Question 2 AEL01 - Assistant Engineer - Limited

The Correct Answer is D **Why option D ("Top dead center") is correct:** In engine performance analysis, an indicator card (like Illustration MO-0108, which depicts an idealized or actual pressure-volume or pressure-crank angle diagram) is used to analyze the combustion process and measure indicated work. Specifically, lines or points on these diagrams are used to denote critical crank positions: * **Line A (Maximum or Peak Pressure Point):** This line usually indicates the point of highest pressure achieved during the combustion stroke. While this point is crucial, it is often *very close* to Top Dead Center (TDC) but occurs slightly *after* TDC due to the time required for complete combustion (combustion usually peaks 5-15 degrees after TDC). * **Line B (Reference Point at Start of Stroke):** This line definitively indicates the position where the piston has reached the absolute limit of its upward travel, which is **Top Dead Center (TDC)**. TDC is the reference point from which all crank angles in the cycle are measured. In the context of typical engine indicator card labeling (where A and B usually bracket the start of the power stroke or the compression end), Line B represents the mechanical definition of the end of the compression stroke and the beginning of the power stroke—the Top Dead Center position. Although the question asks about both lines, the combination of a peak pressure line (A) and the absolute mechanical dead center line (B) is most accurately summarized by "Top dead center," as B *defines* TDC, and A occurs in the immediate vicinity of TDC. **Why the other options are incorrect:** * **A) The end of injection:** While injection timing is critical and occurs just before TDC in diesel engines, neither Line A (peak pressure) nor Line B (TDC) directly marks the *end* of the fuel injection event. * **B) Bottom dead center:** Bottom Dead Center (BDC) is the lowest point of piston travel (the end of the power stroke/start of the exhaust stroke) and would appear on the opposite side of the P-V or P-$\theta$ diagram, far removed from the high-pressure region shown around A and B. * **C) The end of ignition:** The end of ignition (combustion) is defined by the point where the expansion line returns to the initial compression line pressure (or where the rate of pressure rise ceases). Line A shows the *peak* of combustion pressure, not the end of the combustion event. Line B is the mechanical dead center.