Supercharging and Turbocharging

18.1 The Effect of Altitude on Power

• As an aircraft climbs, the air density decreases. For a
• normally aspirated engine (one that breathes air at ambient atmospheric pressure), this
• means less weight of air and fuel enters the cylinders on each intake stroke, causing power to
• decrease with altitude107107107107.
•  Density Altitude: This is the pressure altitude corrected for non-standard temperature. It is
• the altitude in the standard atmosphere at which the prevailing density would occur108. Higher
• density altitude (due to high elevation, high temperature, or high humidity) reduces engine
• performance109.

18.2 Power Augmentation

• To counteract the loss of power with altitude, an engine can be fitted with a supercharger or
• a turbocharger. These are air compressors that force air into the induction manifold at a
• pressure higher than ambient, increasing the weight of the charge and thus increasing or
• restoring engine power110.
•  Rated Altitude: The maximum altitude at which an engine can maintain its rated sea-level
• power111.

18.3 Superchargers vs. Turbochargers

•  Supercharger: An internally-driven compressor, powered mechanically by gearing from the
• engine's crankshaft112.
•  Turbocharger (or Turbo-supercharger): An externally-driven compressor. It consists of a
• turbine and a compressor on a common shaft. The turbine is placed in the engine's exhaust
• stream and is spun at high speed by the hot exhaust gases. This rotation drives the
• compressor, which boosts the intake air pressure113. Turbocharging is more efficient as it uses
• waste energy from the exhaust.

18.4 Turbocharger Operation and Control

•  Waste Gate: Not all exhaust gas is needed to drive the turbine, especially at lower altitudes.
• A
• waste gate is a valve that can bypass some of the exhaust gas around the turbine, sending it
• directly overboard114. Controlling the position of the waste gate controls the speed of the
• turbine, and therefore the output pressure of the compressor115.
•  Automatic Control: The waste gate is typically controlled automatically by an Absolute
• Pressure Controller (APC), which senses the compressor outlet pressure (upper deck
• pressure)116. The APC modulates the waste gate to maintain the manifold pressure selected by
• the pilot's throttle.
•  Operation during Climb: At sea level on takeoff, the waste gate is nearly fully open,
• bypassing most of the exhaust. As the aircraft climbs and the air becomes less dense, the APC
• will progressively
• close the waste gate to direct more exhaust gas through the turbine, spinning it faster to
• maintain the desired manifold pressure117.
•  Critical Altitude: The altitude at which the waste gate becomes fully closed. Above this
• altitude, the turbocharger can no longer maintain its rated sea-level manifold pressure, and
• power will begin to decrease as the aircraft climbs further118.