🔍 Key Concepts

• Carefully identify which colored/hatched band on the polar timing diagram represents the intake valve open period (compare its position to labels A–D). • Relate each quadrant (A, B, C, D) to the corresponding engine stroke: intake, compression, power, and exhaust, based on crankshaft rotation direction. • Use the degree markings (75°, 25°, 55°, 45°) around TDC and BDC to determine how many degrees before or after these positions the intake valve opens and closes.

💭 Think About

• Starting from TDC on the exhaust stroke and following the direction of rotation, which band shows the valve just beginning to open, and how many degrees before TDC does that occur? • Follow that same band through BDC: does it stay open past BDC, and if so, by how many degrees after BDC does it finally close? • Compare the opening and closing angles you read from the diagram with the wording in each choice, paying very close attention to whether it says before/after TDC or BDC and which stroke (exhaust or compression) is mentioned.

✅ Before You Answer

• Verify the direction of rotation shown on the diagram and make sure you are moving around the circle the correct way when reading angles. • Confirm that you are reading angles relative to the correct reference: TDC vs. BDC, and note whether the diagram labels them as before (leading) or after (lagging). • Double‑check that the stroke named in the choice (intake, compression, exhaust) matches the quadrant on the diagram where the intake band lies when it opens and closes.