Definition
The brief period in the engine cycle, near the end of the exhaust stroke and beginning of the intake stroke, when both the intake and exhaust valves are open at the same time. Measured in degrees of crankshaft rotation, valve overlap allows the outgoing exhaust gases to help draw the incoming fuel-air mixture into the cylinder, improving cylinder scavenging and cooling.
Plain English
A short moment when both valves on a cylinder are open together. The exhaust leaving on its way out helps pull the fresh fuel-air mixture in.
Context Anchor
Seen in aircraft engine maintenance, especially when discussing valve timing, camshaft design, and reciprocating engine operation.
Derivation
Overlap simply means two things happening at the same time. Here, the opening times of the two valves overlap by a few degrees of crankshaft rotation.
Why Pilots Care
Proper valve overlap improves scavenging of exhaust gases, increases volumetric efficiency, and supports better power output and fuel economy in piston aircraft engines.
Analogy
It is like one person leaving through a doorway while the next person starts entering before the door is fully clear. For a brief moment, both actions are happening at once.
Intuition Check
Valve overlap does not mean one valve physically lies over another valve. It means the open times of the intake and exhaust valves overlap for a short moment.
Example Sentence 1
During valve overlap, the momentum of the exiting exhaust gases helps draw the fresh charge into the cylinder.
Example Sentence 2
The engine's design uses valve overlap to help clear exhaust gases and draw in a fresh fuel-air mixture more effectively.