Valve timing terminology Valve overlap is defined as the crank/ cam period during which which condition exists?

Introduction / Context:
Valve overlap is a deliberate design choice in four-stroke engines to enhance scavenging and volumetric efficiency. It occurs around top dead center (TDC) between the exhaust and intake strokes.

Given Data / Assumptions:

• Four-stroke timing: exhaust stroke ends as intake begins.
• Cam profiles may keep both valves off their seats briefly.
• Overlap magnitude depends on engine speed target and emissions strategy.

Concept / Approach:
Overlap is specifically the period when the exhaust valve has not yet closed while the intake valve has already opened. This allows exhaust momentum to help draw in the fresh charge and can cool the combustion chamber edges. However, excessive overlap can impair idle quality.

Step-by-Step Solution:
Locate TDC between exhaust and intake strokes on timing diagram.Identify period where intake lift > 0 and exhaust lift > 0.That period is “both valves open,” i.e., valve overlap.

Verification / Alternative check:
Cam cards list IVO/EVC angles; overlap = IVO (degrees BTDC) + EVC (degrees ATDC) at cam or crank scale depending on convention.

Why Other Options Are Wrong:
Both valves closed is compression or power dwell, not overlap.

“Both a and b” is contradictory.

“None of these” is incorrect because a standard definition exists.

Common Pitfalls:
Confusing overlap with lash or with centerline angles; forgetting variable valve timing can change overlap dynamically with RPM/load.

Final Answer:
both valves are open