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NEW ERA UNIVERSITY COLLEGE OF ENGINEERING AND ARCHITECTURE DEPARTMENT OF MECHANICAL ENGINEERING
ME LABORATORY 3
Th; 1:00 PM – 7:00 PM
Performance Heat Balance and Efficiency Test of a
Diesel Electric Power Plant
SUBMITTED BY:
CORPUZ, RALPH MARWIN U. BSME - V
SUBMITTED TO:
ENGR. MOSES MABUTE
August 17, 2017
- The Diesel Electric Power Plant Applications of diesel power plant Diesel power plant’s is in the range of 2 to 50 MW capacity. They are used as central station for small or medium power supplies. They can be used as stand-by plants to hydro-electric power plants and steam power plants for emergency services. They can be used as peak load plants in combinations with thermal or hydro-plants. They are quite suitable for mobile power generation and are widely used in transportation systems such as automobiles, railways, air planes and ships. Now-a-days power cut has become a regular feature for industries. The only solution to tide over this difficulty is to install diesel generating sets.
Layout diesel engine power plant:
Diesel engine: Diesel engines or compression ignition engines as they are called are generally classified as two stroke engine and four stroke engines. In diesel engine, air admitted into the cylinder is compressed, the compression ratio being 12 to 20. At the end of compression stroke, fuel is injected. It burns and the burning gases expand and do work on the position. The engine is directly coupled to the generator. The gases are then exhausted from the cylinder to atmosphere.
Engine starting system: This includes air compressor and starting air tank. The function of this system is to start the engine from cold supplying compressed air. Fuel system: Pump draws diesel from storage tank and supplies it to the small day tank through the filter. Day tank supplies the daily fuel need of engine. The day tan is usually placed high so that diesel flows to engine under gravity. Diesel is again filtered before being injected into the engine by the fuel injection pump. The fuel is supplied to the engine according to the load on the plant.
Beginning with the working medium at state 1, it is first polytropically compressed to state 2, then heat is added during a limited isobaric expansion, after which a polytropic expansion to the
initial volume reduces the pressure to state 4. The ideal work produced by the cycle is represented by its area, and the mean effective pressure is its average height. Polytropic processes 1-2 and 3-4 in the ideal cycle are isentropics with air as the fluid. Thus, for
the air standard performance, n = k = 1. The ideal air standard efficiency,
e = 1 − 1
krk − 1 [
rck − 1
rc − 1 ] (1)
where QA = Qin = mcp ( T 3 – T 2 ) rk = rcre rk = compression ratio = V 1 / V 2 rc = cut-off ratio = V 3 / V 2 re = expansion ratio = V 4 / V 3 = V 1 / V 3
Ideal mean effective pressure,
pm = p 1 r
[
krkk − 1 ( rc − 1 )−( rck − 1 )
( rk − 1 )( k − 1 ) ] (2)
These equations show that high efficiency is promoted by high rk and low rc , but that engine size
(which may be considered proportional to pmi ) is increased as rc decreases. The requirement of adequate fuel combustion imposes a limitation on maximum rc that can be used. Since Diesels are
load-governed by varying point of cutoff, the ideal efficiency increases at part load. This partially offsets other tendencies, and a fairly flat efficiency-load characteristic result. Real engines have cylinder cooling for mechanical reasons; also they work on an open cycle, meaning that the fluid at state 1 is not the same as that which completed the previous cycle. The
products of combustion of each working cycle are discharged as exhaust gas, and fresh air is
inducted for use in the following cycle. Instead of heat being transferred between states 2 and 3, fuel is injected into the air and its heat of combustion provides the energy input. The real engine will
have nonisentropic compression and expansion processes; n = 1 is a fair average in practice. Although Equation (1) has its uses in Diesel engine studies, the actual thermal efficiencies are
considerably less than those of the air standard.
Example No. 1 An air standard Diesel cycle will be analyzed for state of the working fluid and performance. Using the nomenclature of Figure 1, state 1 is at 0 kg/cm 2 ab and 27 C. The volume quantity is that of a
single-cylinder engine with 25 cm bore and 38 cm stroke. It will be assumed that, after a compression sufficient to produce 538 C, heat is added during the first 10% of the working stroke.
Given: State 1 is at 0 kg/cm 2 ab and 27 C Single-cylinder engine with 25 cm bore and 38 cm stroke. Produce 538 C Heat added during the first 10% of the working stroke.
Required: Analysis of the cycle
Solution:
Ratio of compression,
rk =
v 1 v 2
=
(
T 2
T 1 )
1 k − 1
,
p 2 p 1
=
(
v 1
v 2 )
k
T 1 = 27 + 273 = 300 K T 2 = 538 + 273 = 811 K
State 1 2 3 4 Pressure, kg/cm 2 ab
0 29 29 2.
Volume, m 3 0 0 0 0. Temperature, K 300 811 1701 847 Temperature, C 27 538 1428 574
Ideal mean effective pressure
pm = p 1 r
[
krkk − 1 ( rc − 1 )−( rck − 1 )
( rk − 1 )( k − 1 ) ]
pm =(0. 9) ( 12 )
[
(1. 4) ( 12 )1. 4− 1 (2 .097− 1 )−(2. 0971. 4− 1 )
( 12 − 1 ) (1. 4− 1 ) ]
pm =5. 72 kg / cm 2 ab (answer)
Net work done per cycle
= pm ( v 1 − v 2 )=(5 .72 kg / cm 2 )
(
100 cm
1 m )
2 (0. 01865 m 3 )
= 1067 kg ⋅ m (answer)
Ideal Thermal Efficiency
e = 1 − 1
krk − 1 [
rck − 1
rc − 1 ]
e = 1 −
1
(1. 4) ( 12 )1. 4− 1 [
(2. 097)1. 4− 1
2. 097− 1 ]
e =0. 56= 56 (answer)
- Combustion Here the special features of combustion as carried out in the Diesel engine cylinder are to receive attention. However, first it appears desirable to repeat and summarize the equations pertaining to fuel oil. Density scales:
° Be' =
140 S. G. @ 15. 6/15.
− 130 (3)
° API =
- 5 S. G. @ 15. 6/15.
