10-12-2012, 12:07 PM
Boosting Your Knowledge of Turbocharging
Turbocharging.pdf (Size: 184.42 KB / Downloads: 169)
Ashort 15 years after Orville
and Wilbur made their historic
flight at Kitty Hawk,
General Electric entered the
annals of aviation history. In 1918, GE strapped
an exhaust-driven turbocharger to a Liberty
engine and carted it to the top of Pike’s Peak,
CO — elevation 14,000 feet. There, in the crystalline
air of the majestic Rockies, they successfully
boosted this 350 hp Liberty engine to a
remarkable 356 hp (a normally aspirated engine
would only develop about 62 percent power at
this altitude).
An astounding altitude record of 38,704
feet was achieved three years later by Lt. J.A.
Macready.
This new technology began immediately
experiencing a rapid evolution with the full
strength of blowers being tested during
WWII. The B-17 and B-29 bombers along
with the P-38 and P-51 fighters were all fitted
with turbochargers and controls.
Turbocharging had brought a whirlwind of
change to the ever-broadening horizons of
flight.
Turbo-normalized or groundboosted?
Distilled to the most basic of definitions, a
turbocharger is simply an air pump powered by
the unused heat energy normally wasted out the
exhaust. This “air pump” (or more accurately,
compressor), is capable of supplying the engine
intake manifold with greater than atmospheric
air pressures. A collateral benefit is derived as
the turbo also provides air for the cabin pressurization
of certain aircraft.
Some confusion persists as to the difference
between an airplane that is “ground-boosted”
as opposed to one that is “normalized.” Simply
put, turbocharging serves one of two purposes:
either it directly increases (boosts) the
power output of the engine, or it assures that
sea level horsepower performance is maintained
(turbo-normalized) to higher altitudes,
thereby increasing the plane’s potential service
ceiling.
Increased efficiency in a rarified
atmosphere
At sea level, the atmosphere in which we
live and breathe is continually under a pressure
of about 29.92 inches of mercury (Hg).
At 1,000 feet, this “free air” drops in pressure
to about 28.86 inches Hg. Air becomes progressively
less dense at all altitudes above
sea level. Because of this, all naturally aspirated
engines experience a reduction of fullthrottle,
sea level power output as they
increasingly gain altitude. On a “standard
day,” atmospheric pressure at 10,000 feet
hovers at only 20.5inches Hg. In these conditions,
a naturally aspirated engine is
unable to sustain full power performance
due to the lack of air density at these higher
altitudes. Without the aid of a turbocharger,
a loss of 3 to 4 percent of power or approximately
1 inch of MAP per 1,000 foot gain in
altitude seems to be the rule.
Maintenance of the turbocharger
The turbocharger itself is subjected to an
extremely hostile environment. Turbine inlet
temperatures reach a scorching 1,650 degrees.
Some are hotter still. TCM’s liquid-cooled
Voyager is rated at 1,750 degrees continuous
with the capability of 1,800 degrees for 30 seconds
while establishing peak EGTs. Turbine
speeds range from 0 to 120,000 rpm. The T36
in the Malibus and the Lancair 4P is capable of
125,000 at 1,650 F. That’s a screaming 2,083
revolutions per second! Pulsing exhaust gases,
engine vibration and temperature variations all
add to this hellish mix.
The turbo assembly is made of the
strongest of alloys available to withstand these
rigorous conditions. However, these tortuous
duty cycles can shorten the life of a turbo if
proper maintenance is not performed. It is of
the utmost importance to follow the maintenance
and inspection criteria spelled out in
the applicable service bulletins and airworthiness
directives.