DIESEL POWER PLANT
-
A diesel power plant or Standby power station is the plant when we use diesel engine as a
prime mover for the generation of electrical energy.
Diesel plants produce power in the range of 2 to 50 MW, are used as central stations for
supply authorities and work
Uses a diesel engine as prime mover for the generation of electrical energy.
Advantages:
1. Design and installation are very simple.
2. It can respond to varying loads without any difficulty.
3. It occupies less space.
4. For the same capacity diesel power plant is compact and smaller than a thermal power plant.
5. Require less quantity of water for cooling purposes.
6. No problem of ash handling system.
Disadvantages:
1. High operating cost.
2. High maintenance and lubrication cost.
3. The capacity of a diesel plant is limited. They cannot be constructed in large sizes.
4. In a diesel plant noise is a serious problem.
5. Diesel power plants cannot supply over loads continuously where as steam power plants can
work under 25% overload continuously.
Diesel Engine
– is an internal combustion engine which used low grade oil and which burns this fuel inside the
cylinder solely by the heat of compression.
- Is a compression ignition system.
- The 2- stroke cycle engine is more favored for diesel power plants
- The air required for the diesel engine is drawn through the air filter from the atmosphere and
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compressed inside the cylinder.
The fuel(diesel) from the diesel engine is drawn through a filter from the all day tank and injected
into the cylinder through fuel injectors.
Because of the high temperature and pressure of the compressed air, the fuel ignites.
The fuel burns and the burnt gases expand to do work on the moving part inside the cylinder
called piston.
This movement of the piston rotates a flywheel and the engine is directly coupled to electric
generator.
The gases after expansion inside the cylinder is exhausted into the atmosphere and passes
through a silencer in order to reduce the noise.
A. 4 STROKE CYCLE DIESEL ENGINE. A four stroke cycle diesel engine produces power
in 4 piston stroke and in two crankshaft revolution.
1. Intake or Suction Stroke: (Fuel-air mixture is introduced into cylinder through intake valve)
 Piston moves from Top Dead Center (TDC) to Bottom Dead Center (BDC) creating
vacuum in the cylinder
 Intake Valve opens allowing only air to enter the cylinder and exhaust valve remains
closed.
2. Compression Stroke ( Fuel-air mixture compressed)
 Both valves stay closed
 Piston moves from BDC to TDC, compressing air to 22:1.
 Compressing the air to this extent increases the temperature inside the cylinder inside
the cylinder to above 600⁰C.
3. Expansion or Power Stroke (Combustion(constant volume) occurs and product gases
expand doing work )
 Both valves stay closed
 When the piston is at the end of compression stroke (TDC) the injector sprays a mist
of diesel fuel into the cylinder.
 When hot air mixes with diesel fuel an explosion takes place in the cylinder.
 Expanding gases push the piston from TDC to BDC
4. Exhaust stroke (Product gases pushed out of the cylinder through the exhaust valve)
 Piston moves from BDC to TDC
 Exhaust valve opens and the exhaust gases escape
 Intake valve remains closed
B. TWO STROKE CYCLE DIESEL ENGINE. In two-stroke engines the cycle is completed
in two strokes, that s, one revolution of the crankshaft as against two revolutions of fourstroke cycle. These engine have 1 power stroke per revolution of the crankshaft.


Intake begins when the piston is near the bottom dead center. Air is admitted to the cylinder
through ports in the cylinder wall (there are no intake valves). All two-stroke Diesel engines
require artificial aspiration to operate, and will either use a mechanically driven blower or a
turbo-compressor to charge the cylinder with air. In the early phase of intake, the air charge is
also used to force out any remaining combustion gases from the preceding power stroke, a
process referred to as scavenging.
