The introduced error associated with the contact-type detection for piston surface temperature in combustion engines using coaxial thermocouples and its eliminating algorithm

Abstract

Piston surface temperature is critical original data in combustion engines for evaluating its heat load and studying the in-cylinder combustion and heat transfer, where a contact-type measurement method employing coaxial thermocouples is frequently used. But packaging coaxial thermocouples into a piston is bound to change the original temperature field of the piston to be tested, bringing about the introduced error, which is a key problem for the piston surface temperature detection. In this paper, a two-dimensional model of a coaxial thermocouple is established, based on which the introduced error is quantitatively evaluated firstly, followed by a discussion on the mechanisms of the introduced error next. Then three temperature correction methods are proposed aiming to reduce such serious introduced errors, and the correction effects are demonstrated for E, J, and K-type coaxial thermocouples with different film thicknesses. It is found that the introduced errors are remarkable for all the tested thermocouple types and hot junction depths, which are closely related to the corresponding impulse response deviations, more essentially, to the deviations between thermal product values (ρck) of the thermocouple material system and that of the prototype piston material. The introduced errors are effectively reduced by the three correction methods for most cases, among which Method-II behaves best, followed by Method-III, Method-I. Method-II can steadily reduce average relative error, an indicator of the introduced error, by more than two orders of magnitude to 0.09‰ around.