How is the compression ratio defined in a reciprocating engine?

The compression ratio in a reciprocating engine is defined as the ratio of the cylinder volumes when the piston is at the bottom of the stroke (bottom dead center) to the cylinder volume when the piston is at the top of the stroke (top dead center). This measurement is crucial because it indicates how much the air-fuel mixture is compressed before ignition occurs. A higher compression ratio typically leads to greater efficiency and power output, as more energy is extracted from the combustion of the fuel. Understanding the compression ratio is essential for tuning engine performance, optimizing fuel efficiency, and controlling emissions.

In the context of the other options, the pressure exerted during combustion specifically refers to the conditions during the combustion phase, not the ratio that defines compression. The amount of fuel used per cycle relates to fuel efficiency and consumption rather than the physical geometry and mechanics of the engine. The duration of the compression stroke speaks to the timing and mechanics of engine operation but does not provide a definition relevant to the compression ratio itself.