Events That Reduce Volumetric Efficiency – Vertical Compression-Ignition Engine Which valve-timing event is most likely to reduce the volumetric efficiency of a vertical diesel (compression-ignition) engine?

Introduction / Context:
Volumetric efficiency (ηv) determines how much fresh air (in a diesel) the cylinder traps per cycle. Valve timing strongly affects ηv by allowing more or less time for induction and by exploiting flow inertia for extra filling after piston reaches BDC.

Given Data / Assumptions:

• Vertical four-stroke CI engine with conventional swirl-inducing ports.
• Focus on qualitative effect of timing rather than exact crank degrees.
• Steady operation near rated speed.

Concept / Approach:

Keeping the inlet valve open slightly after BDC (ABDC) lets the incoming air, which has momentum, continue to flow into the cylinder even as the piston starts rising. This “ram effect” increases trapped mass, improving ηv. Conversely, closing the inlet valve before BDC truncates the induction period, reducing the amount of charge admitted and thus decreasing ηv. Other timing events have smaller or secondary effects on ηv in this context.

Step-by-Step Solution:

Verification / Alternative check:

Engine breathing models and experimental tuning show ηv peaking with optimized IVC slightly ABDC; advancing IVC toward BBDC reduces airflow significantly at speed.

Why Other Options Are Wrong:

ABDC closing usually helps ηv; IVO BTDC assists with overlap/tuning; EVC ATDC affects residuals but not as decisively as premature IVC; EVO BBDC concerns blowdown and does not directly limit induction duration.

Common Pitfalls:

Assuming any late closing always helps—at very low speed, excessive late closing can cause reversion, but within typical operation, BBDC closing is clearly detrimental.

Final Answer:

inlet valve closing before bottom dead centre (BBDC)