The opposed-piston engine is a reciprocating internal combustion engine. In this engine, each cylinder features a piston at both ends, though there are no cylinder heads.
Opposed-piston engines have two pistons per cylinder instead of one piston per cylinder which would require 4 cycles per firing. Both of the pistons are connected by levers with a single crankshaft. There is no need for cylinder heads because in the combustion process less heat is lost due to the opposite reciprocating action.
This means that for equal performance can be done for less displacement and less emission compared to a four-stroke engine.