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Supercharged Engines
The altitude performance of a piston engine can be improved by supercharging. This involves compressing the air entering the intake manifold by means of a compressor. In earlier supercharged engines, this compressor was driven by a gear train from the engine crankshaft; hence the term gear-driven supercharger. A typical performance curve for a gear-driven supercharged engine is presented in Figure 6.5. Such an engine is generally limited by the manifold absolute pressure (MAP), so the pilot cannot operate at full throttle at sea level. As the pilot climbs to altitude, the throttle is opened progressively, holding a constant MAP. The engine power will increase slightly until an altitude at which the throttle is wide open is reached. Above this altitude, known as the critical altitude, the power decreases linearly with density ratio in the same relative way as a nonsupercharged engine.
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Figure 6.5 Performance curves for a gear-driven supercharged engine. |
Today’s supercharged engines employ a turbine-driven compressor powered by the engine’s exhaust. This configuration is referred to as an exhaust turbosupercharger. The advantage of this type of supercharger as compared to the gear-driven type is twofold. First, the compressor does not extract power from the engine, but uses energy that would normally be wasted. Second, the turbosupercharger is able to maintain sea level-rated power up to much higher altitudes than the gear-driven supercharger.
Turbosupercharged engines are equipped with a regulating system that maintains an approximately constant manifold pressure independent of altitude. A density and pressure controller regulates the position of a waste gate, or bypass valve, which regulates the amount of exhaust gases through the turbine.
A modern, pressurized aircraft used by the general aviation industry is the Piper Navajo, pictured in Figure 6.6. This aircraft uses two Lycoming ТЮ-540 turbocharged engines, each driving a 2-m (6.6-ft) diameter, three- bladed propeller. Performance and fuel flow curves for this engine are presented in Figure 6.7. As shown, the engine is able to maintain its rated power up to an altitude of approximately 7300 m (24,000 ft). The use of these curves is straightforward, so they do not need any detailed explanation. Correction for nonstandard temperature is the same as that for a non- supercharged engine.
