The complete combustion (?)
by
Prof. Dimitris Prapas
Automotive Engineering Dept.
Alexander Technological
Educational Institution
of Thessaloniki
GREECE
[email protected]
SUMMARY
• Unit 1: Combustion theory
• Unit 2: Combustion in petrol engines
• Unit 3: Combustion in Diesel engines
• Unit 4: Some environmental implications
and illusions
Unit 1: Combustion theory
Ideally, the complete combustion of a
generic hydrocarbon gives :
CxΗy + O2  CO2 + Η2Ο + heat
In the resulting exhaust gases of a real internal combustion engine (as well as in any
practical combustion application of solid, liquid or gaseous fuels) the following
products are normally found in abundance:
 CO2 (5-15%)
 Η2Ο (10-15%)
 Ν2 (70-80%)
CxΗy + (O2 + Ν2 )  CO2 + Η2Ο + Ν2 + ???
(air)
What else ?
Energy sources and usages in modern civilization
[ For Greece, approximately :
Fossil fuels: 94%, Biomass: 3.4%, Hydroelectric: 1%, Others: 1.6%]
Why learning about combustion ?
Air
is used for the combustion in a variety of fuels
and applications: heat production, work production,
electricity production
heat production
work production
electricity production
FOSSIL FUELS FORMATION
- Liquid and gaseous: plankton biomass buried under sea floor
- Solid: biomass (i.e. plants) buried under earth’s surface
Ideal formation conditions for both cases: fast burying of biomass,
i.e. rapid sediment deposition, to avoid direct oxidation
Definition:
Combustion (or burning) is the sequence of very
rapid exothermic chemical reactions between a fuel
and an oxidant, accompanied by production of heat.
• Most fuels are organic compounds (especially
hydrocarbons) in gaseous, liquid or solid phase.
• The oxygen contain in air is normally the oxidant
Heat of combustion (or calorific value): energy released as
heat per mass unit after complete combustion with oxygen
-high (or gross) heat of combustion: obtained when, after the combustion, the
water in the exhaust is in liquid form (very rare in practice)
-low (or net) heat of combustion: obtained when the exhaust has all the water in
vapor form (steam)
In practice, using air for the combustion of any fuel
we always (!) get incomplete combustion, which
results additional, usually undesirable, by-products:
Chemical kinetics: the study of chemical reaction rates
Combustion: a series of rather complex rapid chemical oxidation
reactions taking place in various stages, some of them in both directions
A reduced (simplified) kinetic mechanism, consisting of 9 steps and 13 species
reactions, is given bellow for the oxidation of methane:
All reactions can occur in both directions !
A generic chemical reaction A + B → C is evolving
over time t in the following rate, d[C]/dt:
• The reaction rate constant k(T) depends highly on temperature T :
•
Ea is the activation energy, R is the gas constant and A is the frequency factor.
The Arrhenius equation
The reaction rate constant k(T) quantifies the speed of the reaction:
Thermodynamics determines the extent to which
reactions occur
In a reversible reaction, chemical equilibrium is reached when
the rates of the forward and reverse reactions are equal and
the concentrations of the reactants and products no longer
change, macroscopically.
At high temperatures the
“agility” of the chemical
reactions is increased
 formation of various
substances not possible in low
temperatures
Thermal decomposition: Chemical reaction in
which a large molecule breaks up when heated.
(at temperatures of 600–800°C most organic compounds acquire sufficient
vibrational energy to cause breaking of bonds, with formation of free radicals)
• The decomposition of hydrocarbons is call cracking (thermal or
catalytic), otherwise pyrolysis: complex organic molecules are
broken down into simpler light hydrocarbons, e.g:
Liquid fuel:
C22H46 ---> C11H24 + C11H22
Solid fuel:
• Extreme pyrolysis, called carbonization, leaves mostly carbon as the residue.
The strength of carbon-hydrogen bonds
depends on what the carbon is connected to:
Straight chain HCs such as normal heptane have secondary C-H bonds that are
significantly weaker than the primary C-H bonds present in branched chain
HCs like iso-heptane
Differences in the overall reaction rates of these heptane isomers are due to
their molecular structure:
At high temperatures chemistry is more “agile”
 disassociation endothermic reactions
(i.e. decomposition of small molecules)
If the disassociation products cool down rapidly their composition
“freezes”, e.g. in the exhaust gases of internal combustion engines !
