-
Compression Ignition Engines: State-of-the-Art andCurrent
Technologies. Future Trends andDevelopments
Francisco Payri, Jose-Mara Desantes, and Jesus
BenajesCMT-Motores Termicos, Universitat Politecnica de Valencia,
Valencia, Spain
1 INTRODUCTION
Since the invention by Rudolf Diesel in 1892,
thecompression-ignition (CI) engine has been the workhorseof
industry, and has been dominant in applications suchas trucking,
construction, farming, and mining. They havebeen also extensively
used for stationary power generationand marine propulsion and in
large passenger vehicles inmany regions of the world. The main
reason for this result isthat the type combustion in diesel engines
is very effectivein large-size engines, being the main advantage
the highglobal efficiency that can reach values in excess of
50%,considering that the best conventional gasoline engines
areapproximately from 30% to 33% efficient, and then only atwide
throttle openings.
On the other hand, small displacement diesel engines
aredifficult to design and to operate, and consequently the
appli-cation to light-duty vehicles such as vans and cars has
beenvery scarce until some decades ago. The main drawbacks ofthe
diesel engine in automotive applications have been thesmall
power/weight ratio, high levels of noise and harshness,and high
nitrous oxides (NOx) and soot emissions comparedwith other plants,
especially the spark-ignition (SI) enginefuelled with gasoline.
However, during the past decades, andthanks to significant
improvements in injection technology,turbocharging and exhaust
aftertreatment devices, dieselengines have been able to challenge
and partially beat theSI engine in many automotive applications,
changing some
Handbook of Clean Energy Systems, Online 2015 John Wiley &
Sons, Ltd.This article is 2015 John Wiley & Sons, Ltd.This
article was published in the Handbook of Clean Energy Systemsin
2015 by John Wiley & Sons, Ltd.DOI:
10.1002/9781118991978.hces079
historical market trends, especially in Europe, where theglobal
share of new diesel engines attained about 50%,reaching even 80% in
some countries (Figure 1). This routeof conceiving and producing a
competitive diesel engine forautomotive applications has lead to
the current situation inthat diesel engines for passenger cars and
light-duty vehiclesare nowadays the most complex type of internal
combustionengines, compared not only with spark-ignited but also
thewith usual industrial and heavy-duty powerplants.
In many aspects, like the gas management (induction andexhaust
processes), cooling, lubrication, and mechanicaldesign, diesel
engines are similar to SI engines, and someof the ideas exposed in
Internal Combustion Engine (ICE)Fundamentals and Spark Ignition
Engines: State-of-the-Artand Current Technologies. Future Trends
and Developmentsare applicable to this type of engine. However, the
processof fuelair mixture formation and combustion are
radicallydifferent from the SI engines. This fundamental
distinctioninduces also some other characteristics that are not
essentialbut important for practical purposes, which will be
addressedlater.
Moreover, this mode of mixture formation and combustionproduces
some important results in terms of performanceof the diesel engine
and is also responsible for a strongtrend toward the formation of
more soot and NOx than in anequivalent SI engine.
Nevertheless, they have continuously increased their ratedpower
over the past 15 years on the basis of a continuousincrease in the
boost pressure and the improvement of thefuel injection technology.
As shown in Figure 2 (data corre-spond to Spain, but they are not
locally limited), the averagestate-of-the-art Diesel-powered
light-duty vehicles consume
-
2 Reciprocating Engines
2000 2005 2010 2015 2020
Year
Western Europe diesel car share
Die
sel s
hare
of n
ew c
ar s
ales
(%
)
60
55
50
45
40
30
35
Figure 1. Market share evolution of diesel engines in Western
European countries and prospective toward 2020. Source: Reproduced
withpermission from Bedwell, 2013. LMC Automotive Ltd.
Fue
l con
sum
ptio
n (l/
100
km)
2000 2004 2008 20125
5.5
6
6.5
7
Spe
cific
pow
er (
kW/l)
2000 2004 2008 201235
40
45
50
55
60
Year ()Year ()Year ()(a) (b) (c)
Eng
ine
disp
lace
men
t (l)
2000 2004 2008 20121.8
1.9
2
2.1
2.2
Figure 2. Evolution of the averaged fuel consumption (a),
specific power (b), and engine displacement (c) for the light-duty
vehicles withturbocharged direct injection compression ignition
engines marketed in Spain (19992013).
less than 5.5 L/100 km, a level markedly lower than that ofan
equivalent vehicle with a SI engine. Moreover, the tech-nology
breakthrough has pushed the specific power of CIengines beyond 50
kW/L, strongly reducing the performancegap with their competitors.
It should be also noted that duringpast decades, the engines have
suffered an impressive reduc-tion in pollutant emissions of around
a 95% as a boundarycondition that adds value to the significant
improvement inperformance.
2 MAIN CHARACTERISTICS OF DIESELENGINES
2.1 Basic operation of CI engines
Compared with the SI engine, the basic difference of thediesel
engine is the ignition and subsequent combustion ofthe fuel. During
the intake process, only air (or air mixedwith burnt gassee Section
8) is induced into the cylinder.The start of the combustion process
is launched by injecting
fuel directly into the combustion chamber at some instantclose
to the end of the compression stroke. The compressionstroke has
raised density and temperature of the gas and thepresence of oxygen
provoke the auto-ignition of the fueltypically shortly after the
start of the injection, and longbefore the end, so that the
combustion process takes placeat the same time as the
injection.
As the fuel is injected directly into the combustionchamber at
the end of the compression stroke, the fuelmixing with air has as
very short time to happen. Conse-quently, the injection system must
be able to distribute thefuel across the chamber, for optimally
utilizing the mostof the air. In case of using some liquid fuel,
which is themost common case, the jet should be atomized and
thedrops evaporated, as fast as possible, what requires veryhigh
injection pressure. The faster the rotational speed ofthe engine
is, the shorter will be the available time for theinjection and
mixing process; therefore, in some occasions,the injection process
has to be assisted by the air motionin the chamber (swirl, squish,
and turbulence), typicallyin automotive engines. The swirl motion
in the cylinder is
Handbook of Clean Energy Systems, Online 2015 John Wiley &
Sons, Ltd.This article is 2015 John Wiley & Sons, Ltd.This
article was published in the Handbook of Clean Energy Systems in
2015 by John Wiley & Sons, Ltd.DOI:
10.1002/9781118991978.hces079
-
Compression Ignition Engines 3
generated by the geometry of the intake ports. The squishflow is
produced by the bowl-like combustion chamber inthe piston, when the
piston approaches the top dead centerand forces gases into the
bowl. Turbulence can be generatedby the same squish motion or using
a pre-chamber in thecylinder head (indirect injection system). More
detail onthese features will be given in Section 3.
Despite all these measures for enhancing the mixingprocess, and
contrary to SI engines, the conventional CIcombustion mode happens
in completely heterogeneousconditions, with the heat release rate
controlled to a greatextent by the injection process (practically
by the diffusionof the fuel in the combustion chamber). This
simultaneousmixing and burning process has some advantages
anddrawbacks that are explained later, with respect to the
SIengine.
2.2 Control of power
In CI engines the fuelair ratio is the independent variableto
control the engine output. The amount of air induced bythe piston
motion or by the boosting system into the cylinderis the maximum
possible, and the amount of fuel injectedis controlled to produce
the required power. This kind ofpower or load control can be called
a qualitative regulation,as the total gas plus fuel mass changes
very little, but itscomposition or fuelair ratio varies in a very
wide rangebetween 1/18 at full load and 1/900 at idle, when gasoil
isused as a fuel. Unlikely to SI engines, the type of
combustionstart by auto-ignition enables the operation of the
engine atsuch extremely low fuelair ratios. The practical low
limiton the fuelair ratio is set by the fuel quantity requiredto
overcome the friction of the engine while the practicalhigh limit
is set by particulate emissions and smoke (Taylor,1985).
A great advantage of this load-controlling strategy,comparing
with SI engines, is that it is not necessary toreduce the induced
air mass flow rate (typically done bychoking the intake with a
throttling valve) and, consequently,the pumping work is smaller and
the engine efficiency atlow and medium loads is higher.
2.3 Maximum power and efficiency
The characteristics of the combustion in CI engines causea
limitation in the maximum speed of this kind of engines,as the
cycle angle needed for combustion tends to largelyincrease with
engine speed. Besides, the characteristics ofthe mixing process in
CI engines cause that they haveto work with poor equivalence
ratios. This means that CIengines cannot use all the air mass to
burn fuel. Both
restrictions cause that SI engines produce higher specificpower
(power per cylinder capacity) than CI engines. Thismeans that CI
engines produce less power than an equiv-alent SI engine. This has
limited the use of diesel enginesin fast vehicles, where power to
weight ratio is impor-tant.
However, CI engines do not suffer from the typicalcombustion
abnormalities in SI engines, allowing them tooperate with higher
pressures in the combustion chamber(only limited by mechanical
aspects). This means thatCI engines can operate with higher
compression ratiosthat are a potential for obtaining better cycle
efficiency.Moreover, CI tolerate higher boost pressure levels
byturbocharging, which can compensate their lower specificpower
compared with SI engines and contribute to evenbetter
efficiency.
2.4 Pollutants formation
Regulated pollutant emissions in CI engines are basically
thesame as in SI engines: unburnt hydrocarbon (HC), carbonmonoxide
(CO), and NOx, with the addition of soot or partic-ulate matter
(PM).
