Flow Vector Measurements at the Scavenging Ports in a Fired Two-Stroke Engine

The flow vector variations at the transfer port exit in a small two-stroke engine under firing condition were investigated experimentally. A fiber LDV system was used to measure the two-dimensional velocities near the cylinder to obtain the scavenging flow vector. The scavenging flow vector variations at different engine speeds were discussed, and the relation between its vector behavior and the pressure differences between the exhaust pipe and the crankcase was examined. The measurement results show that the velocity profiles at the scavenging port were not uniform and to obtain the representative velocity at the port exit was impossible. But the major features of the scavenging flow can be understood from the pressure difference between the exhaust pipe and the crankcase. The start timing of the scavenging flow was delayed due to the residual gas and high pressure in the cylinder when the scavenging port was opened. The vector had almost constant flowing angle and reached first peak, and the vector rotated due to the reverse flow from the cylinder to the crankcase which caused the negative peak. The second velocity peak was caused by the pressure wave reflection from the exhaust pipe. Finally, the gas leakage between the piston pin and cylinder was found.