Question 8 3AE01 - Third Assistant Engineer

The Correct Answer is A **2. Explanation for Option A (cylinder pressure without injection):** Option A describes the **Motoring Curve** (or No-Firing Curve). When a cylinder indicator diagram is used for performance analysis, the actual firing cycle (the solid line) is often compared against the pressure curve that would exist if the engine were simply being turned over (motored) without fuel injection. The dotted line "L" represents the pressures generated purely by the compression of air and its subsequent expansion, following the principles of ideal gas compression and expansion (adiabatic or polytropic processes), but without the addition of heat energy from combustion. This baseline curve is essential for determining the energy released during the actual firing stroke. **3. Explanation for why other options are incorrect:** * **B) firing pressure at 90 degrees crank angle:** The firing pressure is the actual pressure achieved during combustion, which lies significantly *above* line L. Furthermore, line L describes an entire pressure curve throughout the stroke, not just a single pressure point at a specific crank angle (like 90 degrees). * **C) beginning of compression:** The beginning of compression is a single point on the diagram (where the volume begins to decrease and pressure starts to rise significantly after scavenging/intake closes). Line L represents the pressure behavior throughout the entire compression and expansion strokes. * **D) power expansion curve:** The power expansion curve (or working stroke) refers to the segment of the actual firing diagram where high-pressure gases push the piston down *after* combustion. This actual power curve sits significantly higher than the dotted line L, which is the baseline *non-firing* expansion curve.