Diesel four-stroke valve timing (typical) For a four-stroke Diesel engine, which of the following intake valve timing pairs is most representative of practice?

Introduction / Context:
Valve timing is advanced/retarded to improve cylinder filling and scavenging. Knowing typical ranges helps read timing diagrams and diagnose cam phasing faults.

Given Data / Assumptions:

• Conventional naturally aspirated or mildly boosted four-stroke Diesel.
• Values vary by design but fall within common ranges.
• Cam phasing aims at volumetric efficiency and smooth operation.

Concept / Approach:
Intake valves often open slightly before TDC (end of exhaust stroke) to use scavenging and close after BDC during the intake stroke to leverage inertia of the incoming charge. Representative numbers: open 10°–25° BTDC, close 20°–60° ABDC.

Step-by-Step Solution:
Evaluate options against typical ranges.Option b (20° BTDC open, 35° ABDC close) lies comfortably within norms.Option a (exact TDC/BDC) ignores real-world gas dynamics.Option c (late open, early close) reduces breathing.

Verification / Alternative check:
Manufacturer timing charts confirm early opening and late closing for intake in many Diesel engines.

Why Other Options Are Wrong:
Opening exactly at TDC wastes scavenging benefits.

Late opening and early closing restrict mass flow.

Extreme values like 60°/60° are atypical for standard road Diesels.

Common Pitfalls:
Assuming one exact value fits all engines; cam wear or incorrect belt timing can shift these points and cause hard starting or smoke.

Final Answer:
opens about 20° before TDC and closes about 35° after BDC