🔍 Key Concepts

• Four-stroke diesel cycle (intake, compression, power, exhaust) – what happens in each stroke • How starting air systems spin an engine before fuel is injected • Which stroke has a cylinder volume and valve condition that best allows high‑pressure air to push the piston down and turn the crankshaft

💭 Think About

• In which stroke are both valves in a position that will keep the high‑pressure starting air working directly on the piston instead of escaping out the intake or exhaust? • Think about when, during normal operation, the piston is in a position similar to when starting air should act to push it down and rotate the crankshaft. • Which stroke would be least effective for starting air because the air would just flow straight through and not move the piston much?

✅ Before You Answer

• Verify what valves (intake and exhaust) are open or closed during each of the four strokes. • Check during which stroke the piston is near top dead center (TDC) and enclosed so pressure can build and move it effectively. • Confirm which stroke, if used for starting air admission, would give a strong torque on the crankshaft instead of just blowing air through the cylinder.