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Abstract

A general heat engine model is developed for determining the scaling characteristics of small combustion-driven energy systems. The model is composed of a Carnot heat engine and a combustor operating at a specified temperature dictated by the thermodynamic maximum power point. Considerations have been made for thermal conduction losses to the surroundings and heat recovery from the exhaust stream of the combustor. Modelling of the conduction heat loss is necessary due to the increased importance of this effect upon a reduction in size. This model is used to determine the ideal system efficiency as a function of characteristic length. This length is then varied from mesoscale dimensions to the microscale with the overall system efficiency being determined at each point. The scaling study provides a sense for the ultimate size limitations imposed on combustion-driven engines due to thermal loss mechanisms to the surroundings. Although a high degree of idealization is employed, this analysis shows that submillimetre engine/combustor systems appear impractical, but characteristic sizes in the range of a few millimetres are feasible, at least in regards to the thermal loss mechanisms. This study also shows that systems having a conduction parameter greater than approximately 0.5 do not benefit significantly from heat exchanger NTU values greater than 3 due to the diminishing benefits of heat recovery with larger conduction losses.

Keywords

microengine heat engine scaling

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A scaling study of a combined microcombustor and heat engine system

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