Rachel Schwind and Shaaban Abdallah
With rising fuel costs and new emissions standards for automobiles, automotive manufacturers are turning to new ways to decrease engine size and emissions while still maintaining high power operation. This can be accomplished through the use of turbochargers. The aim of this study is to analyze what is currently being done with turbocharger compressor technology to meet these requirements. This analysis will focus on different impeller blading designs including splitter bladed impellers, tandem bladed impellers, and tandem bladed impellers with a casing blade. There are different advantages associated with each impeller design. The tandem bladed impeller designs are shown to have decreased efficiency and pressure ratios as compared with the backswept and/or splitter bladed designs. It is also shown that the operating range is increased with the tandem design due to a lower surge margin. Further analysis needs to be conducted on refining the tandem bladed and tandem bladed with a casing blade designs to truly see if these designs have the potential for greater performance improvements. Keywords: Centrifugal compressor impeller; Turbocharging; Turbomachinery; Tandem-blading; Compressor surge; Compressor performance; Impeller designs Introduction With rising fuel costs and the depletion of natural resources, automotive manufacturers have been looking for ways to increase fuel economy without significantly decreasing an engine’s power output. Many engine manufactures have turned to the use of turbochargers to meet this need. A turbocharger is able to increase an engine’s power by forcing more air into the combustion chamber thus also allowing it take in more fuel as well. The mass of air entering the combustion chamber can increase the power output as well as allow for leaner combustion. These are an attractive option for modern cars because they allow manufacturers to use smaller engines for the same output power. This can increase fuel economy at idling stages as well as reduce the overall weight of the vehicle. They also function as a way to reduce emissions since a majority of modern turbochargers are driven by an exhaust gas turbine. The turbine is connected to the compressor via a common shaft so when it pulls in the exhaust gases, it generates the rotation necessary for the compressor to operate. A major component of the turbocharger is the compressor which is generally a centrifugal compressor. Flow generally enters the compressor in axial direction. The rotating impeller blades then accelerate the fluid before discharging it radially. This increase in velocity will generally lead to an increase in pressure as well. The accelerated fluid can then be discharged into a collector and then on to a diffuser where the fluid velocity is decreased allowing further conversion of velocity into pressure. Generally, a centrifugal compressor is designed for optimal operation at a single speed. This can be problematic as compressors need to be able to function at off-design speeds, especially in the case of automotive turbochargers which have highly transient operation. Due to the transient nature of engine operation, a turbocharger ideally has a large operating range. Bounding the operating range at low speeds with high pressure ratios is the surge limit. Surge occurs when there is a low flow rate at a relatively high pressure ratio causing flow reversal. This causes fluctuations in the flow resulting in unstable operation. Surge that results in a flow reversal over almost the entire fluid flow, also known
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