🔍 Key Concepts

• Diesel combustion process – what happens between fuel injection and actual ignition • Ignition delay period – how the length of this delay affects pressure rise in the cylinder • Pressure rise / knocking – why a sudden, large pressure increase causes knock

💭 Think About

• Think about what must build up inside the cylinder before a sudden, violent ignition can occur. • Which option would allow a large amount of fuel to be present and then ignite almost all at once, causing a sharp pressure spike? • In a diesel engine, does knock usually come from fuel burning too slowly and gently, or from fuel igniting too quickly after accumulating?

✅ Before You Answer

• Identify which choice leads to a lot of fuel accumulating before ignition, rather than smooth, progressive burning. • Eliminate any option that would cause fuel to burn more smoothly and gradually instead of all at once. • Ask: which condition best matches a sudden, sharp pressure rise in the cylinder, characteristic of knock?