Piston Engines

Understanding Piston Engines in ATPL-Level Aircraft Systems

Piston engines power a large portion of general aviation and remain a core topic in ATPL theory and practical procedures. These engines operate on the four-stroke cycle—intake, compression, power, and exhaust—where only the power stroke delivers useful work. The piston’s reciprocating motion is transformed into rotary motion by the crankshaft. For efficiency, manufacturers optimize valve and ignition timing (including valve overlap and ignition advance before top dead center), improving volumetric and overall efficiency. Ignition is self-contained: rapid flux changes in the magneto’s primary coil occur when the contact breaker points open, and the distributor routes high-tension (secondary) current to the spark plugs in the correct firing order. On normally aspirated engines, the manifold pressure (MAP) gauge reads below ambient whenever the engine is running due to throttle-induced pressure drop in the intake.

Mixture control is central to performance, engine health, and compliance with operating procedures in the POH and aviation regulations. As altitude increases in a normally aspirated engine, air density falls; with a fixed-pitch propeller at constant RPM and the mixture left full rich, both air mass flow and fuel flow decrease, but the mixture becomes too rich relative to the reduced oxygen. Outcomes include power loss, plug fouling, and higher fuel consumption; therefore, pilots must lean progressively in climb and cruise. Applying carburettor heat reduces the density of intake air and thus enriches the mixture further—useful for preventing or clearing carb ice, but it may require re-leaning. Fuel system knowledge is also critical: “vapour lock” occurs when fuel vaporizes in lines, forming bubbles that block flow, typically in hot conditions or after heat soak. From a performance standpoint, specific fuel consumption (SFC) is the mass of fuel required to produce unit power (or thrust) per unit time, a key metric when comparing engines and settings.

Propeller and engine interactions underpin many exam questions and real-world procedures. For a fixed-pitch propeller at constant RPM, increasing true airspeed reduces the propeller blade angle of attack; the propeller’s slipstream effect is most pronounced at low airspeeds and high power, influencing control and yaw. With a constant-speed prop using a single-acting pitch control unit, engine oil pressure drives blades toward a smaller pitch (fine) for takeoff and climb; loss of pressure allows counterweights/springs to move blades toward coarse. Turbochargers, driven by exhaust gases, maintain or restore MAP at altitude, whereas normally aspirated engines lose power in climb as density drops. Use the specified fuel grade—operating with a lower octane can trigger detonation, elevating cylinder head and oil temperatures. Expect about a 100 RPM drop with a single magneto inop on a fixed-pitch engine. Across all operations, manage CHT and oil temperature within limits, adhere to the AFM/POH, and follow approved procedures under applicable aviation regulations.

What This Question Bank Covers

• Four-stroke cycle fundamentals, components (crankshaft, valves), and efficiency improvements.
• Induction and mixture control: altitude effects, leaning techniques, carburettor heat and carb icing.
• Fuel systems: vapour lock, fuel grades/octane, detonation and pre-ignition, temperature management.
• Ignition systems: magnetos, breaker points, distributors, and operational checks (mag drops).
• Propeller theory: fixed-pitch vs constant-speed, single-acting PCU behavior, slipstream effects.
• Engine boosting and instrumentation: exhaust-driven turbochargers, manifold pressure interpretation.
• Performance metrics and procedures: specific fuel consumption (SFC), climb/cruise power and RPM/MAP management.