If your aircraft needs more engine power, you can supercharge it!

Note two things from our "Simple Aerodaynamics" and "How Engines Make Power" lessons:
1) The thrust direction opposes drag.
2) “A mixture of fuel and air is compressed by the pistons…”

Let’s expand on point 1). We can easily imagine that more thrust will give us more airspeed. But imagine aiming the airplane up slightly, as if it were in a climb. The thrust arrow [term: vector] points up slightly but the weight vector will still aim straight down to the center of the earth. Some of the engine thrust will now offset weight. This gives the aircraft the ability to climb. More thrust gives more speed or the ability to climb to a higher altitude plus the ability to climb more quickly to that altitude.

Let’s expand on point 2). If you can shove more fuel-air mixture into the cylinder of a piston engine, you will produce more power and thrust.

Both military and civilian aviation have always had a need for speed, but military aviation in particular, had a need to fly at a higher altitude. (Civil aviation did not need high altitude performance until the development of pressurized cabins eliminated the need for people to wear uncomfortable oxygen masks at altitude.)

You need compressed air [term: boost] to keep maximum engine performance at higher altitudes because the density (compactness) of air decreases with altitude. Air density decreases because, as you fly away from the earth, there is less air to pile on top of the air below it. This concept is easy to visualize by piling the kids onto each other. The bottom kid has the most pressure on top of him; the top kid has the least.

Note how the density of air decreases with altitude in the following chart. You only have half the density by the time you reach 20,000 feet.

How can you shove [term: supercharge] more air-fuel mixture into an engine?

You can put more air into your engine by placing a supercharger between the air intake and the engine, either before or after the carburetor (or injector body). The air blower [term: compressor] in a supercharger resembles the blower in a hair dryer except that diffuser vanes (fan blades aimed backwards) are added outside the blower to straighten the air and raise the air pressure.

The Impeller in a supercharger's compressor is like the impeller in a hair dryer. Stationary vanes are added outside the impeller to make the compressor work better.

There are at least three different ways to drive the compressor.
1) Gear it directly to the engine’s crankshaft (or geared accessory section)
2) Let the gasses that come out the exhaust pipe spin a fan [term: turbine] which in turn, spins the compressor.
3) Use a large hamster to run a treadmill with a shaft hooked up to the compressor.

Let’s start with method 3). This will not work well! First of all, even large hamsters cannot produce much power. Also hamsters are not reliable unless it’s in the middle of the night and you are trying to get to sleep. Perhaps the biggest limitation of hamster power is that, if you use supercharging for altitude performance, the hamster would not be able to breath well at altitude (no company produces oxygen masks for hamsters).

Let’s expand on method 1). Gearing a compressor directly to the engine is a wonderfully simple system. You don’t have to worry about compressing the air too much [term: over-boost] because the amount of pressure boost is directly proportional to engine speed, and therefore limited by the engine speed.

Since the compressor is attached to the engine it is easy to deliver oil to the shaft that the compressor spins on. Another advantage of the gear driven supercharger is that some engine configurations, (mainly a radial engine) allow placement of the compressor in compact space.

A supercharger fits neatly into a radial engine. (Round engine with cylinders "radiating out" from the center.)

The disadvantage of the gear driven supercharger is that as air pressure decreases with altitude, the supercharger, spinning at a constant speed, cannot maintain its high pressure boost.

Method 2) is called turbo-supercharging. Exhaust gages coming out of the engines cylinders are sent through a turbine. The turbine’s shaft turns the compressor.

Turbo-supercharging produces good boost at altitude, the turbine spins faster at higher altitudes because there is less air pressure restricting the flow out of the exhaust pipe. Even if there was no natural tendency for the turbine to spin faster, its speed can be regulated by changing the amount of exhaust that is routed to the turbine.

Besides altitude performance, a big advantage of turbo-supercharging is that it can be added externally to existing engine designs (although it’s a good idea to change to pistons that have less natural compression in the cylinders). Many modern automotive applications use turbo-supercharging for boost, just because of this add-on convenience, even though a geared supercharger would be better in automotive applications).

The turbo supercharger at the left of the picture is added onto an existing engine. You can see the compressor. On the other side of the compressor is the turbine which is spun by hot exhaust gases.

The disadvantages of turbo-supercharging are numerous. The turbine-compressor unit needs to be oiled via extra oil lines, outside of the engine. Also, turbines run extremely hot which makes the engine oil’s job of lubrication, even more critical. Perhaps the biggest problem with turbo-supercharging is the complexity added to give the correct amount of boost and to prevent over-boost. A spring-loaded door [term: waste-gate] located on the exhaust pipe, just before the turbine, is used to release excess pressure.

Now that you know about supercharging an engine, you may wish to add one to your parent's car. You can either add a belt driven supercharger or a exhaust driven turbo supercharger. If you are not an expert mechanic choose the older car because your first project will often be a failure (you have to start somewhere). If by chance you do succeed, get the camera out to record the look of surprise on Mom and Dad's face as they peel out [term: burn rubber] of the driveway. Enjoy it because you're about to be grounded for a year.

Need More Engine Power? Supercharge it!