An internal combustion engine, comprising: an engine block ; a cylinder defining at least one combustion chamber in the engine block ; a piston in the cylinder , the piston being configured to travel in a first stroke from one end of the cylinder to an opposite end of the cylinder , and being sized relative to the cylinder to enable an expansion stroke portion of the first stroke wherein the piston travels under gas expansion pressure, and a momentum stroke portion of the first stroke for the remainder of the first stroke following the expansion stroke portion; at least one piston rod portion connected to the piston and extending from a location within the at least one combustion chamber to an area external to the cylinder , at least one recess in the piston rod portion , the at least one recess forming a passageway configured to communicate gas flow between the at least one combustion chamber and the area external to the cylinder , wherein the at least one recess is configured such that when the piston is in the momentum stroke portion of the first stroke following the expansion stroke portion of the first stroke, the at least one recess is configured to continuously communicate gas flow between the at least one combustion chamber and the area external to the cylinder , wherein the at least one combustion chamber includes a first combustion chamber defined between a first end of the piston and a first end of the cylinder , and a second combustion chamber defined between a second end of the piston and a second end of the cylinder , wherein the cylinder is closed at each opposite end by a cylinder head , wherein the at least one piston rod portion includes a first piston rod portion extending from the first end of the piston through the cylinder head at the first end of the cylinder, and a second piston rod portion extending from the second end of the piston through the cylinder head at the second end of the cylinder , wherein the cylinder head at each end of the cylinder includes an intake manifold , wherein the passageway in the first piston rod portion is configured to communicate gas flow between the first combustion chamber and the intake manifold at the first end of the cylinder, and the passageway in the second piston rod portion is configured to communicate gas flow between the second combustion chamber and the intake manifold at the second end of the cylinder, and wherein substantially the entire expansion stroke portion of the first stroke on the first combustion chamber side of the piston coincides with gas flow between the second combustion chamber and the intake manifold at the second end of the cylinder . an engine block ; a cylinder defining at least one combustion chamber in the engine block ; a piston in the cylinder , the piston being configured to travel in a first stroke from one end of the cylinder to an opposite end of the cylinder , and being sized relative to the cylinder to enable an expansion stroke portion of the first stroke wherein the piston travels under gas expansion pressure, and a momentum stroke portion of the first stroke for the remainder of the first stroke following the expansion stroke portion; at least one piston rod portion connected to the piston and extending from a location within the at least one combustion chamber to an area external to the cylinder , at least one recess in the piston rod portion , the at least one recess forming a passageway configured to communicate gas flow between the at least one combustion chamber and the area external to the cylinder , wherein the at least one recess is configured such that when the piston is in the momentum stroke portion of the first stroke following the expansion stroke portion of the first stroke, the at least one recess is configured to continuously communicate gas flow between the at least one combustion chamber and the area external to the cylinder , <EOS>