The Scotch Yoke makes the pistons dwell very slightly longer at top dead center, so the fuel burns more completely in a smaller volume.

The pistons are connected to a Scotch Yoke mechanism in place of the more usual crankshaft mechanism, thus the piston acceleration is perfectly sinusoidal.

CMC Research designs and develops compact engines with a unique Scotch Yoke Crank Mechanism.

"Comparing Simple Crank/Slider and Scotch Yoke Mechanisms" by Fred Klingener, The Wolfram Demonstrations Project; Active demo.

The use of the Scotch Yoke reduces vibration from the motions of the connecting rod-for example, the peak acceleration in a Scotch yoke is 25% less than the acceleration in a conventional crank and slider arrangement.