Hydraulic jacks in the engine block next to the cylinders manipulate a gear rack to raise or lower the rocker arm gear in order to change the length of the piston stroke, thereby changing the volume of the combustion chamber and, consequently, the compression ratio (Evans 2009). An intermediary gear is added to the other end of the rocker arm. In the Peugeot system, each piston is attached to a rocker arm and the center of the rocker arm is connected to the crankshaft. Peugeot introduced its system at the 2009 Geneva Motor Show. Saab claimed that the technology could change the compression ratio from 8:1 to 14:1 while the engine was running (Evans 2009). The cylinder head can pivot up by four degrees to increase the volume of the combustion chambers, thereby lowering the compression ratio. In the Saab system, one side of the cylinder head with integrated cylinders is attached to the crankcase with a hinge, and a lifting mechanism is placed on the other side of the crankcase. Saab introduced its system at the 2000 Geneva Motor Show. Mercedes has also experimented with a similar concept (Joshi 2012). Ford patented a similar concept in the 1980s (Caswell 1984).

To limit peak cylinder pressure, a pressure relief valve in the piston allowed the piston top to move downward, thereby reducing compression ratio (Grundy et al. The variable compression ratio piston consisted of a moveable top that used engine oil pressure to extend the piston top for the highest compression ratio. tank engines to achieve very high output while limiting peak cylinder pressure by reducing compression ratio. Teledyne Continental Motors developed variable compression ratio pistons for both 1790 cu. Ford and Volvo have evaluated this concept in the past. Adjusting the piston changes the clearance volume at top dead center, thereby altering the compression ratio. An early concept for variable compression ratio was the use of an adjustable piston in the cylinder head. Some of the variable compression ratio engines that have been investigated are shown in Figure 2.14 and include the following: Variable compression ratio engines have subsequently been investigated for decades, but none have been commercially produced due to mechanical complexity, difficulty of control, and cost. This work led to the development of the octane rating system that is still in use today. Variable compression ratio was achieved by raising or lowering the cylinder and cylinder head of the engine relative to the crankshaft.

Harry Ricardo built the first known variable compression ratio engine as a test engine in the 1920s to study knock in aircraft engines.

Compression ratio