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Modern drive units designed for driving non-road machines are characterized by liberal regulations regarding the limits of toxic compounds emission in exhaust gases. These regulations contribute to the low level of technical advancement of this type of drive units. Among the new non-road  small engines (SI) offered in the sale in 2018 in the European Union, the majority of them are equipped with carburetor systems whose fuel supply characteristics do not allow to achieve modern fuel-air mixture control standards. Therefore, action should be taken to develop these drive units towards electronic control of combustion processes in these engines, which will allow the use of innovative control algorithms. One of the basic signals supporting the selection of the air-fuel mixture is the engine temperature. The paper presents an overview of the methods for measuring the temperature of internal combustion engines and presents the results of research on the process of warming up the engine. The tests were carried out with three methods using an oil temperature sensor, a surface temperature sensor and a thermal imaging camera. An attempt was made to indicate construction guidelines taking into account the place of temperature measurement, correction factors were determined in relation to the oil temperature. The developed coefficients can be used to precisely determine the thermal state of the engine, which is an important aspect in the process of controlling the fuel-air mixture and affects its consumption.