The need for a simple and robust engine for natural gas and low quality gas has resulted in the design of a single cylinder, hermetic Stirling engine, which has an electric power output of 9 kW. Two engines have been built. One engine is intended for natural gas as fuel and the other is intended for biogas. The crank mechanism and the alternator are built into a pressurised crank casing. The crank mechanism consists of an upper yoke linked to the working piston and a lower yoke linked to the displacer piston. The design of the yokes results in an approximately linear couple point curve, which eliminates guiding forces on the pistons and the need for X-heads. Grease lubricated needle and ball bearings are used in the kinematic crank mechanism in order to avoid oil penetrating into the cylinder volumes. Working gas is Helium at 8 MPa mean pressure. The engine produce up to 11 kW of shaft power corresponding to approximately 10 kW of electric power. The design target was an efficiency of 26 % based on lower heat content of the gas to electricity, but only 24% were obtained. The decrease of efficiency is caused by inhomogeneous capacity flows in the air preheater and insufficient insulation of the burner. The noise from the engines is low and the small vibrations from the partly balanced mechanism are absorbed by the rubber supports. Until now the engines have been tested for approximately 1000 hours each, but not without a few mechanical problems which have to be solved.