Question 5 AEL01 - Assistant Engineer - Limited

The Correct Answer is D. ### Why Option D is Correct Intake valves and exhaust valves operate under vastly different thermal and chemical conditions. Intake valves handle relatively cool, fresh air (or air/fuel mixture in spark-ignition engines), while exhaust valves handle extremely hot, high-pressure combustion products (exhaust gases). * **Exhaust Gases:** Exhaust gases are corrosive due to the presence of high-temperature combustion products (such as oxides of sulfur and vanadium, depending on the fuel) and are chemically aggressive. They also expose the valve to much higher temperatures (often exceeding 600°C to 700°C on the valve head). High temperatures combined with corrosive agents necessitate the use of high-strength, heat-resistant, and corrosion-resistant alloys (like nickel-based alloys or highly alloyed austenitic steels) for exhaust valves. * **Intake Air:** Intake air is cooler and chemically inert compared to exhaust gases. The temperatures faced by the intake valve are much lower (usually below 450°C). Because the primary requirement for intake valves is strength and wear resistance, and not extreme high-temperature corrosion resistance, they can be reliably fabricated from less expensive materials, such as low-alloy steels (e.g., chrome-silicon steels or similar materials), which would quickly fail if exposed to the exhaust environment. Therefore, the primary reason intake valves can use lower-grade (low-alloy) materials is that they are not subjected to the severe corrosive and high-temperature environment created by exhaust gases. ### Why Other Options Are Incorrect **A) the beveled edges of the intake valves provide for self-centering during seating** This statement describes a standard design feature common to *both* intake and exhaust valves. Self-centering does not relate to the material's ability to withstand high temperatures or corrosion, which is the differentiating factor allowing for the use of low-alloy steel. **B) intake valves utilize stellite-coated valve seat inserts which reduce wear** Stellite (a cobalt-chromium alloy) coatings are used to reduce wear and improve durability, but they are frequently used on *both* intake and exhaust valves, or sometimes more critically on exhaust valves due to the higher loads and heat. Furthermore, the material choice for the valve itself (low-alloy steel) is governed by thermal and chemical resistance, not simply wear reduction provided by the insert/coating. **C) the effective volume of air passing through intake valves is less than the effective volume of air passing through exhaust valves** This statement is factually incorrect for typical engine designs. Intake valves are often larger in diameter (to maximize volumetric efficiency, especially in naturally aspirated diesel engines) and handle the full volume of air required for combustion. The flow volume through the intake must equal the flow volume through the exhaust (mass flow rate), though the timing and pressure differences exist. In any case, flow volume does not determine the necessary heat and corrosion resistance of the valve material.