Analysis of the Effect of Vehicle, Driving and Road Parameters on the Transient Performance and Emissions of a Turbocharged Truck

energies

Article Analysis of the Effect of Vehicle, Driving and Road Parameters on the Transient Performance and Emissions of a Turbocharged Truck

Evangelos G. Giakoumis * ID and George Triantaﬁllou Internal Combustion Engines Laboratory, School of Mechanical Engineering, National Technical University of Athens, 15780 Athens, Greece; ge.triantaﬁ[email protected] * Correspondence: [email protected]; Tel.: +30-210-772-1360

Received: 8 January 2018; Accepted: 23 January 2018; Published: 27 January 2018

Abstract: In this paper, a fundamental analysis of the effects of various influential parameters on the performance and emissions of a turbocharged truck operating under transient conditions is presented. The results derive from a detailed vehicle model that comprises two parts. The first is an engine performance and emissions module that follows a mapping approach, with experimentally derived correction coefficients employed to account for transient discrepancies; this is then coupled to a comprehensive vehicle model that takes into account various vehicle operation attributes such as gearbox, tires, tire slip, etc. Soot, as well as nitrogen monoxide, are the examined engine-out pollutants, together with fuel consumption and carbon dioxide. The parameters examined are vehicular (mass and gearbox), driving (driver ‘aggressiveness’ and gear-shift profile) and road (type and grade). From the range of values investigated, the most critical parameters for the emission of NO and soot are vehicle mass, driving ‘aggressiveness’ and the exact gear-change profile. Vehicle mass, driving ‘aggressiveness’ and road-grade were identified as the most influential parameters for the emission of CO2. A notable statistical correlation was established between pollutant emissions (NO, soot) and vehicle mass or road-tire friction, as well as between fueling/CO2 and vehicle mass, road-tire friction and road grade. It is believed that the results obtained shed light into the effect of critical operating parameters on the engine-out emissions of a truck/bus, underlining at the same time the peculiarities of transient operating conditions.

Keywords: diesel engine; soot; nitrogen oxides; transient operation; turbocharger lag; gearbox; vehicle

1. Introduction The internal combustion engine has dominated the medium and large transport sector (trucks, buses) for many decades now. The major contributing factor for this has been its versatility/ability to operate efficiently at a variety of speed and (mostly) load conditions. In particular, it is the compression ignition (diesel) engine that is primarily used for medium and heavy-duty trucks all over the world. The ability of the diesel engine to operate (highly) turbocharged, as well as its higher fuel efficiency compared to the spark ignition engine are the obvious reasons for its dominance. Nowadays, all diesel engines are turbocharged, with two-stage (series) turbocharged units achieving impressive values of brake mean effective pressure of approximately 30 bar. Nonetheless, there are certain aspects of the turbocharged diesel engine’s operation that have proven problematic over the years, such as its cold starting difficulty, its combustion noise radiation (largely mitigated today by electronically controlled, common rail injection systems with pre-injection) and, mostly, its transient performance and emissions [1,2]. The authorities in many regions in the world have acknowledged the latter by applying transient cycles for the certification of new heavy-duty engines/vehicles. For example, many countries/regions in the world, such as the European

Energies 2018, 11, 295; doi:10.3390/en11020295 www.mdpi.com/journal/energies Energies 2018, 11, 295 2 of 21

