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Quasi-Dimensional Modeling of CI-Combustion with Multiple Pilot- and Post Injections
Abstract:
A new phenomenological CI combustion model was developed. Within this model the given injection rate may contain an arbitrary number of injections during one cycle. Another target was a short computation time of one second per cycle on average. The new approach should also have the ability to simulate a wide engine spectrum from passenger-car engines through to marine engines.
The ignition delay is calculated separately for each single injection. In this way the model depicts the influence of pilot injections on the ignition delay of proximate injections. Each pilot injection is modeled as a single air-fuel mixture cloud with air entrainment. The burn rate of the pilot injection is modeled as a function of flame propagation and of the current local excess air ratio. If the local excess air ratio becomes too lean the pilot combustion stops or does not start at all.
Main and post-injections are calculated by means of a slice approach. The slices are generated isochronously as a function of the injection rate. They propagate through the combustion chamber in the direction of injection. Along the way the entrainment of air into the slices is computed by means of a statistical lambda distribution. The premixed and diffusion combustions are calculated by different approaches. Within the diffusion approach there is a distinction between the fast, nearly stoichiometric combustion and the slow lean combustion, which is responsible for the characteristic burn-out of CI combustion.
The approach will be shown in detail. On the basis of measurements and simulations, the forecast capability and the limitations of the model are discussed in full.