🔍 Key Concepts

• Compression-ignition principle in diesel engines (fuel ignites from heat of compressed air, not from a spark plug) • Sequence of four-stroke cycle events: intake, compression, power (expansion), exhaust / scavenging • Relationship between fuel injection timing and when temperature and pressure are highest in the cylinder

💭 Think About

• In a diesel, when is the air in the cylinder hot enough from compression alone to ignite injected fuel? Think about where this occurs in the stroke. • Which event must be almost finished so that pressure and temperature are at or near their maximum, just before the piston starts back down? • Compare each choice with the basic order of strokes: intake (or scavenging), compression, power (expansion), and exhaust. Which one lines up with when diesel fuel is injected and self‑ignites?

✅ Before You Answer

• Be sure you know the correct order of the four strokes in a typical diesel cycle. • Identify during which stroke fuel is injected in a diesel engine. • Verify that ignition can only occur when temperature and pressure from compression are high enough—match that condition to the correct point in the listed events.