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A limitation currently facing internal combustion engines research is the lack of a direct method of measuring the temperature of the gases inside the cylinder. The rate at which a combustion cycle evolves is too rapid for conventional, direct measurement systems such as thermocouples. Advanced optical systems like laser-induced fluorescence (LIF) require heavy engine modifications to be effective and cannot withstand typical engine operating conditions. The direct measurement of the gas temperature in an engine cylinder, at realistic operating conditions is needed to better understand the combustion cycle and its effects on cycle efficiency. The study presented here uses a Kalman filtering technique to measure the time of flight of an ultrasonic signal to calculate the temperature of the gas in an engine cylinder. Results show that the method is capable of measuring temperatures within an accuracy of 10% and within sample deviations below 0.25% with sufficient data.