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It is necessary to understand wall heat transfer mechanisms in order to mitigate cooling losses in an internal combustion engine. To investigate the turbulent heat transfer on the engine wall, a heat flux sensor has to have a low noise and multi measurement points on comparable scale of gas turbulence. Therefore, the authors have developed a new heat flux sensor with three measurement points by using MEMS (Micro-Electro-Mechanical Systems) technologies. The MEMS sensor has three thin film RTDs (Resistance Temperature Detector) with the size of 315 μm on a 900 μm diameter circle in rotational symmetry. Measurement tests were conducted in a laboratory engine. The noise of the MEMS sensor was evaluated as 13.8 kW/m2, which is small enough to detect instantaneous heat flux. The instantaneous heat flux had oscillation with the amplitude of a few hundred kW/m2. Since the amplitude of the oscillation was much larger than the noise, it was supposed that the oscillation was a meaningful signal reflecting the disturbance of a velocity or temperature field in the gas phase. By a cross-correlation analysis between the three RTDs, it was found that the instantaneous heat fluxes had a moderate correlation with a certain delay time. That can be interpreted as the traveling of a turbulent vortex structure from one RTD to another RTD with the time. Therefore, it can be expected that the turbulent characteristics will be extracted from the instantaneous heat flux data measured with the three RTDs.

To develop a heat flux sensor for internal combustion engines, two metal substrate thin film resistance sensors have been developed as prototypes by using MEMS (Micro-Electro-Mechanical Systems) technologies. In our previous study, a thin film heat flux sensor on a Si chip was developed for combustion fields. To apply the thin film sensor to the engine, a metal substrate sensor technology has to be developed. To begin with, a flat plate shape sensor with a SUS substrate was made in order to confirm the fabrication process and the performance of the metal substrate MEMS sensor. Heat fluxes were successfully measured in laminar premixed combustion fields, and it was confirmed that the SUS substrate flat plate shape sensor has sufficient performance in temporal resolution, measurement noise and temperature durability against requirements. Secondly, a plug shape sensor using an AC8A substrate was produced to be introduced to an engine. The heat from the sensor sidewall has to be taken into account due to the small size of the plug shape sensor, the analytical model for the heat flux calculation was extended to a two dimensional cylindrical system. Heat flux measurement tests under high load conditions with the plug shape sensor were conducted in a rapid compression and expansion machine. As a result, the sensor endured the harsh environment with the maximum pressure of 9.1 MPa and the heat flux load of 8.9 MW/m2. Furthermore, the measurement noise was estimated to 11.0 kW/m2, which was a quite low level compared with a commercially available heat flux sensor. Although the issue in the fabrication process remains, the prospects for introducing the MEMS heat flux sensor in internal combustion engine were obtained.

Heat flux measurement method with thin film resistance sensor in a premixed gas combustion field has been studied to develop an accurate heat flux sensor, and to grasp and improve a heat transfer loss of the engine in SIP (Cross-ministerial Strategic Innovation Promotion Program) innovative combustion technology project. The MEMS technology was introduced to satisfy the requirement of the accurate heat flux measurement with high temporal, spatial and heat flux resolution for a turbulent heat transfer in the engine. The thin film resistance sensors of 250 to 1000 micron scale were fabricated on the Si substrate, and then a calibration method, measurement characteristics and response to the combustion were studied. Since the heat flux was measured through the surface temperature measurement and the transient heat conduction analysis of the sensor, accurate data and an exact thermal model are required. The heat flux calibration using self-heating showed a good agreement between the excited heat flux of 400 kW/m2 level and measured one with an error less than 10 kW/m2 for the wide frequency range from 200 Hz to 8 kHz by introducing an interfacial thermal resistance in the thermal model. The developed measurement system showed heat flux peak of 250 kW/m2 level with noise of 10 kW/m2 level against a butane-air premixed gas combustion in an open chamber. Heat transfer analysis showed that the heat flux trend after the peak can be explained by heat conduction between the burnt gas and the sensor wall. It was also demonstrated that the developed system can measure heat flux down to 10 kW/m2 and up to 5 kHz frequency range. Good prospect of the heat flux sensor for the engine application was obtained with the sufficient accuracy and the resolutions.