Abstract: Improved apparatus for generating a high pressure mixture of steam and combustion gases for utilization by a steam engine or the like. The apparatus includes a combustion cylinder and an accumulator cylinder of substantially equal diameter, with both cylinders interconnected at each end by pipes of substantially equal diameter to form a closed loop. A first pair of opposed free pistons is movable toward and away from each other in the combustion cylinder and a second pair of opposed free pistons is movable toward and away from each other in the accumulator cylinder. The space between the first pair of pistons comprises a combustion chamber, while the space between the second pair of pistons comprises an operating accumulator chamber filled with a compressible fluid.