Thermal boundary layer thickness in the cylinder of a spark-ignition engine

Abstract

The thicknesses of the thermal boundary layers in the cylinder of a spark-ignition engine were measured throughout the complete operating cycle. The engine used for these experiments was a special visualization engine with a square cross-section ‘cylinder’ and two quartz glass walls. Measurements of thickness were taken at different locations, speeds and loads, from schlieren photographs. The results show that the layers on the cylinder wall reach a maximum thickness of about 2 mm at the end of expansion. On the cylinder head and piston top, the layers are two to three times thicker. Reducing engine speed resulted in thicker layers, while changing load showed no significant effects on thickness. It is shown that on the cylinder wall, the thermal boundary layer thickness depends on thermal diffusivity and the time available for the layer to develop. The growth of this layer was correlated with an expression in the form δ T √( αt) ∞ Re 0.2.