Estimation of the performance of turbocharger compressors at extremely low pressure ratios

Constant speed lines on turbocharger compressor performance maps usually have no test data at low pressure ratio and high flow. The engine, however, can force the turbocharger compressor to operate in this region and whole-engine simulation systems need to extend the test data in some way to include this region of the map. A physically based method of estimating the performance at low pressure ratios from measurements made in the rest of the map has been developed and is described in this article. The method requires no detailed geometrical information on the impeller and the stage but extracts the necessary information from the test data available; hence it can be applied to any measured compressor data. The test data over the measured range of speed and flow are used to estimate the work transfer and loss characteristics of the impeller, and the impeller throat area. The Euler equation justifies a linear extrapolation of the work input to higher outlet flow and a new technique based on the density ratio is used to extrapolate the losses. This accounts for choking at the impeller inlet in a similar way to choking at a one-dimensional duct of varying area, where losses are also a function of density ratio. The stage performance at low pressure ratios can then be obtained by recombining the extrapolated loss and work characteristics at higher flows. The method allows the measured performance map to be extrapolated to low pressure ratios on a sound physical basis and to identify physical aspects of the flow in this extreme off-design region, such as the location of increased risk of oil blow-by.