The document discusses variable compression ratio engines. It describes the static compression ratio of a traditional internal combustion engine and introduces the concept of a variable compression ratio that can be adjusted while the engine is running. It then focuses on Nissan's variable compression ratio engine, which uses a multi-link system and harmonic drive actuator to vary the compression ratio between 8:1 and 14:1 depending on the load in order to improve fuel efficiency. The presentation concludes by explaining the main parts and working of Nissan's unique approach to achieving a variable compression ratio.