From an old book of machines, an engine developed by Sturgeon. The green piston compresses and injects the fuel into the chamber.The two orange pistons work together to generate movement on the white shaft, which also move the green piston.
The spark plug is located in the center of the chamber, and the exhaust hole is close to the upper orange piston.
Compared to contemporary two-stroke engines, which used a conventional design of one piston per cylinder, the advantages of the opposed-piston engine were:Eliminating the cylinder head and valvetrain, which reduces weight, complexity, cost, heat loss, and friction loss of the engine.
Creating a uniflow-scavenged movement of gas through the combustion chamber,which avoided the drawbacks associated with the contemporary crossflow-scavenged designs however later advancements have provided methods for achieving uniflow scavenging in conventional piston engine designs.A reduced height of the engine./wikipedia