Because, as commented, CI engines operate with less
thanstoichiometric global equivalence ratios, the emission of HCand
CO is smaller than in the case of SI engines, and ingeneral this is
not a huge problem in the conventional dieselcombustion (CDC).
However, the mixing-controlled combustion leads to reac-tion
conditions in local stoichiometric conditions that leadto high
local temperatures, with a trend to form either NOxor soot,
depending respectively on the excess or shortageof oxygen in the
surroundings of the flame. These forma-tion mechanisms are much
more complex, and next sectionspresent some more details, but here
it can be stated that thetrend to NOx and soot emissions is much
stronger than inSI engines, being very difficult to reduce both of
them, andappearing the well-known soot-NOx trade-off.
The different type of the direst pollutants and also
thedifferent in-cylinder conditions lead to different strategies
toreduce emissions in CI and SI engines, as it will we
explainedlater.
2.5 Noise emissions
Aside from the same sources of noise that are usual in SIengines
(aerodynamic noise through intake and mechanicalnoises), the
particular combustion mode in CI engine, charac-terized by a rapid
rise in in-cylinder pressure, is responsiblefor the characteristic
knock in some diesel engines.
Depending on the engine operating conditions, thiscombustion
noise can be more or less audible; however,
Handbook of Clean Energy Systems, Online 2015 John Wiley &
Sons, Ltd.This article is 2015 John Wiley & Sons, Ltd.This
article was published in the Handbook of Clean Energy Systems in
2015 by John Wiley & Sons, Ltd.DOI:
10.1002/9781118991978.hces079
-
4 Reciprocating Engines
in general, it is louder and more bothersome, than inan
equivalent SI engine, being this one of the importantobstacles for
passenger car applications. However, thedevelopment of new
injection systems and better combus-tion chamber designs, together
with advanced controlstrategies, has allowed to largely mitigating
the typicalnoise and vibrations levels, making the engine
moreacceptable.
2.6 Present and future technological challenges
Technological evolution of heat engines will be imposed
bysociety through the various regulations and the price of
fuel.Although it can be expected that environmental laws appliedto
industrial and marine engines will be as strict as the
envi-ronmental laws applied to automotive engines, nowadays
thedifferences that exist between both environmental require-ments
produce that challenges for CI engines are slightlydifferent
depending on their use.
2.6.1 Challenges for automotive engines
It can be expected that the interest keeps to further improvetwo
basic aspects:
Reduce emissions of pollutants: Especially those regu-lated
substances such as nitrogen oxides, PM, CO, andunburned HCs.
Increase engine efficiency: On the one hand, trying toreduce the
consumption of fossil fuels, either to preservethe worlds reserves,
either for political strategic orcommercial reasons. On the other
hand, the efficiencyimprovement is possibly the most direct way to
reduceCO2 emissions, one of those responsible for the green-house
effect.
In the case of automotive engines, a user requirementis that the
car must be also fun to drive. Technical aspectsto consider are the
power delivery and torque, vibration,noise, and so on. An
additional objective is always reducingmanufacturing and
maintenance costs. However, in thecurrent market situation, these
have a second role in compar-ison to the needs of increased
performance and reducedemissions.
2.6.2 Challenges for industrial and marine engines
The main challenges in the near future are:
Reducing the fuel consumption by increasing the
engineperformance.
Reducing the manufacturing and maintenance costs.
It may be remembered that the possible tightening of
anti-pollution laws applicable to industrial and marine engineswill
cause that the emission reduction will be also an impor-tant
particular demand for this type of engines. However, thisdemand is
more an economical challenge than a technolog-ical challenge, as
the pollutant abatement measures are wellknown and validated in
automotive applications.
2.7 Strategies to overcome CI engine challenges
Strategies applicable to CI engines can be separatedaccording to
the main objective aimed at improving engineefficiency or reducing
pollutant emissions. This situationarises from the fact that the
measures to improve effi-ciency and the ways to reduce emissions
are very oftenincompatible.
Some strategies to improve efficiency are:
Optimization of the thermodynamic cycle: The mainway to achieve
it in CI engines is using new injectionstrategies. Thanks to
implementation of electronics inthe injection system, the injection
process can be adaptedwith high flexibility to every engine
operation mode,for instance, splitting the injection event into
severalshots, or modulating the flow rate of the injected fuel.In
addition, variable valve actuation (VVA) systemsallow changing the
basic processes such as shorteningthe compression stroke for
approaching to a Millercycle.
Reduction in the mechanical losses: Focusing inreducing the
friction between elements, for example,with new lubricants and
changing plain bearings bymore sophisticated ones.
Global energy management: In relation with automotiveengines,
whose operating conditions are fully variable,a strategy is to
obtain always the optimum tempera-ture of the engine by improving
the cooling manage-ment. Moreover, a very interesting strategy is
to recoverheat energy lost through the cooling system and
theexhaust system. For this, it is possible to install a turbinein
the engine exhaust (turbocompound) or thermoelec-tric systems in
order to obtain extra mechanical workor electric energy. This is
applicable to all CI enginesbut, according with Challen and
Baranescu (1999), withmore potential for hybrid vehicles or for
industrial andmarine engines.
Downsizing: This technique consists in reducing the sizeof the
engine (displacement or number of cylinders)while maintaining the
power. For this, higher boost pres-sure and duty cycle conditions
are used. To produce the
Handbook of Clean Energy Systems, Online 2015 John Wiley &
Sons, Ltd.This article is 2015 John Wiley & Sons, Ltd.This
article was published in the Handbook of Clean Energy Systems in
2015 by John Wiley & Sons, Ltd.DOI:
10.1002/9781118991978.hces079
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Compression Ignition Engines 5
same power, a miniaturized engine will work on oper-ating points
with better performance than a larger one.
Among the strategies to reduce emissions include:
Using new fuels: There are two reasons for the searchfor new
fuels, which are the strategic interest in reducingdependence on
oil as energy source and the aim to reduceCO2 emissions. Among
developing new fuels is foundbiofuels, low carbon fuels, or
gasoline-gasoil mixtures(see Section 6).
Exhaust gas recirculation (EGR): Recirculation ofexhaust burned
gases to intake gases aims at reducingemissions of nitrogen oxides
(NOx) owing to a decreasein the combustion temperature. It is a
necessarytechnique in CI engines (see Section 8).
Aftertreatment system: In CI engines, there is not auniversally
adopted technique to reduce pollutant emis-sions. The differences
are in mitigating the productionof particles or the NOx production
and remove the othercontaminant through a post-treatment system.
Particu-late traps or particulate filters (DPF, diesel
particulatefilter) are used to remove particles and reduction
cata-lysts are used for NOx. It is also often included an
oxida-tion catalyst to remove small amounts of CO and HCs(see
Aftertreatment Technologies: State-of-the-Art andEmerging
Technologies).
New combustion modes: New combustion modes arean internal
procedure to reduce particle and NOx emis-sions avoiding their
formation. The key to reduce NOxemissions is to produce low
temperature combustion(lower than 2200K), while to prevent the
formation ofsoot is necessary that the combustion occurs with
poorfuel ratios. However, the advantage of the
simultaneousreduction of NOx and soot is opposed by the tendencyto
a higher emission of CO and unburned HCs, and atendency to produce
more combustion noise. However,the main problem of these combustion
modes is the lowperformance if the auto-ignition is not well
controlled(see Section 4).
Several of these strategies will be explained in more detailin
the following sections in this article.
3 INJECTION
3.1 Requirements of the injection systems
As already commented, the fuelair mixture formation
andcombustion processes are closely related in CI engines, andin
various cases, they occur simultaneously. This lays a set of
limitations and requirements for the fuel injection system
andmixing process so to guarantee the appropriate conditionsfor the
mixture and combustion process. In general, theinjection system
must meet certain demands and bounds thatdetermine the limits to
which the system must be designedto operate:
The injection event must be appropriately timed to theangular
position of the engine and the piston speed.
The fuel mass injected must be controlled in terms oftotal mass
and instantaneous mass flow rate so to prop-erly control the
combustion process.
The injection system must contribute to enhance the fueldelivery
and mixing process.
Injection systems in CI engines can be separated in twomain
concepts: indirect and direct injection systems. In thecase of
indirect injection systems, the combustion chamber isseparated in
two volumes: the pre-combustion chamber andthe main chamber; both
are connected by a small aperture.Piston displacement moves gases
from the main chamberinto the pre-combustion chamber in a highly
swirling andturbulent motion, so gases mix with the fuel being
injected.The gas velocity field plays the key role in the
mixingprocess, and fuel spray characteristics are not so
important;fuel injection pressures can be relatively low and
injectordesigns can be kept simple.
In the case of direct injection systems, on the other hand,fuel
is injected directly into the main combustion chamberwhere the
mixing and combustion occur. The air motion inthis type of chambers
is not as intense as in indirect-injectionsystems, and the injector
plays a major role in the mixingprocess. Therefore, fuel must be
injected at considerably highpressures (HPs), to be conveniently
atomized and spread inthe chamber so to guarantee the appropriate
local conditionsfor the combustion process.
