The Piston Engine

Chapter 18: Supercharging and Turbocharging

Technical General for Aviators — Capt. Pankaj Pahil

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.

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.

 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.

 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.