Abstract: In conventional engines, the compression ratio is a constant. Therefore, cylinder pressure increases proportionately with intake manifold pressure. As manifold pressure changes dramatically according to load, a conventional engine cannot maintain anywhere near constant (let alone optimum) cylinder pressure. Below optimum cylinder pressure, the engine is inefficient—above optimum, it suffers undue stress and (depending on fuel) may emit excess nitrous oxides. My invention makes it possible to manipulate the combustion chamber volume, and therefore the compression ratio, while the engine is running. By changing the compression ratio in a manner inversely proportional to changes in manifold pressure, we can maintain a constantly optimized cylinder pressure.