The steam then expands in a reciprocating engine or a turbine. The low-pressure steam is normally condensed to water before being pumped back into the boiler. In a steam locomotive, however, the expanded steam is blown off. Steam engines are slow, heavy, and inefficient and are rarely used today. Instead, today's large steam power plants use steam turbines, which can operate at much higher temperatures and pressures and can handle more steam.
Steam turbines can supply more power than large diesels at less cost.
Ion engines have been proposed for space flight. Their source of fuel would be an easily ionizable substance, such as cesium metal, to supply ions, or charged particles. A generator or solar batteries would produce an electric field that would repulse the ions strongly enough to eject them from the engine, thereby generating thrust. Such engines would produce very little thrust, but they should be able to operate for long periods in interstellar flight. Electromagnetic fields set up across the air gap between the stator and rotor interact with each other and produce the torque, or turning force, that rotates the motor.