Various embodiments of the present invention are directed toward a linear 
combustion engine, comprising: a cylinder having a cylinder wall and a pair of ends, the cylinder including a 
combustion 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 
combustion section opposite the other 
piston assembly, each piston 
assembly including a spring rod and a piston comprising a 
solid front section adjacent the combustion section and a hollow back section comprising a 
gas spring that directly provides at least some compression work during a compression 
stroke of the engine; 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; wherein the engine includes a variable 
expansion ratio greater than 50:1.