Some products of the pyrolysis may contain
polycyclic aromatic hydrocarbons (PAHs), which
are known mutagens and probable human
carcinogens.
Aroma -> Aromatics : strong smell, they excite strongly our nose
(Not all aromas are polycyclic (carcinogens), but most PAH are smelly)
Pyrolysis also occurs in high temperature cooking (!)
(responsible for the formation of the golden-brown crust in foods)
roasting, baking, toasting, frying, grilling, and caramelizing
• This is because most organic food products are chemically
regarded as solid fuels, the auto-ignition temperature of which is
around 200 oC, i.e. very close to temperatures attained in most
cooking ways (potentially all but boiling).
•
•
Pyrolytic transformation of food begins at temperatures nearing its auto-ignition point !
Pyrolytic transformation of a fuel begins at temperatures nearing its auto-ignition point !
The auto-ignition temperature of most fuels depends on
their physical condition:
- solid fuels auto-ignite more easily than gas fuels
- Diesel oil auto-ignites more easily than petrol
(This may seem intuitively unexpected–exactly the opposite to the fuel volatility)
Consequences for internal combustion engines: Diesel oil auto-ignites when
entering the combustion chamber, Petrol engines require a spark
Because of the rapidity of the various forward and reverse
reactions, chemical equilibrium is hardly reached in an
internal combustion engine.
The final cooling “freezes” the reactions and determines the combustion products
concentrations, macroscopically.
Premixed and diffusion flames
premixed flame
e.g. petrol and air already mixed and then ignited by spark: the flame
front propagates rapidly inside the combustion chamber
diffusion flames
particles of a solid (e.g. coal) or tiny droplets of liquid hydrocarbons
(e.g. Diesel oil) surrounded by air auto-ignite: the diffusive processes
take place in the gaseous film of volatile components adjacent to the
surface of the fuel
In both cases a critical transient zone is temporarily evolved in the vicinity of flames
Unit 2: Combustion in petrol engines
Significance of the transient zone:
a series of cracking reactions will occur in the unburned mixture, just
before the flame front arrives, due to the combination of gradual
compression and both radiant and conductive heating
The hydrogen atoms are removed one at a time from the molecule
by reactions with small radical species (such as OH and HO2),
and O and H atoms. These pre-flame reactions occur at different
thermal stages:
• below 400 oC: addition of molecular oxygen to alkyl radicals,
internal transfer of hydrogen atoms to form an unsaturated,
oxygen-containing species,
• above 600 oC: chain branching and reformation, due to the
reaction of one hydrogen atom radical with molecular oxygen to
form O and OH radicals.
Once ignited, steady burning of the compressed fuel-air
mixture inside the combustion chamber is highly desirable,
i.e. the flame front should propagate out from the spark
plug evenly, until all the fuel is reached and consumed.
Detonation
•
•
the Research Octane Number (RON) of a fuel reflects the ability of the unburned endgases to resist spontaneous and total auto-ignition, properly known as detonation or
knock, since their temperature has already exceeded the auto-ignition point
Knock occurrence depends on the fuel composition and the pressure-temperature history
of unburned end-gas. Knocking results in an extremely rapid pressure rise, rather
“painful” for all engine elements exposed to it.
The octane rating of hydrocarbons (RON) is determined,
among others, by the structure of the molecule:
- long straight hydrocarbon chains produce large amounts of easily auto
ignitable pre-flame decomposition species (smaller octane number),
- branched and aromatic hydrocarbons are more resistant to detonation
Crank angle-dependent propagation of the flame front in
a 4-valve petrol engine
Time-dependent propagation of the flame front:
sequence of turbulent flame propagation in a stoichiometric
CH4/air mixture
The “thorny” tradeoff between:
engine efficiency
and nitric oxides NOx
(due to high temperatures chemistry)
As an example, increase of the ignition
angle in petrol engines (similarly, an
early fuel injection in Diesel engines)
increases both the engine efficiency
and the NOx production, via the
increased temperature.