−131. 5 (4) Ignition quality: Diesel index =0. 018× ° API × tap +0. 32× ° API (5) Heating value: Qh = 41130 +139. 6× ° API kJ / kg (6)
Qh = 51716 −8793. 8( S. G .)
2 kJ / kg (7)
QL = Qh −2442. 7× 9 H 2 (8) Hydrogen content: H 2 = 26 − 15 ( S. G .) percent by weight(9) where tap = aniline point in C.
Combustion in the Diesel engine cylinder begins theoretically at the instant injection starts and
continues, at constant pressure, until injection ceases. The distillate fuel used may be considered to have an average chemical formula of C16H32 for which the ideal air quantity is found as follows: C 16 H 32 + 24 O 2 = 16 CO 2 + 16 H 2 O Considering the numerical prefixes to be mols, the equation of combining weights is written as follows: 224 kg C 16 H 32 + 24 × 32 kg O 2 = 16 × 44 kg CO 2 + 16 × 18 kg H 2 O Since 1 kg air provides 0 kg O 2 ,
Air per kg C 16 H 32 =
24 × 32 0. 232× 224
=14. 8 kg
Early fuel cutoff is necessary to good thermal efficiency, but early cutoff is not possible with the ideal A:F ratio of 14. This is due to the need for limiting maximum temperature of the cycle for
mechanical and thermal reasons, under circumstances as set forth in the following example.
Example No. 2 The ideal maximum temperature of combustion of a fuel of 24 oAPI is calculated, on the assumption of 427 C compression temperature and 14 kg air per kg fuel.
Given: Fuel of 24 oAPI, 427 C compression temperature, 14 kg air per kg fuel.
Required: Ideal maximum temperature
Solution: Specific gravity
S. G .=
- 131 .5 +° API
S. G .=
- 131 .5+ 24
=0.
Heating value:
Qh = 51 , 716 −8793. 8( S. G .)
2 kJ / kg Qh = 51 , 716 −8793. 8(0. 91) 2 = 44 , 434 kJ / kg QL = Qh −2442. 7× 9 H 2 QL = 44 , 434 −2442. 7( 9 ) (0. 26−0. 15×0. 91) QL = 41 , 719 kJ / kg
During isobaric combustion, using cp = 1 kJ/kg, the sensible heat, QL , will raise the products t
degrees, according to the relation
QL = wcpΔt w =14. 8 kg air + 1 kg fuel =15 .8 kg
A hypothetical pressure, known as brake mean effective pressure, bmep , can be employed to
show the magnitude of mean effective pressure. The true pressure, pmep , is higher on account of engine friction losses.
ihp =
pmepLANp
33 , 000
hp (10)
bhp =
2 πWrN 33 , 000
hp (11)
bmep =
bhp × 33 , 000 LANp
lb / ft 2 (12)
in which
pmep = Indicated mep, lb per sqft
L = Piston stroke, ft
A = Piston face area, sqft
Np = Number of power strokes per min (N for two-cycle and N/2 for four-cycle)
N = Rotative speed, rpm
W = Net dynamometer force, lb
r = Dynamometer arm length, ft
As is true of all prime movers, there are a number of efficiency expressions applying to Diesels. Mechanical efficiency is the ratio, bhp/ihp.
Indicated thermal efficiency,
ei =
2545 wiQ (13)
Brake thermal efficiency,
eb =
2545 wbQ (14)
wi , wb = fuel consumption, lb per hr per ihp or bhp
Q = Fuel heating value, Btu/lb, either Qh or QL , according to policy.
Example No. 3
A 6-cylinder Diesel engine on dynamometer test was found to use 84 lb of fuel, having Qh = 19,
Btu/lb, in a one-hour test at steady load. The brake thermal efficiency and the brake mep will be
determined from the following test data and measurements. Cylinder is 8 in x 10 in 4-cycle type.
Speed, 600 rpm. Dynamometer torque, 1809 lb-ft.
Given:
Wr = dynamometer torque = 1809 lb-ft.
Qh = 19,351 Btu/lb
N = 600 rpm
D = 8 in = 0 ft
L = 10 in = 0 ft
Fuel weight = 84 lb
4-cycle type.
Required:
Brake thermal efficiency and the brake mep.
Solution:
bhp =
2 πWrN 33 , 000
hp
bhp =
2 π ( 1809 ) ( 600 ) 33 , 000
=206. 7 hp
Brake thermal efficiency:
Brake thermal efficiency ,ηtb =
2545 wbQ
wb =
84 206.
=0. 4064 lb per bhp hr
Brake thermal efficiency ,ηtb =
2545 (0. 4064) ( 19 , 351 )
=0. 324 or 32. 4
Brake mep:
bmep =
bhp × 33 , 000 LANp
lb / ft 2
A = πD
2
4
× no. of cylinders
Np =
N 2
for 4 − cycles
bmep =
- 7× 33 , 000
(0. 875)
[
π 4
(0. 7083) 2 ( 6 )
](
600
####### 2 )
= 10 , 991 lb / ft 2
bmep =
10 , 991 144
=76 .33 psi
- Heat Balance
Toaz - practice material
Subject: Practical Research
School: Bataan National High School
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