MT 313 IC ENGINES
LECTURE NO: 05
(3 Mar, 2014)
Khurram
engr014@yahoo.com
Yahoo Group Address: ICE14
CALORIFIC VALUE OF A FUEL
It is the amount of heat energy produced on complete combustion
of 1 kg of a fuel. The calorific valve of a fuel is expressed in kilojoule
per kg.
Calorific values of some fuels in kilojule per kg
Cow dung cake
Wood
Coal
Petrol
Kerosene
Diesel
Methane
CNG
LPG
Biogas
Hydrogen
6000 - 8000
17000 - 22000
25000 - 33000
45000
45000
45000
50000
50000
55000
35000 - 40000
150000
Hydrogen has the highest calorific value among all fuels.
CONVENTIONAL FUELS
• Engine converts heat energy
• Heat energy is obtained from chemical
combination of the fuel with Oxygen into
Mechanical Energy
TYPE OF FUELS
• Solid Fuels
• Gaseous Fuels
• Liquid Fuels
TYPE OF FUELS
• Solid Fuels
– Little Application I.C. Engines
– Handling is Difficult
– Storage Problem
• Gaseous Fuels
– Ideal for I.C. Engines
– Handling Problem
– Storage
– Normally use in Stationary Power Plants
TYPE OF FUELS
• Liquid Fuels
–Derivative of Liquid Petroleum
–Three Commercial types are
• Benzyl
• Alcohol
• Petroleum
IMPORTANT FUELS
• Solid Fuels
– Powdered Coal
– Saw Dust
• Gaseous Fuels
– Blast Furnace Gas
– Blue Water Gas
– Coal Gas
– Coke oven Gas
– Natural Gas
– Producer Gas
IMPORTANT FUELS
• Non Petroleum Fuels
– Methyl Alcohol CH3 OH
– Ethyl Alcohol C2 H5 OH
• Liquid Fuels
– Gasoline C8 H17
– Kerosene
– Light Diesel Oil C12 H20
– Medium Diesel Oil C13 H28
– Heavy Diesel Oil C14 H30
CHARACTERISTIC OF A GOOD FUEL
•
•
•
•
•
•
Ignite easily
Burn well, but not explosively
Give out a lot of heat
Have low smoke and ash content
Be Inexpensive
Be easy to store and transport
QUALITIES OF ENGINE FUEL
S.I.
– Volatility
VOLATILITY
• In Chemistry and Physics, volatility is the
tendency of a substance to vaporize. Volatility
is directly related to a substance's vapor
pressure. At a given temperature, a substance
with higher vapor pressure vaporizes more
readily than a substance with a lower vapor
pressure
• Fuels function by releasing combustible gases (vapours)
• Boiling Point is an indicator of volatility: the higher the boiling
point, the less volatile the fuel.
• Vapour pressure is an indicator of volatility: the higher the
vapour pressure, the more volatile the fuel. Vapour pressure
increases with temperature, so the volatility of a fuel can be
increased by raising the temperature.
• A highly volatile fuel is more likely to form a flammable or
explosive mixture with air than a non-volatile fuel. By
definition, gases are volatile.
• Liquid fuels are either sufficiently volatile at room
temperature to produce combustible vapour (ethanol, petrol)
or produce sufficient combustible vapours when heated
(kerosene).
• Solid fuels decompose above the vapourisation temperature
to produce combustible vapours. Solid fuels will have a higher
ignition temperature than liquid or gaseous fuels.
QUALITIES OF ENGINE FUEL
S.I.
•
•
•
•
Volatility
Starting and Warm up
Operating Range Performance
Crankcase Dilution
CRANKCASE DILUTION
• is a phenomenon of I.C. Engines in which
unburned diesel or gasoline accumulates in
the crankcase.
• Excessively rich fuel mixture or incomplete
combustion allows a certain amount of fuel to
pass down between the pistons and cylinder
walls and dilute the engine oil. It is more
common in situations where fuel is injected at
a very high pressure, such as in a direct
injection diesel engine.
CRANKCASE DILUTION
• When a mixture of air and fuel enters the
cylinder of an engine, it is entirely possible for
condensation of fuel to occur on the cooler
parts of the cylinders. The condensate may
wash the lubricating oil from the cylinder
walls, travel past the piston rings and collect in
the oil pan, thus increasing wear and also
diluting the lubricating oil. Since the less
volatile components of the fuel will have the
greatest tendency to condense, the degree of
crankcase-oil dilution is directly related to the
end volatility temperatures of the mixture.
QUALITIES OF ENGINE FUEL
S.I.
– Volatility
– Starting and Warm up
– Operating Range Performance
– Crankcase Dilution
– Vapour Lock Characteristics
VAPOUR LOCK
• The fuel can vaporize due to being heated by
the engine, by the local climate or due to a
lower boiling point at high altitude.
