The twin-wheel engine is a device that could transform heat energyinto mechanic energy. It works through the two wheels driven by the moment of force on the driven wheel, which is produced in the phase-transition process of sma wire or belt. The heat energy comes from hot water with temperature of 65℃~85℃. The shape memory effect is well demonstrated in this device by the incessant turning of the two wheels. Tinicu sma is used in this device. the twin-wheel engines are readily available in five types according to the diameter of the driven wheel: 60mm, 80mm, 100mm, 120mm and 220mm. Other sizes of the engine could also be provided at customer′s order.