Abstract
Keywords Simulation, Model, Emissions, Pollutants, Fluid-dynamic
Abstract The paper reports some typical results of an experimental and simulation work recently carried out by the authors, using the GASDYN code to study the new configuration of a Fiat Fire 1.4L automotive s.i. engine. The complete system formed by the engine together with its intake and exhaust groups has been modeled to predict its main performance parameters not only in steady state conditions, but also during the first transient part of the ECE cycle. Meanwhile the same quantities were measured in a test room to check the predictive ability of the code.
The satisfactory agreement found between predictions and measurements proved that, by coupling suitable kinetic schemes for the formation of pollutants to a multi-zone fractal combustion model, it has been possible to improve the code accuracy in predicting the gas composition at the exhaust valves. Moreover the integration of the complete combustion model with the 1D fluid dynamic code for the calculation of the unsteady reacting flows, has allowed a comprehensive description of the whole engine system, accounting for the transport of chemical species with reactions. The model allows to follow in detail the mass flows through the valves and into the inlet and exhaust ducts, giving an important help in optimizing the effective valve timings. Finally it is possible to predict the total mass fraction of burned gases, trapped into the cylinder from the previous cycle, that is the internal EGR. The burned gases dilute the new cylinder charge, reducing the combustion rate and the exhaust temperature, lowering the NOx emission levels.
The model has been used to simulate the behavior of the complete engine system during the first part (first 90 seconds) of the new European driving cycle. A study of the emission production related to the successive engine operating points (in transient conditions) and of the consequent downstream action of the catalytic converter has been carried out, to predict the effects of both engine parameters and catalyst geometry and position on the conversion efficiency. In order to simulate the ECE driving cycle, by coupling the engine to the vehicle, a simple one degree of freedom model has been developed, to evaluate the torque required by the vehicle to follow the prescribed velocity trend. A closed-loop control algorithm has been introduced in the simulation model to control the throttle valve opening angle and regulate the engine torque, in order to match the required vehicle torque. A simplified modeling of the engine thermal transient from cold start has been considered, since the temperature of piston, cylinder head, cylinder liner, coolant have a significant influence on the production of unburned HC, originated from the crevices and the oil film layer. Similarly, possible partial burn and misfire during the combustion process of the first engine cycles have been taken into account. The variation of A/F ratio, spark timing, valve timing of the ECE driving cycle have been considered as input data, assigned on the basis of the known engine operating points.