Question 48

The Correct Answer is A. ### Explanation for Option A (Engine speed) A Roots-type rotary blower is a positive displacement machine. This means that for every complete rotation of its impellers (or rotors), a fixed, measurable volume of fluid is trapped and moved from the inlet to the outlet. Therefore, the **volumetric flow rate** (the volume of air delivered per unit time) is directly proportional to the **rotational speed** of the blower's rotors. In typical engine applications (like supercharging), the blower is mechanically coupled to the engine, often via a belt or gear drive. Consequently, the speed of the blower's rotors is directly proportional to the engine's speed (RPM). If the engine speed doubles, the blower speed doubles, and thus the volume of air delivered doubles. ### Explanation for Incorrect Options **B) Engine load:** Engine load (e.g., how much power the engine is currently producing) affects the *required* flow rate and pressure, but it does not *determine* the flow rate delivered by the positive displacement blower. The blower's output is governed by its speed, which is set by the engine's RPM, not its instantaneous load. **C) Brake horsepower (BHP):** Brake horsepower is a measure of the engine's mechanical power output. While increasing the volume of air delivered (which requires a higher engine speed) will generally increase the potential BHP, the volume flow rate is a function of mechanical speed (kinematics), not power output (dynamics). BHP is an effect of the delivered air, not its cause. **D) Brake specific fuel consumption (BSFC):** BSFC is a measure of engine efficiency (fuel consumed per unit of power produced). It is a complex thermodynamic characteristic of the engine and has no direct proportional relationship to the volumetric flow rate of the intake air delivered by the blower.