Question 1 DDE01 - Designated Duty Engineer - Unlimited HP

The Correct Answer is C **Explanation for Option C (Power):** The illustration (MO-0206, which depicts a typical four-stroke engine cycle represented in a polar timing diagram) shows the duration of the various strokes in degrees of crankshaft rotation. The Power (or Expansion) stroke begins shortly after Top Dead Center (TDC), where combustion occurs, and continues until the exhaust valve opens, typically before Bottom Dead Center (BDC). In the provided diagram, the Power stroke duration is specifically marked as $103^\circ$. This angle represents the active period during which the expanding hot gases drive the piston down, converting heat energy into mechanical work. **Explanation of Incorrect Options:** * **A) Intake:** The Intake stroke begins near TDC and continues past BDC, covering an angle significantly greater than $180^\circ$ (due to valve timing overlap). It is not $103^\circ$. * **B) Exhaust:** The Exhaust stroke begins when the exhaust valve opens (before BDC) and continues past TDC, lasting significantly greater than $180^\circ$ (due to valve timing overlap). It is not $103^\circ$. * **D) Compression:** The Compression stroke begins near BDC and continues until shortly before TDC, covering approximately $180^\circ$. It is not $103^\circ$. (Note: $103^\circ$ would be the duration of the Power stroke where the expanding gases act on the piston.)