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In order to realize the high compression ratio and high dilution combustion toward improvement in thermal efficiency, the improvement in stability of ignition and initial phase of combustion under the high gas flow field is the major challenge. In terms of the shift on the higher power side of the operating point by downsizing and improvement of real world fuel consumption, the improvement of ignitability is increasingly expected in the wide operating range also including high load and high engine speed region. In this study, the effects of the gas pressure, gas flow velocity near the spark gap at ignition timing, and discharge current characteristics on spark channel formation were analyzed, focusing on restrike event and spark channel stretching in the spark channel formation process. And the relationship between the average discharge current until 1 ms and the EGR combustion limit was considered.

This study examined a method of predicting the piston temperature in reciprocating internal combustion engines with the aim of developing lightweight pistons. Since the piston temperature is strongly affected by the in-cylinder temperature distribution and turbulence, it is necessary to consider the effects of flame propagation, cooling by the intake air, temperature rise due to combustion, in-cylinder flow and the combustion chamber shape. A three-dimensional combustion simulation that can take these effects into consideration was run to calculate the heat transfer coefficient from the piston crown surface and the gas temperature. The results were used as the boundary conditions for an analysis of heat transfer from the piston, and a method was thus developed for analyzing the piston temperature.

The aim of this study is to gain a better understanding of the mechanism of knocking with respect to flame propagation behavior based on 3D simulations conducted with the Universal Coherent Flamelet Model. Flame propagation behavior under the influence of in-cylinder flow was analyzed on the basis of the calculated results and experimental visualizations. Tumble and swirl flows were produced in the cylinder by inserting various baffle plates in the middle of the intake port. A comparison of the measured and calculated flame propagation behavior showed good agreement for various in-cylinder flow conditions. The results indicate that in-cylinder flow conditions vary the flame propagation shape from the initial combustion period and strongly influence the occurrence of knocking.

Three-dimensional combustion simulation tools together with the Universal Coherent Flamelet Model (UCFM), a flame propagation model, have been applied to SI lean-burn combustion to study the influence of the equivalence ratio on the amount of unburned hydrocarbons (HCs). Unburned HCs from piston-cylinder crevices were taken into the consideration by using a calculation grid incorporating the actual crevice volume and shape and by applying an autoignition model to post-flame phenomena. The calculation results show the general tendencies for the total amount of unburned HCs and their distribution in the combustion chamber.

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.

Combustion in engines involves very complicated phenomena (including flame propagation and knocking), which are strongly affected by engine speed, load and turbulence intensity in the combustion chamber. The aim of this study was to develop a flame propagation model and a knocking prediction technique applicable to various engine operating conditions, including engine speed and in-cylinder turbulence intensity. A flame propagation model (UCFM) has been developed that improved the Coherent Flamelet Model by considering flame growth both in terms of the turbulent flame kernel and laminar flame kernel. A knocking prediction model was developed by implementing the Livengood-Wu integral as the autoignition model. The combined model allows evaluation of both where and when autoignition occurs in a real shape combustion chamber. A comparison of the measured and calculated time for the occurrence of knocking shows good agreement for various operating conditions.

Mechanism of suddenly occurring behavior of low speed pre-ignition (LSPI) in boosted spark ignition (SI) engines was analyzed with various experimental methodologies. Endoscope-visualized 1st cycle of LSPI showed droplet-like luminous flame kernels as the origin of flame propagation before spark ignition. With the oil lubricated visualization engine, droplets flying were observed only after enough accumulation of fuel at piston crevice. Also, it was confirmed that subsequent cycles of LSPI occur only after enough operation time. These results indicated that local accumulation of liner adhered fuel and saturation of oil dilution can be a contributing factor to the sudden occurrence of LSPI.