Agricultural Mechanics 2 EOPA Practice Test 2026 – The All-in-One Guide to Exam Success!

Compression ratio is the comparison of the cylinder volume when the piston is at the bottom of its travel (bottom dead center) to the volume when it’s at the top (top dead center). It shows how much the air–fuel mixture is compressed before ignition. A higher compression ratio increases thermal efficiency but also raises the pressure and temperature in the cylinder, which makes the mixture more prone to premature ignition or knocking if the fuel isn’t resistant to detonation. Because that knock can damage the engine, higher-compression engines typically require higher-octane fuels that resist pre-ignition, while lower-compression engines can run on lower-octane fuels. The other options don’t describe what compression ratio measures; they refer to unrelated engine dimensions or settings.