Swaminathan N., Bray K.N.C. (eds.) Turbulent Premixed Flames

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[279] T. Poinsot, Analyse des instabilit

es de combustion de foyers turbulents

elang

es, Th

ese d’etat (Universit

e d’Orsay, France, 1987).

[280] W. Krebs, G. Walz, and S. Hoffmann, Thermoacoustic analysis of annular com-

bustor, presented at the 5th AIAA Aeroacoustics Conference, AIAA paper

99-1971 (1999).

4 Lean Flames in Practice

4.1 Application of Lean Flames in Internal Combustion Engines

By Y. Urata and A. M. K. P. Taylor

In the area of pollution, where there is strong social responsibility, it should be no special

feat to burn materials completely. Success will often be a matter of funds and engineering

design [1].

One of the two main ‘drivers’ for the adoption of lean ﬂames in internal combustion

(IC) engines has been the potential to reduce the emission of some pollutants.

Since Bernard Lewis wrote these words in 1970, the same year as the enactment

of the landmark extension to the Clean Air Act (CAA) – the so-called ‘Muskie’

Act – the design and technology behind IC engines for automobile application

have seen many changes that indeed greatly reduced pollution. These include the

widespread introduction of gasoline injection systems, three-way catalysts, the high-

speed direct injection (DI) diesel engine, turbocharging, and high-pressure common-

rail technology, to name a few at random. The related science has also progressed in

ways amply described elsewhere in this volume: As to whether this has constituted

‘no special feat’, the reader will decide. Be that as it may, research into combustion in

IC engines is now arguably going through one of its most exciting phases in the quest

to ‘burn materials completely’, that is, with as little pollution as possible and with

as much thermodynamic efﬁciency as possible. This part of the chapter describes

the way in which engineering design can exploit combustion, predominantly lean

combustion, in IC engines not only to reduce pollution, but also – by promoting

fuel-conversion efﬁciency, the other main ‘driver’ – to husband the ﬁnite resource of

petroleum-derived fuels [2]. The latter is important not only in the economic sense

but also in the quest to reduce anthropogenic emissions of CO

to the atmosphere.

Road transport is responsible for the release of between 20% and 26% of total CO

emissions in western economies and is the second largest greenhouse-gas- (GHG-)

emitting sector in the European Union (EU), so that, although the EU as a whole

has reduced its emissions of GHGs by just under 5% over the period from 1990

to 2004, the CO

emissions from road transport have increased by 26% [3, 4]. Thus

even modest improvements to indicated fuel-conversion efﬁciency (subsequently we

refer to fuel economy) can have important effects on GHG emissions, although these

244

4.1 Application of Lean Flames in Internal Combustion Engines 245

will be inevitably limited by the time constant associated with the rate of removal of

engines with less efﬁcient technology from the ﬂeet of engines currently in use.

The improvement in fuel economy consequent on lean-burn combustion, based

on the thermodynamics of fuel–air cycle analysis, is well known [5, 6], as is the reduc-

tion in NO

emission that is due to the concomitant lower ﬂame temperature. Both

are desirable ends in principle, but the technical interest in lean-burn combustion

is framed by legislative as well as by commercial requirements: Over the past four

decades, up to three generations of lean-burn engines have been withdrawn from the

market owing to increasingly stringent emissions legislation. Therefore the next sub-

section brieﬂy describes past, current, and likely impending legislation that has been

primarily related to emission standards but is increasingly concerned with limiting

GHGs emissions, which are closely related to improving fuel economy. Subsec-

tion 4.1.2 presents three combustion concepts that make use of lean-burn combustion

in IC engines: One is spark-ignited (SI) and the other two are compression-ignited

(CI). The latter subsection sets the scene for a description of the role of experiments

in Subsection 4.1.3: The ﬁnal section summarises this contribution and outlines the

technological and scientiﬁc challenges.