Union, Japan and S. Korea, apply the WHTC (Worldwide Harmonized Transient Cycle) transient engine-dynamometer cycle for the certification of new heavy-duty engines, whereas in the US and Canada, the similarly transient FTP (Federal Test Procedure) cycle has been in use since the mid-1980s, replacing earlier steady-state test procedures [3,4]. Modeling and experimental investigation of transient diesel engine operation started rather late, in the late 1960s, with the pioneering works from Watson and co-workers [5], and Winterbone and co-workers [6]. In the next decades, various thermodynamic models were developed dealing with transient (diesel engine) operation, and simulating both the engine and turbocharger response during acceleration, load acceptance and starting; these are reviewed in detail in [7]. Such simulation tools were mostly based on a zero-dimensional modeling philosophy, which was considered a good compromise between accuracy and (acceptable) computational time. A few attempts have also been made to simulate a whole driving cycle applying filling and emptying modeling techniques [8,9], although with high burden on the code execution time. In any case, none of the above models has proven capable of accurately predicting the engine or vehicle performance during transients, particularly so for modern engines equipped with complex electronically controlled systems and antipollution devices. In parallel, quasi-steady or mean-value models have also been developed and employed over the years, with obvious benefits in terms of simplicity and code short running time. Such simplified models are usually based on steady-state maps of all the interesting engine properties, applying correction coefficients to account for the peculiarities during transients [10–14]. Although these cannot capture engine properties development on a microscopic (degree crank angle) scale, they have proven very useful owing to their simplicity and real-time execution capabilities. The research group led by the first author has demonstrated the capabilities of quasi-linear approaches in a series of previous works [15–17]. The developed procedure combines simulation and experiment, being thus considerably less costly than pure experimental approaches, and at the same time more versatile. This is based on an initial experimental mapping of the engine under steady-state conditions with correction coefficients applied to account for the transient emission overshoots (detailed in Section2). These coefficients have been derived experimentally, based on a variety of discrete accelerations conducted in the authors’ laboratory. The emissions estimation is then combined with a comprehensive vehicle model that ‘runs’ each time on the requested transient schedule. The analysis presented and discussed in this article is a continuation of these earlier models, which were applied to driving/transient cycles. However, the current work provides three distinct novel features:

(a) it focuses on speciﬁc discrete transient events, a fact that will facilitate better insight into the complex dynamic phenomena an engine encounters during transients (this was not always feasible when 1200 [16] or even 1800 [17] seconds of a cycle were studied); (b) a much more detailed vehicle model has been incorporated in the simulation code, facilitating better evaluation of the vehicle’s true behavior on the road; and (c) a detailed parametric study of the effect of various critical vehicle, driving and road parameters will be conducted in order to assess their importance on both performance and emissions from a typical medium to heavy-duty vehicle equipped with a turbocharged diesel engine.

The paper is structured as follows: Section2 will provide an outline of the model developed and the experimental work it is based on. Section3 will highlight the performance and emissions behavior of the engine/vehicle during the two studied transient schedules. The detailed parametric analysis will then follow in Section4, based on the conclusions from the fundamental analysis of Section3. It is highlighted that in the investigation that follows all emitted pollutants discussed are engine out, and concern fully warmed-up engine operation. Energies 2018, 11, x FOR PEER REVIEW 3 of 21

Energies2. Methodology—Engine2018, 11, 295 and Vehicle Model 3 of 21 The methodology employed for the estimation of performance and emissions during a transient 2.schedule Methodology—Engine has been detailedand in previous Vehicle publications Model [15–17]; only a brief description is provided here, with reference to Figure 1, for the sake of completeness. As was the case with other similar mapping- basedThe approaches methodology developed employed in the for past the [10–14,18–23], estimation of performancethe context of and quasi-linear emissions modeling during a [24] transient is, in schedulegeneral, followed. has been detailed in previous publications [15–17]; only a brief description is provided here,Initially, with reference a thorough to steady-state Figure1, for testing the was sake perf oformed completeness. aiming to formulate As was the the engine case withperformance other similarand emissions mapping-based map. The approaches properties developedunder test inare the soot, past nitric [10–14 monoxide,18–23], the (NO), context and offueling quasi-linear (from modelingwhich CO [2 24emissions] is, in general, can be followed.directly estimated).

Figure 1. Flowchart describing the computational procedure during a transient schedule. Figure 1. Flowchart describing the computational procedure during a transient schedule.

