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Superchargers are mechanically, electrically, or hydraulically driven pumps, compressors, or blowers employed to boost the pressure of the charge air in diesel engines or of the intake charge mixture in spark ignited engines. Most superchargers have traditionally been built around positive displacement compressors. However, with the focus to develop improved drives early in the 21st century, there has been a growth in interest in using superchargers based on centrifugal compressors. A multitude of device types can be used as superchargers, as shown in the classification chart in Figure 1.
Figure 1. Types of Superchargers
The top six devices in the chart are positive displacement, while the centrifugal compressor is classified as an aerodynamic or continuous flow device. Positive displacement devices deliver a specific volume of air per revolution. Since the volumetric efficiency is almost constant, air flow is usually proportional to the supercharger or engine speed. Positive displacement devices can provide high boost pressures without the need for high speed. Therefore, they are well suited for a mechanical connection with the engine, such as through a gearbox or a belt/pulley drive. Each of the particular devices has its advantages and disadvantages, which determine which supercharger is best suited for a specific application.
Centrifugal compressors are well suited to deliver high flow volumes at relatively low pressure ratios. With the boost pressure generally proportional to the square of the supercharger speed, centrifugal compressors must operate at relatively high velocities. In superchargers, they are better suited for coupling with variable speed transmissions or high speed electric motors, rather than for a direct mechanical connection with the engine. Centrifugal devices are also the standard type of compressors that are driven by an exhaust gas turbine in the engine turbocharger.