SeungHwan Keum

An extended multi-zone combustion model for PCI simulation

Novel combustion modes are becoming an important area of research with emission regulations more stringent than ever before, and with fuel economy being assigned greater importance every day. Homogeneous Charge Compression Ignition (HCCI) and Premixed Compression Ignition (PCI) modes in particular promise better fuel economy and lower emissions in internal combustion engines. Multi-zone combustion models have been popular in modelling HCCI combustion. In this work, an improved multi-zone model is suggested for PCI combustion modelling. A new zoning scheme is suggested based on incorporating the internal energy of formation into an earlier conventional HCCI multi-zone approach, which considers a two-dimensional reaction space defined by equivalence ratio and temperature. It is shown that the added dimension improves zoning by creating more representative zones, and thus reducing errors compared to the conventional zoning approach, when applied to PCI simulation.

Assessment of flamelet versus multi-zone combustion modeling approaches for stratified-charge compression ignition engines

The spray-interactive flamelet and extended multi-zone combustion models coupled with multi-dimensional computational fluid dynamics are applied to investigate the effects of charge stratification in a direct-injection compression ignition engine under low load conditions. A parametric study was carried out in order to compare the two approaches for early and late fuel injection timings. Comparison of numerical results with available experimental data shows that for early fuel injection, both models predict the auto-ignition and combustion characteristics with comparable fidelity. As the fuel injection timing is delayed, however, the spray-interactive flamelet model is found to capture the onset of combustion and subsequent heat release with greater accuracy. Further investigation reveals that the better performance of the spray-interactive flamelet model over a wider range of mixture-stratified conditions is mainly attributed to its ability to capture the diffusive transport resulting from small-scale mixing and turbulence–chemistry interaction, which becomes more important when significant mixture inhomogeneities exist in the engine cylinder.

Effects of fuel injection parameters on the performance of homogeneous charge compression ignition at low-load conditions

With the objective of enhancing the effectiveness of late fuel injection strategy in extending the low-load limit of homogeneous charge compression ignition engines, a numerical study is conducted to investigate the effects of fuel injection parameters, such as the injection pressure and spray cone angle, on the overall combustion efficiency and CO/NOx emissions. Closed-cycle engine simulations are performed incorporating detailed iso-octane reaction kinetics and combustion submodel based on the spray-interactive flamelet approach. Extensive parametric studies are conducted to provide a detailed map of the combustion efficiency and emission performance. In general, it is found that the in-cylinder charge stratification can be reduced by both an increased injection pressure and a wider spray cone angle, resulting in substantially lower NOx emissions and reasonably high combustion efficiency simultaneously. The present study demonstrates that an optimal adjustment of the two fuel injection parameters can result in significant extension of the low-load limit of homogeneous charge compression ignition through delayed fuel injection strategy.

Modelling of heat transfer in internal combustion engines with variable density effect

Heat transfer is one of the major factors affecting the performance, efficiency, and emissions of internal combustion engines. As convection heat transfer is dominant in engine heat transfer, accurate modelling of the boundary layer heat transfer is required. In engine computational fluid dynamics (CFD) simulations, the wall function approach has been widely used to model the near-wall flow and temperature field. The present paper suggests a modified wall function approach to model heat transfer in internal combustion engines. Special emphasis has been placed on introducing the effect of variable density and variable viscosity in the model formulation. A non-dimensional temperature corrector is suggested to incorporate the variable density effect on the wall function approach. The suggested model is applied in KIVA-3V and is validated against experimental data of a homogeneous charge compression-ignition engine, showing improved predictions for pressure and emissions compared with the standard wall function model.

A Spray-Interactive Flamelet Model for Direct Injection Engine Combustion

Toward higher efficiency and lower emissions, modern direct injection (DI) engines employ various injection strategies. This leads to more complex in-cylinder spray evaporation and combustion processes, requiring more comprehensive modeling approaches. In this study, an extended flamelet model is developed to describe DI engine combustion over a wide range of injection timings. A key feature of the model is to fully incorporate the interaction between spray evaporation and gas-phase combustion. Additional source terms representing the effect of evaporation were incorporated in the flamelet equation solved in the reactive space. A simple test problem demonstrated that the new formulation successfully accounts for the history of the spray evaporation. The extended formulation was implemented into a multidimensional computational fluid dynamics (CFD) code KIVA3v for full cycle engine simulation. The modeling results were successfully validated against available experimental data obtained from a rapid compression facility.

Modeling of Scalar Dissipation Rates in Flamelet Models for HCCI Engine Simulation

The flamelet approach is considered a viable framework to the modeling of homogeneous charge compression ignition (HCCI) engines under stratified mixture conditions. However, there are several issues that need further improvement. In particular, accurate representation of the scalar dissipation rate, which is the key parameter to connect the physical mixing space to the reactive space, requires further investigation. This involves a number of aspects: (i) probability density functions, (ii) mean scalar dissipation rates, and (iii) conditional scalar dissipation rates, for mixture fraction (Z) and total enthalpy (H). The present study aims to assess the validity of existing models in HCCI environments both in the RANS and LES contexts, and thereby suggest alternative models to improve on the above three aspects.

Comparison of Experimental and Numerical Modeling of Reforming HCCI Combustion

Homogeneous charge compression ignition (HCCI) engines have high potential to provide better fuel economy with low emissions than conventional spark ignition (SI) engines. In an HCCI engine, combustion phasing strongly depends on the initial temperature and composition. Negative valve overlap (NVO) with reforming has been investigated as combustion phasing control strategy. However, the reforming process is not yet fully understood and further research is necessary to fully utilize the NVO reforming strategy. In this research, optically measured reforming process was modeled by 3D CFD simulation and the results were compared to understand the reforming process better. The optical measurement was carried out with sodium additive to enhance the combustion luminosity. Numerical simulation was carried out with state-of-art spray model with chemical kinetics for ignition and combustion. The chemical reaction mechanism was optimized for modeling the reforming process. It was found that the luminosity from the optical measurement correlates well with the chemical reaction source terms from the simulation.Copyright