What is the purpose of scavenging in a two-stroke diesel, and how do scavenging air characteristics influence engine efficiency?

• A. Scavenging removes lubricating oil from the cylinders; scavenging air quality does not affect efficiency.
• B. Scavenging cools the piston surface to reduce wear; air characteristics determine heat transfer.
• C. Scavenging replaces exhaust gases with fresh air; proper scavenging improves volumetric efficiency and combustion quality, while poor scavenging causes residual gas dilution and power loss.
• D. Scavenging compresses the air to increase compression ratio; air characteristics do not affect efficiency.

Answer: (D)

In a two-stroke diesel, scavenging is about clearing the used exhaust and filling the cylinder with fresh air so the next combustion event has enough oxygen. The way the scavenging air behaves—its pressure relative to the cylinder, its flow rate and momentum, and how evenly it distributes into the chamber—directly affects engine efficiency. When scavenging is effective, most of the cylinder volume is replaced with fresh air, boosting volumetric efficiency and providing a good air charge for combustion. This leads to complete or near-complete combustion, better power output, and cleaner exhaust.

If scavenging is poor, residual exhaust gases remain and mix with the new charge. That dilutes the air, lowers available oxygen, and can delay or incomplete combustion, causing power loss, higher fuel consumption, and increased emissions. The characteristics of the scavenging air—pressure, velocity, and distribution—are therefore crucial to achieving high efficiency.

The other options miss the primary purpose: scavenging is not about removing lubricating oil, cooling the piston, or compressing the air to raise compression ratio. While air characteristics can influence cooling and heat transfer, the central role is replacing exhaust with fresh air and how that exchange governs performance.