INSTANTANEOUS HEAT FLUX MEASUREMENTS IN INTERNAL COMBUSTION ENGINES
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High temporal resolution heat flux measurements were performed in two different internal combustion (IC) engines. Measurements in a large compression ignition engine were conducted with a wireless telemetry system utilizing fast-response thermocouples mounted on the piston surface. Parametric studies were conducted over a range of operating conditions and different combustion strategies were investigated and compared to one another including conventional diesel combustion and Reactivity Controlled Compression Ignition (RCCI) combustion. The conventional diesel combustion heat fluxes were characterized by the existence of large heat flux differences between thermocouples that are the result of spray targeting and impingement and spatial stratification within the chamber. In contrast, during RCCI combustion the heat fluxes among the different thermocouples were found to have increased homogeneity, and were of lower magnitude, which gave rise to lower piston temperatures. Measurements performed in a small, air-cooled spark ignition engine were conducted with a spark plug-mounted coaxial thermocouple. The single-point measurements were used to analyze the processing method used for the calculation of surface heat flux, and the fundamentals of the surface temperature measurement. It was demonstrated that the advanced inverse techniques utilizing regularization improved the accuracy of surface heat flux estimates by incorporating the temperature variance data. Regularized surface heat flux estimate are intentionally biased yet the regularization procedure does not decrease the overall accuracy of the solution. Inverse methods were found to increase the solution stability by directly calculating the surface heat flux, thus, avoiding amplification of temperature measurement errors common with finite differences used in analytical inversion techniques. The high-load SI data recorded an anomaly during the expansion stroke, where the heat flux reversed direction turning negative and reaching a negative value that was 8% of the peak magnitude recorded during combustion. A 2-D, axisymmetric numerical model of the coaxial probe with an imposed surface heat flux was used to investigate this phenomenon. The simulation results showed that significant lateral conduction between the adjacent thermoelements occurred at the surface and extended to a depth of 150 microns. The temperature bias produced by radial conduction could account for 3% of the observed heat flux reversal seen in the engine data. The presence of multi-dimensional heat transfer effects led to the design and fabrication, of a new array-based temperature sensor capable of recording rapid transient data without 2-D effects biasing the temperature measurement. The sensor was fabricated on silicon using lithographic techniques, but did not survive an engine test long enough to produce usable data.