🔍 Key Concepts

• Cetane number and what it indicates about diesel fuel ignition quality • Relationship between engine speed (high vs low speed diesels) and required ignition quality • How time available for combustion in the cylinder changes with engine RPM

💭 Think About

• Think about whether high-speed or low-speed diesel engines have more time for the fuel to ignite after injection. • Ask yourself: in which case can the fuel have slightly poorer ignition quality and still burn satisfactorily because of the timing available? • Consider which design change would give the fuel spray more crank angle degrees and milliseconds before peak pressure is needed.

✅ Before You Answer

• Be clear on what "ignition quality" means for diesel (how quickly it auto-ignites after injection). • Compare how the ignition delay time fits into one engine cycle at high vs. low RPM. • Ignore options that deal with things that do NOT directly change the time between injection and required combustion, such as lube oil additives.