What concept did Alphonse Beau de Rochas describe in 1861 regarding engine principles?

Principles of the four-stroke engine.

Before Nicolaus Otto successfully built the compressed-charge four-stroke engine, the theoretical foundation for this highly efficient cycle was laid out by French engineer Alphonse Beau de Rochas. In 1861, Beau de Rochas described the necessary principles for what would become the four-stroke engine, which mandates four specific piston movements—intake, compression, power, and exhaust—for every single power stroke delivered. Although his initial patent application faced later invalidation, his conceptual work provided the necessary framework that made the subsequent mechanical success of Otto's engine possible by recognizing the necessity of pre-compression.