USCG Exam PracticeDiesel engine principles (motor propulsion)
Ignition delay in a diesel engine is defined as the interval between the start of fuel injection and the start of pressure rise. If ignition delay is excessively long, what is the expected result?
- A. Peak cylinder pressure occurs well before top dead center, reducing thermal efficiency and risking a cracked head
- ✓ Too much fuel accumulates in the cylinder before ignition, causing rough running and diesel knockCorrect
- C. The fuel charge is too lean at the moment of ignition, producing white smoke and misfiring
- D. The turbocharger overspeeds because exhaust energy is released too late in the power stroke
Why B is correct
When ignition delay is too long, fuel continues to be injected before combustion begins, so an excess accumulates and then burns all at once. This abrupt pressure rise causes rough running and the characteristic diesel knock.
Cited:DOE-HDBK-1018 Vol.1 §1-8