Medium/Heavy Duty Truck Engines Chapter 11
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Transcript Medium/Heavy Duty Truck Engines Chapter 11
Medium/Heavy Duty
Truck Engines, Fuel & Computerized
Management Systems, 3E
Chapter 16
Chemistry & Combustion
Copyright © 2009 Delmar, Cengage Learning
Introduction
Knowledge of chemistry important to:
Understand fuel composition & combustion
dynamics
Develop ability to work with electricity &
electronics
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Basic Chemistry
Building blocks of all matter are atoms
All atoms are electrical
Electrical charge is a component of all atomic matter
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Elements
An element is any one of
more than 100 substances
that cannot be chemically
resolved into simpler
substances
Elements consist of minute
particles known as atoms
Examples:
• Hydrogen Atom
• Oxygen Atom
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Common Elements
Metallic Elements
Atomic # Non-Metallic Elements
Atomic #
Iron – Fe
26
Hydrogen – H
1
Sodium – NA
11
Carbon – C
6
Magnesium – Mg
13
Helium – HE
2
Aluminum – Al
13
Sulfur – S
16
Nickel – N
28
Silicon – Sl
14
Rhodium – Rh
35
Selenium – Se
34
Silver – Ag
47
Oxygen – O
8
Zinc – Zn
30
Nitrogen – N
7
Gold – Au
79
Argon – Ar
18
Platinum - Pt
78
Radon - Rn
86
Atomic # identifies number of protons in an atom of the element
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Mixtures
A mixture is composed of two or more elements
and/or compounds
For example:
Air = 23% oxygen + 76% nitrogen + 1% inert gases
Both oxygen & nitrogen:
Retain their own identity
Retain their own characteristics
Can take part in reactions independently of each other
Mixture properties depend on the substances in it
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Chemical Bondings
Interactions accounting for the association of atoms
into molecules, ions, crystals
When atoms approach each other:
Their nuclei & electrons interact
Distribute themselves
Their combined energy is lower than in the
alternative arrangement Valency of an atom is simply the
number of unpaired electrons in its
Valency Number:
valence shell
Number of bonds an atom can form
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Compounds
A compound is composed of:
Pure compounds can be
obtained by physical
Two or more elements
separation processes such
Combined in definite proportions as filtration & distillation
Held together by a chemical force
Can be broken down into their elements by chemical
reactions
Carbon atoms are unique
Have ability to form covalent bonds
• With each other
• With other elements
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Covalent bonding occurs
when two electrons are
shared by two atoms.
Molecules
A molecule is:
Smallest particle of a compound
Can exist in a free state
Can take part in a chemical reaction
An
A water
oxygenmolecule
molecule
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Note:
Shared electrons
Atomic Structure
Electron
Carries negative charge
Orbit in shells around atom’s nucleus
Proton
Carries positive charge
Located in atom’s nucleus
Neutron
Electrically neutral
Located in atom’s nucleus
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Balanced Atoms
Electrically balanced atoms have an equal number of
electrons & protons
An atom with either a deficit or surplus of electrons
is called an ion
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Balanced Atoms
Electrons are arranged in circular orbits around the
nucleus
Electrical force attracting the electron to
the positive charge of the nucleus is
offset by the mechanical force acting
outwards on the rotating electrons
keeping them in their orbits.
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States of Matter
Generally classified into one of three states or phases
Solid
Liquid
Gas
Water is the only substance
that is familiar with all three
states:
Ice (solid)
Water (liquid)
Steam (gas)
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Determining State
Difference between solids, liquids & gases can be
explained in terms of kinetic molecular theory
Kinetic = motion
As temperature increases, so does molecular motion
Vaporization: heat applied to liquid, converts to
gaseous state
Condensation: reverse vaporization
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States of Matter - Conclusion
Typical injector pulse:
Fuel directly injected to diesel engine cylinder is
atomized (liquid state)
Exposed to heat of compression (vaporizes)
Gases condensing in exhaust observed as white
smoke
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Properties of Mixtures & Compounds
Each element has:
A special identity
A set of characteristics that make it unique
Chemical Reactions:
Explained by their constituent elements
Combustion is an oxidation reaction
Reactant in engine cylinder is whatever oxygen
present at time of ignition
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Properties of Common Elements
Element
State
Atomic # Properties/Characteristics
Hydrogen
Gas
1
Simplest element, one of the most reactive
Carbon
Varies
6
Combines to form compounds more readily than
other elements
Oxygen
Gas
8
Most common element in earth’s crust
Nitrogen
Gas
7
When oxidized in the combustion process, it forms
several compounds collectively known as Nox
Sulfur
Solid
16
Appears prominently in residual oil
Iron
Solid
26
Used extensively in vehicle technology, mostly as
steel
Aluminum
Solid
13
Excellent conductor of heat & electricity
Most fuels are elementally composed of carbon & hydrogen
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Combustion Reactions
Involved Products & Byproducts:
Air (a mixture)
•
•
•
•
•
•
•
•
•
•
Nitrogen
Oxygen
Argon
Neon
Helium
Methane
Krypton
Hydrogen
Nitrous oxide
Zenon
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N2
O2
Ar
Ne
He
CH4
Kr
H2
N2O
Xe
78.084%
20.946%
00.934%
00.0018%
00.000524%
00.0002%
00.000114%
00.00005%
00.00005%
00.0000087%
Combustion Reactions
Involved Products & Byproducts:
Water vapor
H2O
Ozone
O3
Carbon Dioxide
CO2
Carbon Monoxide CO
Sulfur Oxides
SOx
Oxides of Nitrogen NOx
0 – 7%
0.01%
0.01 – 0.1%
Byproducts of combustion
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Unburned Hydrocarbons
UHCs consist of any emitted unburned fuel fractions
Include:
Paraffins
Olefins
Aromatics
Least volatile elements of a fuel more likely to result
in UHC emissions
Classified as potentially harmful
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Partially Burned Hydrocarbons
PHCs are a result of low-temperature combustion
Include:
Aldehydes
Ketones
Carboxylic acids
Can result from extinguishing the flame front before
a molecule is completely combusted.
