Question 47 UFIV02 - Assistant Engineer - UFIV

The Correct Answer is A ### Explanation for Option A (SAE 10) The question asks for the recommended straight weight petroleum oil to use in a main engine speed control governor, ensuring the oil remains in the acceptable operating range when the governor oil operating temperature may drop as low as $40^\circ\text{F}$. To determine the correct oil, we must consult the provided oil chart (Illustration MO-0161, which typically relates oil viscosity (SAE weight) to ambient or operating temperature ranges). Standard temperature-viscosity charts for engine oil indicate the minimum ambient or operating temperature that an oil grade can safely handle while maintaining fluidity and lubrication. Referring to a standard SAE Straight Weight Oil Selection Chart: * **SAE 10** typically covers the temperature range of $0^\circ\text{F}$ up to approximately $60^\circ\text{F}$ or $90^\circ\text{F}$ (depending on the specific chart design). Crucially, SAE 10 is designed to operate effectively at temperatures well below $40^\circ\text{F}$. * Since the minimum required operating temperature is $40^\circ\text{F}$, **SAE 10** is the lightest grade (lowest viscosity) that safely covers this low temperature while also providing adequate viscosity for the governor's function at higher temperatures within the governor's acceptable range. Choosing a heavier oil (like SAE 20, 30, or 40) might cause sluggish operation or cavitation at $40^\circ\text{F}$. Therefore, SAE 10 is the recommended choice to ensure the governor remains within the acceptable operating range down to $40^\circ\text{F}$. ### Explanations for Incorrect Options **B) SAE 20:** SAE 20 is a heavier grade of oil, typically recommended for minimum operating temperatures around $32^\circ\text{F}$ or $40^\circ\text{F}$ and above. While it might technically operate at $40^\circ\text{F}$, it is close to the lower limit of its effective range. For control systems like governors, which require quick response and light loads, a heavier oil like SAE 20 would provide unnecessary drag and could lead to sluggish response or potentially cause operational issues if the temperature slightly drops below $40^\circ\text{F}$ (or if the oil is already on the colder side of $40^\circ\text{F}$). SAE 10 is preferred for control systems that experience low temperatures. **C) SAE 30:** SAE 30 is significantly heavier than necessary. It is typically intended for minimum operating temperatures above $40^\circ\text{F}$ or $50^\circ\text{F}$. Using SAE 30 at $40^\circ\text{F}$ would result in excessive viscosity, leading to sluggish governor response, increased internal drag, potential cavitation, and difficulty maintaining the acceptable operating range. **D) SAE 40:** SAE 40 is a high-viscosity oil intended for use in high ambient or operating temperatures (e.g., $80^\circ\text{F}$ and above). It would be far too thick at $40^\circ\text{F}$ and would severely impede the proper operation and responsiveness of the speed control governor.