The EcoMotors engine is a module consisting of a single cylinder with four pistons, in two sets of two pistons, one firing against the other, and all linked to a single, central crank (an animated illustration is at http://ecomotors.com).
The Achates layout is more traditional, with two pistons per cylinder, firing away from each other. Each piston is linked to a crankshaft at either end of the block, so there are two cranks. Fuel injection and air movement in and out of the cylinder is through ports in its center. It has no valves, so is also a two-stroker.
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