ENGINES

An engine is a device which transforms the chemical energy of a fuel into thermal energy and uses this energy to produce mechanical work. Heat engines are of two types

1. external combustion engine, and 
2. internal combustion engines.

In an external combustion engine the products of combustion of air and fuel transfer it to a second fuel which is the working fluid of the cycle, as in the case of a steam engine or a steam turbine plant where the heat of combustion is employed to generate steam which is used in a piston engine or a turbine. Another example of an external combustion engine is a closed cycle gas turbine plant in which heat of combustion in an external furnaces transfer to gas, usually air, which is used in a gas turbine. Sterling engine is also an external combustion engine. In an internal combustion engine the products of combustion are directly the motive fluid. petrol, gas, and diesel engines, Wankel engines, and open cycle gas turbine are examples of internal combustion engines. Jet engines and rockets are also internal combustion engines. 

The main advantage of internal combustion engines over external combustion engines like steam plants are greater mechanical simplicity, lower ratio of rate and bulk to output due to absence of auxiliary apparatus like boiler and condenser and, hence lower first cost(except in case of very large units), higher overall efficiency, and lesser requirement of water for dissipation of energy through cooling system. 

The advantage of external combustion plants are: use of cheaper fuels including solid fuels, and high starting torque (internal combustion engines are not self-starting). The steam turbine plant, which is the most important external combustion engine, its mainly used for large electric power generation, whereas internal combustion engines are mainly used for transport vehicles - automobiles, locomotives, aircrafts, etc. External combustion engines are less suitable for transport vehicles because of bulk and weight, and difficulty of transporting the working fluid. 

The revolution in the lifestyle of the people caused by transport vehicles, the extensive use of which became possible due to tremendous development of internal combustion engines.