4.1.1 Legislation for Fuel Economy and for Emissions

Fuel Economy, the Clean Air Act and Greenhouse Gases

Good fuel economy – in principle one of the main advantages of lean-burn com-

bustion – is presumably a desirable attribute for most purchasers of an IC engine,

and in the United States CO

emissions did indeed decrease rapidly from 1975 to

1981 after the 1973–1974 oil embargo. However, emissions increased [7] from 1987

to 2004, decreasing thereafter – currently by 8%. This increase in the United States

arose despite the existence of legislation since 1975 that the ‘corporate average fuel

economy (CAFE)’ must be set at the maximum feasible level, subject to economic

practicability and the effect of other standards on fuel economy as part of the ‘En-

ergy Policy Conservation Act’. In addition, the U.S. Congress established the ‘Gas

Guzzler Tax’, provisions in the Energy Tax Act of 1978, to discourage the produc-

tion and purchase of fuel-inefﬁcient vehicles. The original purpose of CAFE was to

reduce energy consumption: Another purpose may soon be added as a consequence

of a U.S. Supreme Court ruling in 2007 that the CAA covers GHGs. Thereby, in

early December 2009, the U.S. Environmental Protection Agency (EPA) was able

to state that new motor-vehicle engines contribute to GHG air pollution, which en-

dangers both public health and welfare [8]. At the time of writing [9], a proposed

‘National Program’ would require – by 2020 – vehicles to meet an estimated so-called

combined average emissions level of 250 g of CO

per mile, equivalent to 35.5 miles

per U.S. gallon or 6.6 L per 100 kms (note that the 2009 CAFE regulations stand at

27.5 and 23.1 miles per U.S. gallon for cars and light trucks, respectively), assuming

that the automobile industry were to meet this CO

emission level solely through

fuel-economy improvements. Thus, at least in the United States, it seems that likely

future legislation trends may favour improvements in fuel-conversion efﬁciencies to

which lean-burn combustion may be able to contribute. In practice, the importance

of lean-burn combustion relative to other means of improving fuel consumption has

yet to emerge: In the United States, for example, average light-duty vehicle weight

246 Lean Flames in Practice

has increased by about one quarter since 1980 whereas acceleration performance

has become faster by about one third, and reductions in both these metrics could

be a cost-effective method for the lowering of emissions. Power-train hybridisation,

cylinder deactivation, and so-called ’stop–start’ technologies are already making

useful contributions to the lowering of fuel consumption. Although none of these

approaches excludes the use of lean-burn combustion, all add – to a greater or a

lesser extent – to the cost: Lean-burn combustion is an attractive candidate but its

cost-effectiveness has yet to be full established.

In Europe, the need for commitment to measures to reduce CO

emissions from

cars was recognized from the late 1980s [10, 11] and, as party to the UN Framework

Convention on Climate Change in 1992, Europe committed itself to stabilizing CO

emissions by the year 2000 at 1990 levels. As a consequence, in 1999 [12], the Au-

tomobile Manufacturers’ Associations entered into a negotiated self-commitment

for a 25% reduction in CO

emission for the average of new passenger cars sold

in the EU, i.e., to 140-g CO

/km, to be achieved by 2008. The important aspect of

this voluntary commitment, for the purposes of this chapter, was that manufacturers

had to achieve the CO

target ‘mainly’ by technological developments. In practice,

changes in ﬂeet CO

emissions arose to some extent by the substitution of SI engines

by CI engines rather than by any intrinsic improvement in the technology of either

engine type. However, it became clear by 2007 that the revised EU objective of 120-g

/km by 2012 would not be met. As a consequence, a legally binding regulation

has come into force, (EC) No 443/2009, which establishes an average emissions re-

quirement for new passenger cars of 130-g CO

/km to be achieved by means of

improvement in vehicle motor technology: For a gasoline vehicle, that represents

5.4 L per 100 km or 52 miles per Imperial gallon [13]. Thus, from a current average

emissions level of CO

from new passenger cars in the EU of around 160-g CO

/km,

this represents a 19% reduction. It is the emission level currently attained by a two-

seater, 720-kg vehicle, powered by a three-cylinder turbocharged engine [14]. From

2020 onwards, this regulation sets a target of 95-g CO

/km as the average emission

for the new car ﬂeet [15].

