In-Cylinder Heat Transfer Determination Using Impulse Response Method with a Two-Dimensional Characterization of the Eroding Surface Thermocouple

Abstract

Heat transfer from the cylinder of internal combustion engines has been studied for decades, both in motored and fired configurations. Its understanding remains fundamental to the optimization of engine structures and sub-systems due to its direct effect on reliability, thermal efficiency and gaseous emissions. Experimental measurements are usually conducted using fast response surface thermometers, which give the instantaneous cylinder surface temperature. The transient component of heat flux through the cylinder wall was traditionally obtained from a spectral analysis of the surface temperature fluctuation, whereas the steady-state component was obtained from Fourier's law of conduction. This computation inherently assumes that heat flows in one-dimension, perpendicular to the heated surface in a semi-infinite solid with constant thermo-physical properties. Results from this computation are prone to significant uncertainties originating from numerous sources, most of which related to the limitations in the technology of surface thermometry and the method used to convert the surface temperature to heat flux. In this study, a single cylinder version of a 2.0 liter engine operating in the pressurized motored configuration was instrumented at two locations in the cylinder head with surface thermocouples of the eroding type, one fitted along the cylinder central axis and another in the squish region. Three different eroding surface thermocouples were tested on the experimental setup, at the same location. Results from this research showed that heat flux through the surface thermocouple is at least two-dimensional. This paper presents a detailed account of the uncertainties in the determined heat flux, expected from the use of surface thermocouples. The paper also presents an application of a method by which the surface temperature can be converted to heat flux, accounting for multi-dimensional effects. This method is known as the 'Impulse Response', and was coupled to a two-dimensional finite element model of the surface thermocouple.