Test data on several small turbochargers with different levels of heat transfer from the turbine to the compressor have been obtained through cooling of the turbocharger center housing and by testing in hot and cold test stands. This data identifies the strong effect of the heat transfer on the apparent efficiency of the compressor and turbine, particularly at low speeds and low mass flows. A simplified theory is used to explain the apparent effect of the heat transfer on the work input and efficiency. The results confirm that conventional performance maps underestimate the efficiency of the compressor stage and overestimate the efficiency of the turbine by as much as 20% points at low speeds. A correction procedure for this effect is defined which converts performance maps obtained with heat transfer to performance maps for adiabatic conditions (for both compressor and turbine) without any prior knowledge or measurement of the heat transfer. The practical significance of the results with regard to turbocharger performance and the relevance to a broader class of turbomachines is discussed.

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