As the piston rises, the intake charge of air is compressed. Near top dead center, fuel is injected,
resulting in combustion due to the charge's extremely high pressure and heat created by
compression, which drives the piston downward. As the piston moves downward in the cylinder,
it will reach a point where the exhaust port is opened to expel the high-pressure combustion
gasses. However, most current two-stroke diesel engines use top-mounted poppet
valves and uniflow scavenging. Continued downward movement of the piston will expose the air
intake ports in the cylinder wall, and the cycle will start again.
Performance of Diesel Generating Set
1. Heat generated by the fuel/Heat Supplied; Qf
Qf = mfQh ; KW
Where: mf = fuel consumption,
kg/sec
mass flow rate of fuel
Qh =heating value of fuel,
KJ/kg
= 41,130 + 139.6 ° API, KJ/kg
use liquid fuel since this is use in diesel power plant
2. Air fuel Ratio; A/F
- the mass ratio of air to a solid, liquid, or gaseous fuel present in a combustion process. The
combustion may take place in a controlled manner such as in an internal combustion engine
or industrial furnace, or may result in an explosion.
-
Use to define mixtures used for industrial furnace heated by combustion
𝒎
𝑘𝑔𝑎𝑖𝑟
A/F = 𝒎𝒂
𝑘𝑔𝑓𝑢𝑒𝑙
𝒇
𝑃𝑉
ma = 𝑅𝑇𝑎
3. Piston displacement; Vd
- Volume displaced by the piston as it moves from top dead center to bottom dead center
𝝅
Vd = ( 𝟒 )D2LNC
Where: D = diameter of cylinder (bore)
;m
N = speed
;rpm (for 2-stroke)
N/2 = speed
;rpm (for 4-stroke)
L = length of stroke
;m
C = Number of cylinder
4. Piston Speed; Ps
- The total distance of piston travels in a given time.
Ps = 2LN
; m/sec
Whre : 2L = distance travelled by piston
N = speed
5. Indicate Power; I.P.
- Power developed/delivered by an engine as calculated from the average pressure of the
working fluid in the cylinders and the displacement.
Measuring instruments used: Engine indicator traces actual P-V diagram.
Planimeter – measures the area of P-V diagram
Tachometer – measures speed
I.P. = Pmi x Vd
Where:
Pmi = indicated mean effective pressure ; KPa
𝐴𝑟𝑒𝑎 𝑜𝑓 𝑑𝑖𝑎𝑔𝑟𝑎𝑚
= 𝐿𝑛𝑔𝑡ℎ 𝑜𝑓 𝑑𝑖𝑎𝑔𝑟𝑎𝑚 x spring scales
Vd = piston volume displacement
; m3/sec
6. Brake Power; B.P. – power developed by an engine as measured by a force applied to a
friction brake or by absorption dynamometer applied to the shaft flywheel.
- Power output of any engine measured at the engine’s flywheel.
Measuring instruments used: Dynamometer measures the torque.
Tachometer – measures the speed
B.P = 2𝝅Tn ; KW
Calculation of Brake power using Brake mean effective pressure:
B.P. = Pmb x Vd
;KW
Where: Pmb = brake mean effective pressure
; KPa
7. Friction Power; F.P.
- Power dissipated in a machine through friction
F.P. = I.P. – B.P.
8. Mechanical Efficiency; Em
- the ratio of the brake power to the indicated power
𝑩.𝑷.
𝑷
𝒙𝑽
𝑷
Em = 𝑰.𝑷. = 𝑷𝒎𝒃 𝒙 𝑽 𝑫 = 𝑷𝒎𝒃
𝒎𝒊
𝑫
BP = IP x Em
𝒎𝒊
9. Electrical or generator efficiency; Eg
- The ratio of the generator output to the brake power
Eg =
𝒈𝒆𝒏𝒆𝒓𝒂𝒕𝒐𝒓 𝒐𝒖𝒕𝒑𝒖𝒕
𝒃𝒓𝒂𝒌𝒆 𝒑𝒐𝒘𝒆𝒓
Where: generator output = B.P x Eg
= I.P. x Em x Eg
Note: overall efficiency is the ratio of the generator output to the indicated power.
𝑷𝒈𝒆𝒏
Eo = 𝑰.𝑷.
OR
Eo = Eg x Em
10. Thermal Efficiency
- Ratio of the work done by the heat engine for the heat energy absorbed by it.
a. Indicated thermal efficiency; Eti
𝑰.𝑷.
Eti =
𝒎 𝒇 𝑸𝒉
b. Brake thermal efficiency; Etb
𝑩.𝑷.
Etb = 𝒎 𝑸
𝒇 𝒉
c. Overall or combined thermal efficiency; Etc
𝒈𝒆𝒏𝒆𝒓𝒂𝒕𝒐𝒓 𝒐𝒖𝒕𝒑𝒖𝒕
Etc =
𝒎 𝑸
𝒇 𝒉
11. Volumetric efficiency; Ev
- The ratio of the volume of air drawn into the cylinder to the piston displacement
Ev =
𝒂𝒄𝒕𝒖𝒂𝒍 𝒗𝒐𝒍𝒖𝒎𝒆 𝒐𝒇 𝒂𝒊𝒓 𝒆𝒏𝒕𝒆𝒓𝒊𝒏𝒈
𝑷𝒊𝒔𝒕𝒐𝒏 𝒅𝒊𝒔𝒑𝒍𝒂𝒄𝒎𝒆𝒏𝒕
Where : Va =
𝑽
= 𝑽𝒂
𝑫
𝒎𝒂 𝑹𝑻
𝑷
12. Specific fuel consumption – the weight flow rate of fuel required to produce a unit of power
or thrust. It is also known as specific propellant.
a. mi = indicated spec fuel consumption
mi =
𝒎𝒇 𝒙 𝟑𝟔𝟎𝟎
𝑰.𝑷.
𝒌𝒈
; 𝑲𝑾−𝒉𝒓
b. mb = brake spec fuel consumption
mb =
𝒎𝒇 𝒙 𝟑𝟔𝟎𝟎
𝑩.𝑷.
𝒌𝒈
; 𝑲𝑾−𝒉𝒓
𝒍𝒃
; 𝑯𝑷−𝒉𝒓
c. mc = combined or overall spec fuel consumption
𝒎 𝒙 𝟑𝟔𝟎𝟎
𝒌𝒈
𝒇
mc = 𝒈𝒆𝒏𝒆𝒓𝒂𝒕𝒐𝒓
𝒐𝒖𝒕𝒑𝒖𝒕.
; 𝑲𝑾−𝒉𝒓
13. Heat Rate = expression of the conversion efficiency of a thermal power plant or engine as
heat input/unit work output.
a. Indicated heat rate =
b. Engine Heat rate =
𝒎𝒇 𝒙 𝟑𝟔𝟎𝟎 𝒙 𝑸𝒉
𝑲𝑱
; 𝑲𝑾−𝒉𝒓
𝑰.𝑷.
𝒎𝒇 𝒙 𝟑𝟔𝟎𝟎 𝒙 𝑸𝒉
𝑲𝑱
; 𝑲𝑾−𝒉𝒓
𝑩.𝑷.
𝒎 𝒙 𝟑𝟔𝟎𝟎 𝒙 𝑸
𝒇
𝒉
c. Engine generation heat rate = 𝑮𝒆𝒏𝒆𝒓𝒂𝒕𝒐𝒓
𝒐𝒖𝒕𝒑𝒖𝒕
𝑲𝑱
; 𝑲𝑾−𝒉𝒓
14. Generator speed
N=
𝟏𝟐𝟎𝒇
𝑷
Where: N = speed
f = frequency (usually 60 Hertz if not given)
P = number of poles (even)
Typical Full Load Heat Balance of the Engine
Description
Useful output(brake output)
Cooling loss
Exhaust loss
Friction, radiation, etc
Total heat input by fuel
Mass balance of the engine:
ma + mf = mg
Percent share of input
34%
30%
26%
10%
100%
OR miQh
OR mbQh
OR mcQh