Measurement of gas velocity in a combustion chamber of a small-sized, two-stroke S.I. engine by LDA. 1st Report. Make up of a fiber-optic velocity pick-up and measurement of bulk flow velocities.

Abstract

To improve the adaptability of LDA (laser Doppler anemometer) for measuring velocities in an engine, an optical fiber system composed of a couple of single mode, polarization-preserving optical fibers has been made to transmit the incident beams. Though the total transmitting efficiency of this optical system is about 24 %, it has been confirmed that a fiber-optic LDA system with a 25 mW He-Ne laser is applicable to the real-time measurement of gas velocity in the combustion chamber of a small-sized two-stroke spark ignition engine. The ensemble-averaged mean velocities obtained by using this optical system show that the gas velocity rapidly decreases just before the flame front arrival and suddenly increases after passing of the flame. It is also shown that a new bulk flow is generated in the chamber by the gas expansion due to combustion.