If you are not sure of the meaning of such terms as normally aspirated, turbocharged, supercharged or direct drive engines, then perhaps you'll want to read our simplified definition of them.
The Normally Aspirated Engine is one that is not turbocharged or supercharged. If the airplane has a manifold pressure gauge, at full takeoff power at sea level on a standard day it would indicate a MP reading of approximately 29" of Hg. Takeoff power at 5,000 ft. density altitude airport would read about 24" MP. The normally aspirated engine uses atmospheric pressure and is thereby altitude limited.
Direct Drive Engines are those piston powered engines where the propeller is bolted on the end of the crankshaft and the prop turns at the same speed as the crankshaft.
Geared Engines are usually the higher powered, more complex engines using a reduction gear on the nose of the aircraft, and with the prop attached to it. As a result, the prop will turn somewhat slower than the crankshaft, resulting in a lower prop noise level. When the engine is geared, we precede the engine designation with a "G." Thus a geared, opposed (O) normally aspirated Lycoming engine with a 480 cubic inch displacement of the cylinders would be designated a GO-480 model.
Turbocharged Engines as manufactured by Lycoming simply consist of a turbocharger unit with a small turbine wheel attached by a common shaft to a compressor wheel, and utilizes the engine exhaust gas by directing it over the turbine wheel to drive the compressor. The horsepower loss in operating the turbocharger is negligible. Turbocharging can provide greater utility to the piston engine by providing sea-level horsepower, in some models, as high as 20,000 feet; or it can be used to add horsepower to the engine particularly for takeoff. The faster the engine runs, the more air the turbocharger can pack into the cylinder to compensate for the thin air of altitude, or to increase the horsepower. Although this definition is somewhat over-simplified, it is a basic definition of turbocharging of General Aviation power-plants.
Where turbocharging is used with a fuel injected, opposed Textron Lycoming engine with a 540 cubic inch displacement, we designate it as a TIO-540 model. "T" represents the -turbocharging.
Supercharged Engines as manufactured by Textron Lycoming used a compressor wheel to pack air into the cylinders; but the compressor is driven by the crankshaft through an intricate gearing system, which takes considerable horsepower from the engine to operate. In comparison with a turbocharged engine, it is a medium altitude powerplant.
Although supercharged engines could be built by Textron Lycoming, new aircraft designs during the past twenty or more years have used turbocharging instead of supercharging because of the advantages that turbocharging offers.
A supercharged, geared, opposed, fuel injected Textron Lycoming engine with cylinders of 540 cubic inch displacement is designated an IGSO-540 model. "S" represents supercharging.