Improved Modelling of DI Diesel Engines Using Sub-grid Descriptions of Spray and Combustion

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Abstract
Three dimensional CFD tools are commonly used to simulate spray injection and combustion in DI Diesel engines. However typical computations are strongly mesh dependent. By now it is not possible to enhance grid resolution since it would violate the underlying assumptions for the Lagrangian liquid phase description. Besides, a full Eulerian approach with an adapted mesh is not practical at the moment mainly because of prohibitive computer requirements. Based on the Lagrangian-Eulerian approach, new approaches have been developed: the Coupled Lagrangian-Eulerian (CLE) model for the two-way coupling between the spray and the air flow and a new combustion model (CFM3Z) which allows a description of the fuel-oxidizer sub-grid mixing. The previously introduced CLE model consists in retaining vapor and momentum along parcel trajectories as long as the mesh is insufficient to resolve the steep gradients created by the spray. Vapor and momentum are gradually released following specific diffusion laws and criteria. The new combustion model (CFM3Z) is briefly described. In this approach, fuel and oxidizer, originally separated, are continuously mixed by the turbulent flow leading to the creation of a mixed zone. The local structure of the gas phase can then be divided into three zones: the first contains only fuel, the third only air (with EGR) and the second zone, where chemical reactions occur, contains both fuel and air mixed with combustion products. Simulations are performed using the KMB code, a modified version of KIVA-II. Comparisons of computed and experimental results of combustion in a DI Diesel engine are presented for several operating conditions. The influence of the engine speed and load, nozzle tip protrusion, piston bowl shape, EGR and pilot injection is investigated. Calculations show an improved predictivity of the engine performance compared to conventional models. The sensitivity of pollutant emissions to operating conditions is also better reproduced.

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