Natural production of organohalogen compounds handbook of environmental chemistry

Since it is impossible to obtain a 100% complete combustion the exhaust gases always include a great variety of combustion products, the most important are: carbon monoxide, unburnt hydr

Prof Dr D MackayDepartment of Chemical Engineering and Applied Chemistry

University of Toronto Toronto, Ontario, Canada M5S 1A4Prof Dr A.H NeilsonSwedish Environmental Research Institute P.O.Box 21060

10031 Stockholm, Sweden

E-mail: ahsdair@ivl.se

Prof Dr J PaasivirtaDepartment of Chemistry University of Jyväskylä Survontie 9

P.O.Box 35

40351 Jyväskylä, FinlandProf Dr Dr H ParlarInstitute of Food Technology and Analytical Chemistry Technical University Munich

85350 Freising-Weihenstephan, GermanyProf Dr S.H Safe

Department of Veterinary Physiology and Pharmacology College of Veterinary Medicine Texas A & M University College Station, TX 77843-4466, USA

E-mail: ssafe@cvm.tamu.edu

Prof P.J WangerskyUniversity of Victoria Centre for Earth and Ocean Research P.O.Box 1700

Victoria, BC, V8W 3P6, Canada

E-mail: wangers@attglobal.net

Advisory Board

Dr T.A.T Aboul-Kassim

Department of Civil Construction

and Environmental Engineering,

and Air Pollution Research

Technical University Munich

11–13, chemin des Anémones

1219 Châteleine (GE), Switzerland

Portland State University

Science Building II, Room 410

P.O Box 751 Portland, Oregon 97207-0751, USA

Environmental Chemistry is a relatively young science Interest in this subject,however, is growing very rapidly and, although no agreement has been reached

as yet about the exact content and limits of this interdisciplinary discipline, thereappears to be increasing interest in seeing environmental topics which are based

on chemistry embodied in this subject One of the first objectives of mental Chemistry must be the study of the environment and of natural chemicalprocesses which occur in the environment A major purpose of this series onEnvironmental Chemistry, therefore, is to present a reasonably uniform view ofvarious aspects of the chemistry of the environment and chemical reactionsoccurring in the environment

The industrial activities of man have given a new dimension to mental Chemistry We have now synthesized and described over five millionchemical compounds and chemical industry produces about hundred and fiftymillion tons of synthetic chemicals annually We ship billions of tons of oil peryear and through mining operations and other geophysical modifications, largequantities of inorganic and organic materials are released from their naturaldeposits Cities and metropolitan areas of up to 15 million inhabitants producelarge quantities of waste in relatively small and confined areas Much of thechemical products and waste products of modern society are released into theenvironment either during production, storage, transport, use or ultimatedisposal These released materials participate in natural cycles and reactionsand frequently lead to interference and disturbance of natural systems

Environ-Environmental Chemistry is concerned with reactions in the environment It

is about distribution and equilibria between environmental compartments

It is about reactions, pathways, thermodynamics and kinetics An importantpurpose of this Handbook, is to aid understanding of the basic distribution andchemical reaction processes which occur in the environment

Laws regulating toxic substances in various countries are designed to assessand control risk of chemicals to man and his environment Science can con-tribute in two areas to this assessment; firstly in the area of toxicology andsecondly in the area of chemical exposure The available concentration(“environmental exposure concentration”) depends on the fate of chemicalcompounds in the environment and thus their distribution and reaction be-haviour in the environment One very important contribution of EnvironmentalChemistry to the above mentioned toxic substances laws is to develop laboratorytest methods, or mathematical correlations and models that predict the environ-

mental fate of new chemical compounds The third purpose of this Handbook is

to help in the basic understanding and development of such test methods andmodels

The last explicit purpose of the Handbook is to present, in concise form, themost important properties relating to environmental chemistry and hazardassessment for the most important series of chemical compounds

At the moment three volumes of the Handbook are planned Volume 1 dealswith the natural environment and the biogeochemical cycles therein, includingsome background information such as energetics and ecology Volume 2 is con-cerned with reactions and processes in the environment and deals with physicalfactors such as transport and adsorption, and chemical, photochemical andbiochemical reactions in the environment, as well as some aspects of pharma-cokinetics and metabolism within organisms.Volume 3 deals with anthropogeniccompounds, their chemical backgrounds, production methods and informationabout their use, their environmental behaviour, analytical methodology andsome important aspects of their toxic effects The material for volume 1, 2 and 3was each more than could easily be fitted into a single volume, and for thisreason, as well as for the purpose of rapid publication of available manuscripts,all three volumes were divided in the parts A and B Part A of all three volumes isnow being published and the second part of each of these volumes should appearabout six months thereafter Publisher and editor hope to keep materials of thevolumes one to three up to date and to extend coverage in the subject areas bypublishing further parts in the future Plans also exist for volumes dealing withdifferent subject matter such as analysis, chemical technology and toxicology,and readers are encouraged to offer suggestions and advice as to future editions

of “The Handbook of Environmental Chemistry”

Most chapters in the Handbook are written to a fairly advanced level andshould be of interest to the graduate student and practising scientist I also hopethat the subject matter treated will be of interest to people outside chemistry and

to scientists in industry as well as government and regulatory bodies It would

be very satisfying for me to see the books used as a basis for developing graduatecourses in Environmental Chemistry

Due to the breadth of the subject matter, it was not easy to edit this book Specialists had to be found in quite different areas of science who werewilling to contribute a chapter within the prescribed schedule It is with greatsatisfaction that I thank all 52 authors from 8 countries for their understandingand for devoting their time to this effort Special thanks are due to Dr F Boschke

Hand-of Springer for his advice and discussions throughout all stages Hand-of preparation

of the Handbook Mrs A Heinrich of Springer has significantly contributed tothe technical development of the book through her conscientious and efficientwork Finally I like to thank my family, students and colleagues for being sopatient with me during several critical phases of preparation for the Handbook,and to some colleagues and the secretaries for technical help

I consider it a privilege to see my chosen subject grow My interest in mental Chemistry dates back to my early college days in Vienna I receivedsignificant impulses during my postdoctoral period at the University of Californiaand my interest slowly developed during my time with the National Research

Environ-Council of Canada, before I could devote my full time of EnvironmentalChemistry, here in Amsterdam I hope this Handbook may help deepen theinterest of other scientists in this subject.