Despite the disparity between the emissions targets on the two continents, the

planned and existing legislation places an additional premium on ﬁnding ways to

improve fuel economy in the near future. This is likely to be achieved through the

adoption of several different approaches: Research and development on lean-burn

combustion currently constitutes one of these approaches.

Emission Legislation: Why Not Zero Levels?

AMBIENT AIR QUALITY. By 1950, Haagen-Smit’s landmark work at CalTech had iden-

tiﬁed ozone as a major component of smog that caused severe respiratory health

difﬁculties; and by 1952 he had shown that vehicle emissions were a contributor to

the generation of ozone, speciﬁcally nitrogen oxides (NO

) and hydrocarbons (HCs)

(in this text HC is used interchangeably with the term volatile organic compounds,

VOCs), which are both ozone precursors. However, it was only after 1970 that gov-

ernments worldwide enacted coherent legislation to regulate pollutant emissions

from IC engines. In part, this two-decade delay arose from the need to set vehicle

emission regulation within the broader context of other sources of pollution and

the development of associated legislation for the setting of the maximum levels of

4.1 Application of Lean Flames in Internal Combustion Engines 247

pollutants in the ambient air (the CAA [16] in the United States and its European

equivalent, originally [17] and, in its most recent form, [18]). The legislation, broadly

speaking, has become increasingly and signiﬁcantly stringent with each revision of

the related acts and directives, such as the European “Clean Air for Europe (CAFE)”

programme [19] (not to be confused with the same acronym for the U.S. corporate

average fuel economy regulation). To provide the necessary perspective on the ex-

tent to which vehicle emission legislation inﬂuences the development of lean-burn

combustion in IC engines, it is useful to recall the twin purposes of ambient air leg-

islation, which has broad remit. In the United States, it is to protect human health

and the environment: The related standards are as follows:

‘Primary standards’, setting limits to protect public health, including the health

of sensitive populations such as people with asthma, children, and elderly people;

this entails controlling concentrations of air pollutants t hat cause smog, haze,

acid rain, and other problems and reducing emissions of toxic pollutants that

are known to cause, or are suspected of causing, cancer or other serious health

effects.

‘Secondary standards’, to set limits to protect public welfare, including protection

against decreased visibility, damage to animals, crops, vegetation, and buildings

from acidiﬁcation and eutrophication.

European legislation similarly has ‘the aim of reducing pollution to levels which

minimise the harmful effects on human health and on the environment and improving

information to the public on the risks involved’ [20].

In the United States, six principal pollutants are identiﬁed, of which the ﬁrst four

subsequently listed are the more relevant to the emissions from IC engines (lead

having been phased out of gasoline fuel and low-sulphur fuels being increasingly,

though not universally, available):

ground-level (i.e., tropospheric) ozone, O

particle pollution

nitrogen dioxide, NO

carbon monoxide, CO

lead, Pb

sulphur dioxide, SO

For these there exist ‘National Ambient Air Quality Standards’ (NAAQS) [21]. Note

that NO

is the component of NO

of greatest interest and serves as the indicator for

the entire NO

family: Control measures that reduce NO

can generally be expected

to reduce population exposures to all NO

gases [22].

