Handbook of Turbomachinery Second Edition Revised - Ventech!

The overall turbine efficiency includes both the turbine efficiency and
the mechanical efficiency, which includes bearing losses.
combined efficiency ¼
actual enthalpy rise across compressor
isentropic enthalpy drop through turbine expansion ratio
Altitude Effects
The turbine and compressor speed depends on the pressure differential
between the turbine inlet and outlet. The greater this pressure differential
between the exhaust manifold and atmospheric pressure, the higher the
turbine and compressor speed. Since the manifold pressure remains
relatively constant, as atmospheric pressure decreases, such as when
climbing in altitude, the increase in speed will cause the compressor to
increase boost pressure. Thus a turbocharger is largely self-compensating
with respect to altitude.
Boost Control
It is necessary to limit the maximum boost pressure provided by a
turbocharger. For example, the typical turbocharger used on conventional
automobiles and light trucks is designed to provide a maximum boost
pressure of 5–10 psi. By comparison, turbochargers for motorsports
competition (road racing, hill climbing, rallying, oval track, etc.) use boost
pressure up to 25 psi, while short-duration events like drag racing and
tractor pulling can tolerate boost pressures in the 20–75 psi range.
There are three reasons for controlling boost pressure in turbocharged
systems. First, excessive boost pressures can lead to preignition and
denotation in the combustion chamber, leading to severe engine damage.
Second, turbocharger speeds have to be limited to prevent self-destruction.
Finally, speeds have to be controlled so that the turbocharger can be
matched to the engine to obtain the desired performance characteristics over
the wide operating range found in automotive applications.
The earliest boost controls consisted of fixed restrictions in the inlet or
exhaust ducting, or even both. Incidentally, even the air filter or carburetor
choke can provide a measure of boost control. Fixed restrictions provide
progressive control that varies as the square of the exhaust flow. For
example, when the restriction limits the maximum boost pressure, there is
also a corresponding restriction at lower rpm, resulting in less efficient
engine performance and poorer throttle response.
Currently, the most common method of controlling turbocharger
boost is through the use of a wastegate. An externally mounted wastegate is
Copyright © 2003 Marcel Dekker, Inc.