Typical scavenging efficiency for a four-stroke diesel engine (naturally aspirated or mildly boosted) falls within which range?

Introduction / Context:
Scavenging efficiency describes how effectively fresh charge replaces residual gases. In two-stroke engines this is a central design challenge; in four-stroke engines, separate exhaust and intake strokes generally allow near-complete clearing of exhaust products, especially with proper valve timing and moderate boost.

Given Data / Assumptions:

• Four-stroke CI engine with distinct exhaust and intake strokes.
• Conventional valve overlap optimized for breathing.
• No extreme EGR intentionally retained within the cylinder for this conceptual question.

Concept / Approach:
Because exhaust and intake occur on separate strokes with piston-driven displacement, the cylinder has opportunity to expel most burned gases before fresh air induction. As a result, scavenging efficiency approaches unity (near 100%) under typical conditions. This is fundamentally higher than many uniflow or loop-scavenged two-strokes, where short-circuiting and mixing reduce efficiency.

Step-by-Step Solution:

Verification / Alternative check:
Textbook values and engine simulation results typically place four-stroke scavenging efficiency very close to 1.0, albeit volumetric efficiency may vary. This supports the 95–100% selection.

Why Other Options Are Wrong:

• Below 50% or 50–85%: Typical of poorly scavenged two-strokes, not healthy four-strokes.
• 85–95%: Possible under unusual constraints, but well-tuned four-strokes commonly exceed this.

Common Pitfalls:
Confusing scavenging efficiency with volumetric efficiency; the latter may be 70–110% depending on speed and boosting, while scavenging efficiency in a four-stroke is usually very high.

Final Answer:
between 95 and 100%