The difference between a single-acting and an opposed piston engine?
An opposed piston engine is a single-acting engine. All modern
gasoline and diesel engines are single-acting.
The single-acting engine receives force on one side of the
piston, and relies on the engine to push the piston back the other
way. On an opposed engine like a Continental or Lycoming, the
piston on the other side of the engine will do the pushing.
A double-acting engine, which almost always means a steam
engine, receives force on both sides of the piston. Since they run
horizontally, when the piston is to the right the engine will send
steam to push it to the left and when it's to the left the engine
will send steam to push it to the right. A slide valve decides
which side of the piston the steam will go to.