Four-stroke diesel engine valve timing (inlet): In a conventional four-stroke compression-ignition (diesel) engine, which of the following inlet-valve timing pairs best represents typical practice for opening before top dead centre and closing after bottom dead centre?

Introduction / Context:
Valve timing in four-stroke diesel engines is deliberately advanced or retarded relative to ideal dead-centre positions to improve breathing, volumetric efficiency, and scavenging. Recognising typical inlet-valve opening (IVO) and inlet-valve closing (IVC) angles helps interpret timing diagrams and diagnose performance issues.

Given Data / Assumptions:

• Standard single-intake-valve, four-stroke diesel engine.
• Angles are crankshaft degrees relative to top dead centre (TDC) and bottom dead centre (BDC).
• Objective is qualitative identification of typical timing, not an exact spec for a specific model.

Concept / Approach:

To fill the cylinder effectively, the inlet valve often opens slightly before TDC during the exhaust stroke overlap and remains open after BDC during the intake stroke. Opening early uses inertia of exiting gases to start flow; closing late takes advantage of inlet port momentum to pack more air at low-to-moderate speeds.

Step-by-Step Solution:

Verification / Alternative check:

Standard timing diagrams in textbooks show inlet opening slightly before TDC and closing well after BDC for most naturally aspirated and turbocharged diesels at design speed ranges. Manufacturers vary exact angles, but the pattern remains consistent.

Why Other Options Are Wrong:

Opening after TDC and closing before BDC shortens effective intake duration, hurting volumetric efficiency. Exactly-at-dead-centre timing ignores flow inertia. Saying it may open and close anywhere is physically incorrect. An early open with early close (e.g., before BDC) misuses the ram effect.

Common Pitfalls:

Confusing SI and CI timing magnitudes; overlooking overlap; assuming idealised at-TDC/BDC events from simple schematics.

Final Answer:

Opens 20° before TDC and closes 40° after BDC