The Art of the Helicopter John Watkinson - Karatunov.net

Engines and transmissions 213
Fig. 6.11 The turbocharger uses energy from the exhaust gases to increase the mass flow of the induction
process. See text for details.
As altitude increases, atmospheric pressure reduces, so there is less to push the charge
into the cylinder. The air density falls, so the mass of the charge is even further reduced.
Without some help, the piston engine simply cannot breathe at altitude. Any device
which forces more charge into the cylinder is a supercharger. These were originally
mechanical pumps driven by the engine crankshaft. Later it was realized that the residual
energy in the exhaust could be harnessed to drive the pump. This is the principle
of the turbocharger shown in Figure 6.11.
Exhaust gases 1 pass through a turbine 2 before passing to the atmosphere. The presence
of the turbine extracts energy from the exhaust gases, lowering the temperature
and the pressure. The pressure drop needed to make the turbine work means that the
pressure in the exhaust manifold, known as back-pressure, is higher than normal. Heat
insulation 3 is usually needed around the exhaust turbine. The exhaust turbine drives
a centrifugal compressor 4 that takes in air through the air cleaner 10 and forces it into
the induction system. The boosted induction pressure is displayed on the boost gauge 7
and increases the charge entering the cylinder. Back-pressure in the exhaust prevents
the charge blowing straight through during valve overlap.
The turbocharger is a simple device, but it may turn at over 20 000 RPM. The stresses
due to this rotational speed are enormous, yet the material of the exhaust turbine also
has to withstand the elevated exhaust temperature due to raising the power output.