🔍 Key Concepts

• Compression ratio and its effect on peak compression temperature and pressure in a diesel engine cylinder • Meaning of ignition lag (ignition delay) in relation to fuel injection and start of combustion • How higher air temperature at the end of compression affects the time needed for fuel to ignite

💭 Think About

• Think about what happens to the temperature of the air in the cylinder when the compression ratio is increased versus decreased. • If the air is hotter when the fuel is injected, will the fuel need more time or less time to reach self-ignition conditions? • Look at which answer choices correctly keep cause and effect in the right direction: does compression ratio change ignition lag, or does ignition lag change compression ratio?

✅ Before You Answer

• Be sure you know that ignition lag is the time between start of injection and start of combustion. • Confirm that increasing compression ratio raises the final compression temperature for the same fuel and engine speed. • Eliminate any options where the direction of cause and effect is reversed (for example, ignition lag affecting compression ratio).