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Thermal cars are mechanisms that do the conversion between heat and mechanical work. Depending on the meaning of the conversion, it is classified into:
heat engines: convert heat into mechanical work
refrigeration units: convert mechanical work into heat.
A thermal engine is a thermodynamic system that receives energy in the form of heat from a warm source and its purpose is to transform it into mechanical work. There are simple devices that convert heat into mechanical work by a simple thermodynamic process in a single stroke, but these are not considered heat engines. A proper heat engine must perform mechanical work sustainably, and this is only possible through cyclic operation. Inevitably, it is not possible to fully transform the received heat into mechanical work (it would mean 100% efficiency and would violate the 2nd law of thermodynamics). The consequence is that some of the heat received is given up to a cold source.
A refrigeration unit is a thermodynamic system built with the purpose of removing heat from a cold enclosure and giving it to a warm enclosure. According to the second law of thermodynamics, the transfer of heat from a cold body to a hot body is possible only by doing mechanical work (or by using other forms of energy).
