Particle Image Velocimetry (PIV) technique was used to study the in-cylinder flow motion in a single cylinder transparent spark ignition (SI) engine, with 4 valves pent-roof cylinder-head. Experiments have been firstly performed using classical PIV technique in the symmetrical plane of the combustion chamber. The instantaneous two-dimensional velocity fields show that during intake stroke, the flow has strong relatively stable structures with low cycle by cycle (CBC) variations, whereas during compression stroke, the flow consists of large unstable eddies with high CBC variations. In all cases, no large tumble-like motion was observed on instantaneous fields in the measurement plane during the intake stroke.
Therefore, Stereoscopic Particle Image Velocimetry (SPIV) technique was adapted to complete the classical two-dimensional measurements and in order to have a better understanding of the in-cylinder flow during compression stroke. The measurement system uses two double frames CCD cameras with the translation method for imaging the airflow seeding particles. A software coincidence of the images of the two cameras was realized, and a special algorithm was developed for the third component calculation.
The results of SPIV measurements show that during compression stroke, the in-cylinder flow has a fully three-dimensional unstable behavior, with high CBC variations. Moreover, the smooth and quiet mean flow resulting of ensemble averaged of 30 to 70 cycles is not representative of high turbulent instantaneous flow.
PIV and SPIV techniques have been shown to be useful tools to study and to have a good understanding of the real instantaneous aerodynamic in SI engines, that is of most importance to improve the mixture formation and the combustion stability, especially in new gasoline direct injection engines.