Why did anyone design and engine like that? The reciprocating mass must be enormous, the opposite of what you need for power and efficiency.
I believe it is all about reducing vibration. The common cylinder ensured the force of combustion exerted on both pistons was exactly equal, then via the rocker arms and con rods spaced at 180deg on the crank that force was also exerted equally at 180deg on the crank. Hope that makes sense. They also revved at very low speeds in comparison to modern engines, so the high reciprocating mass didn't matter so much.
