Powerplant Theory and Engine Systems
This domain covers reciprocating and turbine engine theory, the systems that keep them running, and the instruments that reveal trouble. Expect cycle questions plus practical limits.
Reciprocating engines
- Four-stroke cycle: intake, compression, power, exhaust. The magneto provides independent ignition; the carburetor or fuel injection meters fuel.
- Detonation is uncontrolled rapid burning after normal ignition, driven by high manifold pressure, high temperature, or low octane. Pre-ignition is ignition before the spark, often from a hot spot. Both raise CHT; detonation can be reduced by enriching the mixture and reducing power.
- Wet-sump and dry-sump lubrication; oil pressure and temperature are the primary health indicators.
Turbine engines
- Brayton cycle: intake, compression, combustion, exhaust. Axial and centrifugal compressors; can-annular or annular combustors; turbine stages drive the compressor and accessories.
- N1 is the fan or low-pressure spool, N2 the high-pressure core. EGT and ITT are the limiting temperatures; trend them, do not just compare to redline.
Propellers
Constant-speed propellers use a governor to hold RPM; blade angle increases with airspeed. Feathering aligns the blade with the relative wind; reverse produces negative blade angle. Geometric pitch minus effective pitch equals slip.
Worked example
A four-cylinder engine has a 5.125 in bore and 4.375 in stroke. Cylinder volume = 0.7854 x 5.1252 x 4.375 = 90.25 in3. Total displacement = 90.25 x 4 = 361 in3, the familiar O-360.
Traps that cost marks
- Mixing detonation and pre-ignition symptoms.
- Using automotive oil or unapproved spark plugs.
- Treating a single EGT reading as normal without a trend.
- Confusing feathering with reverse.