Turbulence Production and Relaxation in Bowl-in-Piston Engines

In order to generate turbulence and thereby improve fuel-air mixing and combustion, direct-injection engines often incorporate high-squish piston bowls and intake-generated air swirl. Here, laser-velocimetry measurements of turbulent air motion in a motored direct-injection engine are examined with power-spectral analysis and with conventional and filtered ensemble-averaging techniques. Results from cylindrical and square piston bowls are interpreted in the context of conventional eddy-cascade concepts of turbulence. In particular, the results show that after intake, as the velocity fluctuations decrease in intensity, their power spectrum (frequency distribution) E(f ) relaxes toward the canonical ƒ^-5/3 form associated with stationary, homogeneous turbulence in the inertial subrange. During the turbulence-production period around compression TDC, however, the power spectrum exhibits increased high-frequency content and (in the square bowl) anisotropy.