Based on the steady-state engine testing, the engine emissions and performance were mapped applyingInitially, a fourth a thorough order polynomial steady-state for every testing studie wasd property performed with aiming respect to to formulate the engine the speed engine and performancetorque [15]. and emissions map. The properties under test are soot, nitric monoxide (NO), and fueling (fromAfterwards, which CO2 aemissions variety of can transient be directly schedules estimated). were conducted, using fast-response soot and NO analyzersBased (Figure on the steady-state2). The transient engine schedules testing, the invest engineigated emissions were discrete and performance accelerations were at mappedvarious applyingloads, such a fourthas the orderones that polynomial will be studied for every in studiedthis paper, property from a with variety respect of initial to the engine engine speeds speed and torquefor a variety [15]. of demanded speed changes. By doing so, we were able to estimate the emission peaks observedAfterwards, for the current a variety engine of transient during schedules accelerati wereons, a conducted, very essential using element fast-response when turbocharged soot and NO analyzersengines operate (Figure [1,2].2). The From transient the engine schedules experimentat investigatedion during were discretetransients, accelerations correction coefficients at various loads, were suchderived as thefor each ones pollutant that will bethat studied were used in this to ‘corre paper,ct’ from (i.e., a increase) variety ofthe initial steady-state engine speedsemissions and [17]; for amore variety information of demanded on the speed transient changes. experimental By doing tests so, is we available were able in [25]. to estimate With reference the emission to Figure peaks 1, observedat each time for step the of current the examined engine during transient accelerations, schedule: a very essential element when turbocharged engines operate [1,2]. From the engine experimentation during transients, correction coefﬁcients were derived for each pollutant that were used to ‘correct’ (i.e., increase) the steady-state emissions [17]; more information on the transient experimental tests is available in [25]. With reference to Figure1, at each time step of the examined transient schedule:

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Keithley Cambustion Data Acquisition Card CLD 500 fast NO analyzer Exhaust Energies 2018, 11, x FOR PEER REVIEW 4 of 21 Filter After-turbo line

Filter 60 AVL 439 50 40 30 PC – Control 20

Keithley0 fast- 0 90 18 0 27 0 3 60 Cylinder exhaustline Panel Cambustionresponse Shaft encoder (engine Data Acquisition Card Filter CLD 500 opacity speed, crank angle) Tektronix fast NO meter Oscilloscope 0 0 analyzer Exhaust Exhaust 0 Amplifier ExhaustTemperature Pressure 0 0 Fuel pump Fuel tank Filter rack position After-turbo line

Filter 60 AVL 439 50 40 30 PC – Control 20

0 fast- 0 90 18 0 27 0 3 60 Boost PressureCylinder exhaustline response Panel To Data Shaft encoder (engine Filter opacity Turbo- speed, crank angle) Tektronix charger Oscilloscope pressure Cylinder Acquisition Card meter T/C 0 0 Speed A/CExhaust Exhaust 0 Temperature Pressure Amplifier 0 0 Fuel pump Fuel tank rack position

Boost Pressure

To Data Fuel Pump Turbo- LVDT charger pressure Cylinder Acquisition Card T/C Speed A/C Turbocharged Diesel Engine

