🔍 Key Concepts

• Four-stroke diesel cycle sequence: intake, compression, power, exhaust • When air temperature and pressure are high enough to ignite fuel (diesel self‑ignition) • Relationship between piston position, valve position, and fuel injection timing

💭 Think About

• In which stroke is the air in the cylinder being squeezed to its highest temperature and pressure so that injected fuel will ignite on its own? • Would it make sense to inject fuel when the valves are open and fresh air is just entering or burned gases are leaving the cylinder? Why or why not? • Think about when you want the main pressure rise from burning fuel to occur: just before, at, or after top dead center, and during which stroke that happens in a four‑stroke diesel.

✅ Before You Answer

• Verify the order of strokes in a four‑stroke diesel: intake → compression → power → exhaust • Identify during which stroke both valves are closed and pressure is rising sharply in preparation for ignition • Confirm that fuel injection must start slightly before top dead center of that stroke so that combustion pressure peaks just after TDC for maximum work