To make an engine to do any work, heat is necessary. That is a thermodynamic law. And heat can be seen as a level of activity of atoms/molecules.

Several types of engines are developed to transform this heat into work. One of the earliest devices is the steam-engine, but automobile-engines, rocket-engines, they all do the same, transform some kind of heat in work. Even a human being does the same, we digest food, produce heat and (sometimes) do some work.

Put the kettle onThis heat can be generated in many ways, for instance by burning coal outside an engine or burning petrol inside an engine. In the last situation the heat is directly transformed into work by the expanding gases. In a steam engine the heat is produced outside the engine and we need some stuff to transport that heat to the engine. And that is where steam comes in. In a boiler we burn coal, heat up water with the hot smoke, the steam is transported to the cylinder, the steam pushes the piston away, and there is the work.

The chemical "water" is known in three aggregation states, ice (solid), water (liquid) and steam (gas). The conversion from ice to water we call melting, from water to gas is evaporate, from gas to water is condensing.

In normal conditions, that is an atmospheric pressure of 1 bar and in an open vessel, ice melts at 0o Celsius, and water boils at a 100o Celsius. When evaporating, the water-molecules get more and more active, and at the end active enough to break through the surface and escape as steam.

In case the boiling takes place in a closed vessel, the space above the water will get saturated with steam molecules, each molecule slowly increasing the pressure. At a certain moment an equilibrium will form, the pressure is that high that new molecules can't escape any more. By increasing the temperature new power is added to the molecules, making them capable to escape again. But that will increase the pressure above the water until a new equilibrium is achieved.

SteamgaugeThere is a relation between boiling temperature and steam pressure. By heating water in a closed vessel the temperature and the steam pressure will increase. This relation is represented in this table.

If an equilibrium is reached in that vessel then the steam is called "saturated", which is, the pressure above the water prevents any more steam-molecules to escape from the water. At the same time it means that every decrease in temperature will immediately invoke condensation.

For the evaporating it is not enough just to raise the temperature to a 100o C. Here we find the term "enthalpy" or latent energy. After reaching a 100o C extra energy is needed to make the molecules actually escape. When condensing the steam this extra energy is released again.



This latent energy is 5.37 times as much as the energy needed to make water reach a 100o C. And this latent energy is the reason Newcomen's engine was so inefficient and Watt's engine was so much better.