Abstract
To develop higher energy-efficiency and lower emission internal combustion (IC) engines, it is important to understand heat loss characteristics and flame-wall interactions since heat transfer through the wall during combustion greatly affects the energy-efficiency and combustion products. Heat loss from the wall depends significantly on velocity boundary layer of IC engine, which has not been fully understood yet. In this study, a micro particle image velocimetry (PIV) has been performed to investigate tumble enhanced flow characteristics near piston top surface of a motored IC engine for three inlet valve timing (-30, -15, 0 crank angle degrees (CAD)). PIV was conducted at 340, 350 and 360 CAD of the end of the compression stroke at the constant motored speed of 2000 rpm. The measurement region is 3.2 mm x 1.5 mm on the piston top including central axis of the cylinder. The spatial resolution of PIV defined by the interrogation region is about 75 micrometers and the vector spacing is about 37.5 micrometers. The first velocity vector is located about 60 micrometers from the piston top surface. The high spatial resolution PIV revealed that the mean flow near the piston top is not close to the turbulent boundary layer, and rather has tendency of the Blasius theorem, whereas turbulent intensity near the wall is not low. This tendency is considered to be possible from the viewpoints of the shortage of flow time and the laminarization due to the adverse pressure gradient through the compression stroke. This result shows that revision of a wall heat transfer model based on an assumption of the proper characteristics of flow field near the piston top is required for more accurate prediction of heat flux in gasoline engines.
