In‐Cylinder Flow

The general properties and evolution of in-cylinder flows are reviewed, from the initial generation of turbulence and large-scale bulk structures during the induction process until the early expansion stroke period, when flow processes continue to aid with the completion of combustion. Flows typical of both spark ignition and compression ignition engine technologies are described. Our primary emphasis is on providing the practicing engineer and engine designer with a physical understanding of the storage of flow energy in large-scale rotating structures; the mechanisms by which this energy can be extracted to enhance turbulent mixing processes; the interaction of these flow structures with the compression, fuel injection, and combustion processes; and the fluid-dynamical origin of cycle-to-cycle fluctuations in the combustion process. We further examine characteristics of the turbulence, with the objective of providing engineers with a natural understanding of how turbulence scales relate to engine parameters (bore, stroke, speed, etc.) and of how turbulence is generated and dissipated throughout the engine cycle. Various methods used to characterize and separate the turbulence from large-scale, coherent cycle-to-cycle fluctuations in the flow, and tools to characterize the topology of the flow, are also briefly reviewed. Lastly, we identify several areas, principally related to turbulence generation and dissipation in engines, to the elucidation of flow structures and their cycle-to-cycle variation, and to near-wall flows, that are still poorly understood and that would benefit from additional investigations by the research community. Keywords: engines; flows; turbulence; tumble; swirl