Metodologia de cálculo de incertezas de medição para uma bancada dinamométrica para motores de combustão interna

Abstract

Measurement uncertainties are related to experiments and measurement instruments. They quantify the reliability of the experiment, that is, the greater the uncertainty, the lower the reliability of the result. According to the information resulting from the measurement process and additional information, it is possible to assign a reasonable range of values to the measuring. Studies of internal combustion engines are complex due to sensitive variations in the analyzed parameters, therefore, to prove possible gains, it is necessary that the results be greater than the uncertainty spectrum. The acquisition of internal cylinder pressure data and crankshaft position are critical parameters in internal combustion engines and require robust methodology and equipment to obtain data. In the present study, a methodology was researched to measure the measurement current of internal combustion engines on dynamometric benches through bibliographical references and the measurement uncertainty expression guide (INMETRO). The measurement uncertainty sources were mapped in order to validate the data obtained in tests and analyze which is the greatest source of uncertainty. A 1.8-liter four-cylinder Otto-cycle commercial engine with fuel injection at the intake ports was used for testing. Ethanol was used as fuel and a FuelTech 450 programmable injection for fuel injection control and ignition control, always seeking for maximum performance. All data were acquired under the same environmental conditions and with controlled oil and coolant temperatures, 110°C and 90°C, respectively, with a maximum variation of 3°C. The tests were performed after the calibration of all sensors, seeking to mitigate all sources of uncertainty. Average data of 200 combustion cycles were obtained, looking for CV (Coefficient of Variation) less than 3.0%. After data acquisition, Type A and Type B uncertainties, combined uncertainty and expanded uncertainty were calculated, with a reliability factor of 95%. After analyzing the data, it was evident that the increase in the CV proportionally influences the increase in the uncertainty of measuring the cylinder pressure data. This parameter is the main source of uncertainty, since the acquisition of cylinder pressure data is complex, due to exposure to heat, cyclic variability and pressure difference. In addition, the cylinder pressure uncertainty directly influences the uncertainty of parameters such as IMEP (Indicate Mean Effective Pressure) and the heat release rate. These parameters presented measurement uncertainties in the order of 10% and 20%, respectively.