Piston Engines - Performance and Power Augmentation — Page 153, Lesson 197

Let’s start with the heart of this section: the difference between a normally aspirated engine and one that’s internally supercharged. A normally aspirated engine is one that relies entirely on atmospheric pressure to draw air into the cylinders — no compressor. Its power is at a maximum at sea level, and it progressively decreases as altitude increases. Why? Because as you climb, the air gets thinner — less density means less oxygen and less air mass per cylinder, so less power. Now take an identical engine but add an internal supercharger — a compressor driven mechanically by the engine itself. At sea level, at the same speed and the same manifold pressure, this supercharged engine actually develops less power than the normally aspirated one. That power loss is exactly the power required to drive the supercharger. The supercharger is a parasitic load — it takes energy from the engine to spin its impeller. But here’s the interesting part. As height increases, the power developed by the supercharged engine at constant throttle settings increases. That’s counterintuitive at first, so let me explain the mechanism. The decreased temperature of the atmosphere at altitude increases the density of the air. So even though the air pressure is lower, the air is colder and denser — and that means a greater weight of air is pumped into the cylinders for the same manifold pressure. More air mass per charge means more fuel can be burned, hence more power. There’s a second effect too. Decreased air pressure at altitude causes less back pressure on the exhaust. Back pressure is the resistance the exhaust gases face when leaving the cylinders. Less back pressure improves scavenging — that’s the clearing of burnt gases from the cylinder after combustion. Better scavenging means the cylinder is filled more completely with fresh charge on the next intake stroke, which also boosts power. Now, the section contrasts this with turbocharged engines. The effect of altitude change on turbocharged and supercharged engines is vastly different. A turbocharged engine’s power output decreases with increase of altitude, while the engine fitted with an internal supercharger increases with altitude. The reason is the variation of exhaust back pressure with each type. A turbocharger is driven by exhaust gas — it sits in the exhaust stream and creates back pressure. As altitude rises, the exhaust gas expands and the turbocharger can’t maintain boost as effectively, so power falls off. The internally supercharged engine, driven mechanically, doesn’t suffer that exhaust-driven limitation — it benefits from the colder, denser air instead. Finally, the section covers engine power checks and reference rpm. When an engine is first installed in an aircraft, a performance check is made and a Reference rpm is established. This rpm is an indication of the engine’s power output with the propeller on the fine pitch stop — that’s the propeller blade angle set for maximum rpm at low airspeed, like during takeoff. The key point: this reference rpm is almost constant, regardless of the airfield altitude or temperature. So it gives you a stable baseline to compare the engine’s health against over time. Let me make sure you’ve got the full picture. Normally aspirated: max power at sea level, decreasing with altitude. Internally supercharged: less power at sea level due to drive losses, but increasing power with altitude due to colder, denser air and better scavenging from reduced exhaust back pressure. Turbocharged: power decreases with altitude because of exhaust back pressure variation. And reference rpm is the fixed baseline power check taken with the propeller on the fine pitch stop, nearly constant regardless of airfield conditions.