Influence of Induction Swirl and Piston Configuration on Air Flow in a Four-Stroke Model Engine

Abstract
Measurements of ensemble-averaged mean, r.m.s. and cycle resolved instantaneous swirl velocities, obtained by laser—Doppler anemometry, are reported for the in-cylinder flow in a four-stroke model engine motored at 200 r/min. Variable induction swirl was created by 30° and 60° vanes located in axisymmetric and off-centre intake ports with flat and re-entrant bowl piston configurations. The results showed that the main features of the initial swirl velocity distribution, which are determined by the intake port/valve geometry, persist through the compression stroke. The swirl centre performed in all cases a helical motion whose development was a function of the clearance volume at top dead centre; the weakness of this motion made it susceptible to cyclic variations which were significantly reduced with the re-entrant piston bowl. High induction swirl resulted in lower turbulence intensity at top dead centre of compression, in the flat piston case, and more complex vortex pattern inside the re-entrant piston bowl. The angular momentum decay from inlet valve closure to top dead centre of compression was shown to depend on initial swirl ratio and velocity distribution and calculated to be about 30 and 45 per cent for the flat and re-entrant bowl piston, respectively.

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