Dynamometer

Fuel Pump Figure 2. Schematic representation of the experimentalLVDT set-up of the engine. Figure 2. Schematic representation of the experimentalTurbocharged Diesel set-up Engine of the engine. • From the vehicle speed,Dynamometer we were able to calculate the actual engine torque and speed based on • Froma drive-train the vehicle model speed, (Figure we were 3) for able the tovehicle calculate concerned the actual [26,27]. engine The torquevehicle andmodel speed employed based in on a drive-trainthis work model hasFigure (Figurebeen 2. Schematicconsiderably3) for therepresentation vehicle upgraded concerned of the from experimental [ the26,27 previously]. set-up The vehicleof the mentioned engine. model employed publications, in this workincorporating, has been considerably among other upgraded things, the from effect the of previously the [28–30]: mentioned publications, incorporating, among• From other the things,vehicle speed, the effect we were of the able [28 to–30 calcul]: ate the actual engine torque and speed based on  a drive-trainInertia of themodel equivalent (Figure 3) rotating for the vehiclemasses co(engine,ncerned driv [26,27].eshaft, The wheels), vehicle model employed in  thisTiresInertia work (inflation ofhas the been equivalent pressure, considerably resistance, rotating upgraded masses slip and from (engine, friction the driveshaft,coefficient),previously wheels), mentioned publications,  Road (gradient and friction coefficient), incorporating,Tires (inflation among pressure, other things resistance,, the effect slip of and the [28–30]: friction coefficient),  Transmission (gear number and ratio, speed-dependent mechanical efficiency etc.),  RoadInertia (gradient of the equivalent and friction rotating coefficient), masses (engine, driveshaft, wheels),  Differential (final drive ratio, inertia, etc.), and finally the  Tires (inflation pressure, resistance, slip and friction coefficient),  DriverTransmission (simulates (gear the number specific anddriving ratio, ha speed-dependentbits, for example gear mechanical shift duration). efficiency etc.),  DifferentialRoad (gradient (final and drive friction ratio, coefficient), inertia, etc.), and finally the • Afterwards, Transmission we perform (gear an number interpolation and ratio, of speed-dthe digitizedependent engine mechanical map to estimateefficiency the etc.), steady-state Driver (simulates the specific driving habits, for example gear shift duration). fueling Differential and emissions (final at drive the exact ratio, load inertia, and etc.), speed and operating finally the point. • • Afterwards, The nextDriver step we is(simulates perform to apply an thecorrection interpolation specific coefficientsdriving of ha thebits, to digitized the for previousexample engine gearmentione map shift to duration).d estimatequasi-steady the steady-stateemissions fueling• evaluating Afterwards, and emissions the we ‘real’ perform attransient the an exact interpolation emissions load and of profile; speedthe digitized operatingthese engine coeffici point. mapents to wereestimate evaluated the steady-state after the • Theextensive nextfueling step andtransient is emissions to apply testing correctionat the of exact the coefficientsengineload and in speed ha tond the operating mentioned previous point. mentionedabove, and quasi-steady are specific emissionsto each evaluating• transient The next event the step ‘real’ is(e.g., to apply transientload correctionincrease emissions or coefficients speed profile; change) to thethese previousthat the coefficients enginementione experiences.d werequasi-steady evaluated emissions after the evaluating the ‘real’ transient emissions profile; these coefficients were evaluated after the extensive transient testing of the engine in hand mentioned above, andBack areaxle specific to each extensive transient testing of the engine in hand mentioned above, and are specific to each transient eventEngine (e.g., load increase or speedN change) that the engine experiences.gear transient event (e.g., load increaseτe or speed change)τL that the engine experiences. Gearbox Wheel Back axle

Engine τe N τL gear Gearbox Propeller Wheel Crankshaft rwheel Clutch shaft Axle Propeller Crankshaft rwheel Figure 3. Engine drivetrain configurationClutch used in the analysis (N correspondsshaft to the engine speed, Τe to the engine torque, and ΤL to the load (resistance) torque; the latter incorporating rolling,Axle aerodynamic, and road grade resistance terms). Figure 3. Engine drivetrain configuration used in the analysis (N corresponds to the engine speed, Τe Figure 3. Engine drivetrain conﬁguration used in the analysis (N corresponds to the engine speed, Te to to the engine torque, and ΤL to the load (resistance) torque; the latter incorporating rolling, aerodynamic, the engine torque, and T to the load (resistance) torque; the latter incorporating rolling, aerodynamic, and road grade resistanceL terms). and road grade resistance terms).

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The last step is to integrate the instantaneous results over the examined transient schedule in order to calculate the total amount of emitted pollutants and CO2. The main technical engine speciﬁcations are provided in Table1; the engine is a six-cylinder, direct-injection, turbocharged diesel engine installed in buses and/or medium-duty trucks. Further, Table2 provides data for the heavy-duty vehicle the engine is installed in, needed for the computational analysis.

Table 1. Engine speciﬁcations.

Six-Cylinder, Four-Stroke, Direct Injection, Engine Model and Type Turbocharged and Aftercooled Diesel Engine Total displacement 5958 cm3 Bore/Stroke 97.5 mm/133 mm Compression ratio 18:1 Speed range 800–2600 rpm Maximum power 177 kW @ 2600 rpm Maximum torque 840 Nm @ 1250–1500 rpm Maximum turbocharger pressure 2.50 bar Fuel Standard automotive diesel fuel

Table 2. Vehicle speciﬁcations.

