Engine lubrication — understanding crankcase dilution What does the term ‘‘crankcase dilution’’ mean in automotive engines?

Introduction / Context:
Crankcase dilution degrades oil viscosity and lubricity, accelerating wear. It commonly arises from incomplete combustion or excessive enrichment during cold starts and idling.

Given Data / Assumptions:

• Four-stroke automotive engine with wet sump.
• Some fuel can bypass rings as blow-by or condense on cold cylinder walls.
• Normal PCV (positive crankcase ventilation) in place.

Concept / Approach:
Unburned fuel can wash past piston rings and enter the crankcase, mixing with lubricating oil. The lowered viscosity thins the hydrodynamic film and reduces boundary lubrication performance, especially at bearings and cam–follower interfaces.

Step-by-Step Solution:

Verification / Alternative check:
Oil analysis shows reduced viscosity and elevated fuel percentage; odor of fuel in oil and rising oil level can be observed.

Why Other Options Are Wrong:

• Water dilution: possible via coolant leak or condensation but not the definition of crankcase dilution.
• Two-stroke mixture flow: not applicable to four-stroke crankcase lubrication.
• Fuel addition intentionally: never recommended; harmful.

Common Pitfalls:
Ignoring warm-up cycles; extended idling in cold weather can exacerbate dilution. Regular oil changes and proper engine management mitigate risk.

Final Answer: