TYPES
OF POWER PLANTS
Normally, most of the engines convert
thermal energy into mechanical work and therefore they are called 'heat
engines'.
Heat engine is a device which
transforms the chemical energy of a fuel into thermal energy and utilizes this
thermal energy to perform useful work. Thus, thermal energy is converted to
mechanical energy in a heat engine.
Heat engines can be broadly
classified into two categories:
(i)
Internal
Combustion Engines (IC Engines)
(ii)
External
Combustion Engines (EC Engines)
Engines whether Internal Combustion
or External Combustion are of two types, viz.,
(i)
Rotary engines
(ii)
(ii) Reciprocating engines
Of the various types of heat
engines, the most widely used ones are the reciprocating internal combustion
engine, the gas turbine and the steam turbine. The steam engine is rarely used
nowadays.
The reciprocating internal combustion engine enjoys some advantages over the steam turbine due to the absence of heat exchangers in the passage of the working fluid (boilers and condensers in steam turbine plant). This results in a considerable mechanical simplicity and improved power plant efficiency of the internal combustion engine.
Another advantage of the
reciprocating internal combustion engine over the other two types is that all
its components work at an average temperature which is much below the maximum
temperature of the working fluid in the cycle. This is because the high
temperature of the working fluid in the cycle persists only for a very small
fraction of the cycle time. Therefore, very high working fluid temperatures
can be employed resulting in higher
thermal efficiency.
Further, in internal combustion
engines, higher thermal efficiency can be obtained with moderate maximum
working pressure of the fluid in the cycle, and therefore, the weight of power ratio
is less than that of the steam turbine plant. Also, it has been possible to
develop reciprocating internal combustion engines of very small power output
(power output of even a fraction of a kilowatt) with reasonable thermal
efficiency and cost.
The main disadvantage of this type
of engine is the problem of vibration caused by the reciprocating components.
Also, it is not possible to use a variety of fuels in these engines. Only liquid or gaseous fuels of given
specification can be efficiently used. These fuels are relatively more expensive.
Considering all the above factors
the reciprocating internal combustion engines have been found suitable for use
in automobiles, motor-cycles and scooters, power boats, ships, slow speed
aircraft, locomotives and power units of relatively small output.
External Combustion and Internal
Combustion Engines:
External combustion engines are
those in which combustion takes place outside the engine whereas in internal
combustion engines combustion takes place within the engine.
example, in a steam engine or a steam turbine,
the heat generated due to the combustion of fuel is employed to generate high
pressure steam which is used as the working fluid in a reciprocating engine or
a turbine.
Principle of engine operation (4 stroke & 2 stroke operating cycles)
In reciprocating engines, the piston moves back and forth in a cylinder and transmits power through a connecting rod and crank mechanism to the drive shaft . The steady rotation of the crank produces a cyclical piston motion. The piston comes to rest at the top center (TC) crank position and bottom-center (BC) [These crank positions are also referred to as top-dead-center (TDC) and bottom-dead center (BDC)] crank position when the cylinder volume is a minimum or maximum, respectively. The minimum cylinder volume is called the clearance volume.
The volume swept out by the piston, the difference between the maximum or total volume Vt and the clearance volume, is called the displaced or swept volume Vd. The ratio of maximum volume to minimum volume is the compression ratio rc.
Typical values of rc are 8 to 12 for SI engines and 12 to
24 for CI engines.
Fig.1.3
:-The f our-stroke operating cycle.
The majority of reciprocating
engines operate on what is known as the four-stroke cycle. Each cylinder requires four strokes of its piston-two
revolutions of the crankshaft-to complete the sequence of
events which produces one power
stroke. Both SI and CI engines use this cycle which comprises
1. An intake stroke, which starts with the piston at TC and ends with the piston at BC, which draws fresh mixture into the cylinder. To increase the mass inducted, the inlet valve opens shortly before the stroke starts and closes after it ends.
2. A compression stroke, when both valves are closed and the mixture inside the cylinder is compressed to a small fraction of its initial volume. Toward the end of the compression stroke, combustion is initiated and the cylinder pressure rises more rapidly.
3. A power stroke, or expansion stroke, which starts with the piston at TC and ends at BC as the high temperature, high-pressure, gases push the piston down and force the crank to rotate. About five times as much work is done on the piston during the power stroke as the piston had to do during compression.
As the piston approaches BC, the exhaust valve opens to initiate the exhaust process and drop the cylinder pressure to close to the exhaust pressure.
4 An exhaust stroke, where the remaining burned gases exit the cylinder: first, because the cylinder pressure may be substantially higher than the exhaust pressure; then as they are swept out by the piston as it moves toward TC. As the piston approaches TC, the inlet valve opens. Just after TC the exhaust valve closes and the cycle starts again.
Though often called the Otto cycle after its inventor, Nicolaus Otto, who built the first engine operating on these principles in 1876, the more descriptive four-stroke nomenclature is preferred.
The four-stroke cycle requires, for each engine cylinder, two crankshaft revolutions for each power stroke.
To obtain a higher power output from a given engine size, and a simpler valve design, the two-stroke cycle was developed. The two-stroke cycle is applicable to both SI and CI engines.
one of the simplest types of two-stroke engine designs. Ports in the cylinder liner opened and closed by the piston motion, control the exhaust and inlet flows while the piston is close to BC.
The two strokes are:
A compression stroke, which starts by closing the inlet and exhaust ports, and then compresses the cylinder contents and draws fresh charge into the crankcase. As the piston approaches TC,
(i) combustion is initiated
(ii)
A
power or expansion stroke, similar to that in the four-stroke cycle
until the piston approaches
(iii)
BC,
when first the exhaust ports and then the intake ports are uncovered. Most of
the burnt gases exit
(iv)
the
cylinder in an exhaust blows down process. When the inlet ports are uncovered,
the fresh charge
(v)
which
has been compressed in the crankcase flows into the cylinder.
(vi)
The
piston and the ports are generally shaped to deflect the incoming charge from
flowing directly into
(vii)
the
exhaust ports and to achieve effective scavenging of the residual gases.
(viii) Each engine cycle with one power stroke
is completed in one crankshaft revolution. However, it
(ix)
is
difficult to fill completely the displaced volume with fresh charge, and some
of the fresh mixture
(x)
flows
directly out of the cylinder during the scavenging process. The example shown
is a cross scavenged
(xi)
design;
other approaches use loop-scavenging or uniflow systems
There
are many different types of internal combustion engines. They can be classified
by:
1. Application.
Automobile,
truck, locomotive, light aircraft, marine, portable power system, power
generation
2 Basic engine design
Reciprocating
engines (in turn subdivided by arrangement of cylinders:
e.g.,
in-line, V, radial, opposed-ref, rotary engines (Wankel and other
geometries)
1. Working cycle.
·
Four-stroke
cycle: naturally
aspirated (admitting atmospheric air), supercharged (admitting recompressed fresh
mixture), and turbocharged (admitting fresh mixture compressed in a compressor driven
by an exhaust turbine),
·
Two-stroke
cycle: crankcase
scavenged, supercharged, and turbocharged, Constant volume heat addition cycle
engine or Otto cycle engine -SI engine or Gasoline engine, Constant pressure
heat addition cycle engine or Diesel cycle engine-CI engine or Diesel engine.
4.Valve
or port design and location.
Overhead
(or I-head) valves, under head (or L-head) valves, rotary valves, cross
scavenged porting (inlet and exhaust ports on opposite sides of cylinder at one
end), loop scavenged porting (inlet and exhaust ports on same side of cylinder
at one end), through- or uni-flow scavenged (inlet and exhaust ports or valves
at different ends of cylinder)
5.
Fuel
Gasoline
(or petrol), fuel oil (or diesel fuel), natural gas, liquid petroleum gas,
alcohols (methanol, ethanol), hydrogen, dual fuel
6.
Method of mixture preparation.
Carburetion,
fuel injection into the intake ports or intake manifold, fuel injection into
the engine
cylinder
7.
Method of ignition
Spark
ignition (in conventional engines where the mixture is uniform and in stratified-charge
engines where the mixture is non-uniform), compression ignition (in
conventional diesels, as well as ignition in gas engines by pilot injection of
fuel oil)
8.
Combustion chamber design.
Open
chamber (many designs: e.g., disc, wedge, hemisphere, bowl-in-piston), divided
chamber (small and large auxiliary chambers; many designs: e.g., swirl
chambers, pre-chambers)
9.
Method of load control.
Throttling
of fuel and air flow together so mixture composition is essentially unchanged, control
of fuel flow alone, a combination of these
10.
Method of cooling.
Water
cooled, air cooled, un-cooled (other than by natural convection and radiation)
. All
these distinctions are important and they illustrate the breadth of engine
designs available from a fundamental point of view. The method of ignition has
been selected as the primary classifying feature. From the method of
ignition-spark-ignition or compression-ignition-follow the important characteristics
of the fuel used, method of mixture preparation,
combustion chamber design, method of load control, details of the combustion
process, engine emissions, and operating characteristics. Some of
the
other classifications are used as subcategories within this basic
classification. The engine operating cycle--four-stroke or two-stroke--is next
in importance.
FOUR-STROKE CYCLE S-I ENGINE - PRINCIPLE OF OPERATION
In Four-stroke cycle engine, the
cycle of operation is completed in four-strokes of the piston or two
revolutions of the crankshaft. Each stroke consists of 180°, of crankshaft
rotation and hence a cycle consists of 720°of crankshaft rotation. The series
of operations of an ideal four-stroke.
1.
Suction stroke
Suction stroke 0-1 starts when the
piston is at top dead centre and about to move downwards. The inlet valve is
open at this time and the exhaust valve is closed. Due to the suction created
by the motion of the piston towards bottom dead centre, the charge consisting
of fresh air mixed with the fuel is drawn into the cylinder. At the end of the
suction stroke the inlet valve closes.
2.
Compression stroke.
The fresh charge taken into the
cylinder during suction stroke is compressed by the return stroke of the piston
1-2. During this stroke both inlet and exhaust valves remain closed. The air
which occupied the whole cylinder volume is now compressed into clearance
volume. Just before the end of
the compression strokes the mixture
is ignited with the help of an electric spark between the electrodes of the
spark plug located in combustion chamber wall. Burning takes place when the
piston is almost at top
dead centre. During the burning
process the chemical energy of the fuel is converted into sensible energy,
producing a temperature rise of about 2000°C, and the pressure is also considerably
increased.
3.
Expansion or power stroke.
Due to high pressure the burnt gases
force the piston towards bottom dead Centre, stroke 3-4, and both the inlet and
exhaust valves remaining closed. Thus power is obtained during this stroke.
Both pressure and temperature decrease during expansion.
A power or expansion stroke, similar to that in the four-stroke cycle until the piston approaches BC, when first the exhaust ports and then the intake ports are uncovered. Most of the burnt gases exit the cylinder in an exhaust blow down process. When the inlet ports are uncovered, the fresh charge which has been compressed in the crankcase flows into the cylinder.
The piston and the ports are generally shaped to deflect the incoming charge from flowing directly into the exhaust ports and to achieve effective scavenging of the residual gases.
Each engine cycle with one power stroke is completed in one crankshaft revolution. However, it is difficult to fill completely the displaced volume with fresh charge, and some of the fresh mixture flows directly out of the cylinder during the scavenging process. The example shown is a cross scavenged design; other approaches use loop-scavenging or uniflow systems
4. Exhaust stroke.
At the end of the expansion stroke the exhaust valve opens, the inlet valve remaining closed, and the piston is moving from bottom dead Centre to top dead Centre sweeps out the burnt gases from the cylinder, stroke 4-0. The exhaust valve closes at the end of the exhaust stroke and some 'residual' gases remain in the cylinder. Each cylinder of a four-stroke engine completes the above four operations in two engine revolutions. One revolution of the crankshaft occurs during the suction and compression strokes, and second revolution during the power and exhaust strokes. Thus, for one complete cycle, there is only one power stroke while the crankshaft turns by two revolutions. Most of the spark-ignition internal combustion engines are of the four-stroke type. They are most popular for passenger cars and small aircraft applications.