Nominal Mass (Refers to Unloaded Vehicle) 7350 kg Frontal area 4.65 m2 Aerodynamic resistance coefﬁcient 0.70 1st—7.72:1 2nd—4.42:1 3rd—2.66:1 Gear ratios 4th—1.79:1 5th—1.28:1 6th—1.00:1 Back-axle ratio 3.70:1 Wheel radius 0.57 m Longitudinal distance between center of gravity and front axle 1.95 m Longitudinal distance between center of gravity and rear axle 1.30 m Vertical distance (height) between center of gravity and ground level 0.89 m

3. Vehicle and Engine Results during Typical Transient Schedules In this section, a description of the vehicle performance and engine emissions during two transient schedules will be provided and discussed. These transient schedules simulate real-world driving, and are typical components of certification driving cycles [4]. The first schedule is a continuous up-gear shift change, typical when entering a highway (Figures4–6); cf. The first seconds of the extra-urban driving cycle (EUDC) of the NEDC (New European Driving Cycle). As past research has indicated [1,2], such an accelerating event is the most demanding dynamic operation a vehicle experiences in terms of emissions. The second transient schedule is a typical urban route, incorporating accelerating, cruising and decelerating phases at low engaged gear and vehicle speeds (Section 3.2—Figure7), cf. The urban segment of the NEDC. The results from the fundamental analysis of this section will be used to interpret the effects of the various examined parameters on the engine/vehicle performance and emissions in Section4. We believe that employing such short schedules with discrete accelerations can provide better understanding of the underlying phenomena. Please notice that the two transient schedules examined in this paper were formulated in such a way so as their accelerations ‘coincide’ to a large degree with the accelerations measured experimentally, in order for the experimental-computational Energies 2018, 11, 295 6 of 21 Energies 2018, 11, x FOR PEER REVIEW 6 of 21 Energies 2018, 11, x FOR PEER REVIEW 6 of 21 procedureIn other words, discussed we designed in Section the2 totwo be examined directly applicable.transient schedules In other to words,be as close we as designed possible theto the two examinedexperimentalIn other transientwords, data we schedulesin designed hand. tothe be two as closeexamined as possible transient to theschedules experimental to be as dataclose in as hand. possible to the experimental data in hand. (%) (%) total /F a total F /F a F (N) r (N) r Rolling Resistance Force, F Rolling Resistance Force, F (N) a (N) a Force, F Aerodynamic Force, F Aerodynamic Gear Selected Gear Selected ) 2 ) 2 (m/s Vehicle (m/s Acceleration Vehicle Acceleration ) m ( ) (km/h) m Distance ( (km/h) Vehicle Speed Distance Vehicle Speed

Figure 4. Development of various vehicle parameters during the first examined transient schedule. FigureFigure 4. 4.Development Development of of various various vehiclevehicle parametersparameters du duringring the the first ﬁrst examined examined transient transient schedule. schedule.

Figure 5. Development of various engine parameters during the first examined transient schedule. Figure 5. Development of various engine parameters during the first examined transient schedule. Figure 5. Development of various engine parameters during the ﬁrst examined transient schedule.

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Figure 6. Development of soot, nitrogen monoxide and carbon dioxide during the first examined Figure 6. Development of soot, nitrogen monoxide and carbon dioxide during the ﬁrst examined transientFigure schedule.6. Development of soot, nitrogen monoxide and carbon dioxide during the first examined transient schedule. transient schedule. 30 25 2030 1525 1020 515 010 5 120 0 80120 80 40 40 0 6 0 6 4 4 2 2 0 0 3200 3200 2400 2400 1600 1600 800 800 0 0 120120 9090 6060 3030 0 0

18001800 16001600 14001400 12001200 10001000 1.51.5 1.0 1.0 0.5 0.5 0.0 0.0-0.5 -0.5-1.0 -1.040 40 30 3020 2010 10 0 0 3 3 2 2 1 1 0 0-1 40 -1 4030 400 3020 200400 2010 10 0 0 200 0 5 10 15 20 25 30 35 40 45 50 55 0 Time (s) 0 0 5 10 15 20 25 30 35 40 45 50 55 Time (s) Figure 7. Development of vehicle and engine parameters and emissions during the second examined Figure 7. Development of vehicle and engine parameters and emissions during the second examined Figuretransient 7. Development schedule. of vehicle and engine parameters and emissions during the second examined transient schedule. transient schedule.

