Power supply systems

A power supply system comprises an internal combustion engine (20) having a pair of opposed pistons (28, 30) and cylinder (32) within which the pistons move in use, the pistons (28, 30) and cylinder together forming a combustion chamber. The pair of opposed pistons (28, 30) and the cylinder (32) are each arranged to rotate, in use, about a common axis of rotation A with respect to an associated electric generator arrangement (22) so as to generate an electric output. The pair of opposed pistons (28, 30) are also arranged to together form a mass-elastic system that is capable of resonance and further arranged such that combustion in the combustion chamber causes the pistons (28, 30) to oscillate relative to each other about the common rotational axis (A).