The main advantages of an indirect injection system
aresimplicity and low cost in both design and manufacturing.As, in
these systems, the injection hardware is not determi-nant to the
combustion quality, the design is simple, injec-tion pressures are
low, and general requirements of thissort permit reliability,
serviceability, while reducing produc-tion costs. They also present
advantages regarding combus-tion noise and particulate emissions,
as the combustionprocess is turbulence-controlled and an adequate
mixing isnot difficult to achieve. For these reasons,
indirect-injectionsystems were dominant in the passenger-car market
for manydecades.
However, direct injection systems present valuableenhancements
regarding fuel consumption, general combus-tion timing, and
development control. Even thoughthe hardware is considerably more
complex in both
Handbook of Clean Energy Systems, Online 2015 John Wiley &
Sons, Ltd.This article is 2015 John Wiley & Sons, Ltd.This
article was published in the Handbook of Clean Energy Systems in
2015 by John Wiley & Sons, Ltd.DOI:
10.1002/9781118991978.hces079
-
6 Reciprocating Engines
design and manufacturing, mass production and yearsof
development have decreased costs to the point thatthe advantages of
these systems significantly out-weightdrawbacks.
These systems have been part of the evolution processthat leads
to electronic control and HP turbocharging.These features
considerably increase power output andreduce fuel consumption and
emissions for a given enginecapacity, thus, they have triggered the
current trend ofengine downsizing. As emission regulations get
moredemanding and fuel consumption standards constantlydecrease,
the automotive industry has strongly movedtoward the electronically
controlled, turbo-charged, directinjection systems.
3.2 Direct injection systems
Various types of direct injection systems have been devel-oped
to meet the particular requirements of each application.Mainly,
direct injection systems can be divided into directaction systems
or accumulation systems.
Direct action systems are those injection systems in whichfuel
delivery is controlled by the HP pump and the injectorjust atomizes
the fuel to create a spray, they are commonlyknown as
pump-line-nozzle systems (Heywood, 1988). Thesesystems consist
mainly of a cam-driven pump, an HP line,and the nozzle. The
injection pressure is proportional to therotational speed of the
fuel pump and thus, the engine, and itis not constant along the
injection event. The actual injectiontiming is controlled by the
phasing of the cam in respect to the
crankshaft, and the start of injection occurs with the
injectionpressure rise, which has to overcome a preloaded spring to
liftthe needle and open the injector nozzle. The fuel
pressure-level control, fluctuations along the injection event, and
poorcontrol of the injection timing are the main disadvantages
ofsuch systems.
These direct action systems were the first type of
directinjection systems implemented, but they have been replacedby
accumulation systems in which the injector controls boththe fuel
delivery and atomization. In accumulation systems,the HP pump
builds pressure that is not immediately relievedbut accumulated, as
the nozzle opening is independentlycontrolled by the injector.
The first of these systems to be introduced is the so-called
pump-injector. In this system, the fuel pump and theinjector are
confined to a single unit bolted to the cylinderhead and driven by
the camshaft. Each unit has its ownsolenoid valve that controls the
injection event timing andduration. Considering that the
pump-injector system offersa great number of advantages over the
pump-line-nozzlesystem, it still lacks features that the ever-more
demandingfuel consumption and exhaust emission standards
require.For instance, although injection timing is controlled
electron-ically, the pressure build-up is still cam-driven and
phased tothe crankshaft position, and this complicates the
implemen-tation of multiple injections per combustion stroke and
thepressure-level control.
The common rail system has become the standard injec-tion system
in light-, medium-, and partially heavy-dutyapplications (Flaig,
Wilhelm, and Ziegler, 1999). Figure 3depicts a standard common rail
system.
Low pressure fuel pump
High pressurefuel pump
Rail pressuresensor
Fuel tank
Fuel filter
Fuel injectors
Crankshaftposition
Camshaftposition
TPS MAP IAT ECT
Fuel pressureregulator
Common-rail
ECU
Figure 3. Main components of a standard common rail injection
system.
Handbook of Clean Energy Systems, Online 2015 John Wiley &
Sons, Ltd.This article is 2015 John Wiley & Sons, Ltd.This
article was published in the Handbook of Clean Energy Systems in
2015 by John Wiley & Sons, Ltd.DOI:
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Compression Ignition Engines 7
Common rail injection systems are constituted by a fueltank, low
pressure (LP) and HP pumps, a fuel rail, an elec-tronic pressure
regulator, common rail injectors, various HPand LP fuel lines and
the electronic control unit (ECU)with its wiring harness and
sensors. The HP fuel pump isdriven by the engine and builds
pressure that is stored in thecommon rail at a constant level. The
pressure level is elec-tronically controlled by the pressure
regulator that bypassesexcess fuel back to the tank, depending on
the pressure setpoint. All injectors are fed from the common rail
throughHP lines and as their actuation is hydraulic, more fuel
thanwhat is injected is needed to drive each injection event.Excess
fuel flows back to the tank through LP return lines.The actual
injection timing and duration is controlled by theECU, which
interpolates values from pre-programmed mapsdepending on the
reading of several control signals. Typicalengine control signals
are those obtained from the crankshaftposition sensor (engine
speed), camshaft position sensor(engine phase in respect to the
four-stroke cycle), throttleposition sensor (TPS), manifold
absolute pressure (MAP),intake air temperature (IAT), and engine
coolant temper-ature (ECT), but many other may be utilized for
furthercalibration.
The injector is certainly the most complex component ofthe
common rail system. A cutaway of a typical common railinjector is
depicted in Figure 4.
This type of injector uses the HP generated by the pumpas a
source of energy to lift the needle or keep it against its
12 3
4
5
6
7
Figure 4. Section view of a typical common rail injector. (1)
Highpressure fitting, (2) fuel filter, (3) control valve, (4)
injector body,(5) needle spring, (6) nozzle, and (7) needle.
seat. This hydraulic control of the injector is the key as
itonly requires a small quantity of energy to operate whilea direct
action on the needle would require hundreds oftimes more.
In newer injector generations, the solenoid has beenreplaced by
a piezoelectric system that offers a better controlfor smaller
injection timings and presents a faster response,thus potentially
increasing the number of injections percycle and timing control
precision.
The common rail injection system presents the sameadvantages of
the pump-injector, but as pressure is constantlybuilt up in the
rail, features such as multiple injectionsare much easier to
implement in comparison to the pump-injector system. In addition,
as the ECU is monitoring a largeset of control signals, a group of
control and correction strate-gies have been developed to help with
fuel consumption,emissions, noise, driveability, and so on.
3.3 Spray structure and development
The very end of the injection system is the nozzle. Theorifice
geometry determines the flow inside the nozzle and,therefore, the
behavior of the flow at the outlet, entering thecombustion chamber.
The main parameter of an injectionnozzle is the discharge
coefficient, which is dependent oninternal features of the orifice
such as lengthdiameter ratio,convergence of the orifice, and
entrance radius.
The phenomenon of cavitation, which reduces thedischarge
coefficient of the injection system, may occurunder certain
conditions but can be controlled or canceledwith the appropriate
internal design of the nozzle orifices.The conditions of the flow
at the outlet, velocity, turbulence,cavitation, and so on determine
the behavior of spraydevelopment (Payri et al., 2008).
When penetrating in the combustion chamber, the liquidflow
injected at high velocity encounters the ambient gasesthat are
comparatively still. Figure 5 depicts the macroscopicspray
structure. Owing to aerodynamic forces principally, theliquid core
atomizes into liquid structures during the firstbreakup process
(primary atomization) and into small andround droplets with the
second breakup (Reitz and Bracco,1986).
Depending on the temperature of the ambient gases, thespray may
experience evaporation. In the case of CI engines,temperatures are
high and thus evaporation occurs and playsa key role. In the
evaporative spray, the liquid spray reachesa certain distance from
the nozzle [referred to as liquidlength, (Payri et al., 2008)] and
then penetrates further inthe chamber as a gas jet. The
characteristics of the spraydepend mainly on the density of the
ambient gases butalso on spreading angle and momentum flux of the
sprayitself.
Handbook of Clean Energy Systems, Online 2015 John Wiley &
Sons, Ltd.This article is 2015 John Wiley & Sons, Ltd.This
article was published in the Handbook of Clean Energy Systems in
2015 by John Wiley & Sons, Ltd.DOI:
10.1002/9781118991978.hces079
-
8 Reciprocating Engines
Fuelflow
Dense spray
Injectornozzle
Liquidcore
Detachment of ligaments(primary atomization)
Formation of dropletsfrom ligaments
(secondary atomization)
Dispersedflow
Dilute spray
Figure 5. Illustration of the macroscopic structure of the
direct injection diesel spray.
As the spray penetrates the chamber, a momentumexchange occurs
between the spray and the ambient gas.This means that the spray
causes air entrainment thatenhances atomization, mixture rates, and
quality. As it isthe spray momentum that causes the air entrainment
andmixing, the injection pressure level is key and thus it hasbeen
increased in the past decade (in some cases up to250 MPa) to
address this subject.
3.4 Injector and spray control strategies
As stated earlier, fuel atomization and air entrainment
arecontrolled by the injector. Reducing nozzle diameter
consid-erably enhances atomization and mixing, so nozzle diam-eters
have been continuously reduced and diameters of80 m are now
commercial. Consequently, this decreasesnominal mass flow rate so
multi-orifice nozzles from 5 to11 orifices have been studied.
