What does the term "compression ratio" refer to in an engine?

The term "compression ratio" refers to the ratio of the maximum cylinder volume to the minimum cylinder volume. In an internal combustion engine, this ratio is a critical factor as it influences the engine's efficiency, power output, and emissions.

When the piston is at the lowest point of its stroke (bottom dead center), the cylinder volume is at its maximum. Conversely, when the piston is at its highest point (top dead center), the cylinder volume is at its minimum. The compression ratio is computed by dividing the maximum volume by the minimum volume. A higher compression ratio typically leads to greater thermal efficiency, allowing the engine to extract more energy from the fuel.

This concept is fundamental in engine design because it impacts how the engine interacts with the fuel being used—higher compression ratios may require higher octane fuels to avoid knocking. Understanding the compression ratio is vital for optimizing engine performance, efficiency, and ensuring compatibility with various fuels.