Twentyone years have now passed since the appearance of the first volumes ofthe Handbook Although the basic concept has remained the same changes andadjustments were necessary

Some years ago publishers and editors agreed to expand the Handbook bytwo new open-end volume series: Air Pollution and Water Pollution Thesebroad topics could not be fitted easily into the headings of the first three vol-umes All five volume series are integrated through the choice of topics and by asystem of cross referencing

The outline of the Handbook is thus as follows:

1 The Natural Environment and the Biogeochemical Cycles,

2 Reaction and Processes,

A recent development is the accessibility of all new volumes of the Handbookfrom 1990 onwards, available via the Springer Homepage http://www.springer de

or http://Link.springer.de/series/hec/ or http://Link.springerny.com/ series/hec/.During the last 5 to 10 years there was a growing tendency to include subjectmatters of societal relevance into a broad view of Environmental Chemistry.Topics include LCA (Life Cycle Analysis), Environmental Management, Sustain-able Development and others.Whilst these topics are of great importance for thedevelopment and acceptance of Environmental Chemistry Publishers and Edi-tors have decided to keep the Handbook essentially a source of information on

“hard sciences”

With books in press and in preparation we have now well over 40 volumesavailable.Authors, volume-editors and editor-in-chief are rewarded by the broadacceptance of the “Handbook” in the scientific community

Dušan Gruden XIII

Introduction

Dušan Gruden 1

Power Units for Transportation

Dušan Gruden, Klaus Borgmann, Oswald Hiemesch . 15

Means of Transportation and Their Effect on the Environment

Hans Peter Lenz, Stefan Prüller, Dušan Gruden . 107

Legislation for the Reduction of Exhaust Gas Emissions

Wolfgang Berg . 175

Fuels

Dušan Gruden 255

Subject Index . 289

Over centuries mankind has pursued technical progress for the benefit ofimproved prosperity without simultaneously taking appropriate steps to ensurethe environmental friendliness of the involved processes However, in the midd-

le of the 20th century environmental episodes drew attention to the negativeimpacts on the environment caused by this progress

As a matter of fact, concern about the influence of human activities on theenvironment is neither a new phenomenon nor a new attribute of modern people but has accompanied human society throughout its existence What isnew, however, is the increasing intensity of man’s efforts to protect his environ-ment as reflected in a multitude of national and international environmentallaws enacted all around the globe

Life as a whole, and human existence in particular, are characterized by stant movement and changes This means that living beings need to be mobile

con-to survive By developing suitable technical means man has enormously sed his mobility – expressed in terms of speed and distance – when comparedwith other living beings on our planet The automobile is one of the inventionsthat has made a decisive contribution to this mobility and it has become an inse-parable part of modern human society In the second half of the 20th century,the automobile developed from a luxury article and prestige object for a few into

increa-a bincrea-asic commodity for millions of people It is through this widespreincrea-ad use thincrea-atnegative impacts on the environment have become clearly visible Therefore,since the late 1960s and early 1970s, automotive development has been accom-panied by an ever increasing number of strict legal standards, e.g., about thereduction of exhaust gas pollutants, noise emissions, hazardous substances andwaste, as well as about improved recyclability of materials and other aspects.Achievements in improving the ecological characteristics of the automobileare highly impressive: A modern car emits only fractions of the amounts ofnoise and exhaust gas pollutants produced by its predecessors 30 years ago.Today, 100 modern passenger cars in total emit less of the legally limited exhaustgas constituents than one single car of 1970 The same trend can be found withall the other ecologically relevant automotive features so that the absoluteimpact of the automobile on our environment is considerably lower today than

it was in the past

The development of the automobile is increasingly linked to deliberationsabout sustainable development.While this term in the recent past was only relat-

ed to the aspect of ecological consequences for the environment, it comprises

today at least two further essential pillars, namely economic consequences andsocial responsibility.

When discussing sustainability in the context of automotive development, itmust be borne in mind that essential technical elements of the automobile –such as safety, power output, torque, fuel consumption, durability, maintenanceintervals, and comfort should not be compromised

The modern automobile has achieved outstanding performance and iority compared to its predecessors in all theses elements and will continue toproceed along this evolutionary development path

super-This book focuses on ecological aspects related to the development and use

of automobiles, leaving many environment-related initiatives towards ments of the automotive production process out of consideration It shall, how-ever, be mentioned in this context that also the production of modern cars is notpossible without the observance of a wide range of stringent environmentallaws Thus, in order to be allowed to enter the market, a car must not only per-form environmental-friendly during its operation but must have been produced

improve-to ecological standards as well Company audits carried out routinely according

to EMAS (Eco Management Auditing Scheme) and ISO 14001 show that motive manufacturers are constantly improving the ecological compatibility oftheir production processes

auto-The contributions to this book were written by experts, most of whom havebeen actively involved in the development of modern automobiles and theircombustion engines for more than 30 years They have participated in all phases

of the ecological development of the automobile – from the basic attempts torespond to the first exhaust gas emission control requirements in the USA(1966) and Europe (1970) to the cost-intensive efforts towards meeting the com-prehensive and highly demanding emission legislations currently existing andfurther anticipated worldwide

As the 20th century ends and the 21st century begins, these experts have marized their experience and know-how in this book which bears witness to thesuccessful implementation of ecological considerations into automotive devel-opment work

sum-In my capacity as coordinator of the preparatory work for this book I wouldlike to thank my colleagues – Prof Dr sc techn Hans Peter Lenz and his colla-borator, Mr Stefan Prüller (Dipl.-Ing.) of Technical University of Vienna, Dr.Klaus Borgmann and Mr Otto Hiemesch (Dipl.-Ing.) of BMW AG and Dr Wolf-gang Berg, Consultant and long-standing collaborator of DaimlerChrysler AG –for their cooperation and valuable contributions

I would like to express particular gratitude to Dr Ing h.c F Porsche AG forpermission to carry out this project

© Springer-Verlag Berlin Heidelberg 2003

The Diversity of Naturally Produced Organohalogens

Gordon W Gribble

Department of Chemistry, Dartmouth College, Hanover, NH 03755, USA

E-mail: ggribble@dartmouth.edu

More than 3700 organohalogen compounds, mainly containing chlorine or bromine but a few with iodine and fluorine, are produced by living organisms or are formed during natural abio-genic processes, such as volcanoes, forest fires, and other geothermal processes The oceans are the single largest source of biogenic organohalogens, which are biosynthesized by a myriad of seaweeds, sponges, corals, tunicates, bacteria, and other marine life Terrestrial plants, fungi, lichen, bacteria, insects, some higher animals, and even humans also account for a diverse col-lection of organohalogens.

