Definition
The point in the engine's compression stroke, expressed in degrees of crankshaft rotation before top dead center, at which the spark plug fires to ignite the fuel-air mixture in the cylinder.
Plain English
The exact moment the spark plug fires inside the cylinder, set so the fuel-air mixture burns at just the right time to push the piston down with maximum force.
Context Anchor
Pilots most often encounter ignition timing in piston-engine maintenance discussions, engine troubleshooting, hard-starting complaints, and magneto-related inspections.
Derivation
Ignition comes from the Latin ignire, meaning 'to set on fire.' Timing refers to when something happens in a sequence. Together: when the fire is set off in the cylinder relative to the piston's position.
Why Pilots Care
Correct ignition timing delivers maximum power and efficiency while preventing detonation or pre-ignition that can damage the engine.
Grounding Statement
In a piston engine, the spark is timed so the fuel-air mixture is already burning when the piston is ready to be pushed down for power.
Intuition Check
Ignition timing is not how long the spark lasts. It is the exact point in the piston’s travel when the spark begins.
Example Sentence 1
The mechanic checked the ignition timing during the annual inspection to make sure both magnetos fired at 25 degrees before top dead center.
Example Sentence 2
Retarded ignition timing reduced power and caused the engine to run hotter than normal.