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3.1. Continuous Acceleration Schedule One of the primary criteria for evaluating truck response, and powertrain performance in general, is based on the ability to accelerate after a sudden increase of driver demand [28]. Figure4 focuses on such a critical transient schedule, illustrating distance covered, vehicle speed and acceleration, selected gear, and resistance forces. In general, the resistance forces a vehicle experiences when moving on a road are the rolling resistance force Fr, the grade-dependent force Fgr, the aerodynamic force Fa, as well as the inertia force Fin; total traction force Ftr is then given by [4,27]:

Ftr = Fr + Fgr + Fa + Fin (1)

For this transient, the road is considered flat (the effect of road grade will be investigated later in Section 4.3), hence the applied resistance forces are only aerodynamic and rolling (as is the case with legislated driving cycles), as well as inertia during the acceleration segments. The transient schedule lasts 100 s, and the vehicle speed reaches the desired value of 120 km/h covering a distance of 2 km. During the low-velocity parts of the transient event, corresponding to gears 1 through 3, the rolling resistance force Fr prevails over the aerodynamic one Fa. As soon as the fifth gear is engaged, and the vehicle speed approaches 60 km/h, the aerodynamic force begins to assume greater values, further supported by the large frontal area of the vehicle and the much smaller (but not negligible) dependence of rolling resistance on vehicle speed [4]. In any case, the rolling resistance contribution, as the upper sub-diagram of Figure4 demonstrates, is, even at high speeds, at least 25% of the total resistance experienced by the vehicle. This is mostly the result of the high vehicle mass, and is typical for heavy-duty vehicles such as trucks and buses. As expected, the aerodynamic resistance graph is patterned after the vehicle speed one. In general, the lower the selected gear (this means high gear ratio ig), e.g., during the low-speed segments (or the urban parts during a driving cycle), the smaller the apparent vehicle moment of inertia. This then leads to higher (vehicle) acceleration values (Figure4); emissions too as will be detailed later in the text [4]. On the other hand, for higher selected gears (hence low gear ratio), the higher the assumed vehicle velocity, whereas the accelerations are (much) lower, since the apparent vehicle inertia is now higher [27]. The specific profile of the engine speed development in Figure5 incorporates the effects of driver ‘aggressiveness’ and gear-shift profile; the influence of both parameters will be investigated in Section 4.2. The absolute values of the engine speed, as Equation (2) shows, are specific to the installed gearbox and wheel radius [26]:

V(km/h) × ib × ig 3 Ne(rpm) = 60 × 10 (2) 2 × π × rwheel(m) where Ne is the engine speed, V the vehicle speed, and ig, ib are the corresponding gear and back-axle ratios, respectively; rwheel represents the wheel radius. The development of torque, power and fueling, on the other hand, follows an identical profile throughout the transient event, as all these engine properties are inter-related with engine loading. During the low-speed, low-gear segments, the engine load is mostly influenced by the respective acceleration. This is due to the fact that the absolute vehicle velocities are low, however the accelerations steeper, resulting in increased inertia terms in Equation (1). In particular, the vehicle mass factor MF (=ratio of apparent mass to vehicle mass, with the apparent mass defined as the vehicle plus rotating masses) is almost triple when the first gear is engaged compared to the sixth. During the high-speed segments, on the other hand, where vehicle acceleration (from Figure4) assumes lower values, it is the elevated vehicle velocities that primarily determine the engine load through the contribution of the aerodynamic resistance [27]; the loading then determines the values for power, torque, and fueling. It is not surprising that during each steep transient, fuel consumption overshoots are noticed. This is mostly due to the extra power required to overcome the vehicle inertia. Energies 2018, 11, 295 9 of 21