Keywords.Organohalogen, Organochlorine, Organobromine, Natural halogen

1 Introduction . 2

2 Sources and Compounds . 2

2.1 Marine Plants 2

2.2 Marine Sponges 3

2.3 Other Marine Animals 6

2.4 Marine Bacteria and Fungi 7

2.5 Terrestrial Plants 7

2.6 Fungi and Lichen 8

2.7 Bacteria 9

2.8 Insects 10

2.9 Higher Animals and Humans 11

2.10 Abiogenic Sources 12

3 Concluding Remarks . 13

4 References 13

Introduction

Thirty years ago some 200 natural organohalogen compounds had been mented (150 organochlorines and 50 organobromines) [1] Nevertheless, the sci-entific community generally considered these compounds to be isolation artifacts

docu-or rare abndocu-ormalities of nature Fdocu-or example,“present infdocu-ormation suggests thatorganic compounds containing covalently bound halogens are found only infre-quently in living organisms” [2] Unfortunately, even today this myth persists andhas entered modern textbooks: “unlike metals, most of these compounds [halo-genated hydrocarbons] are man-made and do not occur naturally …” [3].The striking increase in the number of known natural organohalogens to morethan 3700 is partly a consequence of the general revitalization of interest in naturalproducts as a potential source of new medicinal drugs Furthermore, the relativelyrecent exploration of the oceans has yielded large numbers of novel organohalo-gens from marine plants, animals, and bacteria Much of the success of these ex-plorations is attributed to improved collection methods (SCUBA and remote sub-mersibles for the collection of previously inaccessible marine organisms), selectivebioassays for identifying biologically active compounds, powerful multidimen-sional nuclear magnetic resonance spectroscopy techniques for characterizing sub-milligram quantities of compounds, and new separation and purification tech-niques (liquid-liquid extraction, high-pressure liquid chromatography) Further-more, an awareness and appreciation of folk medicine and ethobotany have guidednatural product chemists to new medicinal leads Although most of the biogenicorganohalogens discovered over the past thirty years are marine-derived, manyother halogenated compounds are found in terrestrial plants, fungi, lichen, bacte-ria, insects, some higher animals, and humans [4–9] As of June 2002, the break-down of natural organohalogens was approximately: organochlorines, 2200;organobromines, 1900; organoiodines, 100; organofluorines, 30 [10].A few hundred

of these compounds contain both chlorine and bromine

pre-The favorite edible seaweed of native Hawaiians is “limu kohu” (Asparagopsis taxiformis), and this delicacy contains more than 100 organohalogens, most of

which were previously unknown compounds [12, 13] Bromoform is the majororganohalogen in this seaweed.A selection of others is depicted in Fig 2.Another

red alga, Bonnemaisonia hamifera, contains several brominated heptanones that

might be precursors to bromoform formed via a classical “haloform reaction”

[14] Bromoform may serve as an antifeedant and/or antibacterial agent for theseaweed.

A vast number of halogenated terpenes and the related C15 acetogenins are

produced by marine plants Nearly 50 species of the red alga genus Laurencia

have yielded hundreds of such compounds; a small selection of recent examples

is shown in Figures 3 and 4 [15 – 22]

Blue-green algae (cyanobacteria) are the source of a large number of

halo-genated, mainly chlorinated, metabolites [23] In particular, Lyngbya majuscula

is prolific in this regard and some recent examples are shown in Fig 5 [24 – 27]

The potent anticancer drug candidate cryptophycin A (1) was isolated from

cultures of a Nostoc sp blue-green alga, and the structurally novel

nostocyclo-phane (2) is produced by Nostoc linckia A detailed study of the brown alga

Cystophora retroflexa reveals the presence of seventeen halogenated phlorethol

and fucophlorethol derivatives, one of which is the complex 3 [28] (Fig 6)

Syn-thetic approaches to cryptophycin are discussed later in this volume

2.2

Marine Sponges

Sponges also rely heavily on chemicals for their survival, and many of these pounds contain halogen In some cases, it is evident that bacteria or microalgaeassociated with the host sponge actually produce the metabolites Recent exam-

com-Fig 1. Some haloalkanes produced by marine algae

Fig 2. Some organohalogens found in the red alga Asparagopsis taxiformis

Fig 3. Some Laurencia terpenes

Fig 4. Some Laurencia C15-acetogenins

Fig 5. Some organohalogens from the blue-green alga Lyngbya majuscula

Fig 6. Some organohalogens from blue-green and brown algae

ples of sponge organohalogens include fatty acid derivatives (4) [29], pyrroles (5) [30], indoles (6) [31], phenol derivatives (7) [32], tyrosine derivatives (8) [33], ter- penes (9) [34], diphenyl ethers (10) [35], and even dioxins (11) [36] These fasci-

nating compounds are illustrated in Fig 7

2.3

Other Marine Animals

Ascidians (tunicates or sea squirts), nudibranchs (sea slugs), soft corals gonians), bryozoans (moss animals), and acorn worms all produce a dazzling col-lection of organohalogens Some recent examples [37 – 40] are shown in Fig 8

(gor-Fig 7. Some organohalogens from marine sponges

Marine Bacteria and Fungi

A new thrust of natural product research is the study of marine bacteria andfungi A number of novel organohalogens have been discovered in this endeavor,

and recent examples (12–14) [41 – 43] are shown in Fig 9 The novel halogenated bipyrroles 15 and 16, which are found in ocean-feeding sea birds [44 – 46], are

most likely produced by marine bacteria These compounds represent the first

case of bioaccumulative natural organohalogens The related “Q1” (17) has been

discovered in a multitude of marine animals and even in the milk of Eskimowomen who consume whale blubber [47, 48] This latter scenario represents thefirst case of the bioaccumulation of natural organohalogens in humans

2.5

Terrestrial Plants

By comparison with marine plants, terrestrial plants are relatively devoid of genated compounds However, many notable exceptions do exist The growth

halo-hormone 4-chloroindole-3-acetic acid (18) and its methyl ester are

biosynthe-sized by peas, lentil, vetch, and fava bean (Fig 10) Bromobenzene has been

de-tected in the volatiles of oakmoss, and the Thai plant Arundo donax contains the

weevil repellent 19 [49] Both chloromethane and bromomethane have several

plant sources Chloromethane is produced by potato tubers [50], and

bro-Fig 8. Some organohalogens from marine animals

momethane, a commercial fumigant and nematicide, is produced by broccoli,cabbage, mustard, pak-choi, radish, turnip, and rapeseed [51] The global annualproduction of bromomethane by rapeseed and cabbage is estimated to be 6600and 400 tons, respectively The authors conclude that “given the ubiquitous dis-tribution of bromide in soil, methyl bromide production by terrestrial higherplants is likely a large source for atmospheric methyl bromide” Some recent plant

organohalogens (20–22) [52–54] are shown in Fig 10 The edible Japanese lily

(Lilium maximowiczii) produces seven novel chlorophenol fungicides in sponse to attack by the pathogenic plant fungus Fusarium oxysporum at the site

re-of infection [55]

