ORIGIN
An Engine can be categorized into two on the basis of combustion and they are:-
i) Internal Combustion Engine
ii) External Combustion Engine
Internal Combustion Engine
With the aid of oxygen (air), fuel and spark plug heat, the combustion is formed in the Engine Cylinder. The energy so generated pushes the piston thereby the engine gets activated. Resultantly, the carbon dioxide (CO2) is emitted as smoke. This is called Open type Engine.
Example – Petrol and Diesel Engines.
External Combustion Engine
The engine activates due to the steam generated while heating coal and water in the boiler. This is open type engine. Example – Steam engine.
Heat Engine
At the time when Steam Engine was in vogue, Mr. Robert Stirling has invented Closed type hot air engine. In this process, the air which is present in the engine cylinder is heated and the pressure emanated thereof activates the engine. However, these types of Engines could not be used for motor vehicles due to the usage of working fluids such as H2, He, N2, air. Besides, water could not be used as working fluid in a closed type engine because of it’s Leidenfrost effect.
If a drop of water is spilled on a hot metal plate, the water so dropped does not get converted into steam immediately because, in between the water dropped and the hot metal plate, a steam layer having is 0.1mm thickness is formed which prevents the water from getting converted into steam immediately. This is called as Leidenfrost effect.
On the other hand, if water is spilled on a hot oil, the water so dropped gets converted into steam immediately, because of two things. The boiling point of the water is lesser than the smoke point of the oil and the density of the water is higher than the oil. As both the hot oil and water are fluids (liquids, gases) by the heat convection current, the respective heat of the water and oil gets shared. As at this point the average temperature of oil is higher than the boiling point of water, water gets converted into steam immediately. By this swift explosion of steam, efficient closed type steam engine could be created.