The complete combustion (?)

by

Prof. Dimitris Prapas

Automotive Engineering Dept.

Alexander Technological

Educational Institution

of Thessaloniki

GREECE

dprap@vt.teithe.gr

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: Solid fuel:

C22H46 ---> C11H24 + C11H22

• 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 end-

gases 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

- 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

Presence of unburned hydrocarbon

after exhaust valve is due to:

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 oxygen-

starving 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 !!

=>

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)

the causality low

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 2000-

30000 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 -1

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. C.F. Taylor, The internal combustion engine in theory and practice, MIT Press,

USA (1985)

2. R. Stone, Introduction to Internal combustion engines, PALGRAVE, 3rd Ed., UK (1999)

3. J.B. Heywood, Internal combustion engine fundamentals, McGraw-Hill (1988)

4. S.R. Turns, An introduction to combustion engineering, McGraw-Hill (1996)

5. 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_deer_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-emissions-technology.pdf

• http://therealglobalwarmingstory.com/GW10.pdf