2.6

Fungi and Lichen

Fungi and lichen produce a variety of organohalogens, from the simplechloromethane and chloroform to exceedingly complex compounds The earliestdiscovered organohalogen compounds are the chlorine-containing fungalmetabolites griseofulvin, chloramphenicol, aureomycin, caldariomycin,

Fig 9. Some organohalogens from marine bacteria and fungi

sporidesmin, ochratoxin A, and others A study of three species of fungi dariomyces fumago, Mycena metata, and Peniophora pseudopini) revealed that

(Cal-they produce de novo up to 70 µg chloroform L–1of culture medium per day [56]

The fungus Mollisia ventosa has yielded several calmodulin inhibitors such as

KS-504d (23), which contains 70 % chlorine by weight [57] The novel

topoiso-merase inhibitors topopyrones A (24) and B (25) were isolated from a Phoma sp.

fungus [58, 59], and a recent study of the white rot fungus Bjerkandera adusta has

yielded bjerkanderol B (26) [60] Experiments with Na37Cl supplied to the culturerevealed incorporation of37Cl in 26 The slime mold Dictyostelium purpureum

produces AB0022A (27), which is the first naturally occurring chlorinated

diben-zofuran [61] These fungal metabolites are listed in Fig 11

2.7

Bacteria

Bacteria are amazing chemical factories and the resulting synthetic metabolites

often possess astounding structural complexity More than fifty Streptomyces

Fig 10. Some terrestrial plant organohalogens

species have yielded organohalogen metabolites The bacterium Amycolatopsis orientalis produces the life-saving glycopeptide antibiotic vancomycin, which has

been used for nearly 50 years to treat penicillin-resistant infections [62, 63] Thetwo chlorine atoms in vancomycin are essential for optimal biological activity

Recent examples of Streptomyces metabolites (28 – 31) [64 – 67] are listed in

of at least a dozen tick species [68] The German cockroach utilizes two nated steroids as aggregation pheromones [69] An extraordinary finding is thatchloroform is produced by termites Six Australian termite species produce chlo-roform within their mounds up to 1000 times higher than the ambient concen-tration [70] The authors conclude that this source may account for as much as

chlori-15 % of the global chloroform emissions

Fig 11. Some fungal and lichen organohalogens

Higher Animals and Humans

Organohalogens are rare in higher animals However, several such compounds

have been identified The Ecuadorian frog Epipedobates tricolor has yielded

epi-batidine (32), and the iodolactone 33 is present in the thyroid gland of dogs cently, several halogenated compounds (34–36) were shown to be products of the

Re-action of human white blood cell myeloperoxidase-induced halogenation on vading pathogens and in various disease processes [71 – 73] (Fig 13) This topic

in-is also the subject of a chapter in thin-is volume Myeloperoxidase from humans

Fig 12. Some Streptomyces sp organohalogens

converts chlorophenols to chlorinated dioxins and dibenzofurans [74], and thus

a human biosynthesis of dioxins is possible The conversion of predioxins todioxins in rats has been demonstrated [75]

2.10

Abiogenic Sources

Natural combustion sources such as biomass fires, volcanoes, and other thermal processes account for a wide range of organohalogens The early stud-ies of volcanic gases and the presence of organohalogens discovered therein byStoiber and Isidorov are well documented [4, 6] A recent study of the volcanoesKuju, Satsuma Iwojima, Mt Etna, and Vulcano has revealed an extraordinarilylarge array of organohalogens, including 100 organochlorines, 25 organo-bromines, 5 organofluorines, and 4 organoiodines, most of which are new com-pounds [76] This topic is discussed further elsewhere in this volume Haloal-kanes have been found entombed in rocks, minerals, and shales Thus, whenrocks are crushed, for example, during mining operations, small quantities of

geo-CH3Cl, CH2Cl2, CHCl3, CCl4, CH3CHCl2, ClCH2CH2Cl, Cl2C = CH2,CH3CH2Br,

CF2Cl2, CFCl3, CHF3, chlorobenzene, 1-chloronaphthalene, and other gens are released [77, 78] For example, 1000 tons of silvinite ore yields 50 g ofchloroform The authors estimate that the potassium salt mining industry aloneaccounts for the annual liberation of 10,000 – 15,000 tons of CHCl3and 100 –

organohalo-150 tons each of CCl4and CFCl3 Several chlorinated benzoic acids, some alkanes, and other chlorinated aromatics, were found in the meteorites ColdBokkeveld, Murray, Murchison, and Orgueil [79, 80]

chloro-While there is no dispute about the emissions of chloromethane and momethane from biomass burning and other natural sources [81, 82], the evi-dence regarding larger organohalogens, such as dioxins, has been more difficult

bro-to obtain and quantify [83] However, numerous recent studies suggest that the

Fig 13. Some organohalogens from higher animals including humans

dioxins in sediments and clays have originated from natural sources [84, 85], andone such obvious source is biomass burning and subsequent deposition [86, 87].Moreover, other studies indicate that dioxins are formed in peat and forest soil,presumably via the enzymatic oxidative dimerization of natural chlorophenols[88, 89].

3

Concluding Remarks

The incredibly large number of marine and terrestrial organisms that are ing exploration for their chemical content virtually guarantees the discovery ofnumerous new natural organohalogens, many of which will doubtless have sig-nificant biological activity It also seems highly likely that additional mammalianorganohalogens will be identified and their role in the biodisinfection processwill become understood The clear and convincing evidence that chlorinateddioxins and dibenzofurans have several natural sources – both abiogenic and bio-genic – is one of the most significant and politically important scientific discov-eries of our age

await-4

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Power Units for Transportation

Dušan Gruden1· Klaus Borgmann2· Oswald Hiemesch2

1 Dr Ing h.c F Porsche Aktiengesellschaft, Porschestrasse, 71287 Weissach, Germany

E-mail: dusan.gruden.@porsche.de

2 Bayerische Motoren Werke Aktiengesellschaft, Hufelandstrasse, 80788 München, Germany

For more than 125 years, gasoline and Diesel engines have prevailed as the exclusive drive unit

in road transportation None of the other power units invented to date has been able to make use of the energy content of mineral oil with the piston engine’s same good efficiency Combustion is the fundamental process by which the chemical energy of fuels is converted into thermal energy and further into mechanical work If hydrocarbon-containing fuels were completely burnt, the resulting products would be carbon dioxide and water vapor only Since

it is impossible to obtain a 100% complete combustion the exhaust gases always include a great variety of combustion products, the most important are: carbon monoxide, unburnt hydro- carbons, nitrogen oxides and particulate matter.

