Piston Engines - Performance and Power Augmentation — Page 136, Lesson 172

Let's start with the fundamental problem we're trying to solve. On a normally aspirated engine — that's an engine that breathes in air at whatever pressure the atmosphere happens to give it — the pressure in the induction manifold at full throttle is slightly less than atmospheric pressure. Why? Because of intake duct losses. The air has to flow through ducts, filters, and bends, and every one of those costs you a little pressure. So even at full throttle, you're slightly below ambient. Now, as you climb, atmospheric pressure drops. So manifold pressure drops with it. And since power output depends on how much air and fuel you can get into the cylinders, power output decreases with altitude. There is one small consolation: some of that loss is recovered in better scavenging of the cylinders. Scavenging means clearing out the burnt exhaust gases from the previous combustion. At altitude, the back pressure on the exhaust is lower — the outside air is thinner — so the exhaust gases leave more easily, and the cylinders fill a little more efficiently. But that's only a partial recovery. The overall trend is still downward. So to fix this, we have to raise the manifold pressure artificially. That process is called supercharging. And the device that does it is a supercharger. Now, there are two distinct reasons you might supercharge, and they lead to two different types of supercharger. First, the Ground Boosted Supercharger. This one is used to increase sea level power — to give you more power at take-off and initial climb than a normally aspirated engine could produce. But here's the catch: if you're boosting power at sea level, you're raising the combustion pressure. The engine has to be strengthened to resist those higher combustion pressures. So a ground-boosted engine is built stronger. Second, the Altitude Boosted Supercharger. This one is designed to maintain sea level values of power up to high altitude. It's not about exceeding sea level power — it's about holding it steady as you climb. But now you have a control problem. If the supercharger is capable of maintaining sea level power at high altitude, then at low altitude it would generate excessive pressure — too much boost, too much stress on the engine. So an altitude-boosted supercharger needs a control system to prevent that excessive pressure at low altitude. That's the key difference: ground-boosted needs a stronger engine, altitude-boosted needs a control system. Now, the superchargers used on aircraft engines are centrifugal compressors. And they can be driven in two ways — internally, externally, or in some installations, a combination of both. Externally driven superchargers are called turbosuperchargers, or more commonly turbochargers. They're driven by a turbine, and that turbine is rotated by the exhaust gases. So the exhaust gas energy spins the turbine, which drives the compressor, which compresses the air. Note the wording carefully: they compress the air. Internally driven superchargers are driven by gearing from the engine crankshaft. The crankshaft turns gears, which turn the compressor. And here's a subtle but important difference: internally driven superchargers compress the mixture — not just the air, but the air-fuel mixture. That's a contrast worth remembering. Turbochargers compress air before it meets the fuel; internally driven superchargers compress the mixture after the fuel has been added. The methods of operation and control of these two types are quite different, and we'll deal with them separately. So hold that thought. Now let's look at the centrifugal compressor itself. Why is it used? Because it's comparatively light, it can run at high speed, it can handle large quantities of air, and it's robust and reliable. Those are the four reasons. A centrifugal compressor is made up of two components. The first is the impeller — that's the rotating part. It accelerates the air. The second is the diffuser — that's the stationary part. It collects the air and directs it into the manifold. Here's the operating principle. Air is drawn into the impeller as it rotates. The air is accelerated as it flows outwards between the vanes. The vanes are the blades on the impeller. As the air accelerates, mechanical energy is converted into kinetic energy — the energy of motion. Then, as the cross-section of the air's path increases — that happens in the diffuser, where the passage widens — some of that kinetic energy is converted into pressure energy. So you go from mechanical energy, to kinetic energy, to pressure energy. That's the whole story of the centrifugal compressor: accelerate the air, then slow it down in a widening passage, and the speed becomes pressure. Let me show you the compressor itself. So to recap what we have: the problem is manifold pressure dropping with altitude. The solution is supercharging. Two types — ground-boosted, which needs a stronger engine, and altitude-boosted, which needs a control system. Two drive methods — external, the turbocharger driven by exhaust gases compressing air, and internal, driven by crankshaft gearing compressing the mixture. And the heart of it all is the centrifugal compressor, with its impeller that accelerates the air and its diffuser that converts that speed into pressure.