Scavenging Flow Measurements in a Fired Two-Stroke Engine by Fiber LDV

The flow characteristics of a small two-stroke engine were investigated. Direct velocity measurements were carried out by a fiber laser Doppler velocimeter (FLDV) developed under the fired conditions at engine speeds of 3000, 4000, and 5000 rpm. The first velocity peak in the firing engine was much higher than that in the motoring engine, and a negative velocity region and a large second velocity peak were observed, which were caused by the back pressure from the exhaust pipe. With increasing engine speed, the spatial velocity distribution varied in the scavenging port due to the reverse flow from the cylinder into the crankcase. The charging flow rate of the fresh air was obtained and compared with that of the motoring case.