The compression ratio of a car engine is the ratio of the largest volume occupied by the air inside one of the cylinders to the smallest volume of the air in that cylinder. Its value for a gas engine is usually around 8 and for a diesel engine around 18. The efficiency of an engine is a direct function of this compression ratio. It would be desirable to vary the compression ratio in a car engine to allow it to perform better under different operating conditions, such as uphill driving or high-speed driving. The first practical variable-compression-ratio engine was developed by Saab in 2000. This engine has a split engine block that is hinged on one side. As the two halves of the engine block rotate away from each other, the compression ratio gets smaller. A hydraulic actuator is used to move the blocks. Paul Corina developed a variable-compression-ratio engine where the two halves of the engine block slide relative to each other. This allows for better sealing of the engine block and the alignment of all moving parts. A four-bar linkage (to be studied in a future design course for mechanical engineering students) is used in Fig. 9.1 to vary the compression ratio, with the highest ratio on the right. These engines represent a significant development in engine design by allowing the performance of the engine to vary with the demands placed on the engine.