In view of what can arguably (see subsequent discussion) be called the over-

riding current health concern, it is perhaps surprising that legislation to control the

adverse effects of ﬁne particulate matter has appeared only in the past 15 years

or so in the United States – and only in the past couple of years in Europe. By

1972, it was known that particle size has a multimode distribution, with the smaller

peak being associated almost entirely with anthropogenic emissions. Nevertheless,

as late as 1982, the U.S. EPA had to conclude [23, 24] – albeit in a heavily qualiﬁed

and guarded review – that poor visibility was still the most quantiﬁable effect on

public welfare. In the United States, it was only in 1987 that standards speciﬁcally

248 Lean Flames in Practice

for inhalable particles (as opposed to ‘total suspended particles’), in the form of

, appeared (see the following dicussion for the deﬁnition of PM

). The exis-

tence of ﬁne particles, PM

2.5

, was recognised but, in 1987, these were the subject

only of an ‘advance notice’ – and even this was issued as a ‘secondary standard’

NAAQS, once again on the grounds of visibility. The main reason was that, in 1987

the ‘. . . available epidemiology information (as opposed to “controlled exposure”

tests), in which aerometry did not clearly separate the two (size categories), could

not be used to support two PM indicators. . . ’ [25]. Particulate matter (hereafter

PM) is currently regulated separately as airborne particles smaller than 10 and

2.5 μm in diameter (also known as PM

, or inhalable coarse particles, and PM

2.5

or ﬁne particles) since 1987 and 1997, respectively, in the United States: Equiva-

lent European legislation for PM

2.5

was enacted only in 2008. Thus, as – and if –

results from new epidemiological and controlled studies appear, a revision of cur-

rent standards may be expected in future. As will be subsequently seen, legislation

on particulate emissions is of particular importance to gasoline SI lean-burn com-

bustion. (There are, of course, many more pollutants: In the United States, there is

a list of 187 such ‘hazardous air pollutants’ [26] – a subset of which forms the ‘air

toxins from motor vehicles’ [27] that extends to more than 100 substances. Although

some (e.g., benzene) of these arise through evaporation or unburned fuel, others are

consequent on incomplete combustion of compounds in the fuel, s uch as toluene,

xylene, formaldehyde, acetaldehyde, and 1,3-butadiene. Model-based estimates sug-

gest that such air toxics may account for up to half of all cancers attributed to outdoor

sources of air toxics. Bachmann [25] tentatively discusses likely future action on these

substances.

Similar air-quality standards exist in European legislation [18, 28]. Of particular

relevance to the subject of this article is t he 2005 European Strategy on air pollution

up to 2020 described in [29], following the CAFE programme in 2001. In particular,

it places the emphasis on the most harmful pollutants to public health, namely

tropospheric ozone, and

ﬁne particles, i.e., PM

2.5

. A similar emphasis on particulates is placed by [30],

(cited by [31]) and by the Scientiﬁc Committee on Health and Environmental

Risks (SCHER) of Directorate General of Health and Consumer Protection of

the European Commission [32] that notes that there is increasing epidemiolog-

ical evidence that PM

2.5

may be related to adverse health effects, especially in

susceptible populations and vulnerable groups.

Both of these pollutants are relevant to the emissions from IC engines, and hence

the associated legislation is important to the research and development of en-

gines, including those based on lean burn, or so-called ‘low-temperature’ (see later)

combustion.

Just how harmful are these pollutants? It has been estimated that, in the year 2000,

there were some 21 000 cases of ‘hastened death’ that were due to ozone [29]. Of

even greater concern, however, is that exposure to PM was estimated t o reduce

the average of ‘statistical life expectancy’ by approximately 8 months in the ‘EU-

25’, equivalent to 348 000 premature mortalities per annum. Even by 2020, [33]

4.1 Application of Lean Flames in Internal Combustion Engines 249

states that the EU will still be a ‘long way’ from achieving the objectives of the

6th Environmental Action Programme [19]: With effective implementation of only

the legislation current in 2005, there would still be 272 000 premature deaths in 2020

that would be due to PM pollution.