Further to the above, each steep acceleration leads also to a substantial increase in emissions, as Figure6 eloquently demonstrates. It is well known that net soot production (fuel consumption and carbon dioxide emissions too) is mainly determined by engine load [31]. When the load increases, more fuel is injected, resulting in an increase of the temperatures in the fuel-rich zones; at the same time, diffusion combustion is prolonged, promoting the production of soot. In parallel, the availability of oxygen decreases; thus, the production of soot is favored [31]. During each acceleration, the above production mechanism is further enhanced by very low air–fuel values owing to turbocharger lag. For the NO emissions in Figure6, the main parameter is the temperature of the burned gas but the availability of oxygen plays an inﬂuential role too [32,33]. NO emissions are increased following the high temperatures during the turbocharger lag thermodynamic cycles; these high temperatures originate in the air-fuel mixture assuming close to stoichiometric values [2]. However, as long as the air-fuel equivalence ratio is lower than unity, the oxygen supply is limited. The combined effect of the above-mentioned two mechanisms produces a not so pronounced NO emission increase compared to soot [2]. This will be quantiﬁed in the parametric analysis (Section4) for various cases. It may be noticed also in Figure6 that soot emissions are rather low at elevated vehicle speeds, owing to the turbocharger operating at high speeds, thus providing adequately high air supply. Conversely, NO continues to assume increased values at high vehicle speeds, as the combination of high loading and oxygen availability promote the production of NO. In general, as is made obvious from Figure6, an abrupt acceleration proves more polluting (regarding engine-out soot) than many seconds or even minutes of cruising. Lastly, following basic combustion principles [32], the CO2 increases in Figure6 follow exactly the fuel consumption ones from Figure5, with peaks noticed at each acceleration. Particularly high values are also observed during the high-speed segment, as the increased aerodynamic resistance from Equation (1) requires high engine loading/fueling in order to be overcome [34].

3.2. Low-Speed, Urban Schedule An additional transient event is depicted in Figure7. This is more typical of urban vehicular operation (cf. The urban segment, or micro-trip, of the European NEDC driving cycle [3,4]), comprising a three-part acceleration, cruising section and deceleration. The whole schedule lasts 52 s, and the vehicle covers a distance a little over 300 m, reaching a maximum speed of 30 km/h. This schedule can be considered, for example, indicative of a bus route between two stops or a truck driving route between two stop lights. The important conclusions from the graphs presented in Figure7 are as follows: Owing to the much lower vehicle speeds involved, the rolling resistance term prevails over its aerodynamic counterpart throughout the whole transient schedule. Thus, ‘external’ or ‘internal’ parameters such as road-type, tire pressure and road gradient assume much greater importance, as will be discussed in Section4. A new property is depicted in Figure7, namely brake force, applicable during the deceleration phase; this, however, is maintained at overall small values (up to 6% of the maximum), owing to the rather mild deceleration profile, typical for a heavy vehicle. Another important finding from Figure7 is that the engine/vehicle emissions are of considerably lesser importance during decelerations. This is due to the fact that the dominating turbocharger lag effect is now absent [1,2], hence the production of pollutants very low (the engine practically operates in naturally aspirated mode and at low loading). Predictably, the same holds true for fueling and CO2 emissions as the resistance and inertia forces are diminishing during deceleration. In general, emissions are low too during the quasi-steady cruising section. The completely different driving (and engine operation) profile between the two examined driving schedules is best illustrated in Figure8 that demonstrates vehicle speed/acceleration distribution (left sub-diagram) and engine speed/power distribution (right sub-diagram). As noticed, the demand from the engine to cope with the prescribed transient is much harder during the first schedule, although the single highest vehicle acceleration is experienced during the second one. Energies 2018, 11, 295 10 of 21 Energies 2018, 11, x FOR PEER REVIEW 10 of 21

Figure 8. Comparison of vehicle speed/acceleration distribution and engine speed/power distribution Figure 8. Comparison of vehicle speed/acceleration distribution and engine speed/power distribution between the two examined transient schedules from Figures 4 and 7. between the two examined transient schedules from Figures4 and7.

Owing toto thethe muchmuch more more pronounced pronounced pollutants pollutant ands and fueling fueling during during the continuousthe continuous up-gear up-gear shift scheduleshift schedule of Section of Section 3.1, this 3.1, is thethis transient is the transien schedulet schedule that will that primarily will primarily be the basis be for the the basis parametric for the investigationparametric investigation that follows. that follows.