• In regions where higher volatility fuels are
used during the winter to improve the starting
of the engine, the use of "winter" fuels during
the summer can cause vapor lock to occur
more readily.
QUALITIES OF ENGINE FUEL
S.I.
– Volatility
– Starting and Warm up
– Operating Range Performance
– Crankcase Dilution
– Vapour Lock Characteristics
– Antiknock Quality
– Gum Deposits
– Sulphur Content
QUALITIES OF ENGINE FUEL
C.I.
– Knock Characteristic
– Volatility
– Starting Characteristics
– Smoking and Odour
– Viscosity
VISCOSITY
• Viscosity is a scientific term describing the
internal friction of a fluid or gas. Both have
adjacent layers, and when pressure is applied,
the friction between those layers affects how
much the substance will respond to external
force. In its simplest form, this friction can be
evaluated by the thickness of a substance. A
general rule is that gases are less viscous than
liquids,
and
thicker
liquids
exhibit
higher viscosity than thin liquids.
QUALITIES OF ENGINE FUEL
C.I.
– Knock Characteristic
– Volatility
– Starting Characteristics
– Smoking and Odour
– Viscosity
– Corrosion and Wear
– Handling Ease
MOLECULES AND COVALENT BONDS
Chemical bonds result from a mutual sharing of electrons between atoms,
the shared electrons are in the outermost shell, known as valence electrons
Lewis notation:
Hydrogen Atomic # 1  1 valence electron
H
Carbon Atomic # 6  4 valence electrons
C
Oxygen Atomic # 8  6 valence electrons
O
Atoms like to have electron configuration like noble gas, usually eight valence
electrons, an octet.
H
H2
H H
CH4
H C H
H
Atoms and molecules with unpaired valence electrons are called radicals
e.g. O, H, OH, N, C
O H
HYDROCARBON FUELS
Common hydrocarbon fuels are grouped as:
Paraffins:
CnH2n+2
n= 1
n= 2
n= 3
n= 4
n= 8
CH4 methane
C2H6 ethane
C3H8 propane
C4H10 butane
C8H18 octane
H
H
C
H
H
H
methane
n=2 C2H4 ethene
n=3 C3H6 propene
H
C
C
H
H
ethane
Olefins:
CnH2n
H
H
C
H
H
C
C
H
H
H
Note: n=1 yields CH2 is an unstable molecule
propene
Acetylenes:
CnH2n-2
n=2 C2H2 acetylene
n=3 C3H4 propyne
H
C
C
acetylene
H
H
ALCOHOLS
An alcohol molecule is simply a hydrocarbon molecule with one of the
hydrogen atoms replaced by a hydroxyl molecule (OH)
The main alcohols used as engine fuels are:
Ethanol – ethyl alcohol (C2H5OH) consists of ethane molecule (C2H6) with
OH substituting one H
Methanol –methyl alcohol (CH3OH) consists of methane molecule (CH4)
with OH substituting one H
Butanol – (C4H9OH) consists of butane molecule (C4H10) with OH substituting
one H
H
H
H
C
C
H
H
H
OH
Ethyl alcohol
H
C
OH
H
Methyl alcohol
IC ENGINE FUELS
Crude oil contains a large number of hydrocarbon compounds (25,000).
The purpose of refining is to separate crude oil into various fractions via
a distillation process, and then chemically process the fractions into fuels
and other products.
A still is used to heat a sample, preferentially boiling off lighter
components which are then condensed and recovered.
The group of compounds that boil off between two temperatures are
referred to as fractions.
The order of the fractions as they leave the still are naptha, distillate,
gas oil, and residual oil. These are further subdivided using adjectives
light, middle, and heavy.
The adjectives virgin or straight run are often used to signify that no
chemical processing has been performed to a fraction.
GASOLINE
Light virgin (or straight run) naptha can be used as gasoline.
Gasoline fuel is a blend of hydrocarbon distillates with a range of boiling
points between 25 and 225oC (for diesel fuel between 180 and 360oC)
Chemical processing is used to:
•
Produce gasoline from a fraction other than light virgin, or
• Upgrade a given fraction (e.g., Alkylation increases the MW and octane
number of fuel: produce isooctane by reacting butylenes with isobutane in
the presence of an acid catalyst)
Octane (C8H18)
The octane molecule is often used to simulate the properties of gasoline
H
H
H
H
H
H
H
H
H
C
C
C
C
C
C
C
C
H
H
H
H
H
H
H
H
H
n-octane
There are 18 isomers of octane, depending on position of methyl (CH3)
branches which replace hydrogen atoms (eg. a side H is replaced with CH3)
CH3
CH3
H
CH3
C
C
C
CH3
H
H
iso-octane
CH3
Reformulated Gasoline (RFG)
In order to reduce emissions such as carbon monoxide (CO) and unburned
hydrocarbons (HC) the oxygen content of gasoline is increased to about 3%
by weight (U.S. oxygenated fuels program, winter only).
