What is the difference between a single-acting and an opposed piston engine is?

Answer 1

Single-acting? You mean like a single cylinder engine, like in my lawn mower? The biggest difference (besides the obvious "an opposed engine has more cylinders" is that an opposed engine has more torque. On a single-cylinder four-stroke, you've got the cylinder moving under input energy--the burning gasoline--for one stroke, and under stored energy from the flywheel for three strokes. If you have a two-cylinder engine, you have two strokes where the engine is moving under input energy--each cylinder has one--and two strokes using stored energy. If you have a four-cylinder engine, each stroke is moving under input energy.