Finding the optimal orificediameter and orifice number combination
is a very complexproblem, which depends on a large group of factors
and theoptimal combination may be very particular for each
appli-cation.
Increasing injection pressure helps to maintain targetmass flow
rates when decreasing nozzle diameter and alsoincreases atomization
quality and air entrainment. Injectionpressures have been also in
rise, and currently, injection pres-sures of up to 300 MPa are
being studied.
Current standard control strategies present multipleinjections
per combustion event. Complete studies onmultiple injections can be
found in the works of Flaig,Wilhelm, and Ziegler (1999) and of
Mendez and Thirouard(2008). With current direct injection engines,
whichexhibit high compression ratios, multiple early
injectionscalled pilot injections are added in order to reduce
the
combustion noise. The noise reduction occurs owing tosplitting
the heat release process, which decreases thepeak heat release. It
is achieved using several injectionsin the appropriate
thermodynamic and auto-ignition delayconditions in order to reduce
the instantaneous fuel burningrate. Moreover, in some operating
conditions, a late injec-tion (usually referred to as
post-injection) may also beemployed during the expansion stroke,
for after-treatmentpurposes.
Multiple injection strategies can also be used to bettercontrol
the spatial fuel distribution to enhance the air use inthe
combustion chamber. Generally, this effect can lead toa reduction
in particulate emissions at intermediate engineloads, allowing for
potentially higher EGR rate. An illustra-tion of the objectives of
every shot is represented qualita-tively later in Figure 22.
It is important to point out that both the actual nozzleopening
and particularly the actual nozzle closing presentsignificant time
delays in respect to their control signals,so this must be
accounted for. Piezoelectric control valveshelp in this regard,
decreasing response times. This is espe-cially important in very
short injections such a pilot orinjections, where the needle never
reaches full lift. For thisreason, a full injector characterization
is common duringthe development phase of a particular engine.
Figure 6illustrates such injection events in a real case (two
pilots,one main, and two post-injections), where both the
injectorcommand electrical current and the actual injection rateare
plotted. It can be observed how there is a nonnegli-gible delay
between the command and the real injectionevents.
The main contribution to the heat release comes fromthe main
injection, which is commonly the longestinjection per combustion
event. The longer the injection
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-
Compression Ignition Engines 9
20
10
0
20
30
40
10
0
0 1 2 3 4 5 6
Time (ms)
Mas
s flo
w r
ate
(g/s
)In
tens
ity (
A)
Figure 6. Real plots of command electrical current and
injectionmass flow rate in a case with two pilots, one main, and
two post-injections events.
duration is, the larger will be the total heat release andthus
the torque output. As exposed earlier, the actualtiming (in respect
to the crankshaft or piston position)and duration of the main
injection are instructed bythe ECU, which interpolates these values
out of pre-settwo or three dimensional look-up tables. The
injectionduration and timing depend mainly on driver torquedemand,
engine rotational speed and phase, but a largeset of correction
factors may be applied to account forthe effects of variables such
as intake air pressure andtemperature, coolant temperature,
electric system voltage,current gear selection, and transient
effects such as suddenacceleration.
3.5 Probable future improvements
The near future of direct injection systems is the
furtherdevelopment of the successful common rail system.
Next-generation systems could feature injection pressures up to300
MPa, for instance. Another innovation in current devel-opment for
this system is the ability to control the needlelift in a
continuous manner. Current common rail injectorsoffer only full
lift or close conditions, but direct-acting piezo-electric
injectors that permit partial needle lifts are beingstudied. This
enables not only multiple injection rate possi-bilities for a
single injector (through partial needle lifts) butalso injection
rate shaping, both of which open a series ofpossibilities for
combustion control that could lead to next-generation fuel
consumption and emission commercial stan-dards. Figure 7
illustrates the real operation of a direct-acting
60
50
40
30
20
40
20
Mas
s flo
w r
ate
(g/s
)S
pray
tip
pene
trat
ion
(mm
)
10
0
0
0 0.5 1.51 2.52 3
Time (ms)
SquareBoot
Liquid phaseVapor phase
Figure 7. Injection rate shapes (square and boot) produced by
adirect-acting injector and the corresponding spray tip
penetration.Injection is produced in a test rig without wall
impingement.
piezoelectric injection, producing a two-step injection
rateevent (boot-shape) and the corresponding effects in the
spraytip penetration.
Another realistic development for the future of injectionsystems
is the dual fuel setup. Many applications are beingdeveloped where
two fuels are utilized to better controleach phase of the
combustion process and thus enhanceconsumption capabilities and
reduce exhaust emissions. Aninteresting development for heavy-duty
diesel engines is theWestport concept, based on an injector with a
double fuelcircuit, able to inject natural gas and gasoil
simultaneouslyor sequentially (Ouellette and Douville, 2001).
4 COMBUSTION
4.1 Conventional diesel combustion
In the previous sections, it has been already implied that
thecharacteristic combustion in CI engines, based on the burningof
a fuel spray in an oxidizing atmosphere, is a very complexprocess
involving closely interrelated physical and chemicalphenomena.
However, nowadays, the most relevant aspectsof this combustion
process are well known, and a detaileddescription is easily found
in the classic internal combustionengine literature (Heywood,
1988).
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10 Reciprocating Engines
10 0 5 10 15
HR
L/R
oHR
Inje
ctio
n ra
te
20
CAD
Injector controlelectrical signal
Auto-ignitiondelay
Premixedcombustion
Fastdiffusion-controlled
combustion
Late slowdiffusion-controlled
combustion
RoHRHRL
SoI EoI
Injectionrate
SoC EoC
Figure 8. Temporal description of the injection-controlled
dieselcombustion, with the four main stages defined from the
injectionand heat release events (Heywood, 1988).
In the CDC concept, the liquid diesel fuel spray is injectedat
HP into the previously compressed gas trapped insidethe combustion
chamber delimited by the cylinder head,liner, and piston walls.
From this moment, a sequence ofprocesses develop including the
atomization of the liquidvein, the evaporation of the fuel, the
turbulent mixingbetween the fuel and the surrounding gas, and
finally thefuel oxidation.
The usual temporal description of the CDC concept shownin Figure
8 is based on following the time evolution of thefuel injection and
the fuel burning (or the equivalent heatrelease) rates. From the
start of injection, four well-definedsequential stages are easily
identified with different intrinsiccharacteristics.
The first auto-ignition delay stage corresponds to the
timebetween the start of injection and the start of combustion.
Itis during this initial stage when all the physical and chem-ical
processes required to ignite a suitable air/fuel gaseousmixture
happen. Therefore, this stage comprises the phys-ical delay related
to the time spent mainly by the atom-ization and evaporation
processes to generate an ignitablegaseous air/fuel mixture, and the
chemical delay accountingfor the kinetics of the auto-ignition of
this air/fuel mixtureat the given thermodynamic conditions. In
state-of-the-artCI engines, the physical processes are much faster
thanthe diesel auto-ignition kinetics so the auto-ignition
delaystage duration is essentially controlled by the chemicaldelay.
This is the reason explaining the correlation observedbetween this
auto-ignition stage duration and the combustion
chamber thermochemical conditions (pressure, temperature,and
oxygen concentration) according to an Arrhenius expres-sion, being
the temperature the most influential parameter asusual in chemical
processes.
The next premixed combustion stage is in fact closelyrelated
with the previous auto-ignition delay stage as thefuel already
mixed within the auto-ignition limits burns ina very short time, so
the heat release rate usually shows asharp and narrow profile. This
fast energy release results ina sharp cylinder pressure rise. The
fuel quantity burnt inthis premixed combustion stage and then the
total energyreleased depend fundamentally on the duration of the
auto-ignition delay stage and the amount of fuel injected
duringthis time, but also to some extent on the mixing
strengthduring this time.
The third and fast diffusion-controlled combustion stagedevelops
if fuel is still being injected after the premixedcombustion stage.
This condition occurs normally exceptat very light loads, when the
injected fuel mass is verysmall. In this stage, the combustion
process adopts thespatial structure characteristic of a burning
spray flame as itwill be described in detail later. The fuel
burning and heatrelease rates are basically controlled by the
physics asso-ciated to the spray mixing process, which is mainly
drivenby the spray momentum flux, while the chemical
kineticsprocesses are much faster, and are not a limiting
factor.Finally, the late slow diffusion-controlled combustion
startsafter the end of injection, when fuel mixing rate decaysas
the spray momentum flux dissipates and the combustionchamber volume
grows rapidly owing to the piston motionin the expansion stroke.
Consequently, the fuel burning andheat release rates progressively
decrease and the spray flamestructure is lost.
Each one of these stages influences engine
performance,emissions, and noise. Current technologies (boosting,
injec-tion, EGR, and combustion chamber design) change to
someextent the four combustion phases, and thus the
enginebehavior.
4.2 Burning diesel spray structure
The spatial description of the CDC concept was developedmuch
later, in the 1990 decade, by means of the application ofadvanced
optical techniques (Dec, 1997). A recent exampleof the burning
spray visualization by the Schlieren techniqueobtained by the
authors is given in Figure 9.