Mechanisms of formation of unburned hydrocarbons
inside the combustion chamber of a petrol engine
Presence of unburned hydrocarbon
after exhaust valve is due to:
- Flame quenching and extinction on walls
- Mixture trapped in crevices
-
Mixture rich-lean oscillations (e.g. under rapid load changes)
Hydrocarbons absorbed-desorbed by oil and soot
Leakages from exhaust valve
Flame quenching inside cylinder
Incomplete combustion of lubricating oil
Ignition faults
Detailed course of unburned HC formation
in a 4-stroke petrol engine
The flame goes off as it
approaches the chamber wall
→ increased airborne unburned
hydrocarbon emissions, particularly during
engine warm-up
Pressure trace in the combustion chamber
(under constant load)
The cause of cyclic pressure variations is probably due to:
- Variation in turbulence/charge motions (Specially close to spark plug)
- Variations in charge composition
- Variation in the amount of charge in cylinder
Concentrations of various elementary radicals inside the
combustion chamber varies greatly during combustion
Typical changes in the composition of a petrol engine
exhaust dry gases (the remaining percentage is Ν2),
for various values of the air-fuel ratio (A/F)
The triodic catalyst reduces the emissions of HC, CO
and NOx approximately 10 times (!),
providing engine operates strictly at air ratio of λ=1 (i.e. A/F=14.6)
The complex rapid chemical reaction inside the
catalyst are taking place together, in simultaneously
oxidizing and reducing conditions
The reforming efficiency of a three-way catalyst is kept high
only within a very narrow air ratio window of
approximately λ=0.98-1.01
Yearly evolution of the gradually reduced upper emission
limits for the polluting emissions of a petrol car
Unit 3: Combustion in Diesel engines
Schematic representation of
fuel spray dispersion
The resulting situation is complex, both space- and time-wise:
- rich and lean mixture are both formed locally
(Macroscopically, the air–fuel ratio is always weak of stoichiometric. Still, the
fuel–air equivalence ratio is highest on the center-line of the fuel jet, decreasing
to zero outside boundaries)
- we get the emission “demons” of both the rich and lean combustion !
Sub-processes of mixture formation and combustion in diesel engines
Characteristics of Diesel engine combustion
•
•
•
•
•
a variety of non-uniform physical and chemical processes
limited time available for mixture preparation
fuel fine atomisation and spray penetration needed, to enable mixing of the fuel with air,
heating and evaporation via a diffusion process
preparation: the droplets at the outer edge evaporate first, creating a fuel–air vapour film
(sheath) around the still liquid cone jet.
pre-flame oxidation and localized ignition occur, preparing chemically the fuel–air mixture for
burning by cracking of the heavier hydrocarbons, forming combustion radicals
Summarizing diesel engine combustion evolution
(for single injection strategy)
• ignition delay period (cetane number), where the injected fuel is physically and
chemically prepared and mixed with air,
• a rapid premixed burning phase of the already evaporated fuel during the
previous ignition delay period; this stage is usually characterized by a high
rate of gas pressure increase and is, mainly, responsible for the combustion noise,
• a slower mixing-burning (or diffusion-controlled) phase, where the burning
rate is not governed by chemical kinetics but, rather, by the fuel initial dispersion
and subsequent turbulent mixing rates.
The two main mechanisms of combustion of fuel droplets
in the chamber of a Diesel engine
(average diameter of droplets: d=0.1 mm)
Approximate time needed for combustion:
for d= 0.1 mm 0.01 s
for d= 0.2 mm 0.04 s
At the end of fuel injection complete
closing of the injector should be
achieve, because unintendedly
post-injected fuel remains near
the nozzle, not reaching the
region inside the diffusion flame.
This highly undesirable dripping
fuel undergoes partial oxidation
and produces soot that cannot be
consumed by the diffusion flame
any more.
