Modeling of Biofuelled HCCI Engines with a Parallel Multizone Model

With growing concerns over emissions, homogeneous charge compression ignition (HCCI) engines offer a promising solution through reducing NOx and particulate emissions and increasing efficiency. However, this technology is not without its challenges and numerical modeling of these engines can offer some insight into addressing these challenges. This study uses domain decomposition with FORTRAN MPI to subdivide computationally intensive sections of a 10 zone simulation model. Using an Intel i7 quadcore workstation the parallelized model reduced runtimes by half compared to serial computations. From here, two sets of biofuel experimental data were used to improve the validation base of the model. The fuels used were a simulated biomass derived gas (consisting of H2, CH4, CO, CO2, and N2) and a butanol/n-heptane blend. Once calibrated, the model showed good pressure, heat release, and products of incomplete combustion prediction for biogas. NOx emissions were high, however the overall trend was captured. Similarly, once calibrated to the butanol/n-heptane data to account for some of the effects of negative valve overlap (NVO), excellent pressure and heat release predictions were obtained. However, products of incomplete combustion and NOx were low and this was attributed to the inability of the model to properly account for inhomogeneity and all the effects of NVO. Once again though, the overall trend in NOx levels was captured by the model. It was also found that the model does not operate very well near the misfire limit of the engine as it cannot capture the cyclic variability that can occur here. Based on the two new validation cases, it is concluded that once calibrated, the model can be used as a predictive tool for pressure, heat release, and combustion phasing of biofuelled HCCI engines. Furthermore, to improve its predictive capabilities, it is recommended that the model be restructured to incorporate mass transfer between zones, a fixed crevice volume and variable thermal boundary layer, and a CFD solver to improve emissions predictions and reduce reliance on calibration. Finally, changing the zone distribution from ring like zones to lumped stirred reactors is recommended to allow for more realistic modeling of actual experimental HCCI conditions

An adapted heat transfer model for engines with tumble motion

In the last years, a growing interest about increasing the engine efficiency has led to the development of new engine technologies. The accurate determination of the heat transfer across the combustion chamber walls is highly relevant to perform a valid thermal balance while evaluating the potential of new engine concepts. Several works dealing with heat transfer correlations that consider the swirl motion are found in the literature; however, there is a lack of works dealing with heat transfer correlations which take into account the effect of the tumble movement. In this work, a new heat transfer model accounting for the tumble motion is presented. A two stroke HSDI Diesel engine with high tumble and no swirl is used to perform the theoretical study, the model development and its final calibration. Initially, a theoretical analysis of the gas movement phenomena is carried out based on CFD results and then, a model is developed and calibrated based on a skip-fire testing technique. Finally, a sensitivity study focused on evaluating the model robustness is performed. The results confirm an average RMSE reduction of 70% with respect to the Woschni model, being this consistent improvement qualitatively evidenced in the instantaneous heat transfer evolutionThe support of the Spanish Ministry of Economy and Competitiveness (TRA2013-41348-R) is greatly acknowledged.Olmeda González, PC.; Martín Díaz, J.; Novella Rosa, R.; Carreño, R. (2015). An adapted heat transfer model for engines with tumble motion. Applied Energy. 158:190-202. https://doi.org/10.1016/j.apenergy.2015.08.051S19020215