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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.

The flow velocity in a scavenging port of a small two-stroke engine was measured directly by a specially developed fiber LDV. The measurement was carried out under motored conditions at engine speeds of 1500 to 5000 rpm, and with throttle-opening ratios of 100, 50, and 20 %. The performance of the FLDV was improved for measuring the scavenging velocity in the backscatter mode. The flow in the scavenging port changed significantly from -11 m/s to 47 m/s with the engine cycle, and the pressure difference between the crankcase and the exhaust pipe provided the typical features of the flow but not the absolute values. The results show that the scavenging flow entered the cylinder just before scavenging port opening (SO) and reached a maximum at the crank angle of 145°, which was constant for all conditions. A second velocity peak was formed by the back pressure from the exhaust pipe. The charging rate of the fresh air into the cylinder was obtained to evaluate the engine performance.

The purpose of this study was to examine the cause of fluctuations in combustion. It is important to understand the changes that occur in flame kernel development and in flame propagation during cyclic variation. In this study, a comparison was made between time-series variations in OH emission with THC concentration, and the intensity of the combustion reaction and the direction of flame propagation are also discussed. Early flame development and cyclic variation at an early stage of combustion were demonstrated by simultaneously measuring a two-dimensional image of flame emission and the time-series variation of local flame emission. The instantaneous intensity at Cassegrain measurement point agreed with the intensity of time-series variation in local flame propagation at CCD recorded timing. Variations in THC concentration in the cylinder were compared with time-series variations in local flame emission.

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 purpose of this study was to detect a misfiring cycle in terms of the transfer-passage and the exhaust-pipe flow rate by experimental measurements. Simultaneous measurements of flow rates and in-cylinder pressure were carried out. The flow rate data were grouped into the different combustion classes by the in-cylinder pressure. A large flow rate of exhaust blow-down and a large reverse flow rate were observed in the cycle before misfiring, compared with in the cycle before firing. It showed that high concentration of the residual burnt gas in the cylinder was the main source of misfiring, this feature was also demonstrated by the complementary measurement of CO and CO2 concentrations.

The purpose of this study is to examine the cause of combustion fluctuation in a partially loaded two-stroke engine with respect to the hydrocarbon (HC) concentration in the cylinder. HC concentration in the cylinder, exhaust gas velocity and pressure were simultaneously measured in order to determine the influence of HC concentration on combustion fluctuation. A correlation between cyclic variation in HC concentration in the cylinder and IMEP was confirmed. The way in which the HC concentration influenced the combustion states in the next cycle made clear. A decrease of HC concentration cause the delay of early flame development and combustion, the decrease of HC concentration had an great influence on the combustion states. The relationship between combustion states and HC concentration was discussed. The relative value of IMEP and HC concentration were closely related to the HC concentration in the cylinder.