Fast-Running Autoignition Model for Diesel Combustion Modeling and Control, Based on Detailed Reaction Kinetics Simulation

摘要

Detailed and reduced kinetic mechanisms have been proposed for description of the complex chemistry of autoignition processes of n-heptane, as a representative diesel fuel. These kinetic models are attractive for a detailed 3-D CFD or multi-zone simulation, however the simulation time is normally not affordable for phenomenological engine process modeling. For phenomenological combustion models, typically single-to multiple-step Arrhenius equations are used to model the autoignition processes. Based on the number of Arrhenius equations and model structure the low-temperature, high-temperature and the negative temperature coefficient (NTC) behavior can be modeled. For diesel engine simulation modeling the ignition delay using Arrhenius equation(s) and a Livengood-Wu integration can deliver fairly good results, depending on the number of equations and calibration of constant parameters. However, it needs integration, as in-cylinder pressure and temperature change in each time step, due to e.g. piston movement. The aim of this study is development of a novel autoignition model for diesel fuel combustion which can be used for efficient and fast-running combustion modeling. The presented model is based on simulation results using realistic diesel engine geometry under various operating conditions. Detailed chemical reactions of the ignition processes are solved by a n-heptane mechanism which is coupled to the thermodynamic simulation of in-cylinder processes during the compression and autoignition phases. All relevant engine operating conditions, like engine speed, in-cylinder charge mass and temperature as well as the EGR ratio are varied and ignition delay times are calculated. Using a large number of simulation results, a very-fast running ignition delay model is trained and validated against detailed reaction kinetics simulation results. The developed autoignition model can reproduce the results using engine and detailed reaction kinetics simulation with a very good accuracy. As next step, the developed autoignition model is implemented into a phenomenological combustion model. Experimental investigations are carried out on a single-cylinder heavy-duty diesel engine for validation of the developed model. Finally, advantages of using the proposed novel ignition delay model for combustion control in the next generation of the engine control units are discussed.