Multidimensional Optimization of DI Diesel Engine Process Using Multi-Zone Fuel Spray Combustion Model and Detailed Chemistry NO_x Formation Model

摘要

A previously developed multi-zone direct-injection (DI) diesel combustion model was implemented into a turbocharged diesel engine full cycle simulation tool DIESEL-RK. The combustion model takes into account the following features of the spray dynamics: Detailed evolution process of fuel sprays. Interaction of sprays with the in-cylinder swirl and the walls of the combustion chamber. Evolution of a Near-Wall Flow (NWF) formed as a result of a spray-wall impingement as a function of the impingement angle and the local swirl velocity. Interaction of Near-Wall Flows formed by adjacent sprays. Effect of gas and wall temperatures on the evaporation rate in the spray and NWF zones. In the model each fuel spray is split into a number of specific zones with different evaporation conditions. Zones, formed on the cylinder liner surface and on the cylinder head, are also taken into account. The piston bowl in the modeling process is assumed to have an arbitrary axisymmetric shape. The combustion model supports central, non-central and side injection systems. A NO_x calculation sub-model uses detailed chemistry analysis which considers 199 reactions of 33 species. The soot formation calculation sub-model used is a phenomenological one and takes into account the distribution of the droplets Sauter Mean Diameter (SMD) during the injection process. The ignition delay period is estimated using relevant data in the pre-calculated comprehensive 4-D map of ignition delays. This 4-D map is developed using CHEMKIN detailed chemistry simulations and takes into account effects of the temperature, the pressure, the Fuel/Air ratio and the Exhaust Gas Recirculation (EGR). The noted above sub-models were integrated into full-cycle engine simulation software together with library of non-linear programming procedures, allowing multidimensional optimization of DI diesel engine working parameters to reach prescribed emissions regulations norms. List of optimized parameters includes: CR, EGR, injection profile shape, fuel injection pressure, port timings (IVC), boost pressure, power for turbocharger assistance, injection timing, nozzles hole number, diameter and inclination angle of nozzles. Two variants of piston bowl were investigated. In the research there was done an optimization of working parameters of medium speed diesel engine at few operating points with account of weighting coefficients of the points. At each operating point the problem of optimization has individual peculiarities and an individual set of independent variables and restrictions. The expression for objective function of conjoint optimization of SFC, NO_x and PM was proposed. Procedures of Rosenbrock, Powell and other were used for optimum search. Restrictions were accounted by penalty function method. Controlling algorithms for EGR booster driving, injection timing, Common Rail pressure, turbocharger assist for locomotive performance were obtained. To provide a required injection profile shape being obtained in optimization a modification of injector was carried out. There were optimized fuel pipe line diameter and dimensions of internal elements of injector: control valve, orifice and internal volume. The injection profile was simulated with hydrodynamic simulation software INJECT.