1.Lec1 Fuels for Internal Combustion Engines
-
Upload
meronaruto -
Category
Documents
-
view
219 -
download
0
Transcript of 1.Lec1 Fuels for Internal Combustion Engines
-
7/28/2019 1.Lec1 Fuels for Internal Combustion Engines
1/41
Fuels for Internal
Combustion
Engines
-
7/28/2019 1.Lec1 Fuels for Internal Combustion Engines
2/41
Introduction.
Heat required in I.C.E. is generally
produced by chemical reaction of fueland air inside the engine cylinder. The
reaction time is of the order ofhundreds to thousands of a second
-
7/28/2019 1.Lec1 Fuels for Internal Combustion Engines
3/41
In engines where the fuel air
mixture is formed outside thecylinder ( carburetor engines )
the fuel must evaporate easily
at atmospheric temperature
and form an inflammable
mixture with incoming air.
-
7/28/2019 1.Lec1 Fuels for Internal Combustion Engines
4/41
In engines where the fuel is
directly injected into thecylinder (diesel engines ), the
atomized fuel must evaporatequickly and must be well mixed
with air so that he combustionprocess may be completed
within reasonable time.
-
7/28/2019 1.Lec1 Fuels for Internal Combustion Engines
5/41
The reaction characteristics of the
fuel should be such that the rate ofpressure and temperature rise
inside the cylinder due tocombustion should be moderate;
this decreases the possibility oflarge mechanical and thermal
stresses
-
7/28/2019 1.Lec1 Fuels for Internal Combustion Engines
6/41
The fuel at the completion of
combustion should not leave
appreciable carbon, coke, sulfur or
gum deposits which may cause
excessive wear and corrosion on
cylinder and piston rings. The
composition of the fuel should notlead to the evolution of harmful
combustion products.
-
7/28/2019 1.Lec1 Fuels for Internal Combustion Engines
7/41
Influence of Fuel
on Engine
Performance
-
7/28/2019 1.Lec1 Fuels for Internal Combustion Engines
8/41
1. Output:.
The output break h.p. Nbr of an ICE isexpressed as follows
)4500427(
)1(thcabr Q
FFVN
-
7/28/2019 1.Lec1 Fuels for Internal Combustion Engines
9/41
-
7/28/2019 1.Lec1 Fuels for Internal Combustion Engines
10/41
Qv is a fuel characteristic and affects
the engine output so does the thermalefficiency. The inlet temperature of
mixture which also vary with fuel
characteristics affects the engine
output
cav QF
FQ
)1(
-
7/28/2019 1.Lec1 Fuels for Internal Combustion Engines
11/41
2. Thermal efficiencyDue to combustion of a certain fuel the
change of number of moles nm
=nmproducts -nmmixture affects thepressure of the products as follows:
mixture
products
mixture
products
mixtureproductsnm
nm
T
Tpp
-
7/28/2019 1.Lec1 Fuels for Internal Combustion Engines
12/41
Hence an increase in the number of moles
increases the pressures of the product for
the same value of TP which is dependenton Qc . Therefore, for fuels where there is
an increase in the number of moles due to
combustion the thermal efficiency will behigher than for fuels where there is no
such increase.
mixture
products
mixture
products
mixtureproductsnm
nm
T
Tpp
-
7/28/2019 1.Lec1 Fuels for Internal Combustion Engines
13/41
3. Inlet temperatureIf the prevailing atmospheric temperature
is higher than that necessary for efficientevaporation and distribution of fuel, inlet
temperature of the mixture in carburetor
engines will be reduced due to the
evaporation of fuel. Fuels with higher
evaporation heat needs will reduce theinlet temperature to a greater extent.
Using such fuels will increase the engine
output due to the increase in inlet density.
-
7/28/2019 1.Lec1 Fuels for Internal Combustion Engines
14/41
4. Specific Fuel consumption
in case of petroleum fuels the heat of combustiondoes not vary very much. Therefore SFC mainly
depends on the thermal efficiency. For alcohols or
benzols whose heat of combustion is much lower
than those of Petroleum fuels the SFC will be much
higher compared to petroleum fuels for the same
power output and thermal efficiency
cthermal QCFS
1
...
-
7/28/2019 1.Lec1 Fuels for Internal Combustion Engines
15/41
Properties of
Internal CombustionEngine Fuels
-
7/28/2019 1.Lec1 Fuels for Internal Combustion Engines
16/41
.1. Volatility
.
For engines with carburetor the liquid fuelmust be volatile enough to produce a
combustible fuel vapor air mixture at
intake temperature and to producecomplete fuel vapor air mixture inside the
cylinder before combustion. In case of
diesel fuels the evaporation rate at the
average gas temperature should be
sufficiently high.
-
7/28/2019 1.Lec1 Fuels for Internal Combustion Engines
17/41
2. Anti-knock value
and ignitability.
.
.
