🔍 Key Concepts

• Compression ratio in internal combustion engines • Relationship between cylinder volume when the piston is at the very top vs. the very bottom of its stroke • How compression affects air temperature and pressure before fuel injection in a diesel

💭 Think About

• Think about what physically changes inside the cylinder as the piston moves from bottom dead center (BDC) to top dead center (TDC). Which quantity is being ‘compressed’? • Which answer choices talk about volumes inside the cylinder, and which talk about dimensions or counts that don’t directly describe compression? • For a diesel engine that ignites fuel by heat of compression, what ratio best describes how much the air is squeezed before fuel is injected?

✅ Before You Answer

• Identify which option describes a ratio of volumes inside the cylinder rather than lengths, areas, or counts. • Make sure the ratio is set up between conditions at top dead center (TDC) and bottom dead center (BDC), not between unrelated engine parts. • Eliminate choices that do not clearly involve the space available for the air in the cylinder during the stroke.