At the beginning of combustion, during the premixedcombustion
stage, the reaction locates inside the fuel sprayin between the
length where the flame stabilizes in quasy-steady conditions,
widely known as lift-off length, and thespray tip. The local
conditions in terms of equivalence
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-
Compression Ignition Engines 11
(a) (b)
Figure 9. (a) Schlieren sequence of images describing the
temporaland spatial evolution of a reacting diesel spray during the
auto-ignition and (b) the flame stabilization until reaching a
quasi-steadymixing-controlled combustion stages.
ratio where this premixed combustion develops are criticalfor
pollutant formation as NOx and soot formation dependbasically on
this mean local equivalence ratio. In conven-tional diesel
operating conditions, the premixed combustionprogresses in rich
conditions, in zones with equivalence ratiobetween 2 and 4,
although isolated regions with lower orhigher equivalence ratios
can be also observed (Espey et al.,1997).
Along the premixed combustion stage, the diffu-sion flame
enveloping the spray starts to form from thereacting zones. From
here and during the fast diffusion-controlled combustion, the flame
front consolidates, beingsupported by the convective and diffusive
supply of fuel andoxygen.
At this moment, the diesel spray shifts to a quasy-steadystage
in which the general characteristics of the spray andflame
preserve, but their length progressively increases.Nowadays, the
most widely accepted conceptual model fordescribing the diesel
diffusive flame in quasy-steady condi-tions was proposed by Dec
(1997) and completed later byFlynn et al. (1999) to define the
structure shown in Figure 10.
According to this model, it exists a first zone between
thenozzle exit and the minimum axial distance where the
flamestabilizes (lift-off length) in which the conditions are
similarthan those observed for the nonreacting spray. In this
region,all processes related to atomization, air entrainment,
andevaporation take place, but they are affected by the
diffusiveflame evolving downstream.
From the lift-off length, the spray shifts to reacting
condi-tions, beginning by a premixed reaction zone just after
thislift-off length where the oxygen already entrained into
thespray along the first inert zone is consumed. In conven-tional
diesel operating conditions, this premixed combus-tion happens in
rich mixture conditions, at local equivalenceratios about 4, so the
main products are partially oxidizedHCs flowing along the spray and
acting as soot precur-sors.
After this premixed reaction zone, the spray adopts thetypical
diffusive flame structure, with an internal zoneincluding nonburnt
fuel, partially oxidized HCs and soot,all enveloped by the reaction
surface stabilized around thelocal stoichiometric equivalence
ratio. Thus, thermal NOare mostly formed following the thermal path
owing tothe oxygen availability at the periphery of the very
hightemperature flame, while soot precursors appear insidethe fuel
spray owing to both high temperature and lackof oxygen (Dec and
Canaan, 1998). The key parametercontrolling soot formation is the
local equivalence ratio atthe lift-off length; therefore, the lower
this equivalence ratiois, the lower will be the soot precursors
formation duringthe premixed combustion (Pickett and Siebers,
2006). Theequivalence ratio at the lift-off length is controlled
mainlyby the temperature and density of the gas in the chamber,
bythe injection pressure and by the reactivity and
molecularcomposition of the fuel. The oxygen concentration
increasesthe lift-off length but does not affect the local
equivalenceratio.
Finally, after the end of injection and during the
slowdiffusion-controlled combustion stage, the flame progres-sively
loses its structure, the premixed reaction zone disap-pears, and
several pockets of fuel and soot burning indiffusive conditions
form. During this stage, thermal NOis still being formed and soot
is oxidized. Both processesdepend on the rate at which the
combustion chamber gasdecreases its temperature, but following
opposite trends, so
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12 Reciprocating Engines
Maximumliquid length
Lift-off length
100
50
0
2
1
0
0
0Mixture formation
Rich premixedcombustion
(Fr 4)
Diffusionflame
(Fr 1)
T 700 K
T 2700 KT 1600 K
RoHR 1015%R
oHR
(%
)T/
100
(K)
NO
x (r
el)
Soo
t (%
fuel
)
1
30
20
10
0.5
Soot formation zone (YO2 = 0)
Sootprecursorsformation
Post-flame
Fuel-rich premixed flameInitial soot formation
Soot oxidation zoneThermal NO production zone
Figure 10. Sketch of the structure of the quasi-steady flame
during the fast mixing-controlled combustion stage according to the
conceptualmodel described by Dec (1997) and Flynn et al.
(1999).
this explains the NOx and soot trade-off characteristic ofdiesel
engines.
From previous description of the CDC concept, presenttrends in
diesel engine design are evident, so the current tech-nology
include a pilot injection or rate shaping to controlauto-ignition
delay and the premixed combustion stage inan attempt to decrease
cylinder pressure gradients and noise.Concerning the NOx and soot
emissions control by internalmeasures, the path followed is based
on introducing externalcooled EGR to control NOx by reducing the
oxygen concen-tration of the gas inside the combustion chamber,
slowingdown the chemical reactions involved in the thermal
NOformation. This action promotes soot emissions by wors-ening late
soot oxidation, so it should be counterbalancedwith other measures
such as decreasing nozzle orifice diam-eter and increasing
injection and boost pressures to enhancethe soot late oxidation
processes.
Aside from these strategies, new advanced combustionconcepts are
being investigated with the aim of avoidingthermal NO formation as
usual, but also controlling soot
by avoiding its formation. These combustion conceptsare still
far from being applied in production engines,but great research
efforts are being carried out owingto the impressive results
reported in terms of pollutantcontrol.
4.3 New combustion modes and their challenges
Looking at the combustion process from the local equiva-lence
ratio and temperature conditions inside the combustionchamber as
shown in Figure 11 (Kamimoto and Bae, 1988),it is clear how
different suitable options arise for avoidingboth NOx and soot
formation processes. A comprehensivereview of the advanced
combustion concepts recently devel-oped in the frame of CI engines
is already available inthe literature (Dec, 2009; Musculus, Miles,
and Pickett,2013).
Research works performed in the past two decades haveconfirmed
how promoting a lean premixed combustion bydetaching the fuel
injection event from the combustion
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Compression Ignition Engines 13
10
9
8
7
6
5
4
3
2
1
0600 1000 1800 2200 2600
Temperature (K)
Equ
ival
ence
rat
io (
)
30001400
CDC
MC-LTC
HPC HCCI
COoxidation
Soot
NOx
Figure 11. Schematic description of new combustion modes interms
of local conditions plotted in the equivalence ratio
versustemperature map introduced by Kamimoto and Bae (1988).
process is an interesting alternative for reducing these
pollu-tant emissions. This combustion concept based on
attainingsufficiently lean and homogeneous local equivalence
ratios,well below the stoichiometric value, is widely known
ashomogeneous charge compression ignition (HCCI). Thislean
combustion slows down or even avoids the chemicalreactions leading
to thermal NOx formation owing to thedrastic reduction of the local
temperatures inside the combus-tion chamber, while soot formation
is also hindered by theabsence of high local equivalence ratios
during the combus-tion process.
The injection strategies commonly reported in the litera-ture as
suitable for implementing a highly premixed combus-tion (HPC)
concept, with different levels of local air/fuelmixture
homogeneity, are the port-fuel injection, where thefuel is injected
at the intake port and mixes with the air beforeentering into the
cylinder, and the direct injection charac-teristic of current CI
engines. However, despite producing aperfectly homogeneous lean
air/fuel mixture, port fuel injec-tion of usual fuels for CI
engines is not a realistic alternativebecause of its limited
efficiency, high HC and CO emissions,early onset of the combustion
process, lack of combustionphasing control and high noise. In
addition, as diesel fuelshave poor evaporation characteristics,
they create a wall filmthat does not evaporate from the intake port
walls becausethe temperatures there are not high enough.
The direct injection strategy comprises two different
alter-natives suitable to produce an HPC, consisting of
injectingthe fuel early during the compression stroke or late
duringthe expansion stoke. In the late direct injection
alternative, asin the modulated kinetics (MK) or the highly
premixed lateinjection (HPLI) concepts, the injection is placed
just afterthe TDC and the fuel should ignite also relatively close
tothe TDC as displacing the combustion toward the expansionstroke
produces combustion instability, high levels of COand HC, and the
sharp decrease on engine efficiency causedby a delayed combustion
phasing observed in Figure 12(Benajes et al., 2004). Then, the
practical application of thelate direct injection alternative is
limited by the availablemixing time and the high sensitivity of the
engine efficiencyto combustion phasing, especially at high engine
speed orloads, where it requires an extremely fine tuning and
controlof different engine parameters, such as the EGR rate and
theswirl level.
In the early injection alternative, the injection event canbe
arbitrarily advanced toward the compression stroke whilecombustion
starts relatively close to the TDC, increasingthe mixing time
available for producing a suitable premixedcombustion without
intrinsically compromising the engineefficiency. However, injection
timing is usually set close tothe TDC as in the case of the
premixed charge compressionignition (PCCI) concept, and the lack of
homogeneity causedby a shortened mixing time is compensated by
introducingEGR to reduce the temperatures in those zones of the
mixturethat reacts in locally stoichiometric combustion. This
earlydirect injection represents the most promising alternative
forimplementing the HPC concept, as it is also confirmed bynumerous
investigations reported in the literature. However,the HPC concept
attained by advancing the injection timingis still under
investigation as it presents important challengesmainly related to
avoiding liquid fuel impingement onto thecylinder liner surface,
controlling the combustion phasingand burning rates, and extending
the range of operation ofthe concept in terms of engine load.
Figure 13 evidencesthe differences between the burning rates
generated with theCDC and the early injection HPC concepts, which
are muchshorter and faster.
