Chapter 18: 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.