The four strokes

Learning objective: Name the four strokes with valve position, piston direction, 720° crank / 360° cam, ignition near TDC (practically BTDC).

60 seconds: Intake ↓ inlet open → Compression ↑ both closed → ignition → Power ↓ both closed → Exhaust ↑ exhaust open. One cycle = two crank revolutions.

The Otto four-stroke engine performs four strokes per combustion cycle with two complete crankshaft revolutions:

→ Order: 1. Intake — 2. Compression — 3. Power (Ignition) — 4. Exhaust.

Cylinder arrangement in light aircraft engines

The four strokes of the Otto cycle: intake, compression, power, exhaust. Two crankshaft revolutions per cycle.

The most common cylinder arrangement in light-aircraft engines and helicopter engines is the boxer with horizontally opposed cylinders — e.g. Lycoming O-235, O-320, O-360, Continental O-200, IO-360. Features:

• 4 or 6 cylinders, paired opposite.
• Low CG, compact form.
• Good cooling via airflow (air-cooled).
• Mechanically balanced — low vibration.

Touring Motor Gliders (TMG) such as Grob G109, Diamond HK36 Super Dimona or Stemme S10 typically use 4-cylinder 4-stroke engines (often Limbach L2000/L2400 or Rotax 912) — compact, light, sufficient power for TMG use.

Valve timing diagram

Real valve timing is more complex than the ideal 4-stroke model:

• Intake valve opens before TDC (valve lead), closes after BDC (valve lag) — better cylinder filling.
• Exhaust valve opens before BDC, closes after TDC — better scavenging.
• Valve overlap — region where both valves are briefly open; uses gas inertia.
• Ignition occurs BEFORE TDC (e.g. 20–30° before TDC), as combustion takes time and peak pressure should act after TDC on the descending piston.

Idle system — independent idle circuit

A carbureted engine continues to run at closed throttle in idle — because the carburettor has an independent idle system:

• A separate nozzle in the main carburettor circuit supplies a small fuel flow even with throttle closed.
• The engine keeps running at ~700-900 RPM in idle.
• Without the idle system the engine would stop at every throttle reduction.

Maximum take-off power duration

The maximum time during which the engine may be operated with take-off power is specified in the flight manual (AFM/POH) — typically:

• 5 minutes for many standard light-aircraft engines (Lycoming, Continental).
• No limit for lower power settings (climb, cruise).

→ On extended climb the pilot watches CHT and reduces power to climb setting (cruise climb) as needed.

Hazards — engine can start even with ignition off

Glow ignition in a hot engine

When restarting a still-hot engine, the engine may start even with the ignition switch off if glow ignition occurs:

• Hot carbon deposits or glowing cylinder spots ignite the mixture without a spark.
• A faulty ignition switch can also cause uncontrolled start.

→ Consequence: after shutting down a hot engine, pull mixture fully to idle cut-off, set ignition OFF, then leave the cockpit. Never assume "ignition OFF" alone makes the engine safe.

!! Mortal danger !!: Ignition ON + manual propeller movement

If the ignition switch is on and someone moves the propeller by hand before start, the engine can start:

• Magnetos generate ignition voltage even at slow rotation of the crankshaft.
• Result: uncontrolled start with engine running — severe or fatal injuries.

→ Never hand-rotate the propeller without verifying that the ignition switch is OFF.

!! Mortal danger !!: Missing ground connection of the ignition system

A missing ground connection (missing ground) in the ignition system causes the engine to be able to start even with the ignition switch in "OFF". Reason:

• The ignition switch works by grounding the magneto's primary winding (P-lead / short-circuit wire) to disable it — not the secondary (plug lead).
• With a broken P-lead (wire break, corrosion), the magneto is not silenced → ignition voltage is generated despite the "OFF" position.
• A pilot or mechanic who turns the propeller will get an uncontrolled engine start.

→ Standard procedure: a short-circuit test is performed before engine shutdown (briefly switch ignition OFF → RPM should drop / engine slow). If not, the P-lead is faulty → maintenance immediately.

Magneto check with constant-speed propeller

For an aircraft with constant-speed propeller (CSU), the magneto check (during run-up) is only correctly performed when:

• The brake test at the specified RPM (POH) is performed, AND
• The RPM selector lever (propeller pitch control) is at maximum RPM (fine pitch).

Reason: at coarser pitch the CSU governor would hold the RPM constant and a mag-drop could not be cleanly detected.

Aviation Maintenance Technician Handbook — Powerplant*; Lycoming O-235 / O-320 / IO-360 Operator's Manual; Continental O-200 / IO-360 Operator's Manual; FAA AC 20-103A Aircraft Engine Crankshaft Failure.*

Exam knowledge

DE/EN Ansaugen (Induction/Intake), Verdichten (Compression), Arbeiten (Power), Ausstoßen (Exhaust). Ignition is not a fifth stroke. The P-lead grounds the primary winding.

Check yourself

1. Otto-cycle four-stroke order?

• A) Compression — intake — exhaust — power
• B) Intake — compression — power — exhaust
• C) Intake — power — compression — exhaust
• D) Compression — power — intake — exhaust

2. During compression, piston and valves?

• A) Piston down, intake open, exhaust closed
• B) Piston down, both valves closed — ignition near TDC
• C) Piston up, both valves closed — mixture compressed
• D) Piston up, exhaust open, intake closed

1. B — intake, compression, power, exhaust.
2. C — stroke 2. A is intake, B power, D exhaust.