Once the pollutants of importance are identiﬁed, how are standards set? The NAA QS

pollutants were originally regulated by developing health based or environmentally

based criteria for setting permissible levels (these are guidelines that are drawn up on

so-called science-based criteria: Hence the NAAQS six are also known as ‘criteria

pollutants’). This required the painstaking gathering of evidence of undesirable

effects and is a ‘risk-based’ category of environmental protection. The alternative

approach to emissions standards requires controls on ‘best available technology’ or

‘maximum achievable control technology’ (BAT and MACT, respectively) [25] and

it is this which has held sway since the revision of the CAA in 1990. Bachmann [25]

provides an interesting perspective on the development of standards in the United

States.

A particular challenge is set by ozone and PM

2.5

, because no safe levels have

yet been identiﬁed for ozone or for an unambiguous threshold for the PM

2.5

dose:

It appears to depend on the health-effect endpoint, populations, and vulnerabil-

ity [34]. So what is the legislator to do? This challenge and its consequences are

discussed in [25] in detail: ‘...The O

preambles also give EPA’s ﬁrst statement

on the fundamental issue of why the agency believed the CAA requirements for

safety and restrictions against considering costs did not require levels approaching

zero:’

The decision is made more difﬁcult by the fact that the Clean Air Act . . . does not permit

him to take factors such as cost or attainability into account in setting the standard; it is to

be a standard which will adequately protect public health. The Administrator recognizes,

however, that controlling Ozone to very low levels is a task that will have signiﬁcant

impact on economic and social activity. It is thus important that the standard not be any

more stringent than protection of public health demands.

‘Subsequent articulations of this view would stress the actual wording of [the] stan-

dards must be “requisite” (i.e., no more than necessary) to protect public health with

an adequate margin of safety. . . ’. As a consequence, for PM, Bachmann [25]also

relates that the review of the standards was ‘. . . also among the most contentious,

fraught with intrusive litigation, dueling expert panels, and a little intrigue ...’and

describes how the standards had been arrived at by 2007. In particular, he describes

‘...thefocus on the more general issue so many academic reviews had raised over

the years. That is, given a wide range of scientiﬁc opinion and no clear thresholds

[for PM and ozone], what criteria do EPA use for making the ultimate normative

choice on the standards ...’.Intheend, a judgement was made by the U.S. Supreme

Court, which in 2001 issued a ‘. . . unanimous decision upholding EPA’s position that

the statutory requirement that NAAQS be “requisite” to protect public health with

an adequate margin of safety sufﬁciently guided EPA’s discretion, afﬁrming EPA’s

approach of setting standards that are neither more nor less stringent than neces-

sary...’.As furtherevidence becomes available, the standards are reassessed. For

250 Lean Flames in Practice

example, in early 2010, the standards for ozone and NO

were made more stringent,

and those for ﬁnes were revised in 2006, the latter being indicative of the current,

active concern with this particular pollutant of the ambient, which has implications

for vehicle exhaust emission levels. In addition, concerns have been raised about the

health impact of ultraﬁne particles (commonly deﬁned as those below 0.1 μm) but,

as yet, no legislation addresses these concerns.

In any event, it is clear that a s igniﬁcant reduction of these substances in the

ambient will have beneﬁcial effects in terms of public health and will also generate

beneﬁts for ecosystems. In the EU, the process by which objectives were established

is summarised in [33]: Brieﬂy, consideration was given to the cost and beneﬁts of

closing the gap, by 2020, between the projected baseline at 2020 in the absence of

further measures and the ‘maximum technically feasible reduction’ (MTFR) by 2020.

The MTFR would still result in 96 million EU-wide cumulative years of life lost by

2020 as opposed to 137 million for the baseline. Control costs were found to increase

rapidly at about 75% between the baseline and the MTFR. Accordingly, compared

with the situation in 2000, the strategy [33] set speciﬁc long-term objectives (for

2020):

47% reduction in loss of life expectancy as a result of exposure to particulate

matter;

10% reduction in acute mortalities from exposure to ozone;

reduction in excess acid deposition of 74% and 39% in forest areas and surface

freshwater areas, respectively;

43% reduction in areas or ecosystems exposed to eutrophication.

