Definition
In reciprocating engine cylinder operation, the lowest position of the piston travel within the cylinder, commonly referred to as bottom dead center (BDC). It marks the end of the intake and power strokes and the beginning of the compression and exhaust strokes in a four-stroke cycle.
Plain English
The point where the piston has traveled as far down inside the cylinder as it can go before reversing direction and moving back up.
Context Anchor
Seen in piston engine discussions about piston travel, valve timing, and engine position.
Derivation
Bottom comes from an Old English word meaning the lowest part or foundation of something. In an engine cylinder, it keeps that same basic idea: the lower end of the piston’s path.
Why Pilots Care
Engine timing events — when valves open and close, and when ignition fires — are described relative to piston position. Knowing what 'bottom' refers to is essential for understanding timing diagrams, valve overlap, and troubleshooting engine performance issues.
Intuition Check
Do not read bottom here as just the underside of the engine. In this powerplant context, it means the low end of the piston’s travel inside the cylinder.
Example Sentence 1
When the piston reaches bottom, the intake valve closes and the compression stroke begins.
Example Sentence 2
Wear on the cylinder wall is often greatest near bottom due to side loading.