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The quest for high efficient internal combustion engines has intensified in the last years due to, among other reasons, increasing fuel costs and the pressure to reduce environmental deterioration. One of the possible alternatives capable of providing the sought efficiency gains is the recovery of the energy wasted in the exhaust gases and turbo-compounding is one obvious option. Compression ignition engines are usually the target of turbo-compounding, however, the rising interest for alternative fuels could result in the use of turbo-compounding for spark ignition engines as well. Due to its different flame propagation mechanism, spark ignition engines may force operation at higher fuel-air ratios, eventually creating a quite distinctive operational behavior.

The main challenge of the internal combustion engine industry is strongly related to the development of products that satisfy both the current and future legislation standards for pollutant emission and the increasing market demand for high efficiency energy solutions [1, 2]. In this sense, the usage of fuels such as natural gas and ethanol along with the development of high-efficiency combustion systems represent an important option to meet those requirements. In this study, three-dimensional numerical calculations were done considering a regular piston geometry and the AVL tri-flow® system. Three levels of swirl were generated during the intake stroke aiming to evaluate its influence over the flow features at spark timing, comparing the initial flame kernel development and flame front propagation.

A computational fluid dynamics (CFD) analysis of the M366LAG in-cylinder flow was performed to explain what are the reasons for the measured engine slower combustion which results from the replacement of the intake valve seat angle of 45° by 20°. Simulations showed that a quite different in-cylinder flow resulted from the different designs. The 20° geometry was found to generate a weaker turbulent field at the end of the compression stroke, specially around spark plug, which was identified as the cause of the slower combustion. The weaker turbulence is a result of a less coherent flow structure which is less capable of both storing the main flow kinetic energy until the compression stroke end and of taking advantage of squish to generate turbulence.

Despite a real progress since their introduction in early sixties by NSU, Wankel engines are still lagging behind their equivalent reciprocating counterparts as far as fuel economy and emissions are concerned. Fuel stratification during combustion promises further progresses, but a similar move is expected for reciprocating units. A type of stratified combustion system, which was called hybrid fuel energy conversion system, is outlined in the text. One special characteristic of such a system is the dilution of the already burned gases by the air which does not participate in combustion, when engine is operating at partial loads. The hybrid system features a deep recess in the rotor housing which has the function of a prechamber. The resultant geometry produces a significant recirculation of unburned gases, late in the expansion process of the leading chamber to the trailing chamber performing compression, with a major impact on engine thermodynamic cycle.