Medium/Heavy Duty Truck Engines Chapter 11

Report
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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