During its 125 years of existence, the Otto (gasoline) engine – as it was called after its tor – has been developed into a mature combustion engine which is characterized by an excel- lent efficiency and low pollutant emissions The properties of the gasoline engine strongly depend

inven-on the compositiinven-on of the air-fuel mixtures and ignitiinven-on parameters The influence of the called engine design parameters on combustion and exhaust emission is no less important The emission of many of the exhaust-gas constituents can be influenced and minimized at their place of origin, that is in the combustion chamber by correctly selecting and adapting the relevant engine design and operating parameters If optimization of engine-internal parame- ters for further reducing of the exhaust gas emissions are not enough anymore, so-called en- gine-external measures must be additionally taken It was found that so-called three-way cat- alyst reduces the three aforementioned pollutants by clearly more than 90%, provided that a precisely stoichiometric A/F-ratio is used.

so-Thanks to the strict maintenance of a precise stoichiometric air/fuel mixture the three-way catalyst allows very low HC, CO and NOx pollutant emissions to be achieved However, in this operating range, fuel consumption is 8 to 15% higher (with a resulting higher CO2 emission) than during lean-burn operation.

One of the technically most useful solutions to reduce the fuel consumption and CO2 sion of gasoline engines is to make them tolerate lean air/fuel mixtures The future of the lean- burn gasoline engines will almost exclusively depend on the successful development of NOx- exhaust-gas after-treatment technologies for lean air/fuel mixtures.

emis-Diesel engines are internal combustion units with the highest thermal efficiency Mixture formation is achieved through high pressure fuel injection The fuel leads to self-ignition in the highly compressed air of the engine cylinder The power and torque characteristics of modern Diesel engines are comparable with those of spark ignition (Otto) power units of equal ca- pacity, the fuel consumption however is approx 20% lower.

The Diesel power unit has achieved a high status in transport The world wide share of Diesel engines in passenger vehicles is now approx 20%, whereas in freight transport on the roads and

by water the share is approaching 100%, diesel being the only cost effective alternative Increasingly, new methods for injection combustion, exhaust gas recirculation and after treatment (NOx-Cat, Diesel particle filter) are being pursued to meet the ever stricter emission legislations, aimed at limiting the effects on the environment.

Ever since its invention, the 4-stroke reciprocating piston engine has been considered as a rather complex thermal unit which should better be replaced by far less complicated designs.

© Springer-Verlag Berlin Heidelberg 2003

When summing up all the properties required to smoothly operate cars over wide speed and load ranges and a long lifetime, all alternative concepts have never succeeded in edging the Otto and Diesel engines out of their top positions Further optimized versions of gasoline and Diesel engines will continue to prevail in the automotive domain in the coming 15 to 20 years Due

to their theoretically high efficiency and low pollutant emissions, fuel cells are among the most promising alternative energy sources of the future.

Keywords.Combustion process, Otto engine, Gasoline engine, Diesel engine, Fuel/air mixture, Power output, Fuel consumption, Exhaust gas emission, Carbon monoxide, Unburnt hydrocar- bons, Nitrogen oxides, Particulates, Operating parameter, Design parameter, Ignition, Injection, Compression ratio, Combustion chamber, Valve timing, Exhaust gas after-treatment, Catalyst, Particulate filter, Turbo charging, 2-stroke engine, Alternative engine, Fuel cell, Hybrid drive

1 Combustion Fundamentals and Combustion Products

(D Gruden) 181.1 General Issues 181.2 Carbon Monoxide (CO) 201.3 Unburnt Hydrocarbons (HC) 201.4 Nitrogen Oxides (NOx) 211.5 Particulate Matter (PM) 221.6 References 24

2 The Otto (Gasoline) Engine (D Gruden) 252.1 General Issues 252.2 Power Output and Fuel Consumption 272.3 Exhaust Gas Emission 302.4 Engine-Internal Measures for Pollutant Reduction 302.4.1 Operating Parameters 302.4.1.1 Air-Fuel Mixture 302.4.1.2 Ignition 322.4.2 Design Parameters 332.4.2.1 Combustion Chamber Shape 342.4.2.2 Compression Ratio 342.4.3 Limitation of Pollutant Reduction by Engine-Internal Measures 342.5 Engine-External Measures for Pollutant Reduction 352.5.1 Fuel-Independent Measures 362.5.1.1 Secondary Air-Injection 362.5.1.2 EGR (Exhaust-Gas Recirculation) 362.5.1.3 Portliners 362.5.1.4 Thermal Exhaust-Gas After-Treatment 372.5.2 Fuel-Dependent Measures 382.5.2.1 Oxidation Catalyst 382.5.2.2 Reduction Catalyst 382.5.2.3 3-Way Catalyst Plus Oxygen Sensor 382.6 The Lean-Burn Engine – the Ultimate Target of Otto-Engine

Development 412.6.1 Problems of Lean-Burn Operation 41

2.6.2 State of the Art 432.6.3 Exhaust Gas After-Treatment for Lean-Burn Engines 432.6.3.1 DeNOxCatalyst 442.6.3.2 NOxStorage Catalysts 452.7 References 47

3 The Diesel Engine (K Borgmann, O Hiemesch) 483.1 General Issues 483.1.1 Formation of the Fuel Mixture, Combustion Process 503.1.2 Power Unit 533.1.3 Charge Cycle and Turbocharger Technology 543.1.4 Fuel Injection Systems 573.1.5 Injector Support and Injection Nozzle 593.2 Current Status of Modern Diesel Engines and Future Trends 623.2.1 Passenger Car Diesel Engines 633.2.2 Utility Vehicle Diesel Engines 673.2.3 Marine Diesels 673.2.4 Future Trends in the Use of Diesel Engines 683.3 Fuel Consumption 713.4 Exhaust Emissions 733.5 Engine-Internal Measures for Reducing Exhaust Emission 743.5.1 Development of the Combustion Process 753.5.2 Exhaust Gas Recirculation 793.6 Exhaust Gas After-Treatment 813.6.1 Oxidation Catalyst 813.6.2 DeNOxCatalyst 823.6.3 Particle Filter 893.7 Exhaust Gas Concepts and Outlook 883.8 References 89

4 Alternative Propulsion Systems (D Gruden) 914.1 Introduction 914.2 Thermal Engine with Discontinuous Combustion 914.2.1 Two-Stroke Engine 914.2.2 Wankel Engine 924.3 Thermal Engine with Continuous Combustion 934.3.1 Gas Turbine 934.3.2 Stirling Engine 934.3.3 Steam Engine 944.3.4 Common Characteristics of Continuous Combustion Engines 944.4 Electric Motor 954.5 Flywheel Storage System 964.6 Outlook on the Future 974.6.1 Hybrid Drive 974.6.2 Fuel Cell 994.6.3 Powerplants Using Alternative Fuels 1054.7 References 106

by the appearance of the railway train – a means of transportation which offeredone essential advantage over the preceding ones:

Steam engine-powered trains were much faster than all the previous means oflocomotion This attribute was so attractive that it triggered a people movementfrom slow individual vehicles to this speedy and more comfortable means ofmass transportation

