Piston Engines - Performance and Power Augmentation — Page 143, Lesson 175

I want to walk you through how a centrifugal supercharger actually builds pressure, because this is the heart of power augmentation on a piston engine. Let's start with the impeller itself. The impeller is the spinning part that does the first stage of compression. The proportion of pressure gained in the impeller depends on three things: the impeller's diameter, its speed of rotation, and the shape of its vanes. Those vanes are the curved blades that fling the air outward. So bigger diameter, faster spin, and the right vane shape all mean more pressure rise. Now, here's the key part. The air leaves the impeller with considerable tangential and radial velocity. Tangential means it's moving around the circumference, and radial means it's moving outward from the centre. That air is moving fast, but it's not yet at high pressure. To convert that speed into pressure, it passes into the diffuser. The diffuser consists of a number of vanes fixed between the walls of the supercharger casing. Those vanes form divergent passages. Divergent means the passages get wider as the air travels through them. And here's the physics: when a passage widens, the air slows down, and as it slows, its pressure increases. So the diffuser's job is to decrease the velocity and increase the pressure of the air passing through it. But there's a catch. Compressing air rapidly increases its temperature. And that temperature rise reduces some of the increase in density that the pressure rise would otherwise give you. Think of it this way: you've gained pressure, but the air is hot, and hot air is less dense, so you've lost some of the benefit. That loss of density can be partially recovered in one of two ways. Either you pass the air through an intercooler, which is a heat exchanger that cools the compressed air, or you spray fuel into the eye of the impeller. The eye is the centre inlet of the impeller. When the fuel vaporizes there, it absorbs heat and reduces the air temperature. Both methods recover some of that lost density. Now, let's talk about the limits. At a particular speed of rotation, a centrifugal supercharger increases the pressure of the air passing through the impeller in a definite ratio. That's the pressure ratio. But physical constraints limit the speed of rotation and the size of an impeller. So those constraints limit the pressure rise, or pressure ratio, and consequently limit the power output or the maximum operating altitude of the engine to which it's fitted. In practice, pressure ratios up to 3:1 are generally obtainable. That means the outlet pressure is three times the inlet pressure. If you need any further compression beyond that, you'd have to fit two compressors in series. In series means the outlet of the first compressor feeds into the inlet of the second, so the compression stages add up. Now, let's move to manifold pressure, because this is what the pilot actually manages. Any engine with a supercharger will also be equipped with a variable pitch propeller controlled by a constant speed unit. Let me unpack that. A variable pitch propeller can change the angle of its blades, and the constant speed unit automatically adjusts that pitch to keep the engine rpm constant. So the rpm of the engine is therefore controlled by the propeller pitch lever, not by the throttle. That leaves the throttle to control something else: the amount of pressure entering the cylinders. To properly set the power and prevent the engine being overboosted, the pilot must have an indication of the amount of pressure he or she is allowing into the cylinder with the throttle. That indication is the manifold pressure. Overboosting means forcing more pressure into the cylinders than the engine is designed to handle, which can cause detonation or mechanical damage. So the manifold pressure gauge tells the pilot how much boost is actually going in, and the pilot uses the throttle to set it correctly while the constant speed unit holds the rpm steady. So the whole picture is this: the impeller and diffuser build the pressure, the intercooler or fuel spray recovers density lost to heat, the pressure ratio is capped at about 3:1 before you need two compressors in series, and on the flight deck, the pilot manages that boost through the throttle while watching the manifold pressure gauge, with the constant speed unit holding rpm constant through the propeller pitch.