A variable geometry diffuser was developed for a centrifugal compressor with the objective of meeting the surge regulation and air flow requirement of the Army's Variable Area Turbocharger Program. Two types of compressor rotors were built and tested: a radial-bladed impeller design and a backswept impeller.
Results of the performance test program indicated that the variable geometry compressor met the range and efficiency goals over most of the required operating conditions. The backswept impeller was found to be superior to the radial-bladed impeller over the entire operating map. Compressor efficiencies as high as 80% were demonstrated in the high airflow operating regime (rated engine speed). At very low airflows corresponding to the lowest engine speed operation, the onset of impeller stalling caused an efficiency loss. It is believed that the complexity of variable geometry is justified where maximum efficiency and range is mandatory.