When, at the end of the 19th century, the piston internal combustion enginewas invented which was so much smaller and more compact than the big un-wieldy steam engine the obvious consequence was to fit it into a horse carriage

In 1886, the first motorized carriage was built in Stuttgart (Fig 1) which wentdown in history as one of the first combustion-engine-equipped vehicles It was

Fig 1. First passenger car with internal combustion engine built in Stuttgart (1886)

then that the automobile was born Since, from the very start, motorized mobiles were able to travel at the same speed as – if not faster than – trains theygave rise to another people movement, this time away from mass transportationand back to individual transportation.

auto-Despite intensive research and numerous efforts aimed at developing native propulsion systems, the piston engine has prevailed as the exclusive pow-erplant unit in road transportation For more than 125 years, gasoline and Dieselengines have been the best answers engineers could find to the cheapest andmost convenient terrestrial energy source

alter-None of the other power units invented to date has been able to make use

of the energy content of mineral oil with the piston engine’s same good ciency Gasoline and Diesel oil being regular by-products of oil refining, Otto and Diesel engines have never been mutually exclusive alternative concepts buthave always ideally complemented each other in the efficient employment ofmineral oil

effi-Combustion is the fundamental process by which the chemical energy offuels is converted into thermal energy and further into mechanical work neededfor locomotion

Combustion in a heat engine consists in the rapid chemical oxidation of containing fuels This reaction is accompanied by the release of major amounts

HC-of heat and luminous radiation The released heat energy is then transformedinto mechanical work by the reciprocating-piston mechanism

Even though combustion is the basic functional principle of a heat engine, ithas not been possible, to date, to define a satisfactory combustion theory whichdescribes the phenomena of combustion in every detail What we have not gotyet is a mathematical method allowing us to precisely calculate all phases of thecombustion process taking place in the cylinder of an engine This lack is due

to the fact that combustion is a complicated chemical process characterized byrapidly changing temperatures and pressures and varying concentrations of thereactive substances The chemical conversions taking place in a combustion en-gine have little to do with simple chemical reactions The burning of hydrocar-bons triggers chain reactions which are both consecutive and competing witheach other The fuels burnt in the cylinder of a combustion engine are not ho-mogeneous simple hydrocarbons but rather consist of mixtures of hydrocarbons

of different structures and highly varying percentages At the present time, weare far from knowing the whole range of elementary processes going on duringcombustion

The velocity of the chemical reactions strongly depends on the chemical andphysical properties of the reactive substances The relationship between the re-action velocity (K) and temperature is given by Arrhenius’ law:

To simplify matters, combustion can be represented as follows:

Fuel (CxHy) + oxygen (air) Æ chain reaction (combustion) Æ

Fuel combustion consists of chain reactions.According to the chain-reaction ory, the initial substances pass through a number of intermediate states beforereaching the end-product condition A chain reaction mainly depends on so-called active centers (free atoms, radicals, peroxides) which do not enter into con-tact with the initial compounds or intermediate products

the-If hydrocarbon-containing fuels were completely burnt, the resulting productswould be carbon dioxide (CO2) and water steam (H2O) Combustion productsalso contain excess oxygen (O2) and nitrogen (N2) Since it is impossible to ob-tain a 100% complete combustion the exhaust gases always include a great vari-ety of other products, too

1.2

Carbon Monoxide (CO)

Carbon monoxide results from incomplete combustion of the carbons contained

in fuel hydrocarbons Theoretically – in the presence of sufficient oxygen stoichiometric, “lean” mixtures) – the carbon monoxide should be completelyburnt to non-poisonous CO2and not be present in the combustion products anylonger

(over-However, as CO measurements have shown, the carbon monoxide

concentra-tion in the exhaust gas is about 1 vol.-% with stoichiometric mixtures (l=1,0) with small amounts of CO being detectable also if lean mixtures (l>1,0) are used.

The percentage of carbon monoxide contained in the exhaust gas strongly pends on the reaction temperature: At high temperatures, permanent counter re-actions (CO2dissociation) take place Sudden cooling of the combustion gases inthe expansion phase “freezes” the balance created at high temperatures thus caus-ing carbon monoxide to be present in the exhaust gas under all operating con-ditions and A/F ratios

de-1.3

Unburnt Hydrocarbons (HC)

Most of the unburnt hydrocarbons an automobile releases into the atmospherecome from the combustion process The place in the cylinder and the moment atwhich unburnt hydrocarbons are generated has not yet been precisely deter-mined They occur even if there is sufficient oxygen for complete combustion, ifflame propagation in the combustion chamber is perfect, if there is little resid-ual gas and if there is an efficient distinct charge turbulence

Most scientists believe that the unburnt hydrocarbons result from incompleteflame propagation, causing the flame to be quenched at the cool walls of the com-bustion chamber (wall quenching) But the theory of flame quenching explainsonly part of the generation process of unburnt hydrocarbons A major portion

is generated through incomplete fuel combustion caused by residual gases which

strongly dilute the charge or by low cycle temperatures etc Unburnt bons are also created in those cylinder areas where the mixture cannot bereached by the flame, such as the space between the piston top land and the cylin-der wall or the piston ring grooves (Fig 2) [6].

hydrocar-In the expansion and exhaust phases, the unburnt hydrocarbons mix with theproducts resulting from complete combustion thus continuing their oxidationthe intensity of which depends on temperature, the hydrocarbon and oxygen con-centrations and the time available

The overall amount of unburnt hydrocarbons in the exhaust gas consists of amultitude of individual hydrocarbons The exhaust gases of gasoline and Dieselengines contain several hundred hydrocarbon compounds with 1 to 9 (and more)

C atoms Unburnt hydrocarbons include paraffins, olefins, aromatic compounds,acetylene and their isomers, partly oxidized hydrocarbons (aldehydes, ketones,alcohols) as well as organic nitrogen and sulfur compounds Some of these comeunchanged from the fuel whereas others are combustion products Each indi-vidual hydrocarbon compound needs a specific temperature to be generated.Anychange of the operating conditions will automatically change the respective com-pound’s share in the overall amount of hydrocarbons

1.4

Nitrogen Oxides (NO x )

The atmospheric air used for combustion essentially consists of nitrogen andoxygen molecules Under normal conditions, it is chemically well balanced andvery stable

Under temperatures of several hundred degrees, the two-atom nitrogen andoxygen molecules dissociate into their respective atoms and partly combine to