The US Clean Air Act requires certain large US cities to use RFG year-round
in order to reduce ozone by requiring a minimum oxygen content of 2% by
weight and maximum benzene content of 1%.
The primary oxygenates are MTBE (CH3)OC(CH3)3 and ethanol (C2H5OH)
As part of the reformulated gasoline program sulfur is restricted to 31 ppm
Renewable Fuels
Currently most automotive IC engines use fossil fuels (gasoline or diesel)
Due to the ever increasing cost of oil, due to diminishing oil reserves and
accessibility to the oil reserves, and environmental concerns such as global
warming, alternative fuels have become very attractive. The 2007 US Energy
Bill set a 36 billion gal target for renewable fuels to be used in autos by 2020
In Canada the Renewable Fuel Mandate that took affect Dec. 2010 requires
an average of 5% ethanol content for gasoline and 2% biodiesel
Alcohols such as ethanol, methanol, and butanol are receiving a lot of
attention because they can be synthesized biologically, i.e., bioalcohols or
biofuels.
Since there is oxygen in the fuel, combustion of alcohols produces no CO
but more greenhouse gas carbon dioxide (CO2) than fossil fuel combustion.
Alcohol Fuels
However, since the fuel is derived from plant matter the CO2 produced is
extracted from the atmosphere during the growth of the plant, i.e.CO2 neutral
6CO2 + 6H20 + solar energy → C6H12O6 + 6O2
photosynthesis
sugar
Ethanol used for fuel is obtained by fermentation. Yeast metabolizes sugar
(C6H12O6) in the absence of oxygen to produce ethanol and carbon dioxide
C6H12O6 → 2C2H5OH +CO2
In Brazil ethanol is derived from sugar cane whereas in the US and Canada
corn is used as the feed stock (sugar cane has 30% more sugar than corn).
Sugars for ethanol fermentation can also be obtained from cellulose
(C5H10O5)n that makes up agricultural byproducts, such as corn cobs, corn
stalks, straw, switch grass, and wood, into renewable energy resources
Pros and Cons of Alcohol fuels
Main advantage of alcohol fuels is they are derived from renewable biomass
which is CO2 neutral when burned
Alcohol fuels have a higher heat of vaporization (hfg) than gasoline → cools
the air during mixing, resulting in a higher volumetric efficiency
Alcohol fuels have several drawbacks compared to fossil fuels:
- lower energy density (kJ/m3) than gasoline
(10% less for butanol, 27% less for ethanol, 55% less for methanol),
- corrosive to fuel systems (methanol > ethanol > butanol)
- poor cold temperature start up due to low vapor pressure
- toxicity (methanol)
- use of food crops (i.e., corn, wheat) drives up world prices of food
- production of ethanol is very energy intensive and thus expensive
NEB of corn grain ethanol and soybean biodiesel production
Hill J. et.al. PNAS 2006;103:11206-11210
NEB: net energy balance
36
©2006 by National Academy of Sciences
Ethanol as a Fuel
Ethanol is quickly becoming the alternative fuel of choice for IC engines
An IC engine can run on gasoline with up to 10% ethanol (E10) without any
modifications, the use of higher blends requires changing certain components
in the fuel system, i.e, use stainless steel fuel lines and tank.
In Brazil half of the cars can run on 100% ethanol including flex-fuel engines
that can run on all ethanol, all gasoline, or any combination of the two
Gasoline with up to 85% ethanol (E85) is now starting to enter the US and
Canadian markets. Flex-fuel engines in this market can run on E85 or all
gasoline, 100% ethanol not yet permitted
Other Alternative Fuels
Biodiesel – made from vegetable oils (soybeans), waste cooking oil, animal
fats. It is produced by reacting the oil with an alcohol (usually methanol)
and a catalyst (sodium hydroxide). The resulting chemical reaction
produces glycerine and alkyl esters (biodiesel). It is a liquid at RTP and
has 9% lower energy content than regular diesel, usually mixed with diesel.
Propane has been used for many years as a fuel for IC engines, especially
in Europe where the price of gasoline has been historically high. In N.A.
mainly used on fork lifts and golf carts. Stored as a liquid in steel tanks
Pv(25oC)= 10 bar. Cleaner burning than gasoline.
Dimethyl ether (CH3OCH3 same as ethanol) – good diesel fuel because
of good autoignition quality. Is a gas at RTP, produced from syngas
(biomass) or methanol.
