Opposed Pistons (op) – Piston Speed & Size
There is a practical limit to piston speed, above which engine performance begins to suffer from things such as increasing friction. Piston speed is related to the distance the piston must travel. In an opposed piston (op) engine, each piston travels about half the distance of a piston in a conventional engine, and travels at about half the speed. It is desirable to design an engine to operate near the piston speed limit. So, when an opposed piston engine is designed with the same piston speed as a conventional engine of the same specified power a very good thing happens – the size of the engine can get dramatically smaller (as measured by the required volume displacement of the engine).
Opposed Cylinders (oc) – Friction & Efficiency
By orienting two identical cylinders, each containing two opposed pistons, symmetrically around a central crank shaft, the forces generated during engine operation are almost entirely balanced. One result of this opposed cylinder (oc) architecture is very low bearing loads, which leads to less friction when compared to conventional engines that do not run in such a balanced fashion.
The long outer connecting rods of the opposed cylinder architecture contribute significantly to lowering outer piston side loads. The lower the piston side loads the less the pistons tend to rub against the walls of the cylinder, and thus the lower the friction in the system.