In electric power generation a combined cycle is an assembly of heat engines that work in tandem from the same source of heat, converting it into mechanical energy, which in turn usually drives electrical generators. The principle is that after completing its cycle, the temperature of the working fluid engine is still high enough that a second subsequent heat engine may extract energy from the waste heat that the first engine produced. By combining these multiple streams of work upon a single mechanical shaft turning an electric generator, the overall net efficiency of the system may be increased by 50–60%. That is, from an overall efficiency of say 34% to possibly an overall efficiency of 62.22% in net Carnot thermodynamic efficiency. This can be done because heat engines are only able to use a portion of the energy their fuel generates. In an ordinary heat engine the remaining heat from combustion is generally wasted.