Effects of Intake Port Structures and Valve Timings on the Scavenging Process in a Two-Stroke Poppet Valve Diesel Engine

The two-stroke operation is one of the most effective approaches to significantly increase the torque and power of a 4-stroke engine without the necessary requirement of intensifying the engine. Scavenging process is one of the key factors determining the performance of the two-stroke engine. In this work, a structure of top entry intake ports with poppet valves was employed on a 2-stroke single cylinder diesel engine with the conventional horizontal intake ports replaced. By this way, the reversed tumble flows in the cylinder were formed during the intake process to improve the scavenging performance of 2-stroke operation. In the meanwhile, the effects of valve timings and intake port structures on scavenging processes were estimated respectively through the1D and 3D simulation of the gas exchange process.

Results show that compared to the conventional horizontal intake port case, the reversed tumble flow created by the top-entry intake port led to a lower air short-circuiting rate and a higher scavenging efficiency. Furthermore, by advancing the exhaust valve opening the exhaust gas was discharged more sufficiently and the intake backflow was significantly reduced. Therefore, the charging efficiency and scavenging efficiency were improved with the reversed tumble. It was also found that with the constant valve timings by implementing the reversed tumble intake ports structure, the scavenging efficiency was increased from74% to 86%, and the trapping efficiency from 72% to 76%.