US Patent 10273927 Controlling variable compression ratio with a pressure-reactive piston

Methods and systems are provided for adjusting an ignition timing of an internal combustion engine responsive to a biasing force of a pressure-reactive piston. In one example, an engine may include a pressure-reactive piston having a top wall biased away from a piston base by a pressure of gas contained within the base. An ignition timing of a combustion chamber including the piston is adjusted responsive to an estimated biasing force of the gas against the top wall.