Valve timing in four-stroke petrol engines: The approximate valve overlap (simultaneous opening of intake and exhaust valves around top dead center) is generally about how many crankshaft degrees?

Introduction / Context:

Valve overlap is the period near top dead center (TDC) between the exhaust and intake strokes when both valves are open. Overlap improves scavenging and charge exchange at certain speeds but can hurt idle quality if excessive. Textbook values provide a sense of typical design choices for naturally aspirated petrol engines.

Given Data / Assumptions:

• Conventional small/medium petrol engines without extreme race cams.
• Factory street-tuned camshafts prioritizing drivability and emissions.
• Approximate values suitable for academic MCQs.

Concept / Approach:

Production engines often use modest overlap, commonly around a few tens of crank degrees. While performance and racing cams may show 50–80° or more overlap, classroom references typically cite ≈30° as a representative figure for general gasoline engines, balancing idle stability and mid-range performance.

Step-by-Step Solution:

Verification / Alternative check:

Valve-timing diagrams in textbooks frequently show intake opening slightly before TDC and exhaust closing slightly after TDC, summing to roughly 20–40°, often simplified to 30° for MCQs.

Why Other Options Are Wrong:

• 60°, 90°, 120°: typical of aggressive performance profiles; not representative of standard road engines in foundational questions.
• 15°: lower than common production baselines; some engines may approach this, but 30° is the standard teaching value.

Common Pitfalls:

• Confusing crank degrees with cam degrees (camshaft rotates at half crank speed).
• Assuming a single fixed value across all engines—here we choose the widely accepted approximate figure.

Final Answer:

30°