分会场

控制与测试

摘要

Compared to the 4-stroke engines, 2-stroke engines have the advantage of higher power density due to their doubled firing frequency. In order to improve the power output in the traditional 4-stroke diesel engines, running in 2-stroke operation was proposed. In this study 2-stroke operation in a single cylinder poppet valve diesel engine was investigated. In the meanwhile, 2-stroke scavenging process with the traditional poppet valves was analyzed. The 1-D and 3-D CFD simulation were carried out to improve the scavenging efficiency and trapping ratio of 2-stroke operation. The effects of valve timings and operating parameters, such as EVO, intake pressure, and so on, on 2-stroke operation were studied. In addition, an online scavenging efficiency measurement based on CO2 concentration variations in the cylinder was demonstrated. According to its cycle-resolved measurement results, the simulation models were verified. The results show that by advancing the timing of the exhaust valve open, the cylinder pressure during intake process was reduced, and thus less back flow to the intake port led to the increase in scavenging efficiency and trapping ratio. The increase in the exhaust valve opening duration not only effectively improved the scavenging process, but also caused the increase in short-circuiting and thus the reduction in trapping ratio due to the longer valve opening overlap. The effects of exhaust valve lift were quite similar to that of the exhaust valve duration. Finally, with the original 4-stroke engine intake/exhaust port structures, by optimizing the scavenging system, the air flow rate was reduced by 27.1% at the engine rated operating condition (120kW @ 3000rpm), while the scavenging efficiency increased by 0.8-2% and trapping ratio increased by 2.6-3.6%.

关键词

气阀式二冲程;扫气效率;捕获率;计算流体仿真

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