Introduction to Internal Combustion 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. Engines normally convert thermal energy into mechanical work and, therefore, they are called heat engines. When fuel burns in the presence of atmospheric air, a tremendous amount of heat energy is released. The products of combustion attain a very high temperature. A heat engine converts the released heat energy into useful work with the help of a working fluid.
Classification of Heat engines
Heat engines are broadly classified into:
- (a) External Combustion Engines (EC Engines)
- (b) Internal Combustion Engines (IC Engines)
In external combustion engines, the combustion of the fuel in presence of air takes place outside the engine cylinder. The heat energy released from the fuel is utilized to raise the highpressure steam in a boiler from water.
Steam is a working fluid, which enters into the cylinder of a steam engine to perform mechanical work. Here, the products of combustion of fuel do not enter into the engine cylinder and hence they do not form the working fluid.
The steam turbine in a steam power plant is another example of an external combustion engine. The steam engine may be called an intermittent external combustion engine and the steam turbine a continuous external combustion engine.
A closed cycle gas turbine plant is also an example of an external combustion engine. Here, normally the air is a working substance which completes a thermodynamic cycle.
It receives heat from products of combustion of fuel in a heat exchanger and rejects heat from another heat exchanger to the surroundings.
Here also the products of combustion do not enter into the turbine. Stirling engine is also an external combustion engine.
In internal combustion engines, either the combustion of the fuel takes place inside the engine cylinder or the products of combustion enter into the cylinder as a working fluid. In reciprocating engines having cylinder and piston, the combustion of the fuel takes place inside the cylinder and such engines may be called intermittent internal combustion engines.
In an open cycle gas turbine plant, the products of combustion of fuel enter into the gas turbine and work is obtained in the form of rotation of the turbine shaft. Such a turbine is an example of a continuous internal combustion engine.
Fundamentals of Internal Combustion Engines
The intermittent internal combustion engines are most popular because of their use as the primemover in motor vehicles, and usually these engines are reciprocating engines. The reciprocating engine mechanism consists of a piston which moves in a cylinder and forms a movable cas tight seal. By means of a connecting rod and a crankshaft arrangement, the reciprocating motion of the piston is converted to rotary motion at the crankshaft.
The steam turbine plant is the most popular continuous external combustion engine used for large electric power generation. The essential components are boiler, steam turbine, condenser and feed pump
Advantages Of An Internal Combustion Engine Over An External Combustion Engine
The main advantages of an internal combustion engine over an external combustion engine are:
a) Greater mechanical simplicity
(b) Higher power output per unit weight because of the absence of auxiliary units like boiler. condenser, and feed pump
(c) Lower initial cost
(d) Higher brake thermal efficiency as only a small fraction of heat energy of the fuel is
dissipated to the cooling system.
The advantages of internal combustion engine
The advantages of internal combustion engine accrue from the fact that they work at an average temperature which is much below the maximum temperature of the working fluid in the cycle.
The disadvantages of the internal combustion engine over the external combustion engine are:
(a) The IC engines cannot use solid fuels which are cheaper. Only liquid or gaseous fuels of
given specifications can be efficiently used. These fuels are relatively more expensive.
(b) The IC engines are not self-starting whereas the EC engines have a high-starting torque.
(c) The intermittent IC engines have reciprocating parts and hence they are susceptible to the rigid problems of vibration.
Wankel engine is a rotary intermittent internal combustion engine. Jet engines and rockets are also internal combustion engines. They fall under the category of rotary continuous internal combustion engine. This book deals with the intermittent internal combustion engines, mainly of reciprocating type and thus excludes the gas turbine. Reciprocating IC engines are used in automobiles, motorcycles and scooters, power boats, ships, locomotives, and small aircraft. Due to scarcity of electric power, the use of IC engines as a portable power output unit has gained tremendous momentum. These engines are also used in farm tractors, lawnmowers and in many other devices.