An engine that uses intake air at atmospheric pressure is said
to be normally aspirated. The amount of air a normally aspirated engine
can use is limited by the air density (barometric pressure) in which the engine
is operating, and pressure losses in the intake system. As most pilots know,
under normal conditions, air density decreases with altitude, resulting in
reduced power output. With a normally aspirated engine, most airplanes (that are
also equipped with a constant speed prop) have a manifold pressure gauge. At
full takeoff power, at sea level, on a standard day the manifold pressure gauge
would indicate a MP reading of approximately 29" of Hg. Takeoff power at
5,000 ft. density altitude airport would read about 24" MP. The normally
aspirated engine uses atmospheric pressure and is thereby altitude limited.
In order to get more air into an engine, small compressors are
sometimes used to pressurize the intake air. The amount of pressurization is
referred to as "boost." If the compressor is driven off the
engine crankshaft, the process is called supercharging. Another way to
power the compressor is to put it on a common shaft with a turbine driven by the
engine's exhaust gas. This process is called turbocharging. For most
aircraft engines, turbocharging is preferred since it extracts energy from
exhaust gases that would otherwise be wasted, so it can be more efficient than
supercharging, which takes energy from the crankshaft. In some cases, two
turbochargers are installed on a single engine, one in the exhaust ducting on
each side of the engine.
Recently, turbo-charging has become rather common in the
automotive industry, but because turbochargers can augment combustion air by boosting the intake
manifold pressure at higher altitude, they have been been used in aircraft
applications for several years.** Turbocharging can
provide greater utility to the piston engine by providing sea-level horsepower,
in some models, as high as 20,000 feet; or it can be used to add horsepower to
the engine specifically for takeoff. The faster the engine runs, the more air
the turbocharger can pack into the cylinders to compensate for the thin air of
altitude, or to increase the horsepower.
To maintain boost pressure at a relatively constant amount over
a wide range of engine speeds, some sort of pressure regulation is needed. This
pressure regulation is usually accomplished by using a variable waste-gate
that bypasses excess pressurization from the engine intake. There are also
turbochargers that use fixed, or non-variable waste-gates as well. As expected,
they are not quite as efficient as the controllable versions.
Some precautions are required for turbocharged engines.
Over-boosting, or over-pressurization, especially during take-off, can severely
damage an engine. Turbocharger control systems may include linkages or controls
to prevent over-boost, but if not, strict operational guidelines must be
Turbocharged engines as manufactured by Lycoming consist of a turbocharger
unit with a small turbine wheel attached by a common shaft to a compressor wheel.
They utilize the engine exhaust gas by directing it over the turbine wheel to
drive the compressor. The horsepower loss in operating the turbocharger is
When turbocharging is used with a fuel injected, opposed Textron Lycoming
engine with a 540 cubic inch displacement, it is designated as a TIO-540 model.
"T" represents the turbocharging, "I" represents fuel
injection, and "O," the opposed cylinder configuration.
Lycoming makes a TIO-540 that is perfectly suited to the
Seawind. Most of the turbocharged Lycoming engines have cooling schemes that are
different than our standard K1H5.
These engines are most commonly used on Piper Navajos. The airflow direction is
from the bottom side of the cylinders up through the top, opposite that of the
engine usually used on the Seawind. The exhaust ports are also located on top of
these engines. Next time you see a Navajo, take a look at the cooling inlets and
course, turbochargers have been used with the bottom exhaust Lycomings as well.
There are a few turbocharged Seawinds flying. The ISPA is
seeking content for this page. Please submit photos and articles to the ISPA
editor. Thank you.
If you would like to know more about turbo-charging, you may
find the following links interesting:
A Brief History
Ellison Fluid Systems
Technical Bulletin, Aviation Fuels
*The ISPA has no formal affiliation with
Devlin Aviation, SNA
Inc., nor any of those above.
**Around 1920, Dr. Sanford Moss
of the General Electric Co. was spearheading turbocharger research in an effort
to maintain sea-level performance in aircraft engines in the thinner air of high
altitudes. Dr. Moss eventually become known as the "Father of Turbocharging."