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Theory 2 - Buffer Pressure The buffer pressure is the pressure in the outside of the mechanism, it is suppose to be a constant value. If the engine is not enclosed in a vessel, the buffer pressure is the atmospheric pressure. The quantity of negative work through the cycle (W-) i.e. the work given by the mechanism to the working fluid, is defendant on the shape of the thermodynamic cycle and also on the buffer pressure. The thermodynamic work of any cycle does not depend on the buffer pressure. Nevertheless when accounting for the mechanical efficiency of the engine we will see that the buffer pressure is a key parameter to determine the overall efficiency of the engine. They negative work should be as small as possible to minimize the overall losses of the engine. The negative work (W-) may occur at different location around the cycle, depending on the buffer pressure. Some specific cylces may also have zero negative work. Some examples are summarized in following images showing the case of a perfect stirling cycle as well as a real cycle. The grey areas correspond to negative work W-
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