The heat transfer occurring within a turbocharger is closely coupled to the internal combustion exhaust. This process is highly complex and it is not taken account either in the choice of components nor is it reflected in the efficiency maps. In particular the flow within the compressor is usually treated as adiabatic. This is not the case since the heat fluxes from both the turbine and the engine must be taken into account in the compression process. Tests at Imperial College enabled the heat fluxes on a turbocharger installed on a 2.0 litre diesel engine to be quantified. The main aim was to evaluate the heat transfer impact on the compressor efficiency. In addition to this a simplified 1-D model was then developed and validated with experimental measurements. Based on the data generated by the model, a correlation for the compressor non-adiabatic efficiency is presented in the paper. A multiple regression procedure was developed; the work is based on a statistical description of the different parameters that affect the heat transfer model. The regression analysis showed that the compressor non-adiabatic efficiency can be evaluated with good degree of confidence by three main independent parameters: Mach number, pressure ratio and “temperature parameter”. The Mach number had the largest effect on the compressor efficiency, while the temperature factor was revealed to be significant only at low speeds. The impact of the turbocharger size on efficiency was also assessed. The length of the bearing housing and the compressor diameter turned out to be significant within a few percentage points of the compressor efficiency.

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