8.1 Reciprocating Engine Cycles, Components, and Operating Boundaries

The Four-Stroke Otto Cycle

The certificated aircraft reciprocating engine is an internal-combustion, four-stroke engine operating on the Otto cycle (a constant-volume cycle). A complete cycle requires four piston strokes, two crankshaft revolutions (720 degrees), and one camshaft revolution. The camshaft turns at one-half crankshaft speed because each valve opens once per cycle. The four strokes in order are intake, compression, power, exhaust — a useful exam mnemonic is "Suck, Squeeze, Bang, Blow."

• Intake: the piston travels down from top dead center (TDC); the intake valve is open and a fuel-air charge (or air, in fuel-injected designs) is drawn in.
• Compression: both valves close and the piston rises, compressing the charge; ignition occurs before TDC so peak pressure builds just after TDC.
• Power: the burning charge drives the piston down; this is the only stroke that produces useful work.
• Exhaust: the piston rises with the exhaust valve open, scavenging burned gas.

Valve Timing, Overlap, and Compression Ratio

Real engines open and close valves several degrees away from dead center to use gas inertia. A typical light-aircraft pattern is: intake opens about 15 degrees before TDC, intake closes about 60 degrees after bottom dead center (BDC); exhaust opens about 60 degrees before BDC and closes about 15 degrees after TDC. Valve overlap is the interval near TDC of the exhaust stroke when both valves are open at once (here about 30 degrees). Overlap improves cylinder filling and cooling at high RPM but can cause rough idle. )

Compression ratio is the ratio of total cylinder volume with the piston at BDC to the volume with the piston at TDC. Certificated avgas engines run roughly 6:1 to 8.7:1; higher ratios increase efficiency but raise detonation risk and demand higher-octane fuel. Excessive compression ratio, lean mixtures, high cylinder-head temperature, retarded timing, or low-grade fuel can cause detonation (uncontrolled near-instantaneous burning) or preignition (ignition before the spark from a hot spot), both of which can destroy a cylinder.

Major Components and Safe Boundaries

The internal parts include the crankcase, crankshaft, connecting rods, pistons, wrist (piston) pins, cylinders, valves, camshaft, lifters/tappets, bearings, gears, and accessory drives. The crankshaft converts reciprocating motion into rotation; connecting rods transmit piston force; the camshaft and valve train control breathing.

Maintenance decisions require boundaries, not theory alone. A rough-running engine may stem from fuel metering, induction leaks, ignition faults, low compression, valve problems, cooling, lubrication, or bad indications. A safe mechanic does not swap magnetos just because the engine is rough — they isolate whether the symptom follows a cylinder, an ignition source, fuel flow, a control setting, temperature, or oil pressure. Propeller movement is a major hazard: an engine can fire unexpectedly if ignition, fuel, and compression coincide.

Treat every propeller as if it could start the engine, follow the approved procedure, verify ignition and mixture configuration, and keep your body out of the arc. The ACS explicitly lists propeller movement during maintenance as a risk-management element.

Diesel-Cycle and Two-Stroke Variants, and Troubleshooting by System

Not every aircraft piston engine is a gasoline Otto-cycle engine. Compression-ignition (diesel-cycle) aircraft engines, burning Jet-A, have appeared on the certificated market; they compress air to a much higher ratio (often near 16:1 to 18:1) so the heat of compression ignites injected fuel without spark plugs or magnetos. The constant-pressure burn and high compression give better fuel economy and remove the lead-fouling and magneto-timing concerns of avgas engines, at the cost of higher peak pressures and heavier construction.

Two-stroke designs (rare in certificated aircraft, common in light-sport and ultralight powerplants) complete a power event every crankshaft revolution using ports rather than poppet valves, trading mechanical simplicity for higher fuel and oil consumption.

For every cycle type, the written test rewards thinking like a troubleshooter rather than reciting theory. Build the habit of separating the eight functional boundaries: air (induction), fuel (metering), ignition (spark/timing), compression (cylinder sealing), valves/timing (breathing), lubrication, cooling, and controls/rigging, plus the indication system itself.

A symptom such as low static RPM is not a single cause — it can be propeller load, induction restriction, fuel metering, ignition timing, low compression, exhaust restriction, control rigging, or simply density altitude. A high cylinder-head temperature could be a lean mixture, damaged baffles, retarded timing, reduced airflow, or an over-rich power setting. The best exam answer stays within approved data, avoids unapproved shortcuts, and preserves the separation between theory, indication, and actual mechanical condition before any part is replaced.