Powerplant: Reciprocating and Turbine Engines
Reciprocating engine fundamentals
Expect questions on the four-stroke cycle (intake, compression, power, exhaust), valve timing and overlap, compression ratio, ignition timing, and the difference between detonation and preignition. Detonation is the rapid, uncontrolled burning of the charge after normal ignition, driven by high manifold pressure, lean mixture, or high CHT. Preignition is ignition of the charge before the spark, usually from a hot spot such as a glowing carbon deposit or overheated plug. Both raise CHT; detonation usually shows a pressure spike and rough running, preignition often shows a rising temperature trend that continues after power reduction.
Fuel metering, induction, and cooling questions usually test cause and effect: carburetor heat richens the mixture and slightly reduces RPM because heated air is less dense; carburetor ice forms most readily at moderate temperatures with high humidity. Magneto questions hinge on the P-lead - a broken P-lead lets the magneto keep firing even with the switch OFF, which is why the grounded-lead check matters before shutdown.
Worked example: torque to horsepower
Brake horsepower = (torque in lb-ft × RPM) ÷ 5,252. An engine producing 300 lb-ft at 2,400 RPM makes (300 × 2,400) ÷ 5,252 = 720,000 ÷ 5,252 ≈ 137 hp. The classic trap is using propeller RPM instead of engine RPM, or forgetting the reduction gear ratio.
Turbines and propellers
Turbine questions focus on the compressor–combustion–turbine sequence, N1 versus N2, and EGT. EGT peaks during start; exceeding the start limit is the classic hot-start damage. Propeller questions test blade angle versus RPM and why a constant-speed propeller needs a governor.
Rule of thumb
If an answer choice fixes a temperature or mixture problem by changing ignition timing, it is almost always wrong - timing does not cure detonation.