Red Seal Motorcycle Technician Practice Exam

What does "compression ratio" refer to in a motorcycle engine?

1. The size of the engine

2. The ratio of maximum to minimum volume in the combustion chamber

3. The speed of the engine's pistons

4. The amount of fuel that can be stored

The correct answer is: The ratio of maximum to minimum volume in the combustion chamber

Compression ratio is an important parameter in a motorcycle engine that quantifies the engine's efficiency and performance potential. It refers specifically to the ratio of the maximum volume of the combustion chamber when the piston is at the bottom of its stroke (bottom dead center) to the minimum volume of the combustion chamber when the piston is at the top of its stroke (top dead center). This measurement is crucial because it directly influences the amount of air-fuel mixture that can be compressed within the cylinder before ignition. A higher compression ratio generally allows for a more efficient engine, as it can produce more power and better fuel economy. Additionally, higher compression ratios can lead to increased thermal efficiency, meaning the engine can turn more of the fuel's energy into usable work, though they may also require higher octane fuel to avoid knocking. While size, speed of pistons, and fuel storage are relevant aspects of an engine, none of these factors accurately define the compression ratio. Thus, the definition of compression ratio as the ratio of maximum to minimum volume in the combustion chamber is the most precise and relevant choice regarding motorcycle engine operation and performance.