The compression ratio is a key parameter in understanding how efficient an Otto cycle engine can be. In simple terms, the compression ratio (\( r \) ) is the ratio of the volume of the engine's cylinder when the piston is at the bottom of its stroke (maximum volume) to the volume when it’s at the top of its stroke (minimum volume). This ratio is crucial because it influences the efficiency and power output of an engine.
The higher the compression ratio:
- The greater the volume change during the compression stroke, leading to more energy being converted from heat into mechanical work.
- The more the air-fuel mixture is compressed, which typically results in higher temperatures and pressures, thus increasing the thermal efficiency of the engine.
For example, in the exercise given, the Mercedes-Benz SLK230 has a compression ratio of 8.8, while the Dodge Viper GT2 has a ratio of 9.6. A higher compression ratio means the Viper should, theoretically, have a higher thermal efficiency.