The document discusses turning moment diagrams, which graphically represent the turning moment or crank effort for various positions of a crank. It provides details on plotting turning moment diagrams, including representing turning moment on the vertical axis and crank angle on the horizontal axis. The document also discusses how the area of a turning moment diagram represents the work done per revolution. It further discusses turning moment diagrams for engines with multiple cylinders and how flywheels help control fluctuations in speed caused by variations in engine turning moment during each operating cycle.