Diesel engine combustion “knocking”
The rate of heat release in the combustion chamber is far from constant:
a “disturbingly knocking” peak is present, soon after the initial ignition at point
b, due the simultaneous burning of the fist evaporated fuel layers in almost
all injected droplets
Effects of various operation
parameters on heat release rate
EFFECTS OF AIR MOVEMENT (SWIRL AND TUMBLE)
INSIDE THE COMBUSTION CHAMBER OF A DIESEL ENGINE
a) during the “premixed” phase:
Condition
Consequence
More air movement
NΟx,
η,
soot
Less air movement
NΟx,
η,
soot
b) during the “mixing controlled” phase:
Nearly the opposite consequences !
Phase I: premixed combustion
- After the ignition delay period, the already formed nearly homogeneous mixture
burns very quickly, exhibiting a steep pressure peak (similar to SI engine knocking).
- The combustion noise typical for the diesel engine is caused by the rapid pressure
increase speed dp/dt at this phase
- This pressure increase speed can be inadequately influenced by changing
the injection timing:
an early injection start leads to a "hard" and a later to a "soft" combustion
Phase II: diffusion combustion
Mixture formation processes continue during this main combustion phase,
influencing both the combustion course itself and pollutant formation.
Phase III: post-combustion
Further oxidized, as a result of local lack of oxygen during the previous
diffusion phase
This is a decisive phase for the oxidation of soot: over 90 % of the previously
produced soot is broken down during this phase
Soot formation
Soot is primary carbon which has escaped combustion, typically in oxygenstarving combustion regions. The final form of grown soot is more complex,
having absorbed various heavy hydrocarbons
Soot-NOx tradeoff
The problem of reducing both soot and NOx formation in Diesel engine raw
emissions, the so called soot-NOx tradeoff, becomes obvious: these two pollutants
show opposite behaviour, since conditions that reduce the formation of nitric oxides
increase the production of soot, and vice versa.
The soot production is normally smaller:
- the smaller the molecule (easier atomic cracking)
- the smaller its content in carbon (fair enough, soot contains
mostly clusters of carbon!)
- the larger the air quantity available
Continuous variations of the combustion
products inside the cylinder
Common rail fuel injection system
Multi-injection
strategy
• The pilot injection(s) is
intended to shorten the
ignition delay of the
main fuel injection.
• The post injection(s) is
able to enhance the soot
oxidation, by raising
exhaust temperatures
and activating
regeneration of diesel
particulate filters.
The piezoelectric injectors are “high tech” components of the
common rail fuel injection system in modern Diesel engines
(they are the most rapid electro-mechanical actuators)
Multi-injection strategy in a common rail Diesel engine
Influences of a pre-injection and a post-injection
on a common-rail Diesel engine operation
The successful timing of the pilot pre-injection is judged by
whether it manages to initially ignite just a few drops of the main
injection, not many (late injection) neither none (early injection)
New Diesel engine combustion technologies
(applied mostly to partial load)
- modulated kinetics (MK)
- premixed charge compression ignition (PCCI)
- homogeneous charge compression ignition (HCCI)
- addition of oxygenates to reduce soot
The effects of ignition angle and cetane number on some
operation parameters of typical DI diesel engine
A common example of
trading off emissions
applied to the reaction engines of jet planes
Combustion improvement measures
The quality of combustion can be generally improved by:
• proper design of combustion devices, e.g. burners, internal
combustion engines,
• avoiding combustion instabilities, typically due to violent
pressure oscillations in a combustion chamber
• catalytic or non-catalytic after-burning devices (e.g. catalytic
converters),
• partial recirculation of the exhaust gases into the combustion
chamber (EGR),
• additives in the fuel or in the exhaust gases,
• more specific measures, e.g. injection of urea.
Such measures are required by environmental legislation for
cars and thermal power plants, to meet emission standards.
A series of exhaust gas after-treatment
measures in a modern Diesel engine
EMISSIONS REDUCTIONS: NOx: up to 98%, PM: more
than 85%, CO: up to 95%, HC: up to 90%.
Petrol-Diesel oil engines comparison
Petrol engines (average efficiency, η ≈ 0.24)
• combined with a conventional three-way catalyst system for exhaust gas
after-treatment, the conventional port-fuel-injected (PFI) petrol engine
will be still able to fulfill future legislation regarding the emission of soot,
NOx, unburned hydrocarbons and CO.
