The pressure-volume (P-V) diagram shown in the illustration represents a theoretical diesel engine cycle which ideally represents all diesel engines, including those used in the towing industry. What statement is true concerning the combustion cycle event represented between points "2" and "3" in figure "B" of the illustration? Illustration MO-0205
• Ideal diesel engine (compression-ignition) cycle on a P-V diagram • What a horizontal line on a P-V diagram means for pressure and volume • Difference between constant-pressure and constant-volume combustion in ideal cycles
• Look closely at the line between 2 and 3. Does the pressure change, the volume change, or both? What does that tell you about the type of process? • Recall how the ideal diesel cycle is usually described: which stroke is compression, which stroke is expansion (power), and during which part does fuel burn? • Compare this with the ideal gasoline (Otto) cycle: in which one is heat added at constant volume, and in which one is it added at constant pressure?
• Verify whether point 2 to 3 is drawn as horizontal (constant pressure) or vertical (constant volume) on the P-V diagram. • Confirm that, in the ideal diesel cycle, the heat addition during fuel combustion is modeled at either constant pressure or constant volume. • Check that choices mentioning compression and expansion match the actual direction of piston motion between 2 and 3 in an ideal diesel P-V diagram.
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