Irreversibility analysis of a downsized gasoline engine

The concept of a multi-mode engine is introduced to modern engine designs in order to obtain improved engine performance by having the combined advantages of different engine technologies. Since last decade, Engine Downsizing" was becoming the most well-known technology that was applied to commercial vehicles. It achieves the desired performance with a smaller displacement engine whilst providing improved fuel economy and reduced emissions. In order to further improve the performance of a downsized engine, additional engine technologies, such as direct injection and the Miller cycle are also applied. However, current studies of a downsized engine with Miller cycle operation focus more on the reduction of emissions. The studies of the performance improvement of an engine in terms of its potential utilization are still limited. In addition, a combination of two different technologies (e.g. Miller cycle with engine downsizing) requires a careful pro vs. cons analysis in order to find out a proper trade-off. This analysis is challenging and complicated when more technologies are involved. In order to deal with the energy shortage and low emissions targets, the overall efficiency of a modern engine need to be improved further and this requires a detailed understanding of the engine performance potentials and an innovative engine operation strategy to utilize these potentials. The aim of this project is to determine the improvement potential of a downsized engine by using the entropy generation method. An optimized engine performance strategy is proposed based on the minimum amount of generated entropy This thesis presents the new methodologies to assess the improvement potential of a downsized engine using entropy generation minimisation. It analyses the engine performance in terms of the wasted work potential. Firstly, the entropy generation of various engine cycles is investigated. This is followed by a complete entropy generation model of a turbocharged, downsized engine. The results show that the irreversibility is proportional to the amount of generated entropy, and thus entropy generation is able to address the system performance directly. In terms of an internal combustion engine, the main causes of entropy generation are due to heat transfer from the gas to the coolant, combustion, exhaust flow, and friction. Results show that the boost pressure ratio will reach a limit where further increase beyond this limitation will lead to an increase in entropy generation, and thus worsen the engine performance, eventually. Further analysis shows that this increase in entropy generation is caused by the heat transfer from the gas to the coolant. Finally, relevant analyses show that entropy generation due to combustion and exhaust flow are reduced with further downsizing, although those two factors still contribute to a large percentage of the overall entropy generation. As a result, the causes of the wasted fuel energy potential are in the following order, from strong to weak: combustion, heat transfer from the gas to the coolant, exhaust flow, and friction.