-
7/28/2019 1.Lec1 Fuels for Internal Combustion Engines
18/41
2.1 . Spark ignition engine
fuel.In SIE fuel air mixture is taken inside
the cylinder where aftercompression, combustion is
initiated in this mixture. Thus thefuel must resist abnormal burning
or detonation Ab l b i d i
-
7/28/2019 1.Lec1 Fuels for Internal Combustion Engines
19/41
Abnormal burning or detonation
can cause:
*too rapid rate of energy release*excessive temperature and pressure
inside the cylinder*decreases the efficiency of
conversion of heat into useful work
**this will adversely affect the
efficient and smooth operation of the
engine
-
7/28/2019 1.Lec1 Fuels for Internal Combustion Engines
20/41
The fuels used for petrol
engines must offer sufficientresistance to detonation, i.e.
it must have a suitable anti-
knock property. The anti-knock property is expressed
by the Octane number
-
7/28/2019 1.Lec1 Fuels for Internal Combustion Engines
21/41
2.2. Compression ignition engine
fuelIn case of diesel engine the fuel is
injected into compressed air with
increased temperature. There is a time
interval between the moment the fuel
is injected to the moment it is ignited.This interval is known as the ignition
lag or ignition delay
-
7/28/2019 1.Lec1 Fuels for Internal Combustion Engines
22/41
A long period of ignition lag will
result in a high rate of pressurerise in the cylinder and this will
result in rough running of theengine. Ignition lag depends on
the operating conditions as wellas on the physical and chemical
properties of the fuel
-
7/28/2019 1.Lec1 Fuels for Internal Combustion Engines
23/41
-
7/28/2019 1.Lec1 Fuels for Internal Combustion Engines
24/41
3. Heat of Combustion..The quality of a fuel in
general is determined by the
amount of heat evolvedupon complete combustion
-
7/28/2019 1.Lec1 Fuels for Internal Combustion Engines
25/41
4. ChemicalComposition and
Molecular Structure
of Fuels
-
7/28/2019 1.Lec1 Fuels for Internal Combustion Engines
26/41
5. Anti-knock
Rating of Fuel
5 1 F l f S k I i i E i
-
7/28/2019 1.Lec1 Fuels for Internal Combustion Engines
27/41
5.1. Fuel for Spark Ignition Enginesresistance to detonation is an extremely
important characteristic of the S.I. fuel. Inaddition to the effect of the chemical
characteristic of the hydrocarbon, other
factors such as fuel-air ratio, ignition
timing, dilution, effectiveness of jacket
cooling, atmospheric conditions,compression ratio, etc. , affect the
tendency of the fuel to detonate in engine
cylinder.
-
7/28/2019 1.Lec1 Fuels for Internal Combustion Engines
28/41
Hence a scale of detonation
intensity is necessary and a
particular intensity denotedby a point on the scale must
be selected for comparisonof fuels being rated.
-
7/28/2019 1.Lec1 Fuels for Internal Combustion Engines
29/41
The anti-knock value of a fuel is
determined by comparing its anti-
knock property with a mixture of tw
reference fuels. The iso-octane
(C8H18) is observed to have very highresistance to detonation. On the
other hand normal heptane(C7H16)was found to have extremely
poor resistance to detonation.
-
7/28/2019 1.Lec1 Fuels for Internal Combustion Engines
30/41
A suitable scale known asthe Octane number scale
was evolved for specifying
the knock resistanceproperty of fuels.
-
7/28/2019 1.Lec1 Fuels for Internal Combustion Engines
31/41
Iso-octane was given an octane
number of 100, and normalheptane was rated at 0 octane
number. The octane number of a
fuel is defined as the percentage by
volume of iso-octane in a mixture
of iso-octane and normal heptane,
which will match the detonation
intensity of the fuel.
-
7/28/2019 1.Lec1 Fuels for Internal Combustion Engines
32/41
Knock InhibitorsChemical compounds like tetra-ethyl
lead Pb(C2H5)4 , iron carbonyl, and
nickel carbonyl appreciably increasesthe knock resistance of spark ignition
engine fuel when a small quantity isadded to the fuel (0.5 ml to 1.5 ml per
liter of gasoline)
-
7/28/2019 1.Lec1 Fuels for Internal Combustion Engines
33/41
A few milliliters of tetra-ethyl lead
Pb(C2H5)4 added per liter of
gasoline increase the octane
number. The tetra-ethyl lead
Pb(C2H5)4 is widely used but it alsoproduces cylinder deposits and
poisonous emissions. Therefore theamount of compound added is
restricted
-
7/28/2019 1.Lec1 Fuels for Internal Combustion Engines
34/41
5.2 Fuels for compression
ignition engines.In the case of diesel fuels the
knock resistance depends on
chemical characteristics as well
as on operating conditions and
engine design
-
7/28/2019 1.Lec1 Fuels for Internal Combustion Engines
35/41
-
7/28/2019 1.Lec1 Fuels for Internal Combustion Engines
36/41
Cetane number of a fuel is the
percentage by volume of normalcetane in a mixture of normal
cetane and alpha methylnapthelene, which has the same
ignition characteristics (ignitiondelay) as the test fuel when
comparison is carried out
-
7/28/2019 1.Lec1 Fuels for Internal Combustion Engines
37/41
Knock InhibitorsKnock resistance property of diesel fuel can be
improved by using small quantities of
compounds like amyl nitrite, ethyl nitrate, ethyl
nitrite or ether. These compounds are generally
introduced in small amounts with the intake air.
They ignite early in the compression stroke and
thereby increase the temperature of
compressed air at the time when fuel injection
starts. The ignition delay of the fuel is thereby
reduced
-
7/28/2019 1.Lec1 Fuels for Internal Combustion Engines
38/41
-
7/28/2019 1.Lec1 Fuels for Internal Combustion Engines
39/41
h l
-
7/28/2019 1.Lec1 Fuels for Internal Combustion Engines
40/41
Chemical
Reactions in
Fuel
Combustion
-
7/28/2019 1.Lec1 Fuels for Internal Combustion Engines
41/41
Analysis ofProducts ofCombustion