Other Alternative Fuels
The following fuels are in the gaseous state at 25oC and thus have low
energy density and engines using them have low volumetric efficiencies:
Hydrogen (H2) is the “new” natural gas with similar issues. The main benefit
is no CO, CO2 and HC emissions. Biggest problems with hydrogen is safety,
lack of distribution infrastructure, onboard storage, and production energy
(hydrogen gas is not found in nature it must be produced). Can be stored as
a gas (compressed), a liquid (cryogenic, Tbpt=20K), in a metal hydrides
Natural gas (CH4, C2H6,..) Normally stored in tanks at 16-25 MPa. Cleaner
burning than gasoline. Limited use in automobiles, mainly used for running
pipeline compressors and fixed power generation gas turbines
Producer gas (syngas) engines run on the gaseous products from thermal
gasification of biomass such as wood. The carbon reacts with steam, or a
limited amount of air, at high temperature (>700C) to produce a mixture
consisting of roughly 25% CO, 15% H2, and 5% CO2 , 50%N2 ….
Drawback is very low energy content, ten times lower than natural gas.
Benefit CO2 neutral. Used to power vehicles during WWII.
Ideal Gas Model
The ideal gas equation of state is:
R
PV  mRT  m
M

T  nR T

where R
is the Universal Gas Constant (8.314 kJ/kmol K),
weight and n is the number of moles.
is the molecular
M
Specific internal energy (units: kJ/kg)
u(T )   cv (T )dT
Specific enthalpy (units: kJ/kg)
h(T )   c p (T )dT
Specific entropy (units: kJ/kg K)
s ( P, T )  s o (T )  R ln( P Po )
(Po = 1 bar, so entropy at Po)
Ideal Gas Model for Mixtures
The mass m of a mixture is equal to the sum of the mass of n components
n
m   mi
i 1
The mass fraction, xi, of any given species is defined as:
m
xi  i
m
n
and
 xi  1
i 1
The mixture internal energy U and enthalpy H (units: kJ) is:
n
U  mu   mi ui
i 1
n
H  mh   mi hi
i 1
whereui and hi are mass specific values (units: kJ/kg)
The mixture specific internal energy u and enthalpy h is:
U n mi ui n
u 
  xi u i
m i 1 m i 1
H n mi hi n
h 
  xi hi
m i 1 m i 1
Ideal Gas Model for Mixtures
The total number of moles in the mixture is:
n
n   ni
i 1
The mole fraction, yi, of any given species is defined as:
n
yi  i
n
n
and
 yi  1
i 1
The mixture internal energy U and enthalpy H (units: kJ) is:
n
U   ni ui
i 1
n
H   ni hi
i 1
whereui and hi are molar specific values (units: kJ/kmol)
The mixture molar specific internal energy and enthalpy (units kJ/ kmol) is:
n
u   yi ui
i 1
n
h   yi hi
i 1
Ideal Gas Model for Mixtures
Mass specific entropy (kJ/kg K) molar specific mixture entropy (kJ/mol K) :
s   x s
n
i 1
i
o
i
s   yi sio  Ri ln yi   R ln P Po 
 R ln(P / P)   R lnP P 
i
i
n
o
i 1
The mixture molecular weight, M, is given by:
n
mi n
ni M i n
m i
1
M 

  yi M i
i 1 n
i 1
n
n
The partial pressure of a component, Pi, in the mixture (units: kPa) is:
PiV
yi 
ni
RT  Pi

n PV
P
RT
or
Pi  yi P
Composition of Standard Dry Air
Air is a mixture of gases including oxygen (O2), nitrogen(N2), argon (Ar),
carbon dioxide (CO2), water vapour (H20)….
For combustion dry air is taken to be composed of 21% O2 and 79% N2
by volume (yO2=0.21, yN2=0.79).
nN
nN
ntot y N 0.79




 3.76
ntot nO
yO
0.21
2
nO
2
2
2
2
2
For every mole of O2 there are 3.76 moles of N2.
Molecular weight of air is
n
M air   yi M i  yO2  M O2  y N 2  M N 2
i 1
 0.21(32)  0.79(28)  28.84 kg/kmol
The amount of water in moist air at temperature T is specified by the
specific humidity (w or the relative humidity (F) defined as follows:
w
mH 2O
mair
F
PH 2O
Psat (T )
0  F 1
Combustion Stoichiometry
If sufficient oxygen is available, a hydrocarbon fuel can be completely
oxidized, the carbon is converted to carbon dioxide (CO2) and the hydrogen
is converted to water (H2O).
The overall chemical equation for the complete combustion of one mole of
propane (C3H8) with oxygen is:
C3 H8  aO2  bCO2  cH 2O
# of moles
Elements cannot be created or destroyed, so
C balance:
H balance:
O balance:
3=b
→ b= 3
8 = 2c
→ c= 4
2a = 2b + c → a= 5
Thus the stoichiometric reaction is:
C3 H8  5O2  3CO2  4H 2O
species