Abstract: Testing an engine having a rotary power output shaft by mechanically-coupling the output shaft of the engine to the rotor of a rotary electrical machine, operable alternatively in motor and generator modes, by connecting output/input conductors of the machine to conductors of an electrical supply main from which electrical power will be taken by the machine, when the machine is operating in the motor mode, and to which electrical power will be supplied by the machine, when the machine is operating in the generator mode, the electrical connections between the machine conductors and the supply main conductors being effected by a power converter including power-carrying devices of a semi-conductor type, e.g.