The Scotch Yoke is a reciprocating motion mechanism, converting the linear motion of a slider into rotational motion, or vice versa. The piston or other reciprocating part is directly coupled to a sliding yoke with a slot that engages a pin on the rotating part.
source.image: Ft-share International
In this type of engine the piston and slider are a single part, and the crankpin is constrained to move within a track.The Scotch yoke is not used in most internal combustion engines because of the rapid wear of the slot in the yoke caused by sliding friction and high contact pressures.
This is mitigated by a sliding block between the crank and the slot in the piston rod.n an engine application, less percent of the time is spent at bottom dead centre when compared to a conventional piston and crankshaft mechanism, which reduces blowdown time for two-stroke engines. Experiments have shown that extended dwell time does not work well with constant volume combustion Otto cycle engines.
The term scotch yoke continues to be used when the slot in the yoke is shorter than the diameter of the circle made by the crank pin. For example, the side rods of a locomotive may have scotch yokes to permit vertical motion of intermediate driving axles.