To achieve these objectives, it was estimated that SO

emissions must decrease by

82%, NO

emissions by 60%, VOCs by 51%, ammonia by 27%, and primary PM

2.5

(particles emitted directly into the air) by 59% compared with those in the year 2000.

In terms of health, the number of premature deaths should fall from 370 000 that are

due to PM

2.5

and ozone in 2000 to 230 000 in 2020 (compared with 293 000 in 2020

without the strategy).

In conclusion to this subsection, it should be noted that, despite the existence of

air-quality standards for up to four decades, there remain several regions and areas

in both the EU and the United States that do not attain these standards.

Vehicle Emissions

The attainment of ‘levels of air quality that do not give rise to signiﬁcant negative

impacts on, and risks to, human health and the environment’ [19], especially in urban

areas and densely populated regions, implies legislation limiting the emission of

atmospheric pollutants from motor vehicles. Of pollutants in the ambient, European

road transport was responsible for 41% of total NO

emission and 22% of total

non-methane volatile organic compounds (NMVOCs) emissions in 2004 [31]. Road

transport is held to be the dominant source of ozone precursors, contributing 36%

in the European Environment Agency’s (EEA) 31 [31]. The transport sector as a

whole (i.e., including aviation, rail, and shipping, as well as road) contributed 29%

of total PM

2.5

emissions in 2000 [33](citedby[31]): Note that most diesel exhaust

particles are considerably smaller than 2.5 μm. Similarly, in the United States, motor

vehicles are responsible for nearly one half of smog-forming VOCs, more than half

4.1 Application of Lean Flames in Internal Combustion Engines 251

of the NO

emissions, about half of the toxic air pollution, and 75% of CO emissions.

This is despite the fact that emissions from new cars are more than 90% cleaner than

in 1970 [16].

This apparent ineffectiveness of legislation has to be placed in perspective. In

the United States, the total vehicle distance that people travel has increased (by

178% in the United States between 1970 and 2005) and the type of vehicle driven

has changed (by 2000, ‘light-duty trucks’ accounted for half of all new passenger car

sales). Thus for this reason, if no other, legislation continues to require a reduction

in emissions. Similarly, European NO

(and HC) emissions must be reduced because

many member states will otherwise be unable to fulﬁl the requirements of the Na-

tional Emissions Ceilings Directive (2001/81/EC) and the proposal for revision of the

air-quality directives [34, 35]. In addition, new – and generally, although not always,

lower – emission standards are required as research shows new cause for increasing

concern for health: PM

2.5

is a case in point.

Standards for ‘tailpipe-out’ emission levels exist for CO, HCs, NO

, and PM. Hy-

drocarbons are regulated as non-methane organic gases (NMOGs or VOCs) mainly

because these are precursors for ozone. However, since about 2004 in the state of

California, formaldehyde is subject to a separate pollutant category, the latter de-

velopment being potentially important for the future as ethanol biofuel is being

introduced. Although NO

is the regulated pollutant in European air-quality legis-

lation, currently NO

is the regulated emission for engines, with NO

being formed

from the atmospheric oxidation of nitric oxide, NO, as well as by direct emission, de-

pending on the conditions of combustion. Although about 90% of the NO

mixture

emitted by road vehicles was NO, in the future this emphasis may change [31]for

three reasons, thereby contributing to the expectation that the regulated ambient

level will be exceeded. First, diesel engines are an increasing fraction of the ﬂeet,

and these engines emit a greater proportion of NO

. Second, some diesel particulate

ﬁlters work on the principle of deliberately converting NO in the exhaust stream to

over an oxidation catalyst unit and using the NO

to oxidise the particulates

held on the ﬁlter, thus regenerating the trap. Third, NO

increases in slow-moving

trafﬁc [36]. Particulate emission may be either primary or be added to, secondarily,

by chemical reaction with NO

(as well as ammonia and SO

), and the increasing

concern with the PM

2.5

contribution has already been noted. To ensure that emis-

sions of ultra-ﬁne particulate pollutants (0.1 μm and below) are controlled, it is

proposed in the future to introduce a number-based approach to emissions of partic-

ulate pollutants in addition to the mass-based approach that is currently used. The

number-based approach to emissions of particles should draw on the results of the

