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High-speed flow measurements in a two-stroke engine by photon-correlation laser velocimetry.

Three-dimensional mapping of the spatial structure and the temporal evolution of the air flow field in a commercially available two-stroke engine requires the ability to resolve a wide range of flow velocities (±250 m/s) at measurement locations near the cylinder wall and the piston crown. Laser-Doppler velocimetry that uses photon-correlation signal processing and Fourier-transform analysis permits ensemble-averaged velocity-distribution functions to be extracted from scattered-light signals that are sampled at nearly the Nyquist limit and whose strength is typically less than 1 photon per Doppler cycle. Selected results from a systematic study illustrate the impulsive jet-like flow through the transfer ports into the engine cylinder, the complexity of the resultant in-cylinder flow field, and the likelihood of large-scale flow-field variations from one engine cycle to another.