As discussed, HPC concepts have been widely investi-gated as
combustion technologies to avoid soot and NOxengine-out emissions.
However, despite the research effortsand promising results obtained
by means of these HPC strate-gies, ignition timing control and load
limits are still the mainchallenges for its practical application.
Owing to these draw-backs, the mixing-controlled low temperature
combustion(MC-LTC) strategy arises as an alternative to overcome
thelack of ignition timing control of the highly premixed
strate-gies as well as the NOx-soot trade-off characteristic of
theconventional diffusive combustion.
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14 Reciprocating Engines
0
4
8
12
16
6 7 8 9 10 11 12 13 14
sNOx (g/kWh)
0
0.01
0.02
0.03
0.04
0.05
Dry
soo
t (g/
kwh)
BS
FC
(%)
2 aTDC
4 aTDC
51%
Figure 12. Pollutant emissions and fuel consumption trends
observed while retarding the injection event for achieving a late
injection HPCconcept. Source: Reproduced with permission from
CMT-Motores Termicos.
20 10 00
10 20 30 40
40
80
120
160
200
RoH
R (
J/ca
d)
Crank angle (cad aTDC)
Diesel low NOx
Diesel high NOx
PPC gasoline triple injection
Figure 13. Different RoHR profiles comparing the CDC concept for
low NOx (with DeNOx catalyst), CDC concept for high NOx
(withoutDeNOx catalyst), and early injection HPC concept.
Three different alternatives to attain
mixing-controllednon-sooting low flame temperature diesel
combustion havebeen reported from the research results obtained in
anoptically accessible, quiescent constant-volume combustionvessel
(Pickett and Siebers, 2004). The first is based on theuse of
reduced nozzle hole diameters; the second consists ofsharply
decreasing the ambient gas temperature; and the thirdneeds the use
of extensive EGR to reduce the gas oxygenconcentration (YO2) as
shown in Figure 12.
Different investigations confirmed the feasibility of theMC-LTC
concept for avoiding NOx and soot emissionsformation in an HSDI
diesel engine (Benajes et al., 2010),as shown in Figure 14. The
MC-LTC concept was imple-mented with success by introducing massive
EGR rates, sofollowing the third alternative, but the sootless and
zero-NOx
combustion process was proven to intrinsically generate
highlevels of HC and CO emissions, together with lower
engineefficiency.
5 POLLUTANT EMISSIONS
5.1 Regulated pollutants in CI engines
The main contribution of pollutant emission from anengine is due
to exhaust gases released to the atmo-sphere, especially in CI
engines running on little volatilefuels. Health studies show that
exposure to diesel exhaustprimarily affects the respiratory system
and worsensasthma, allergies, bronchitis, and lung function. There
is
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2015 by John Wiley & Sons, Ltd.DOI:
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Compression Ignition Engines 15
0.00
2
4
Tint = 40C
611%O2
12%O210%O2
9%O2
8
Soo
t (g/
kgfu
el)
0.5 1.0
NOx (g/kgfuel)
40 kg/m3
30 kg/m3
26 kg/m3
35 kg/m3
Figure 14. Pollutant emissions trends observed during the
imple-mentation of the mixing-controlled LTC concept. Source:
Repro-duced from Benajes et al., 2010. Elsevier.
some evidence that diesel exhaust exposure can increasethe risk
of heart problems, premature death, and lungcancer.
The combustion process produces many substances thatfind their
way to the atmosphere, but during normal opera-tion, the proportion
of those considered toxic is very smallcompared with the rest of
products from the clean combus-tion (Figure 15). In addition to
this, very few of thesesubstances are considered legally pollutants
and regulated bythe standards (Turns, 1996).
Non pollutants substances. Water (H2O), carbon dioxide(CO2), and
oxygen appear in clean combustion. ConsideringCO2 as a not
polluting gas is questionable, as it is themain potential precursor
of the so-called greenhouse effect.In the cases of incomplete
combustion, hydrogen (H2) isformed too.
Regulated pollutants. Their origin varies greatly. Theincomplete
combustion produces CO and unburned HC.
0
0.2
0.4
0.6
0.8
1
(%)
0.85
0.08 0.050.005
NOx CO HC PM
Figure 16. Typical composition of pollutant emissions in a
dieselengine.
There may also be oxidation products of the intake airnitrogen
(NOx), and pollutants from fuel sulfur (SOx).Finally, there is PM,
containing solid (ISF) and solubleorganic fractions (SOF) of
particles from elemental carbonformed during combustion.
Figure 16 shows the typical percentage of the more impor-tant
pollutants in the exhaust gas of a light-duty diesel
enginefollowing one of the standard cycles.
The increasing importance in reducing pollutants emissionfrom CI
engines has been stronger on automotive and heavy-duty
transportation engines, owing to their greater numberand proximity
to living beings. Other engines, such as thosein railway or marine
applications, are bounded by less severelimitations.
5.2 Pollutants formation
The basic pollutant formation chemistry is very similar inSI and
CI engines, but the global operating conditions andlocal phenomena
are very different, owing to the typicalmixture formation and
combustion processes (Heywood,
Nitrogen67
Carbon dioxide12
Pollutants1
Water11
Oxygen9
Figure 15. Typical composition of diesel combustion
products.
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16 Reciprocating Engines
Soot41
HC32
SOx + H2O14
Others13
Figure 17. Typical composition of particulate matter.
1988). Therefore, while in a CDC, CO and unburned HCsare not
very problematic, NOx and soot or PM are the mainchallenges.
5.2.1 Nitrous oxides (NOx)
In CI engines, NOx formation is due mainly to the
so-calledthermal mechanism, caused by the high local
temperaturesduring combustion process and lean mixtures with
excessof oxygen. It leads in the oxidation of the nitrogen of
air.As, in the combustion chamber of the CI engine, thereare wide
regions with lean mixture, NOx formation is verysensible to the
increase in combustion temperature. Hence,all the measures that
produce an increase in the gas temper-atures (high compression
ratio, turbocharging) or in the rateof heat release (high injection
pressure, advanced injectiontiming, and so on) will probably
produce an increase in NOxemissions.
5.2.2 Soot and particulate matter
Soot is basically carbon particles of certain size and color
thatmake them visible. PM is a more general term that includessoot
(visible or not), but also other small particles (solidor liquid).
The older emissions standards used the opacityof the exhaust gases
as an indirect measurement of sootconcentration, while current
regulations focus on the mass,number, and size of the particles
collected by some filteringsystem.
Soot emission is the final result of a formation phasefollowing
by an oxidation process. The formation isproduced mainly by a very
rich mixture entering the flameat the lift-off section of the
diesel spray (see Figure 10).The high temperature and default of
oxygen leads to adehydrogenation of the HCs. If the resulting soot
particlesare not burned later when they cross the flame around
thespray envelop, they will exit the engine. In CI engines,
soot is produced mainly when global mixture is very
rich(excessive fuel injected), or when the mixing conditionsare bad
(low injection pressure, low in-cylinder gas density,injector
malfunctioning, and so on).
Soot particles or PM in general are the result of
complexphenomena of agglomeration and nucleation, but also
ofadsorption of other substances in their surface. Figure 17shows
the typical composition of the particles emitted in CIengines.
In general, those conditions that lead to a reduction in
NOxemissions produce an increase in soot and PM, as it will
beillustrated later.
5.2.3 Carbon monoxide (CO)
The generated CO at the end of the diesel combustiondepends on
the balance between formation processes (fastreactions) and
oxidation (slow reactions), being both veryactive at high
temperatures. In general, if temperature is highenough, the main
cause for high CO emissions is the exces-sively rich mixtures, that
is, the default of oxygen. This isnot a common situation in CI
engines that operate with leanmixtures, with excess of oxygen, but
a small CO amount canbe produced as the recombination process has
some inertiathat there is not enough time for the entire CO to
oxidize toCO2, as expansion and exhaust processes are relatively
fast.In general, the CO emission in CI engines is smaller than inSI
engines. A different situation appears in the case of CIengines
operating at any of the low temperatures combustionmodes,
especially in HPC. In these circumstances, the exces-sively lean
mixtures and the low combustion temperaturesare responsible for
high CO emissions.
5.2.4 Unburned hydrocarbons (HC)
In diesel engines, the formation of HC takes place mainlyby
incomplete combustion in those inner spray regions with
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article was published in the Handbook of Clean Energy Systems in
2015 by John Wiley & Sons, Ltd.DOI:
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-
Compression Ignition Engines 17
HC
CO
Soot
NOx
Concentration (ppm-Vol) Soot (g/m3)
0.15 0.3 0.45 0.60 10
0.05
0.1
0.151500
1000
500
0
Figure 18. Typical trends in pollutant emissions as a function
ofthe global equivalence ratio.
very rich mixture, and that cannot be oxidized later owingto
defective mixing or reduction in the chamber temperatureduring
expansion stroke. Another eventual source of HC isthe impingement
of the spray on the piston, especially if thefuel wets the
piston/cylinder walls. Aside from the gaseousemission of HC, some
HCs can be adsorbed in the particlematter after condensation on the
particles surface, adheringto them and being included in their
structure.
