Performance test and analysis of lateral swirl combustion system and separated swirl combustion system in diesel engines

A diesel engine has been widely used in ground transportation, agricultural machinery, and sea shipments, due mainly to its excellent reliability and high thermal efficiency. However, the fuel emission and consumption of diesel engines have posed a great threat to the ecological environment in recent years. Therefore, it is very necessary to improve the combustion and emission performance of diesel engines, further to meet the specific requirement of national environmental regulations. Diffusion combustion can generally dominate the combustion process in the direct injection diesel engine. The mixing quality of fuel and air can be expected to serve as the most effective means for the higher performance of diffusion combustion. Different techniques have also emerged to optimize the combustion quality of the fuel/air mixing process in modern diesel engines. For instance, the special geometry structure of the combustion chamber has been adopted to utilize the momentum of the high-pressure fuel jet in the wall-flow-guided combustion system, where the diffusion can be guided in the chamber by the effect of the spray jet hitting the wall. As such, the air utilization rate can be improved for the high combustion and emission performance of a direct injection diesel engine. Both lateral swirl combustion system (LSCS) and separated swirl combustion system (SSCS) have been specially designed for the wall diversion structure in the combustion chamber, where the spray in the cylinder can be guided to form a swirl motion for the high quality of fuel/air mixing process. Nevertheless, the two combustion systems behave in different in-cylinder fuel spray strategies. In this study, a single-cylinder diesel engine was selected to verify the combustion performance of the LSCS and SSCS under different loads and excess air coefficients. AVL Fire software was used to explore the fuel/air mixing mechanism of the combustion system. The test results show that the fuel consumption rate, soot emission, and combustion duration of LSCS under various loads were reduced by 2.4-7.8 g/(kW·h), 2.7-3.9 FSN, and 5.6-10.7 °CA, respectively, compared with the SSCS. Under different excess air coefficients, the fuel consumption rate, soot emission, and duration of combustion were reduced by 3.2-9.8 g/(kW·h), 2.3-3.8 FSN, and 7.9-10.0 °CA, respectively. The maximum drop of LSCS in the fuel consumption and soot emission were 4.3%, and 87.0%, respectively. The simulation results show that the spray jet impinged the convex edge in the LSCS, leading to the wall jets scrolling to form two lateral swirls. The two adjacent spray jets produced the wall jet interference, when flowing out of the convex edge, further improving the air utilization rate. By contrast, there was the fuel aggregation, as the position of the spray jet touched the wall, particularly when the piston was moving down in the SSCS, which was not conducive to the fuel/air mixing process. That is why the air entrainment quantity of the LSCS increased by 9.3%, compared with the SSCS. There was a smaller fuel mass ratio in the interval of equivalent ratio 2-4, whereas, the larger in the interval of equivalent ratio <1-2, indicating the better uniform fuel/air mixing of the LSCS. Therefore, the LSCS can be widely expected to outstandingly improve the fuel/air mixing process, and the combustion performance of the direct injection diesel engine with less soot generation. The finding can provide a technical reference for the structural design and optimization of the combustion chamber in the direct injection diesel engine. © 2022 Chinese Society of Agricultural Engineering. All rights reserved.