Comparison of Motored and Fired Velocities in a Two Stroke Engine 982012
Measurements were made of the in-cylinder fluid velocities in a crankcase compression, piston ported two-stroke engine under both motored and fired operating conditions to investigate the effect of combustion on the scavenging process. The engine was modified to allow optical access to the clearance volume and was operated in a burst fired manner, where the firing sequence was controlled by modulating the fuel delivered to the crankcase. The burst fired sequence consisted of 30 cycles, of which there were twelve consecutive cycles where fuel was injected into the crankcase. The engine was operated at a speed of 500 RPM, with a delivery ratio of 0.54, and a fuel-air equivalence ratio of 1.28. Laser Doppler Velocimetry was used to measure two components of the in-cylinder fluid velocity at five locations in the cylinder head cup during the burst fired operation of the engine. These measurements were sorted into groups representative of the motored and fired operation of the engine based on the cycle number within the burst fired sequence in which the measurements were taken. The groupings were based on an analysis of cylinder, crankcase, and exahust port pressure measurements made while the engine was being operated in the burst fired mode.
The velocity measurements show a significant increase in the peak velocity of the scavenging flows under fired conditions when compared to the scavenging flows under motored conditions. These results indicate that under motored conditions the backflow from the exhaust system into the cylinder at the time of exhaust port opening retards the incoming jets from the intake ports. Under firing conditions, the flow out of the cylinder does not impede the incoming fresh charge, and the momentum of this outflow may help draw the fresh charge into the cylinder. A comparison of the RMS velocities under both motored and fired conditions indicated that for the period between bottom dead center and ignition both components of the RMS velocities were equal. This result indicates that the main effect of firing on the scavenge flow was to induce mean flow patterns which purge the burned gases, as opposed to enhancing in-cylinder mixing.