compression ignition engines- state-of-the-art and current technologies. future trends and...

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

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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.

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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,



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



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



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

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Figure 3. Main components of a standard common rail injection system.



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

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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.



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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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

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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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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



(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



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



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.



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



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.



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



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.



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.



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.



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



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


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

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