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Particulate Matter
Any liquid or solid matter emitted from exhaust stack
Can be detected in light extinction test apparatus
(i.e. smoke opacimeter)
Classified as particulate matter (PM)
The term PM is more appropriately applied to
emitted ash & carbon spots in the solid state
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Combustion
Fuel
To ignition temperature!
+ Oxygen
+ Heat
= Chemical Reaction
Fuel
Energy
Oxygen
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Ignition
The reaction causes
the energy in the fuel
to be liberated
resulting in a large
volume of hot gases!
Combustion with Ambient Air
Combustion in an engine cylinder uses the oxygen
available in the ambient air mixture
Proportionally the largest ingredient of the reaction
is always nitrogen
Ideally nitrogen should remain inert, unaffected by
Noxious emissions
the oxidation of the fuel
When nitrogen is oxidized, NOx are produced
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Combustion in an Engine Cylinder
Pressure volume curve in a
diesel engine.
The large volume of hot gases
produced as a result of the
combustion reaction creates this
pressure.
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Cylinder Gas Dynamics
Injected fuel is:
Dispersed
Mixed
“Swirl”
Combusted in the cylinder
Intent is to create cyclonic turbulence in the cylinder
as the piston is driven upwards
Behavior governs:
Engine’s performance efficiency
Noxious emissions
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Stoichiometry
Actual ratio of the reactants in any reaction to the exact
ratios required to complete the reaction
Stoichiometric ratio or lambda () factor is dependent on
actual chemical composition of the fuel to be burned
> = greater than, < = less than
= Actual air supplied
Stoichiometric requirement
> 1 lean burn
< 1 rich burn
= 1 stoichiometric AFR
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Calculating Air-Fuel Ratio
Petroleum contains
by mass:
Calculating
an Air-Fuel Ratio
With a hypothetical diesel
by mass; 86% carbon,
Carbon
84 –fuel
87containing
%
13% hydrogen & 1% sulfur the oxygen required to completely
Hydrogen
11would
– 15be:%
oxidize 1 Kg. of the fuel
Carbon (2.66 X .86) + Hydrogen
Sulfur
0 -- 2 %(8 X .13) + Sulfur (1 X .01) = 14.5
Kg.
For oxidization:
The air fuel ratio for this example would be 14.5:1
1 Kg. of carbon (C) requires 2.66 KG. of Oxygen (O)
1 Kg. of hydrogen (H) requires 8.0 Kg. of Oxygen (O)
1 Kg. of Sulfur (S) requires 1.0 Kg. of Oxygen (O)
Air contains approximately 23% by mass
1 Kg. of air would contain .23 Kg. of Oxygen
1 Kg. of oxygen is contained in 4.35 Kg. of air
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Combustion Cycle
injector nozzle
opening.
1. Ignition delay or ignition lag
Occurs between start of ignition & the moment
ignition occurs
2. Period of rapid combustion
Fuel that evaporated & mixed during ignition delay
period is burned, the rate & duration of rapid
combustion are closely associated with the length of
Available fuel is oxidized.
the delay period
3. Third phase of combustion
Begins at the moment of peak cylinder pressure &
ends when combustion is measurably complete
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Combustion Cycle
4. Afterburn phase
A period in which any unburned fuel in the
cylinder may find oxygen & burn
5. Dosing Injection
Final shot of fuel into the cylinder, not intended
to be combusted in the cylinder. Shot is injected
with intention of discharging into the exhaust
system as raw fuel to be combusted in exhaust
gas aftertreatment systems
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Combustion Cycle
6. Detonation
Multiple flame front condition that causes an
abnormally high rate of combustion & resultant
pressure rise in the cylinder block
“Diesel knock”
“Ping”
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