Fig 2. Sources of unburnt hydrocarbons in combustion chamber

form nitrogen monoxide (NO) The degree of dissociation depends on the perature and pressure levels and is accompanied by strong energy consumption.Provided that there is a sufficiently high amount of oxygen, the high cylindertemperatures in a combustion chamber further the partial oxidation of nitrogenfrom the air forming nitrogen monoxide The NO concentration in the combus-tion engine mainly depends on the maximum combustion temperatures, thecomposition of the air/fuel mixture (A/F ratio) and the reaction time available.

tem-It is generally assumed that the combustion process produces NO only andthat other nitrogen oxides such as NO2, N2O, N2O3, N2O4and N2O5are generatedthrough continued NO oxidation in the expansion and exhaust phases and in theatmosphere Nitrogen monoxide which has been generated and is then cooleddown to ambient temperature will quickly oxidize in the atmospheric air to form

NO2 Further atmospheric oxidation of NO2into N2O4, for example, is ably slowed down at ambient temperature Low temperatures and high dilutionwith air allow nitrogen oxides to continue to exist in the atmosphere for a longtime

consider-1.5

Particulate Matter (PM)

Besides the gaseous CO, HC and NOxemissions, Diesel engines also emit ulate matter (PM) Particulates have been defined as solid matter which is detected by diluting the engine exhaust gases with air, passing them through a fil-ter at a temperature of less than 52°C and weighing the resulting residue.Thus as soot described particulates contained in the exhaust gas is the mostobvious form of air pollution caused by combustion engines The amount of sootmeasured in the exhaust gas from Diesel engines is a criterion of the quality ofboth the combustion process and the mixture control

partic-Soot is an inevitable constituent of exhaust gases resulting from the tion of organic fuels Its amount and properties, however, depend on how thecombustion process goes

combus-In the past, distinction was made between three types of Diesel engine smokeemission: white, blue and black smoke

White smoke is generated if the combustion temperatures are low or if the

ig-nition delay is too long This kind of smoke occurs after the engine has beenstarted and when the cylinder temperatures are high enough to evaporate but not

to self-ignite the fuel

Blue smoke usually occurs when small amounts of lubricating oil penetrate

and are burnt in the combustion chamber

Black smoke emitted under higher engine loads almost exclusively consists of

carbon and other solid combustion products The smoke is black if less than 1%

of the carbon contained in the fuel is emitted in the form of soot

When analyzing the soot phenomenon, consideration must be given above all

to the type of flame used for combustion In the premixed flame of an gasolineengine, for example, the fuel vapors and the oxygen of the air are closely mixedand in direct contact with each other, so that no soot is generated if the amount

of oxygen is sufficiently high (l≥1.0).

The extremely heterogeneous combustion of a Diesel engine is characterized

by the simultaneous existence of a mixture of gases, vapors and liquid fuel in thecombustion chamber whose concentrations vary continuously These heteroge-neous conditions (diffusion flame) result in incomplete chemical reactions al-lowing solid particles as well as unburnt or only partly burnt hydrocarbons to oc-cur in the exhaust gas

Soot is generated at the flame front under high pressures and temperaturesthrough various chemical and physical processes It has not been possible to date,

to scientifically determine the mechanisms of soot formation with sufficient cision There are many hypotheses as to the particulates-forming reactions dur-ing Diesel-engine combustion none of which is able to provide a complete description of the processes involved Quite frequently, polymerization is thought

pre-to be the primary source of soot formation in a diffusion flame Other erating reactions are dehydration, condensation and graphitization An exem-plary soot formation model is shown in Fig 3 [10]

soot-gen-Particulates mainly consist of soot (black smoke) Soot is elementary carbonresulting from incomplete Diesel combustion The organic compounds (hydro-carbons) settled down on the soot particles – also known under the designation

of SOF (Soluble Organic Fraction) – consist of unburnt, partially cracked or merized hydrocarbons coming from the fuel and the lubricating oil In addition,there are sulfates caused by the burning of the sulfur contained in the fuel Par-ticles also include residues of lubricants and fuel additives as well as settled-downwater Figure 4 shows the typical particle mixture of a Diesel engine at full load.The results of the particulates analysis suggest that all carbon-containing fu-els are susceptible to forming particles.With aromatic compounds, this tendency

poly-is greater than with olefins and paraffins A low hydrocarbon saturation level creases the particle formation trend This means that the C/H ratio of the fuel is

in-an essential parameter when it comes to evaluating the soot-formation sity of fuels

propen-Fig 3. FVV Project “Soot oxidation model” – soot formation and oxidation in Diesel engines

The first particles have an almost spherical shape with diameters ranging tween 0.002 and 0.01 µm These particles agglomerate very quickly to formchains.

be-A typical soot particle has a size of about 0.1 to 0.2 µm.With this scatter, Dieselsoot is in the same range as numerous other particulates so that it is extremelydifficult from a measuring point of view to clearly separate Diesel soot particlesand particulates from other sources in the atmosphere (Fig 5)

1.6

References

1 Woinov AN (1965) Verbrennungsprozesse in schnellaufenden Kolbenmotoren (russ.) Moskau

2 Fristrom RM, Westenberg AA (1965) Flame Structure McGraw-Hill, New York

3 Bradley JN (1965) Flame and Combustion Phenomena Methuen & Co Ltd., London

Fig 4. Composition of particulates

Fig 5. Size ranges of different types of particulate matter

4 Gaydon AG,Wolfhard HG (1970) Flames Their structure radiation and temperature man and Hall, London

Chap-5 Taylor CF (1985) The Internal Combustion Engine in Theory and Practice The MIT Press, Cambridge

6 Stone R (1992) Introduction to Combustion Engines The Macmillan Press, London

7 Warnatz J, Maas U, et al (2001) Verbrennung Physikalisch-Chemische Grundlagen Springer, Berlin, Heidelberg, New York

8 Warnatz J (1995) Probleme bei der Simulation von motorischen Verbrennungsprozessen Symposium Kraftfahrwesen und Kraftfahrzeuge, Stuttgart

9 Polycyclic aromatic hydrocarbons in automotive exhaust emissions and fuels CONCAWE Report No 98/55, 1998

10 Pischinger S (1998) Rußbildung und Oxidation im Dieselmotor FVV-Vorhaben dationsmodell” FVV Frankfurt

“Rußoxi-11 Moser FX, Flotho A, et al (1995) Entwicklungsarbeiten an Dieselmotoren für den fahrzeug- und Industrieeinsatz zur Erfüllung der zukünftigen Emissionsanforderungen Symposium Kraftfahrwesen und Verbrennungsmotoren, Stuttgart

to the fact that these two power plant concepts will continue to prevail also in theforeseeable future and far into the 21st century Of the 750 odd million passengercars registered world-wide more than 90% are powered by gasoline engines – anindication of the enormous importance this type of propulsion system has hadfor mankind

The configuration of an Otto engine depends on the fuel type (gasoline) forwhich it has been laid out According to the current state of knowledge, gasolinescan only be efficiently burnt in a homogenous gasoline/air mixture That is why,

in the Otto engine, the fuel is injected into the intake manifold (or cylinder) inthe suction phase already (Fig 1)

The intake and compression strokes (360° C.A.), which account for 50% of theworking cycles, provide sufficient time to evaporate the fuel and intensively mixthe air and fuel vapors

As a matter of fact, homogeneous air/fuel mixtures need an external ignitionsource triggered by a spark-plug in order to be able to burn in a controlled reg-ular manner Following the ignition of the A/F mixture, the flame spreadsthroughout the combustion chamber at a velocity of 30 to 50 m/s.