United Nations Economic Commission for Europe (UN/ECE) Particulate Measure-

ment Programme (PMP) and be consistent with the existing ambitious objectives for

the environment. The standards ‘...wouldbesetsothattheybroadly correlate with

the mass standards of the current proposal . . . ’. If appropriate, there should also

be speciﬁcation of the value of the admissible level of NO

component in the NO

limit value. However, the intention is that the use of a particle-number standard is

a means to ensure that developments in ﬁlter technology continue to focus on the

removal of ultra-ﬁne particles. Consequently, measuring the number of particulates

instead of their mass could be considered a more effective means of regulation in the

future.

252 Lean Flames in Practice

The legislation, worldwide, is speciﬁed in great detail, as is to be expected given

its commercial implications and the considerable public interest. Many references

exist that describe the following items:

Vehicle classiﬁcations (e.g., passenger cars, light-duty trucks, heavy-duty trucks),

test procedures (which – currently at least – are different for Japan, Europe,

and the United States, with California applying more stringent limits than apply

federally),

emission standards (which are currently common for diesel and gasoline-fuelled

vehicles in the United States, but not so for Europe. Note also that the standards

may be in units of either grams per kilometer or grams per kilowatt hour),

emission categories, compliance, in-ﬁeld monitoring, the increasingly important

requirement for ‘on-board diagnosis’ systems [37, 38] and, particularly, for the

important effects of transients [39].

Introductions more closely related to lean-burn engines are to be found in [13,

40–43] and, particularly for biofuels, [44]; [45] is a more detailed research monograph.

The point of interest here, however, is the current and likely future general inﬂuence

of this and similar legislation on the development of lean-burn engines. As will

be seen later, this section focuses on three applications of lean-burn combustion,

namely, SI engines and homogeneous charge-compression-ignition (HCCI) engines

fuelled by gasoline or diesel, the latter particularly in the guise of ‘low-temperature

combustion’ (LTC). Thus we consider the legislation for fuels and both SI and CI

engines.

European Emission Standards – Passenger Cars and Light

Commercial Vehicles

The standards will have undergone six major revisions by 2014 [46] since the original

directive in force in 1992 (91/441/EEC). The latest requirements take effect in two

stages, Euro 5 starting from 2009 and Euro 6 from 2014. For this section, the changes

in the emission limits for stoichiometrically fuelled gasoline vehicles are not of

interest, other than to note that the limits were lowered, partly to prevent the price

differential relative to the more fuel – and hence CO

– efﬁcient diesel engine from

widening excessively.

Gasoline – lean burn SI. Until the introduction of the ‘Euro 5a’ emission stage [46]in

late 2009, particulate emissions were unregulated for gasoline engines. However, as

subsequently described at greater length, lean-burn DI (LBDI) engines can provide

not only large improvements in fuel economy but also can, unfortunately, produce

levels of particulates that are signiﬁcantly higher than those of diesel engines ﬁtted

with ﬁlters. As a consequence, from late 2009 onwards, particulate emissions from this

particular class of engine are regulated, initially by mass per unit distance travelled,

at a level broadly comparable with limits in other countries. This is in view of the

expectation that this technology might account for only 10% of market share by

2015. The PM limit for LBDI gasoline cars was carefully selected so as to enable

the technology to develop without the need for any form of particulate ﬁlter to

be installed [31]. This is arguably an important concession to the development of

lean-burn combustion.