The 4th Workshop of the Engine Combustion Network (ECN) was held September 5-6, 2015 in Kyoto, Japan. This manuscript presents a summary of the progress in experiments and modeling among ECN contributors leading to a better understanding of soot formation under the ECN “Spray A” configuration and some parametric variants. Relevant published and unpublished work from prior ECN workshops is reviewed. Experiments measuring soot particle size and morphology, soot volume fraction (fv), and transient soot mass have been conducted at various international institutions providing target data for improvements to computational models. Multiple modeling contributions using both the Reynolds Averaged Navier-Stokes (RANS) Equations approach and the Large-Eddy Simulation (LES) approach have been submitted. Among these, various chemical mechanisms, soot models, and turbulence-chemistry interaction (TCI) methodologies have been considered.

Nowadays 1D fluid dynamic models are widely used by engine designers, since they can give sufficiently accurate predictions in short times, allowing to support the optimization and development work of any prototype. According to the last requirements in terms of pollutant emission control, some enhancements have been introduced in the 1D code GASDYN, to improve its ability in predicting the composition of the exhaust gas discharged by the cylinders and the transport of the chemical species along the exhaust system. The main aspects of the methods adopted to model the combustion process and the related formation of pollutants are described in the paper. To account for the burnt gas stratification, two different approaches have been proposed, depending on the expected turbulence levels inside the combustion chamber. The reliability of the simulation of the pollutant formation process has been enhanced by the integration of the thermodynamic module with the Chemkin code. The so-called super-extended Zeldovich mechanism has been introduced to perform a more detailed description of the NOx production process. Separate modules describe CO and HC formation, considering individually the crevices, the oil film layer and the partial burn contributions. The comparison between predictions and experimental measurements carried out on a new configuration of a Fiat-Alfa Romeo four-cylinder, 2.0 L automotive s.i. engine, has pointed out a satisfactory agreement in terms of pressure trends inside the cylinder and gas composition at different locations in the exhaust system.

A correct prediction of the initial stages of the combustion process in SI engines is of great importance to understand how local flow conditions, fuel properties, mixture stratification and ignition affect the in-cylinder pressure development and pollutant formation. However, flame kernel growth is governed by many interacting processes including energy transfer from the electrical circuit to the gas phase, interaction between the plasma channel and the flow field, transition between different combustion regimes and gas expansion at very high temperatures.In this work, the authors intend to present a comprehensive, multi-dimensional model that can be used to predict the initial combustion stages in SI engines. In particular, the spark channel is represented by a set of Lagrangian particles where each one of them acts as a single flame kernel. Each particle is convected by the gas flow and its growth is governed by flame speed and thermal expansion due to the energy transfer from the electrical circuit. From particle positions and size it is then possible to reconstruct the flame surface density distribution, that is then used by the gas phase to compute the fuel reaction rate. A simplified model for the secondary electrical circuit was applied to estimate the amount of energy transferred as function of the circuit properties (equivalent resistance and inductance), discharge energy and time. To compute the flame kernel expansion velocity, the heat conduction equation was solved accounting for real gas properties in the 5000 - 50000 K temperature range. All the effects of the flame kernel growth are grouped into a single source term, that is added to the flame surface density transport equation, solved following the Extended Coherent Flamelet Model (ECFM).The proposed model has been extensively validated with experimental data provided by Herweg et al., illustrated in [1, 2]. A computational mesh reproducing the geometrical details of the optical, pre-chamber SI engine was built, including the electrodes. Initially, cold-flow simulations were carried out to verify the validity of the computed flow-field and turbulent distribution at ignition time. Then, the combustion process was simulated accounting for the effects of different engine speeds, air/fuel ratio, ignition systems and spark-plug position. Validation was performed by comparing computed and experimental evolution of the burned gas volume. Encouraging results were achieved for a wide range of operating conditions.