On Heat Transfer Measurements in Diesel Engines Using Fast-Response Coaxial Thermocouples

Finite element models of fast-response CO-AX thermocouples typically used for heat transfer measurements in diesel engines have been developed. Due to the small differences in thermal properties between the thermoelements and the iron engine components, CO-AX thermocouples are capable of measuring transient temperatures of iron components within an accuracy of 98 percent. However, these relatively small errors in temperature measurement result in as high as 25 percent errors in peak surface heat flux calculations. This implies that heat flux results depend not only on the temperature of the surface thermocouple junction, but are also sensitive to its time rate of change. Increasing the thin film thickness can significantly alter the heat flux profile deduced from surface junction temperatures.