Fascinating looking design, though I cant figure out how it works...
Hi Chris, page 2 off my above linked PDF may be will give you a better idea about the function of these 3 pistons, playing with each other.
Excellent document, though I got worried when it said that the two figures were self explanatory, thats usually a sign that they are not!
If I follow it correctly, the center main piston is connected to the crankshaft and moves the most, with combustion happening on both ends while the two exhaust pistons at either end move together a small distance to open and close the ports? Quite clever, I have not studied the internals of a double acting IC engine before.