Abstract
The modern light duty Diesel combustion systems, based on the use of Common Rail Fie technology and high values of EGR rates, and adopting multiple injection for reduction of soot emission and combustion noise, are characterized by the design parameters and in addition by a large number of adjustable parameters of the engine electronic management system, that are to be tuned to achieve the required emission levels. The multidimensional combustion models begin to be useful to develop the new combustion system arrangements: in fact the availability of parallel computer architectures , the reduced computational costs and the increased speed up of the modern codes make possible to simulate the combustion with more and more detailed kinetic schemes. The present paper reports the results of numerical simulations obtained with a modified version of the KIVA-3V code and the comparisons of the predicted Soot-NOx trade off with the experimental results. The engine is a single cylinder D.I. prototype equipped with a third generation Common Rail FIE, able to meet the EUROV emission demands. The kinetic model consists of 271 reactions and 59 chemical species. It is a modified version of a mechanism for the N-heptane combustion: in our case we use N-dodecane as primary fuel, adding a set of equations to model the reduction of the primary fuel to heptyl-radicals. Handling detailed chemical kinetics involves dealing with a high number of intermediates: their concentration is very low but they typically exhibit very high creation and destruction rates. Therefore, the model is characterized by a high degree of stiffness, indeed during the simulation the ratio between the minimum and the maximum characteristic times of the species destruction rates reaches the order of about 1015. In this condition the choice of the ODE solver may be very important, not only in terms of the computing time, but also as concerns the accuracy of the solution. More in details, when combustion starts, due to the high stiffness, stability and shape-preserving properties of the numerical methods for solving the ODE systems are crucial. On the other hand, when the combustion reaches the smooth heat-release phase, the efficiency of the ODE solver is the main request. Therefore a multi-method ODE software component, based on different general-purpose ODE solvers, has been implemented and tested, comparing the numerical predictions with the experimental measurements.
Keywords: Combustion, Emissions, Kinetics, Parallel, Computing