• The most important challenge remains the reduction of specific fuel
consumption (gr/kWh or lt/100 km), to lower energy consumption, via:
direct injection (DI), downsizing+supercharging, throttle valve abolition
etc.
Diesel oil engines (average efficiency, η ≈ 0.36)
• Diesel engine has an excellent efficiency but suffers from rather high soot
and NOx emissions, so various exhaust treatment devices have to be used
• Parameters involved in the combustion event have to be carefully varied in
the engine design and operation to achieve lower emissions, while
maintaining high engine performance in a wide range of operating
conditions
Emissions measuring technologies
for engine exhaust gases
• CO2 and NO: Non-dispersive infrared (NDIR)
Fourier transform spectral analysis
• NOx: chemiluminescent reaction, catalyst converter
• Unburned hydrocarbons: flame ionisation sensor,
(NDIR), heated flame ionization detector (HFID)
• Particulates ( by filtering a portion of the exhaust
gas): weighing the mass increase of the filter, optical
measurement of filter opacity (nebulosity)
Unit 4: Some environmental
implications and illusions
A Low-Carbon Economy (LCE) is a concept of an
economy with minimal output of greenhouse gases
emissions, obviously referring to the greenhouse gas
carbon dioxide (CO2)
• Recently, most of scientific and public opinion has come to this questionable
conclusion: due to anthropogenic causes, there is such an accumulation of
greenhouse gases (especially CO2) in the atmosphere that the climate is
changing. Over-concentrations of these gases are producing global warming,
with negative impacts on humanity and life in the foreseeable future.
• Globally implemented LCE measures are therefore needed and are currently
applied, as a means to avoid catastrophic climate change, and hopefully as a
precursor to an advanced zero-carbon society and renewable-energy economy.
The Low-Carbon Economy seems to:
- teem with misunderstandings, illusions, and loose “scientific” analyses
- have certainly succeeded in persuading most scientists and the layman that
CO2 (atmospheric concentration of ≈390 ppm) is mainly to blame for the
global warming
- have made people indifferent about the worst greenhouse gas of all, i.e.
water vapor H2O (variable atmospheric concentration of 2000-30000 ppm) !
- point, directly and indirectly, to a “promising” hydrogen (H2) future
.
The hydrogen reserves on the Earth are zero (!),
thus the highly promoted future of hydrogen looks uncertain
How to get rid of CO2 !!
=>
the causality low
From this chart it looks like temperature changes precede carbon dioxide
changes by at least 100 years
It seems that the temperature rise of the Earth’s surface causes the
increase of the CO2 concentrations in the atmosphere – most people think
it is vice-versa ! (some overzealous scientists/officials may even have been
willing to tweak the truth, when the evidence is inadequate to support it)
Greenhouse gases -1
Selective (narrow) absorption and scattering of electromagnetic
radiation is exhibited by any gas consisting of 3 or more atoms, at
different wavelength bands specific for each gas, both in the solar
and in the thermal (infrared) spectrum.
These are called greenhouse gases, as opposed to the
symmetrical gas molecules of 1 or 2 atoms, which are totally
transparent to all radiation wavelengths.
Typical greenhouse gases of the Earth’s atmosphere (in order of importance):
1. Water vapour Η2Ο (triatomic gas), present at varying concentrations of 200030000 ppm. *
2. Carbon dioxide CO2 (triatomic gas), present at constant concentration of ≈390
ppm.
3. Others: CΗ4, O3, SO2, CFCs, CΗx, NOx, at smaller concentrations
* clouds are liquid water droplets and not vapour of Η2Ο, they also act as heat blanket
Greenhouse gases -2
Total transmission of
electromagnetic radiation
in a narrow infrared
spectrum, due to the
presence of some
“greenhouse gases”,
which are shown to
absorb quite selectively,
approximately as follows:
Η2Ο at 1.4 μm and 1.9 μm,
HCx at 3.4 μm, SO2 at 4.0
μm, CO2 at 4.3 μm, CO at
4.7 μm and ΝO at 5.3 μm.
