Steam engines

A steam engine is a type of heat engine. It takes heat from the hot steam, converts some of this heat into useful work and dumps the rest into the colder surrounding air. The maximum fraction of heat that can be converted into work can be found using the laws of thermodynamics, and it increases with the temperature difference between the hot steam and the surrounding air. The hotter the steam and the colder the air, the more efficient is the steam engine at converting heat into work.

In a typical steam engine a piston moves back and forth inside a cylinder. Hot, high-pressure steam is produced in a boiler, and this steam enters the cylinder through a valve. Once inside the cylinder, the steam pushes outward on every surface, including the piston. The piston moves. The steam does mechanical work on the piston and the piston does mechanical work on the machinery attached to it. The expanding steam transfers some of its thermal energy to this machinery, so the steam becomes cooler as the machinery operates.

When the piston reaches the end of its range, the valve stops the flow of steam and opens the cylinder to the outside air. The piston can then return easily. In many cases, steam is allowed to enter the other end of the cylinder so that the steam pushes the piston back to its original position. Once the piston is back at its starting point, the valve again admits high-pressure steam to the cylinder and the whole cycle repeats. Overall, heat is flowing from the hot boiler to the cooler surrounding air and some of that heat is being converted into mechanical work by the moving piston. The maximum efficiency of a steam engine is e_max = (T_steam - T_air)/T_steam. The actual efficiency is usually much lower.