One way of globally understanding the pollutant formationtrends
in CI engines is representing the emission concentra-tion as a
function of the global equivalence ratio, as repre-sented in Figure
18. The plotted trends evidence that thereis not an optimal range
of equivalence ratio, where all theemission are low, except perhaps
at very lean mixtures, whichcorrespond with low load operating
conditions of the engine,being CO and HC relatively high in this
zone.
As commented earlier, smoke opacity was substituted byPM mass as
the evaluating parameter for assessing the envi-ronmental impact of
CI engines. However, the hazard onhealth is more linked to the
particulate size than on the totalmass. Smaller particles are more
dangerous, as they staylonger suspended in the air, and after
inhalation, they reachdeeper in the airways. The typical size of
the particles emittedfrom a diesel engine varies from a few
nanometers to about30 m (Giechaskiel et al., 2014). Figure 19 shows
a typicalsize distribution of particles and their contribution to
totalmass. It can be observed that the many small particulateshave
a small share in the total mass.
5.3 Present and future trends in emissionsreduction
As already commented, there is not an easy way of
reducingsimultaneously the generation of all the emission fromCI
engine by controlling the usual operating conditions.Moreover, some
of the actions that lead to the reductionof a particular pollutant
may have a negative impact onfuel consumption or on engine noise
and durability (Heck,Farrauto, and Suresh, 2009). However, as it
has beenmentioned earlier, along the past decades, CI engines
emis-sions have been greatly reduced, and so has been the
fuelconsumption. The success in this pursuit has been mainlydue to
two kinds of actions:
Internal measures: based on the optimized design ofthe engine
and the control of the air management andinjection systems, aiming
at preventing the production
Nucleation mode Accumulation mode
0.25
0.15
0.05
0
Particle diameter (nm)
Con
cent
ratio
n N
/Nm
ax (
%)
1 10 100 1000 10,000
0.2
0.1
MassNumber
Figure 19. Typical distribution of exhaust particulate size and
their contribution to total mass.
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article was published in the Handbook of Clean Energy Systems in
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-
18 Reciprocating Engines
of the pollutants in the combustion chamber, that is,limiting
engine-out emissions.
External measures (aftertreatment): based oninserting devices
that can extremely reduce pollu-tants leaving the cylinder, thus
reducing the tailpipeemissions.
Internal measures, despite dealing with the source of theproblem
without requiring additional equipment, are not ableto fulfill the
severe limits imposed by current and upcomingregulations. Hence, in
automotive engines and heavy-dutyvehicles, some kind of
aftertreatment device has been neces-sary since several years.
As discussed earlier, it might be concluded that there is
aconflict between the formation of different pollutants,
mainlybetween NOx and soot. As explained earlier, NOx own
theirorigin mainly to high combustion temperatures and highoxygen
content, favorable conditions to soot formation andCO and HC
reduction. Figure 20 illustrates the achievementsof the different
techniques used currently in the inexorableNOx-soot trade-off.
Finally, it should be noted that although CO2 is not consid-ered
a limiting pollutant emission, there is a growing pres-sure to
reduce the emission of this gas, especially fromthe passenger car
fleets. There are two basic strategies toachieve this goal:
reducing fuel consumption and burningfuels that generate less CO2
in his cycle life (from well towheel). As far as the first
strategy, there is a linear rela-tion between fuel burnt and CO2
emissions. Hence, all themeasures that allow reducing fuel
consumption will be favor-able. However, the expected results from
applying enginedesign and control techniques may not be enough, and
here acomplete world of vehicle design and management
strategies
are being developed. On the other side, using low carbonfuels or
biofuels can contribute to the reduction of thewell-to-wheel
emissions. In this sense, new generations ofbiodiesel fuel are
being developed, as well as the combi-nation of different fuels. It
should be considered that someof these new fuels with typically
higher contents in oxygentend to produce a reduction in soot but an
increase in NOxemissions.
5.3.1 Internal techniques
These techniques are known as active solutions and basicallyare
always affected by the trade-off between NOx and soot,with the
exception of the new combustion modes.
Combustion chamber design. In direct-injection dieselengines,
the combustion chamber is shaped as a bowl onthe piston head. The
smaller the diameter of the bowl is,the faster the air motion will
be when piston approaches topdead center and during the injection
process. This increase inflow velocity is due to the squish of the
gas into the cylinderand to the acceleration of the swirl motion
produced duringthe intake process. In all, the mean velocity field
and theturbulence improve the fuelair mixture, which helps
inshortening the combustion process, and can improve
fuelconsumption. This measure tends to reduce soot formationand to
increase NOx emissions. Moreover, the high gasvelocities increase
heat transfer and this can counteract thebenefits in terms of
efficiency improvement by combustionacceleration.
In large and slowly rotating CI engines (industrial andmarine
applications), where combustion does not need tobe extremely fast,
the trend has been toward a quiescentchamber, leaving to the
injection system the role of a good
NOx emission
DeNOx-SCR
Injection + combustion+ EGR
Injection +combustion +
air management
State ofthe art
New combustionconceptsP
artic
ulat
e tr
ap
Soo
t em
issi
ons
Target
Figure 20. Possible internal and external measures for tailpipe
NOx or soot reduction.
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article was published in the Handbook of Clean Energy Systems in
2015 by John Wiley & Sons, Ltd.DOI:
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Compression Ignition Engines 19
mixing, and so attaining a high fuel efficiency by reducingheat
losses.
In automotive, high speed engines, the usual objective hasbeen
the opposite; however, in the past decade, for a betterfuel
efficiency, the trend has been reducing the gas motion byopen and
shallow combustion bowl designs, exploiting thepower of the new
injection systems for the fuelair mixtureformation.
Injection system upgrade. Increase in injection pres-sure. As
already commented in Section 3, the injectionsystems have been
improved for higher injection pres-sures, and a better control of
the fuel delivery, resultingin general in better fuel atomization.
The increase ininjection pressure enhances both fuel atomization
and airentrainment into the spray, speeding up combustion.
Theimmediate consequences are the reduction in soot, CO,and HC, and
an increase in efficiency. However, the NOxemissions tend to
increase owing to the higher combustiontemperatures achieved.
Figure 21 shows the commentedeffects of increasing injection
pressure in a heavy-dutyengine, at different EGR rates, which will
be commentedlater.
These injection strategies, despite producing a smalleramount of
soot mass, tend to produce a larger number ofparticulates with
smaller size, with their worse impact forliving beings. This is
moving to establish new regulationsthat limit not only total
particulate mass but also the numberof particulates.
210
205
200
195
0.2
0.15
0.05
02 4 6 8 10 12
SNOx (g/kWh)
0.1
Dry
soo
t (g/
kWh)
BS
FC
(g/
kWh)
19%
20%
20%0%
13%8%
EGR
BP = 3.45 bar
840 bar
970 bar1100 bar
IP
Figure 21. Effects of increasing EGR and boost pressure on
theNOx-soot trade-off in conventional diesel combustion.
Pilot I Pilot II Main After
Combustion noise and NOx reduction Soot oxidation
NOxsoot trade-off optimization
Figure 22. Multiple injections strategy for control of emissions
andnoise.
Other improvements made in the injection process are
thecapability of modulating the injection rate, especially in
thecases of common-rail systems and direct-acting injectors
(seeSection 3). A common application is the multiple
injectionevent, which splits the injection process in several
pulses, asillustrated in Figure 22.
Pilot injection (or pre-injection). It is a techniquecommonly
used in light-duty engines in order to reducethe combustion noise.
It involves injecting a small quantityof fuel few degrees before
the main injection. In this way,the amount of fuel burned is
reduced during the premixedcombustion phase. Its impact on exhaust
emissions is scarce,but reduces the noise that is one of the
classic problems ofthe diesel engine.
Post-injection. It involves injecting a small amount of fuelfew
degrees after the end of the main injection. This smallamount of
post-injected fuel will not burn under optimumconditions, thus fuel
efficiency will decrease. However, ifproperly timed, the last shot
of fuel that has been detachedfrom the trailing edge of the burning
spray can benefit from abetter mixing with fresh air and it will
burn at higher temper-ature, thus promoting to soot oxidation. The
consequence isthen a lower soot emission.
A different strategy of injection modulation is the
so-calledinjection rate shaping, which is usually referred to
changingfuel injection velocity in the same shot, as it is
illustrated inthe boot-shape in Figure 7. This boot shape, with a
slowervelocity at the beginning of the injection, produces a
similareffect to the pilot injection depicted in Figure 22.
Another way of affecting the pollutant formation is by
thegeometry of the injector nozzle. Therefore, small orificestend
to improve atomization and mixing of fuel with air,while a large
number of nozzles contributes to spreading thefuel in the
combustion chamber and enhancing the fresh airutilization. Both
measures contribute in general to reducesoot and to increase
NOx.
Handbook of Clean Energy Systems, Online 2015 John Wiley &
Sons, Ltd.This article is 2015 John Wiley & Sons, Ltd.This
article was published in the Handbook of Clean Energy Systems in
2015 by John Wiley & Sons, Ltd.DOI:
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20 Reciprocating Engines
EGR. A general and widely used measure for the reduc-tion in NOx
emissions is the EGR that introduces gasesfrom the exhaust into the
intake line, replacing and mixingwith fresh air, and so reducing
the oxygen concentration ofthe gas that later mixes with the fuel
during the injectionprocess. There are different effects affecting
the NOx forma-tion, but the most important in usual combustion
conditionsis the lower oxygen concentration that reduces the
flametemperature. As a counter effect, the less oxygen
contributesto higher soot emissions by reducing the soot
oxidationrate. Moreover, the slower reaction rates are
responsiblefor a trend to increase fuel consumption, and a
proportionalincrease in the production of CO2 (Ladommatos et
al.,1996a, b). The EGR strategy is currently always combinedwith
some degree of cooling of the recirculated gases, asthis measure
contributes further to the reduction in the flametemperature and
NOx formation.
