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The Art of the Helicopter John Watkinson - Karatunov.net

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Engines and transmissions 213<br />

Fig. 6.11 <strong>The</strong> turbocharger uses energy from <strong>the</strong> exhaust gases to increase <strong>the</strong> mass flow <strong>of</strong> <strong>the</strong> induction<br />

process. See text for details.<br />

As altitude increases, atmospheric pressure reduces, so <strong>the</strong>re is less to push <strong>the</strong> charge<br />

into <strong>the</strong> cylinder. <strong>The</strong> air density falls, so <strong>the</strong> mass <strong>of</strong> <strong>the</strong> charge is even fur<strong>the</strong>r reduced.<br />

Without some help, <strong>the</strong> piston engine simply cannot brea<strong>the</strong> at altitude. Any device<br />

which forces more charge into <strong>the</strong> cylinder is a supercharger. <strong>The</strong>se were originally<br />

mechanical pumps driven by <strong>the</strong> engine crankshaft. Later it was realized that <strong>the</strong> residual<br />

energy in <strong>the</strong> exhaust could be harnessed to drive <strong>the</strong> pump. This is <strong>the</strong> principle<br />

<strong>of</strong> <strong>the</strong> turbocharger shown in Figure 6.11.<br />

Exhaust gases 1 pass through a turbine 2 before passing to <strong>the</strong> atmosphere. <strong>The</strong> presence<br />

<strong>of</strong> <strong>the</strong> turbine extracts energy from <strong>the</strong> exhaust gases, lowering <strong>the</strong> temperature<br />

and <strong>the</strong> pressure. <strong>The</strong> pressure drop needed to make <strong>the</strong> turbine work means that <strong>the</strong><br />

pressure in <strong>the</strong> exhaust manifold, known as back-pressure, is higher than normal. Heat<br />

insulation 3 is usually needed around <strong>the</strong> exhaust turbine. <strong>The</strong> exhaust turbine drives<br />

a centrifugal compressor 4 that takes in air through <strong>the</strong> air cleaner 10 and forces it into<br />

<strong>the</strong> induction system. <strong>The</strong> boosted induction pressure is displayed on <strong>the</strong> boost gauge 7<br />

and increases <strong>the</strong> charge entering <strong>the</strong> cylinder. Back-pressure in <strong>the</strong> exhaust prevents<br />

<strong>the</strong> charge blowing straight through during valve overlap.<br />

<strong>The</strong> turbocharger is a simple device, but it may turn at over 20 000 RPM. <strong>The</strong> stresses<br />

due to this rotational speed are enormous, yet <strong>the</strong> material <strong>of</strong> <strong>the</strong> exhaust turbine also<br />

has to withstand <strong>the</strong> elevated exhaust temperature due to raising <strong>the</strong> power output.

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