A linear reciprocating engine may include a cylinder having first and second combustion chambers at opposing ends, first and second cylinder heads at an end of the first and second combustion chambers, respectively, and a double-faced piston slidably mounted within the cylinder. The engine may further include an exhaust port located in a peripheral cylinder wall and at least one combustion gas inlet in a location other than the peripheral cylinder wall. The combustion gas inlet and the exhaust port may be configured to cooperate such that combustion gases introduced through the inlet are evacuated from the cylinder through the exhaust port. The double-faced piston may have an axial length from one face to an opposite face of the piston less than or equal to of a distance from at least one of the first cylinder head and the second cylinder head to the exhaust port.

An internal combustion engine may include an engine block, a cylinder within the engine block, and a piston within the cylinder. The piston may have an outer peripheral wall, and a groove in the outer peripheral wall of the piston may have a first edge and a second edge spaced from the first edge. The piston may have a piston ring in the groove, and the piston ring may have a shape that meanders within the groove, such that the shape of the piston ring differs from a shape of the groove and such that the piston ring does not substantially fill the groove. The piston ring may be constructed of a material that when subjected to heat causes a shape of the meanderings to change, thereby enabling the piston ring to expand in an axial direction of the piston, between the edges of the groove.

A linear electrical machine (LEM) comprising: at least one stator mounted in a housing, the housing and stator defining a working cylinder; a two-section central core within the working cylinder, wherein the two sections of the core are co-axial, separate and cantilever mounted within the working cylinder, a cylindrical stator bore cavity between the working cylinder and the two central core sections; and one or more hollow translators, each translator being axially movable within the stator bore cavity, such that each section of the central core is traversed by part of the one or more translators, thereby forming an exterior magnetic circuit airgap between the respective translator and stator.

Various embodiments of the present invention are directed toward a linear generator, comprising: a cylinder having a cylinder wall and a pair of ends, the cylinder including a reaction section disposed in a center portion of the cylinder; a pair of opposed piston assemblies adapted to move linearly within the cylinder, each piston assembly disposed on one side of the reaction section opposite the other piston assembly, each piston assembly including a spring rod and a piston comprising a solid front section adjacent the reaction section and a gas section; and a pair of linear electromagnetic machines adapted to directly convert kinetic energy of the piston assembly into electrical energy, and adapted to directly convert electrical energy into kinetic energy of the piston assembly for providing compression work during the compression stroke.

A linear electrical machine (LEM) comprising: at least one stator mounted in a housing, the housing and stator defining a working cylinder; a two-section central core within the working cylinder, wherein the two sections of the core are co-axial, separate and cantilever mounted within the working cylinder; a cylindrical stator bore cavity between the working cylinder and the two central core sections; and one or more hollow translators, each translator being axially movable within the stator bore cavity, such that each section of the central core is traversed by part of the one or more translators, thereby forming an exterior magnetic circuit airgap between the respective translator and stator.