Gasoline engine combustion is represented by the Otto cycle (Fig 2), ing of adiabatic compression (T1–T2), isochoric heat supply (T2–T3), adiabatic ex-pansion (T3–T4) and isochoric heat removal and/or gas exchange (T4–T1).The homogenous A/F mixtures in an Otto engine can be burnt efficiently only

consist-in a relatively narrow A/F-mixture range about the stoichiometric ratio prox 1.0, A/Fª14.5) and require a quantitative engine load control (throttling).

(l=ap-With decreasing load, both the amount of fuel and the amount of air must be duced in order to maintain the A/F ratio at a constant level This means that thepressure and temperature levels in the combustion chamber at the moment of ig-nition keep dropping while the engine load diminishes (Fig 3)

re-In a Diesel engine, the amount of air sucked in and compressed is practicallyalways the same regardless of the engine load The pressure and temperature

Fig 1. Mixture formation in Otto engine

Fig 2. Thermodynamic cycle (Otto engine)

levels reached at the end of the compression stroke are high and completelyindependent of the load which is controlled qualitatively (unthrottled) by reduc-ing the amount of fuel injected.

Besides the throttling losses, the unsatisfactory efficiency of the gasoline gine at part load is mainly due to the low pressures and temperature levels dur-ing the combustion process In a Diesel engine, the combustion process takesplace always at constantly high energy level The differences between the part-load behaviors of the Otto and Diesel engines are caused, among others, by thedifferences between their inherent energy potentials at which the combustionprocesses take place

en-2.2

Power Output and Fuel Consumption

The properties of the gasoline engine strongly depend also on the composition

of the A/F mixture or the A/F ratio l Figure 4 illustrates the dependence of the

specific work we(mean effective pressure) and the specific fuel consumption be

on the A/F ratio (l).

In the event of an air deficiency, homogeneous A/F mixtures can always be

safely ignited and burnt in what is called the “rich” mixture range (l=0.8–0.9) It

is in this range that Otto engines reach their highest mean pressures or poweroutputs That was also the reason why the early generations of gasoline engineswere exclusively operated on rich A/F ratios over the entire operating range fromstarting through idling to full-load

These operating conditions made no major demands on engine control Therequired amounts of fuel and air were metered in the carburetor; the ignitiontiming was adjusted via the engine speed by means of a flyweight-controlled reg-ulator in the ignition distributor and via engine load by means of a intake-man-

Fig 3. P-V diagrams at different loads

ifold-pressure-controlled vacuum advance unit Exemplary l- and

ignition-tim-ing maps of a former carburetor engine are shown in Fig 5

The low CO, HC and NOxexhaust emission limits prescribed by tal legislation as well as the engine manufacturers’ constant efforts to reduce fuelconsumption resulted in the development of highly complex electronic A/F-mix-ture and ignition control and regulation systems for modern gasoline engines(Fig 6)

environmen-Fig 4. Influence of air/fuel-ratio on specific work (power) and fuel consumption

Fig 5. Air/fuel ratio and ignition timing maps of former gasoline engines

To reach their maximum power output and torque levels, modern Otto enginesuse slightly enriched A/F ratios under full-load conditions only, yielding as nat-urally aspirated Otto engines specific power outputs of as high as

Fig 6. Ignition timing map of modern gasoline engines

Fig 7. Map of specific fuel consumption

and specific torques of

In the part-load range, advanced Otto engines are operated on stoichiometric A/F

ratios (A/F=14.5, l=1.0), in order to create optimum operating conditions for the

3-way catalyst (see Chapter 2.5)

The lowest fuel consumption levels realized with modern Otto engines areabout bemin=230–240 g/kWh (Fig 7)

2.3

Exhaust Gas Emission

The explosive increase of the vehicle population in the industrial countries afterWorld War II resulted in a new problem in the big population centers – withawareness starting in Los Angeles, USA: air pollution through exhaust emissionsfrom combustion engines First, it was the carbon monoxide (CO) and unburnthydrocarbons (HC) which were rated as being noxious Shortly thereafter, nitro-gen oxides (NOx) were added to this group of pollutants Since that time, the sur-vival of the gasoline engine has depended and will continue to depend on its abil-ity to comply with all the existing and planned regulations meant to reduce theburden on environment

2.4

Engine-Internal Measures for Pollutant Reduction

For both Otto and Diesel engines, so-called engine-internal measures are the firstchoice when it comes to reducing pollutant emission The emission of many ofthe exhaust-gas constituents can be influenced and minimized at their place oforigin, that is in the engine cylinder or in the combustion chamber, by correctlyselecting and adapting the relevant engine design and operating parameters

ratio or l) (Fig 8).

The A/F ratio influences the composition of the exhaust gas far more stronglythan any of the other combustion parameters, because it determines with rela-

tively great precision whether the Otto engine is operated on a rich (l<1.0) stoichiometric (l=1.0) or a lean (l>1.0) mixture.

The high levels of CO and unburnt hydrocarbons resulting from rich air/fuelmixtures are due to the fact that the mixture cannot be completely burnt for lack

of oxygen The only way of noticeably reducing these pollutants at their place oforigin in the cylinder would be to increase the A/F ratio (mixture enleanment).The lack of air prevents excessive amounts of NOxfrom being generated eventhough the maximum combustion temperatures are high

Contrary to the results of corresponding equilibrium calculations, using

sto-ichiometric mixtures (l=1.0) does not completely eliminate the CO contained in

the exhaust gas, the residue being about 0.5 to 1.0 vol.% Due to the reaction netics of the CO combustion, the exhaust gas contains a certain amount of CO

ki-even when l>1.0.

The lowest HC levels are obtained with lean mixtures (lª1.1–1.3) or, in other

words, with those A/F ratios at which the highest engine efficiency is reached.The high combustion temperatures and amounts of air required for the oxi-dation of CO and HC result in a steeply increasing NOxconcentration The max-imum NOxlevel occurs in the same A/F ratio range in which the concentration

of unburnt hydrocarbons is lowest

Fig 8. Influence of air/fuel ratio on fuel consumption and exhaust gas emission