Methods and metrics to assess the accuracy of heat flux data from a spark ignition IC engine

Abstract

The measurement of in-cylinder heat transfer can be a valuable diagnostic tool for quantifying and improving IC engine performance. Increased heat transfer out of the combustion chamber negatively impacts overall efficiency, while heat transfer from the metal to the charge can increase knock propensity. The heat flux at various locations in an engine cylinder can be measured using heat flux probes consisting of two thermocouples – a surface thermocouple exposed to the changing in-cylinder temperatures, and a far field thermocouple which has a constant temperature at steady state operating conditions. The buildup of carbon deposits on the surface thermocouple can affect the heat flux measurements and lead to errors in the data. Heat flux measurements were performed on a proprietary single-cylinder research engine with instrumented heat flux probes in the cylinder head and cylinder liner. The engine was run rich with a high PMI fuel to expedite the buildup of carbon deposits. A metric was developed based on the output of the surface thermocouple to determine the cleanliness of the heat flux probes non-intrusively. Heat flux calculations were done using the FFT method and validated using the Cook-Felderman method. An uncertainty analysis was conducted on the heat flux data to verify that the negative heat flux observed was not due to errors in the distance between the two thermocouples or the measurement of temperature. Finally, to bolster confidence in the results, heat flux measurements were made for motoring conditions with varying coolant and intake air temperatures to gauge whether the trends in the output were in the expected direction.