Any “greenhouse atmospheric gas” acts not indiscriminately as a
heat blanket (i.e. the thicker, the more isolating) but as a narrow
electromagnetic filter: once the gas has absorbed or scattered all
rays contained in its respective narrow band(s), any further increase
in its concentration does not induce any significant absorption.
Greenhouse gases -3
The filtering role of the atmospheric CO2 has almost reached its
saturation point, so doubling its concentration will not result doubling
of its IR radiation absorption but only a small increase, e.g. <10 % !
CxΗy + air (O2 + Ν2 )  CO2 + Η2Ο + Ν2
• The combustion of any hydrocarbon produces both CO2 and Η2Ο
• With regard to their energy content, both CO2 and Η2Ο are
situated at the “bottom”: no more energy can be extracted from
them (best energy exploitation has already taken place).
• CO2 and Η2Ο are the “bricks and mortar” for all forms of life on
Earth
• Both CO2 and Η2Ο are greenhouse gases but, since the act as
narrow filters, at their present concentrations they have done
most of their job – any envisaged further increase is expected to
be of small significance !
Why then consider today CO2 seemingly
as the sole unwanted greenhouse gas of combustions?
Some further thoughts:
- the presence of clouds, may be more important: the more fuels we burn the
more heat we release to heat up the planet, the more water evaporation
occurs on the earths surface, the more water mass and latent heat is
transferred to atmosphere, the more the clouds up there, the less the
incoming solar radiation….. → final effect on earths temperature?
-
nearly all fuels emit both CO2 and Η2Ο when burned, so the single
“option” to reduce greenhouse gas emissions (and partly atmospheric
pollutants, of course) is to become an even more energy-efficient
civilization, by consuming less energy or/and by achieving higher
efficiencies in any necessary energy usage.
For road vehicles it may all come down to:
• reducing specific fuel consumption (gr/kWh or lt/100 km)
• Reducing the most unhealthy emissions
A final look:
Combustion of a generic hydrocarbon:
CxΗy + air (O2 + Ν2 ) 
CO2* + Η2Ο* + Ν2 : normally expected complete combustion products
+ HC
: products of incomplete combustion (could be eliminated)**
+ C + CO
: products of highly incomplete combustion (could be eliminated)**
+ CO + ΝOx + O3 : products of high temperature kinetics (cannot be eliminated !!)++
Converting all carbon to CO2 and all hydrogen to Η2Ο is the best a
“bright” engineers could ever hope for, in terms of best energy
conversion and least pollution
** By using stoichiometric and lean mixtures – still, there exist many
design and combustion challenges (eliminating combustion chamber crevices,
wall quenching, oil desorption, exhaust valve leakage, ignition failures, cold start
problems etc.)
++ Unfortunately, sometimes Nature seems to enjoy playing havoc and
posing contradictions, e.g. the higher the combustion temperature, the
better the thermal efficiency of a combustion engine (desirable) but
the more increased the NOx emissions (undesirable)
Thank you
for your attention !
REFERENCES
1.
2.
3.
4.
5.
•
•
•
•
•
•
•
C.F. Taylor, The internal combustion engine in theory and practice, MIT Press,
USA (1985)
R. Stone, Introduction to Internal combustion engines, PALGRAVE, 3rd Ed., UK
(1999)
J.B. Heywood, Internal combustion engine fundamentals, McGraw-Hill (1988)
S.R. Turns, An introduction to combustion engineering, McGraw-Hill (1996)
G.L. Borman and K.W. Ragland, Combustion engineering, McGraw-Hill (1988)
WEB PAGES:
http://en.wikipedia.org/wiki/Pyrolysis
http://www1.eere.energy.gov/vehiclesandfuels/pdfs/deer_2003/session7/2003_d
eer_pickett.pdf
http://dictionary.sensagent.com/fuel+efficiency/en-en/
http://dare.ubn.kun.nl/bitstream/2066/18774/1/18774_nitroxina.pdf
http://www.cumminscalpacific.com/pdf/Emissions/CPG-468-low-emissionstechnology.pdf
http://therealglobalwarmingstory.com/GW10.pdf