🔍 Key Concepts

• In a pressure-volume (P–V) diagram, atmospheric pressure is shown as a line of constant pressure (horizontal line). • Compare the positions of the various dashed reference lines to the plotted engine cycle to see which one represents zero gauge pressure (atmosphere). • Remember that compression and expansion curves must lie above atmospheric pressure for a working engine cycle.

💭 Think About

• Which line on the diagram stays at the same pressure value while volume changes along the bottom axis? • Which reference line is located just below the lowest part of the working cycle curves, representing outside air pressure rather than cylinder pressure? • Look at where the exhaust or intake processes approach atmospheric conditions—what line do they line up with?

✅ Before You Answer

• Identify which candidate line is perfectly horizontal, indicating constant pressure regardless of volume. • Confirm that the atmospheric line is below all the cycle curves (compression/expansion) but above the zero-pressure baseline, if shown. • Verify that intake or exhaust points that occur at nearly atmospheric conditions lie directly on or very close to this line.