Turbulence-combustion interaction in direct injection diesel engine

Abstract

The experimental measures of chemical species and turbulence intensity during the closed part of the engine combustion cycle are today unattainable exactly. This paper deals with numerical investigations of an experimental direct injection Diesel engine and a commercial turbocharged heavy duty direct injection one. Simulations are carried out with the kiva3v2 code using the RNG (k-?) model. A reduced mechanism for n-heptane was adopted for predicting auto-ignition and combustion processes. From the calibrated code based on experimental in-cylinder pressures, the study focuses on the turbulence parameters and combustion species evolution in the attempt to improve understanding of turbulence-chemistry interaction during the engine cycle. The turbulent kinetic energy and its dissipation rate are taken as representative parameters of turbulence. The results indicate that chemistry reactions of fuel oxidation during the auto-ignition delay improve the turbulence levels. The peak position of turbulent kinetic energy coincides systematically with the auto-ignition timing. This position seems to be governed by the viscous effects generated by the high pressure level reached at the auto-ignition timing. The hot regime flame decreases rapidly the turbulence intensity successively by the viscous effects during the fast premixed combustion and heat transfer during other periods. It is showed that instable species such as CO are due to deficiency of local mixture preparation during the strong decrease of turbulence energy. Also, an attempt to build an innovative relationship between self-ignition and maximum turbulence level is proposed. This work justifies the suggestion to determine otherwise the self-ignition timing.