🔍 Key Concepts

• Diesel combustion phases: ignition delay, rapid (uncontrolled) combustion, controlled combustion, and afterburning • Relationship between piston position (top dead center vs. bottom dead center) and maximum cylinder pressure • Why excessive pressure rise too early or too late can damage the engine or waste energy

💭 Think About

• Think about when in the stroke you want the peak pressure to push hardest on the piston for best power output—near the top, halfway down, or near the bottom? • If the most rapid pressure rise continues too long into the power stroke, what happens to efficiency? If it’s finished very early, what happens to stress on engine parts? • How does the term injection lag relate to when the fuel actually starts burning compared to when it is injected?

✅ Before You Answer

• Identify where top dead center (TDC) and bottom dead center (BDC) occur during the power stroke in a diesel engine • Mentally sketch a typical diesel cylinder pressure vs. crank angle diagram and note where the rapid combustion phase starts and ends • Verify which choice places the completion of the rapid pressure rise just after the piston has started moving down on the power stroke, not far into the stroke or before TDC