🔍 Key Concepts

• compression ratio definition in a diesel engine (relationship between total cylinder volume and clearance volume at TDC) • effect of main bearing wear on the crankshaft position and how far the piston can travel toward TDC • how a change in clearance volume at TDC affects compression pressure and temperature

💭 Think About

• Visualize the piston at top dead center (TDC) in a new engine: what are the volumes involved in the compression ratio formula? • If the main bearings wear, does the crankshaft centerline move closer to or farther from the cylinder head, and what does that do to how close the piston can get to TDC? • When the piston no longer reaches as high at TDC, does the trapped volume above the piston get larger or smaller, and how does that affect the compression ratio value?

✅ Before You Answer

• Write out the formula: Compression ratio = (Swept volume + Clearance volume) / Clearance volume and decide which term changes with worn main bearings. • Confirm that bore and stroke do not change with bearing wear; only the effective TDC position (and thus clearance volume) is affected. • Be sure you understand that if clearance volume increases, the compression ratio moves in one specific direction only—identify that direction before choosing.