first direct measurements of integral length scales by LDV in an I.C. engine. A controversy persists as to what may be the best way of defining turbulence in an I.C. engine, therefore, both an ensemble analysis (which assumes that the mean flow is the same for all engine cycles) and a low—pass—filtered cycle-nresolved analysis (which identifies a mean flow for each engine cycle) of the data were performed. The methods of analysis define fluctuation and turbulence quantities, respectively, and were found to be complementary, each revealing different aspects of the same flow. In general, the TDC turbulence integral length scale was on the order of 1 mm and the TDC fluctuation integral length scale was on the order of 2 mm. The fluctuation velocities were shown to have significant underlying trends, both oscillatory and monotonie. Turbulence, being random, should not show such trends. On the other hand, the small difference between the turbulence integral length scales (order 1 mm) and the cycle dependent mean velocity (i.e. bulk velocity) integral length scales (order 5 mm) was also demonstrated and confirms a conceptual limitation of the low—pass—filtered cycle-resolved analysis.

Integral length scale investigations were made of the influence of the TDC clearance height, of the extent of anisotropy, and of the extent of inhomogeneity. The influence of TDC clearance height was found to be dependent on the particular orientation of the lateral integral length scale being measured. To permit a quantitative comparison of the extent of anisotropy and inhomogeneity, anisotropy and inhomogeneity coefficients of intensity and length scale were defined. The anisotropy coefficient revealed that the anisotopy of the turbulence intensity in the engine is only slightly larger than that found in grid turbulence and significantly smaller than that found in free jet turbulence. Finally, from comparisons between measured length scales and computed length scales, it is concluded that some re-evaluation of current in-cylinder flow models would seem warranted.