Past and Future
There are other outdated methods of boost control, such as intake restriction or bleed off. For instance, it is possible to install a large butterfly valve in the intake to restrict airflow as desired boost is approached. It is also possible to actually release large amounts of already compressed air similar to a blowoff valve but on a constant basis to maintain desired boost at the intake manifold. The currently popular exhaust gas bypass via wastegate is superior to creating intake restriction or wasting energy by releasing air that has already been compressed. These methods are rarely used in modern system due to the large sacrifices in efficiency, heat, and reliability.
Other methods may come into widespread use in the future, such as variable geometry turbochargers. With a sufficiently large turbine, no wastegate is necessary. Low speed response and faster spool up are then obtained using variable turbine technologies rather than a smaller turbine. These systems may replace or supplement typical wastegates as they develop. Control methods for the variable mechanical controls, such as the principles of closed loop will still apply even if they no longer involve pneumatics.
Read more about this topic: Boost Controller
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