HANDS ON
FIREWALL FORWARD
Engine Exhaust Gas Flow
Upperdeck Pressure
Manifold Pressure
Temperature values are given for the following flight
conditions; TSIO-550-C 25,000 ft. MSL during
standard day conditions.
exit the turbo system much hotter—even
hundreds of degrees—than when it... entered
the compressor. That hot air is then forced
into the cylinder so the entire combustion
process starts out at a radically elevated
temperature. The turbo engine suffers the
double whammy of more heat from the extra
power and even more heat because the
induction air is heated by the compressor.
To help control the heat entering the
engine, most turbocharged engines—at least
the higher output ones—use an intercooler.
An intercooler is really a radiator that uses
the typical cooling fins to cool the induction
air as it travels from the compressor to the
cylinders. Intercoolers are effective at lowering induction air temperature and help
control the total amount of heat the engine
creates, but they are not 100 percent efficient, so the turbo engine will always need to
get rid of more engine operating heat than
the natural engine. Careful cowling design is
essential for a turbocharged engine to cool
effectively, and pilots need to pay attention
to keep the airspeed up so sufficient cooling
air is flowing through the cowling.
Because of the hotter conditions, fuel
mixture is more critical in a turbo because
heat makes detonation more likely.
Detonation is an explosion of the air and fuel
mixture in the cylinder instead of the
desired rapid and even burning of the
charge. Detonation is called “knock” because
that is the sound it makes in a solid block
engine. Anybody who remembers the cars of
50 or 60 years ago has probably heard that
“pinging” or “knocking” sound that came
from the engine under load if the ignition
system was not properly tuned, or the gasoline was less than adequate. Car engines
with their heavy construction could tolerate
at least a little knocking, but the aluminum
cylinder heads and pistons of an airplane
engine cannot. You can’t hear an airplane
engine detonate like in the old car, but if detonation occurs, it will take only minutes, or
even seconds, to destroy the cylinders and
pistons and kill the engine.
The good news is that turbocharged
engines use lower compression than the
same size and type of naturally aspirated
engine, and that increases detonation margin. Also, when the fuel system is properly
set up, the turbo engine is fed extra fuel for a
