Late-Cycle Turbulence Generation in Swirl-Supported, Direct-Injection Diesel Engines

Cycle-resolved analysis of velocity data obtained in the re-entrant bowl of a fired high-;speed, direct-injection diesel engine, demonstrates an unambiguous, approximately 100% increase in late-cycle turbulence levels over the levels measured during motored operation. Model predictions of the flow field, obtained employing RNG k-ε turbulence modeling in KIVA-3V, do not capture this increased turbulence. A combined experimental and computational approach is taken to identify the source of this turbulence. The results indicate that the dominant source of the increased turbulence is associated with the formation of an unstable distribution of mean angular momentum, characterized by a negative radial gradient. The importance of this source of flow turbulence has not previously been recognized for engine flows. The enhanced late-cycle turbulence is found to be very sensitive to the flow swirl level. Furthermore, experiments conducted in a N₂ atmosphere, in which the injected fuel does not burn, indicate that buoyant turbulence production likely plays a role between approximately 10 and 20 CAD, but is not primarily responsible for the late-cycle increase. Finally, combustion induced gas expansion is found to produce a brief increase in turbulence near the time of the premixed burn.