System Simulation of Combustion in Direct-Injection Spark-Ignition Engines

Future constraints on pollutant emissions pushed car manufacturers towards Direct-Injection Spark-Ignition (DI-SI) technologies to improve engine performances and reduce both fuel consumption and emissions. New challenges are then introduced in terms of combustion optimization, due to a more complex phenomenology and a larger number of degrees of freedom, while system models require developments to approach such new engine architectures with the sufficient level of detail.

A combustion model that works toward a more comprehensive approach

In order to achieve the latter objective, a PhD work performed at IFPEN^[1] hosted the development and validation of a Zero-Dimensional (0D) model of DI-SI combustion for system simulation. The proposed model focuses on physics of atomization and drop evaporation, fuel/air mixing, flame propagation in heterogeneous charge and mutual interaction between these phenomena, Figure 1.
 

In particular, it is characterized by the following aspects:
  

• The liquid phase is discretized in parcels grouping drops of the same size. An empirical atomization model based on injection velocity, fuel characteristics and thermodynamic conditions provides initial diameters. A Lagrangian model including drag-inertia dynamics, heat-up and forced convection describes drop parcel penetration and evaporation.
     
• Fuel / air mixing is described using a discrete Probability Density Function (PDF) approach, based on constant-mixture-fraction classes interacting with each other and with the drop parcels^[2].
     
• Flame propagation takes into account mixture heterogeneity effects on flame speed and pollutant production is modeled.

A validation method based on experimental results and simulation tests

The approach was validated against experimental results, when available, and 3D CFD*  RANS** simulations^[3], Figure 2 and Figure 3.

*      CFD  = Computational Fluid Dynamics
**    RANS  = Reynolds-Averaged Navier–Stokes
***  270 CAD =  Crank Angle Degree 
**** BTDC =   Before Top Dead Center

The implemented model already opens up multiple perspectives

The model was implemented in the Simcenter Amesim platform for multi-physical modelling of the Siemens Digital Industries Software and integrated in the CFM Spark Ignition combustion chamber submodel of the IFP-Engine library.

This PhD work opens various perspectives for future works:
  

• The discrete PDF, only applied so far to the fresh mixture, could be employed to describe the mixing-controlled post-oxidation reactions in the exhaust gas, thus improving the gaseous pollutant emissions prediction capability of the model;
    
• The parcel-based liquid phase model could be used to predict the mass of liquid fuel colliding with the cylinder walls, information that can be used to develop a liquid film model;
    
• Predicting mass and drops size of liquid fuel present in the cylinder during combustion is also highly relevant in the context of soot formation modelling.
   

Scientific Contact: Alessio Dulbecco

Publications

^[1] F. Pellegrino, System Simulation of Combustion in Direct-Injection Spark-Ignition Engines, CentraleSupelec PhD thesis, 2019.
  
^[2] F. Pellegrino, A. Dulbecco, D. Veynante, Development of a Quasi-Dimensional Spray Evaporation and Mixture Formation Model for Direct-Injection Spark-Ignition Engines, SAE Technical paper, 2015
>> DOI: 10.4271/2015-24-2471
   
^[3] F. Pellegrino, A. Dulbecco, D. Veynante, Development and validation of a quasi-dimensional spray model for DI-SI engines, Thiesel conference on Thermo-and-Fluid Dynamic Processes in Direct Injection Engines, poster session, 2018.