Figure 21 shows some results of the clear effect ofincreasing
EGR in a heavy-duty engine. In this case, intro-ducing an EGR rate
of about 20% can reduce NOx emissionsby a factor of 4. In modern
engines, EGR rates can range upto 40% and 50% at low load operation
conditions. EGR is anecessary measure for controlling the
alternative combustionmodes based on a premixed charge
auto-ignition. Moredetails on the techniques for producing EGR are
given inSection 8.
Increase in boost pressure. Increasing boost pressure isa
desirable measure that has an already commented potentialfor
largely increasing engine power if fuel mass is increasedin
proportion to the increase in intake air. However, if equiva-lence
ratio is reduced, the general effect is a reduction in
sootformation, owing to the excess in air. The faster
combustionwith plenty of available oxygen produces a benefit in
fuelefficiency and so in CO2 reduction. The familiar repercussionis
an increase in NOx emissions.
New combustion modes. The trend in future active solu-tions
focuses mainly in new combustion modes, which havebeen introduced
in Section 4. These combustion modesare focused on shifting the
combustion curve illustratedin Figure 11 into areas where NOx and
soot formationdoes not occur. On the one hand, systems known as
PCCI,which perform the injection process at a lower
temperature,thus increasing the delay period. This controls the
combus-tion evolution below the NOx-forming temperatures. In
thissense, this type of combustion reduces NOx emission but
mayproduce a tendency to not to oxidize the CO and HC owingto the
decrease of temperatures.
5.3.2 External measures
These techniques are also known as passive solutions, and
aremainly based on some aftertreatment device. Aftertreatment
Technologies: State-of-the-Art and Emerging Technologiesdeals in
detail with this subject, and only some commentsare made here
focusing on the effects on the engine operationand interrelation
with other measures.
Despite being the most important pollutant emissionsimilar to SI
engines, the same type of aftertreatmentdevices cannot be used,
owing to the excess of air in theexhaust gases of CI engines
(Eastwood, 2000). These condi-tions limit the use of any concept
based on the reductionreactions (for instance, for eliminating
NOx). On the otherhand, the lower exhaust gas temperature and the
commonuse of turbocharging yields lower exhaust temperaturesin the
point where the aftertreatment system is placed,compared with the
equivalent SI engine.
The most common system used currently in CI engines isthe
oxidation catalyst, which is able to abate simultaneouslyCO and HC
emissions.
The catalytic reduction of NOx is not easy in an ambientwith
excess of oxygen. The most commonly used techniquetoday is the
selective catalytic reduction (SCR), which needsto introduce urea
in the exhaust gas flow upstream of thedevice for generating
ammonia (NH3), which will react withthe NO2 to produce N2 and H2O.
An alternative are thechemical filters, the latter being called NSR
(NOx storage-reduction) or LNT (lean NOx trap). These are
characterizedby their ability to hold NO2 from the exhaust gas
duringlean operation conditions, and release it during rich
operationconditions.
The current technology for reducing soot and PM is theinsertion
of a particulate filter (DPF), which simply retainsmost of the
particles in the exhaust flow. When the filter getsclogged, some
regeneration strategy must be introduced toburn the particles.
As already commented, current engines are not able tomeet the
pollutant limits with only internal measures, andprobably the same
will happen in the future, hence somecombination of aftertreatment
devices will be required. Asillustrated in Figure 20, there are
three ways of meeting lowerpollutant limits:
Accelerating combustion (high injection pressure andboost
pressure, high turbulence, little, or no EGR), whichleads to low
soot and high efficiency and reduce theexcessive NOx emissions by
aftertreatment.
The second alternative is the opposite: reducing
injectionpressure and especially introducing high rates of EGR.This
leads to low NOx emissions but to high soot. Sootis then reduced by
a particulate filter. The aftermath ofthese systems is the trend to
reduce the efficiency.
The third way of improvement would be based onsome technological
breakthrough, such as successfullyimplementing some new combustion
concept that would
Handbook of Clean Energy Systems, Online 2015 John Wiley &
Sons, Ltd.This article is 2015 John Wiley & Sons, Ltd.This
article was published in the Handbook of Clean Energy Systems in
2015 by John Wiley & Sons, Ltd.DOI:
10.1002/9781118991978.hces079
-
Compression Ignition Engines 21
lead to simultaneous reduction of NOx and soot ideallywithout
the need of aftertreatment device. However,current state of the art
allows applying these strategiesonly at low load operation
points.
In addition, it must be considered that the presence of
someaftertreatment equipment will interact with the engine
oper-ation, and with the other systems such as the EGR circuitsand
the turbine, described in later sections. Negative effectson the
engine operation are mainly due to the increasedbackpressure (that
can be somehow mitigated by combiningthe design of the silencing
devices), and to the requirementof some more or less frequent
ineffective engine operationmodes for the regeneration of the
particulate filters or for thecatalyst light-off in cold
starting.
6 FUELS
6.1 Suitable fuels for CI engines
For the development of the conventional combustionprocess
described in Section 4, involving the fast injectionand mixing of
the fuel, it is necessary that the used fuelaccomplishes a broad
list of requirements involving ther-mophysical and chemical
properties closely related withvolatility, injectability, and
combustibility in this particularapplication (Chevron Corporation,
2007). The usual valuesof these properties for a commercial gasoil
and other fuels,commented later, are given in Table 1.
One of the first stages of injection is atomization, andin order
to produce a huge amount of droplets, the fuel isinjected through a
narrow nozzle with a diameter of aroundone hundred of microns. A
very important property for thiscondition is fuel viscosity, as a
high viscosity is a commoncause for a deficient atomization,
leading to poor combus-tion. Moreover, the design of the injection
system impliesthat some moving parts of diesel fuel pumps and
injectorsare protected from wear exclusively by the fuel. Hence,
thefuel must be able to lubricate by itself the moving parts,and
the determinant property is lubricity. The lubrication
mechanism in the injection systems is a combination
ofhydrodynamic lubrication and boundary lubrication. Thesephenomena
are closely related with the fuel viscosity, andhere there is a
compromise between adequate atomization,which requires low
viscosity, and proper hydrodynamiclubrication, which means the
opposite. On the other hand,boundary lubrication occurs when the
liquid film is notcontinuous and small areas of the opposing
surfaces get incontact. Although lubricity-enhancing substances
(mainlytrace amounts of oxygenated, nitrogenated, and
aromaticcompounds) are naturally present in diesel fuel derivedfrom
petroleum crude by distillation, the increase of therequirements of
fuel regarding to pollutant emissions has ledto severe distillation
processes and to a loss of this property.
Once the fuel is atomized and droplets in vapor phasemixed with
air, the state of combustion is dependent on theignition quality of
the fuel. In the conventional combustionprocess, smoothness of
operation, misfire, smoke emissions,noise, cold start performance,
and ease of starting can beimproved using a fuel with good
auto-ignition quality. Thecetane number is a measure of how readily
the fuel startsto burn, comparing the fuel to a scale made of two
knownchemical substances, in tests carried out in a special
engine.Increasing the cetane number implies a shorter delay
incombustion, which leads to an improvement of the processand
performance on startup, and a reduction of NOx and sootemissions.
Cetane number varies systematically with the HCstructure, and some
fuel processing can reduce this param-eter, so that a series of
fuel additives have been developed toimprove the cetane number.
The energy released in the combustion of a certainamount of fuel
is directly dependent on the chemical energycontained in the fuel,
which is evaluated by the heatingvalue. As plain CI engine fuels
are stored and used inliquid phase, the density is also an
important parameter, asthe injection systems operate on a
volumetric basis. Fuelconsumption is related to the heating value
of the fuel, whilethe size of the relevant devices (pumps and
injectors) isaffected by fuel density.
As the usual conventional fuels are distilled from crudeoil,
some relevant contents of sulfur present in fossil fuels
Table 1. Properties of several fuels for CI engines.
Ultra-Low Sulfur Gasoil Biodiesel Fischer-Tropsch
Specific gravity 0.830.87 0.870.89 0.770.79Cetane rating 4055
4570 >70Viscosity at 40C (mm2/s) 1.93.3 3.55.0 2.12.8Sulfur
(ppm) 715 024
-
22 Reciprocating Engines
will be found in the gasoil. Sulfur is a substance
contributingto the lubricity of the fuel, but aside from producing
pollu-tant oxides of sulfur, it can disturb the operating of
theaftertreatment devices in the exhaust. Therefore, the
increas-ingly stringent emissions standards in the world have
forcedto reduce the amount of sulfur in the fuel to the level
ofseveral parts per million.
An extensive use of additives has been applied to ensurethe
performance of the fuel and to broaden the range ofdistillation
products that can be used in diesel combustion.Sometimes applied in
parts per million concentrations, thesechemical compounds improve
significantly the performanceof the fuel used. Related to the
performance of the fuelinjection system, the mainly used additives
ar