Piston Engines - Performance and Power Augmentation — Page 147, Lesson 190

Let's start with the internally driven supercharger. This is the type of supercharger that is mechanically driven by the engine itself, and it's generally used on medium and high powered piston engines — that's roughly 250 brake horsepower and above. The key point here is where it sits in the induction system: it's fitted downstream of the throttle valve. So the air passes through the throttle first, and then it reaches the supercharger, which compresses it before it goes into the induction manifold. Now, in the past, high powered engines often drove the supercharger at two different speeds. The idea was to save power at low altitudes. At low altitude, the air is already dense, so you don't need as much compression — you'd use the low speed gearing. At high altitude, where the air is thin, you'd switch to the high speed gearing to get more compression. Some high powered superchargers even had two impellers working in series — that means one impeller feeding into another — to raise the overall compression ratio. But current engines generally use a single impeller driven at a fixed speed ratio to the crankshaft, and that ratio is usually between 6:1 and 12:1. So the impeller spins six to twelve times for every revolution of the crankshaft. This type of supercharger is usually capable of maintaining sea level manifold pressure up to an altitude of 5000 to 10,000 feet, at Rated Power 1 settings, depending on the gear ratio. So at sea level, the manifold pressure is what you'd expect at sea level, and the supercharger can hold that pressure even as you climb — up to that altitude range, as long as you're at the rated power setting. Now let's look at how the drive works. There's a shaft that is splined into the rear of the crankshaft — that's the initial drive to the supercharger impeller. This shaft may incorporate a Spring Drive Unit. The spring drive transmits the drive through intermediate gears to the impeller pinion. Its job is to limit the torque transmitted to the supercharger impeller during high rates of propeller or engine acceleration or deceleration. Think of it as a shock absorber — it smooths out the sudden changes in torque so the impeller doesn't get damaged. It may also include a centrifugal clutch. Finally, let's talk about the controls. The supercharger is designed to compress air and provide sea level pressure, or greater, in the induction manifold when atmospheric pressure is low. But here's the problem: when atmospheric pressure is high — like at low altitude on a cold day — the supercharger could produce excessive manifold pressures. So it's necessary to restrict throttle opening below full throttle height. Full throttle height is the altitude at which the throttle can be fully open without exceeding the manifold pressure limit. Below that altitude, you have to limit how far the throttle opens. To relieve the workload on the pilot, this is often done automatically. There are two controls that affect the pressure developed by the supercharger — and that's where we'll pick up next.