Question 42 3AE01 - Third Assistant Engineer

The Correct Answer is C ### Why Option C (Oxidation inhibitors) is Correct Lubricating oil in a running diesel engine is subjected to high temperatures and the presence of air, which promotes a chemical reaction called oxidation. This process breaks down the base oil and additives, leading to the formation of acidic byproducts and insoluble contaminants (like polymers and resins). These insoluble materials are the primary components of **sludge** (thick, black deposits) and **varnish** (thin, hard, lacquer-like films) that stick to engine parts. **Oxidation inhibitors** (also known as antioxidants) are chemical additives designed to slow down or stop the oxidation process. They work by scavenging free radicals and neutralizing hydroperoxides that initiate and propagate the chain reaction of oil degradation, thereby significantly reducing the formation of sludge and varnish. ### Why the Other Options are Incorrect * **A) Flash point improvers:** There is no common additive category called "flash point improvers." The flash point is primarily a characteristic of the base oil and the volatility of the components. Additives are generally designed to maintain performance, not specifically adjust the flash point upward (though overall oil quality affects volatility). This option is irrelevant to sludge and varnish control. * **B) Pour point improvers:** These additives (also known as depressants) modify the way wax crystals solidify at low temperatures. They ensure the oil remains fluid and can be pumped during cold starts. They have no significant function related to inhibiting chemical oxidation or preventing sludge and varnish formation at operating temperatures. * **D) Foam suppressors:** These additives (usually silicone-based polymers) are used to reduce the oil's tendency to hold air bubbles, which can cause excessive aeration, reduce lubrication effectiveness, and promote localized oxidation. While they maintain oil integrity, their primary purpose is strictly to manage foam and air release, not to chemically inhibit the fundamental oxidation process that leads to sludge and the formation of varnish.