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In the current investigation, the authors identified conditions under which increased in-cylinder turbulence can be used to improve diesel emissions. Two separate regimes of engine operation were identified; one in which combustion was constrained by mixing and one in which it was not. These regimes were dubbed under-mixed and over-mixed, respectively. It was found that increasing mixing in the former regime had a profound effect on soot emission. Fuel injection characteristics were found to be extremely important in determining the point at which mixing became inadequate. In addition, the ratio of the fuel injection momentum flux relative to that of the gas injection was found to be important in determining how increasing mixing would effect soot emissions.

Laser Doppler velocimetry has been used to measure velocity and turbulence intensity profiles in the wall boundary layer of a spark-ignited homogeneous-charge research engine. By using a toroidal contoured engine head it was possible to bring the laser probe volume to within 60 μm of the wall. Two different levels of engine swirl were used to vary the flow Reynolds number. For the high swirl case under motored operation the boundary layer thickness was less than 200 μm, and the turbulence intensity increased as the wall was approached. With low swirl the 700-1000 μm thick boundary layer had a velocity profile that was nearly laminar in shape, and there was no increase in turbulence intensity near the wall. When the engine was fired the boundary layer thickness increased for both levels of swirl.

The quasi-steady assumption is often used to determine the flow resistance of highly compressible critical or near-critical (approaching sonic velocity) pulsating flows through engine valves, EGR system and other flow restrictions for modeling and control. The quasi-steady assumption utilizes steady (non-pulsating) flow results where the discharge coefficient (Cd) of flow nozzles/orifices is solely a function of Reynolds number (Re), and Cd is constant at high Re. There exists some literature addressing the flow resistance of incompressible pulsating flows and also for compressible steady flow, but virtually no literature for the highly compressible, critical/near-critical pulsating flow typical in engines. In this work, the Cd of a square edged orifice placed in the exhaust stream of a four-cylinder diesel engine was measured and found not to be a sole function of Re, but correlated to Re.