Combustion Bowl Heat Transfer Analysis in Diesel and PPC Engines
Abstract: This thesis concerns a numerical investigation on heat transfer in internal combustion engines, with the aim of increasing engine efficiency. The efficiency gains are to be extracted from reduced heat transfer losses, by increasing the knowledge on how the heat transfer process is affected by various hardware and operational parameters in the engine. The engines concerned are both conventional diesel engines and engines operated in partially premixed combustion mode. In the thesis, heat transfer results for these two engine modes are compared and discussed. In addition, evaluation of the engine performance and emissions is done in order to confirm that reduced heat losses contribute to increased efficiency. The numerical investigation is based upon three-dimensional computational fluid dynamics simulations using RANS based models, where transport equations for turbulent reacting flows inside the engine cylinder are solved. The engine is represented by an engine segment model, where a single spray enclosure for the closed volume part of the cycle is simulated. This provides information on the compression, combustion and expansion part of the cycle, where the interaction between combustion and heat transfer are studied for the two combustion modes. The results showed that heat transfer can be affected by both operational and geometrical parameters, while the results for emission values differed between the engine concepts. Changing the piston geometry in the engine lead to changes in the engine flow-field and, thereby, the amount of heat transfered from the engine cylinder. Another efficient tool for affecting engine heat transfer was the manipulation of the injection strategy. A study showed that with a favorable injection strategy, the high-temperature in-cylinder gases could be stratified so the engine walls would be sheltered from the high-temperature gases. This lead to a reduced temperature gradient in the near-wall region and reduced heat transfer. Other parameters that had an effect on engine heat transfer were inlet pressure and temperature values. These were optimized for optimal trade-off between engine heat transfer, engine performance and emission levels. Comparing the conventional diesel combustion concept and the partially premixed combustion concept, while moving towards adiabatic conditions, revealed that even though engine performance was improved for both combustion concepts, emission levels were quite different. Lower temperature during combustion in the PPC mode resulted in a more modest increase in emission levels, while conditions were moved towards adiabatic wall conditions.
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