Question 26 DDE01 - Designated Duty Engineer - Unlimited HP

The Correct Answer is C ### Why Option C is Correct: **C) Excessively restricted exhaust silencer/muffler.** A restricted exhaust system (due to a clogged silencer, a collapsed internal muffler baffle, or excessive carbon buildup) prevents exhaust gases from leaving the cylinder quickly and completely. This restriction causes high back pressure. The high back pressure traps residual exhaust gas (burnt air/fuel mixture) inside the cylinder, displacing the fresh air charge needed for the next combustion cycle. This effectively reduces the amount of clean, oxygen-rich air available for combustion (lowering the Air-Fuel Ratio). When the engine is under load, more fuel is injected, but since there isn't enough air to burn it cleanly, the combustion is incomplete, resulting in high levels of soot (unburned carbon). This manifests as **gray to black smoke**, and the effect worsens significantly (darker smoke) as the load increases because more fuel is being injected into the air-starved cylinder. ### Why Other Options Are Incorrect: **A) Excessively worn exhaust valve guides.** Worn valve guides allow lubricating oil to be drawn past the valve stem and into the combustion chamber or exhaust stream. Burning lube oil typically produces **blue or bluish-white smoke**, not the gray to black smoke associated with incomplete fuel combustion (soot). **B) Leaking exhaust piping expansion joints.** Leaking exhaust expansion joints allow hot exhaust gases to escape into the engine room or surrounding atmosphere. While this is a safety hazard and reduces the efficiency of turbocharging (if applicable), it does not create back pressure or directly interfere with the combustion process inside the cylinder. Therefore, it would not cause excessive black smoke under load. **D) Leaking exhaust manifold cooling water jackets.** A leak in the cooling water jacket would introduce water into the exhaust stream. This water would flash to steam immediately upon contact with the hot exhaust gases. The primary symptom of water entering the exhaust system is **white smoke** (steam), not the gray to black smoke indicative of fuel-rich combustion.