4. Parametric Study In the followingfollowing paragraphs,paragraphs, a comprehensivecomprehensive parametric analysis will be conductedconducted focusingfocusing on engine-out pollutant emissionsemissions (NO and soot), CO2 and fuel consumption over the previously described transienttransient schedules schedules (mostly, (mostly, the one of Sectionthe one 3.1 ). Toof doSection so the computational/experimental3.1). To do so the procedurecomputational/experimental discussed in Section procedure2 will be applied. discussed Overall, in Section six influential 2 will be parametersapplied. Overall, will be six investigated, influential groupedparameters into will three be investigated, categories. The grouped first category into three will categories. cover two The basic first vehicle category attributes, will cover namely two massbasic andvehicle gearbox attributes, (Section namely 4.1). Themass second and gearbox will focus (Secti on theon driver,4.1). The investigating second will driver focus ‘aggressiveness’ on the driver, andinvestigating the specific driver gear-shift ‘aggressiveness’ change schedule and the (Section specific 4.2 gear-shift). Lastly, change Section schedule 4.3 will present(Section the 4.2). effects Lastly, of roadSection characteristics, 4.3 will present namely the road-typeeffects of androad gradient. characteristics, Each parameter namely willroad-type be studied and separatelygradient. soEach as forparameter its effects will to be be studied readily discernible.separately so as for its effects to be readily discernible.

4.1. Effect of Vehicle Attributes (Mass and Gearbox) Perhaps the most obvious parameter inﬂuencinginfluencing a vehicle’s performance (and(and emissions) is the latter’s mass. mass. Two Two cases have been investigated apart from the nominal (corresponds to an unloaded vehicle). One with 1500 kg loading (e.g., bus with 20 passengers or half-loaded truck), and another with 3000 kg loading (e.g., fully-loaded busbus oror truck);truck); thethe emissionemission resultsresults areare illustratedillustrated inin FigureFigure9 .9. Predictably, thethe higherhigher thethe vehiclevehicle mass,mass, thethe higherhigher thethe amountamount ofof emittedemitted pollutants,pollutants, COCO22 and fueling. TheThe obviousobvious culprit culprit here here is theis the rolling rolling resistance resistance term, term, which which increases increases in proportion in proportion to vehicle to mass,vehicle hence mass, leadshence to leads higher to higher engine engine loads loads throughout throughout the transient the transient schedule, schedule, as the as the engine engine has has to overcometo overcome higher higher ‘internal’ ‘internal’ resistance; resistance; Equation Equation (3) (3) is is helpful helpful here, here,describing describing thethe rollingrolling resistance term [[4,26]:4,26]: Fr = cfrW cos θ = cfr mV g cos θ (3) FcWcosrfrθ cmgcos frV θ (3) where mV (kg), W (N) are the vehicle mass and weight, respectively, θ is the road grade (=0; investigated 2 laterwhere in m theV (kg), text, W in Section(N) are 4.3the), vehicle g is the mass gravitational and weight, acceleration respectively, (9.81 m/sθ is ),the and road cfr thegrade rolling (=0; resistanceinvestigated coefﬁcient, later in the studied text, in in Section more detail 4.3), ing is Section the gravitational 4.3 too. acceleration (9.81 m/s2), and cfr the rolling resistance coefficient, studied in more detail in Section 4.3 too.

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Figure 9. Effect of vehicle mass on fueling and emissions (NO, soot, CO2) during the first 64 s of the Figure 9. Effect of vehicle mass on fueling and emissions (NO, soot, CO2) during the ﬁrst 64 s of the first transient schedule. ﬁrst transient schedule.

FromFrom Figure Figure9, it9, isit revealedis revealed that that it it is is the the sootsoot productionproduction that that is is mostly mostly influenced influenced by bythe the increased vehicle mass. The higher the vehicle mass, the more intense the turbocharger lag phases increased vehicle mass. The higher the vehicle mass, the more intense the turbocharger lag phases during each acceleration in the cycle (owing to the need to overcome greater inertia), hence the lower during each acceleration in the cycle (owing to the need to overcome greater inertia), hence the the experienced air–fuel ratios. Supportive here is Figure 10 that presents the acceleration profile of lower the experienced air–fuel ratios. Supportive here is Figure 10 that presents the acceleration the three trucks as well as the instantaneous soot emissions and corresponding engine load. The profilehigher-mass of the three vehicles trucks experience as well as theharsher instantaneous accelerations, soot particularly emissions and at correspondinglow speeds, where engine the load. Theturbocharger higher-mass lag vehicles is more experiencepronouncedharsher as the turbocha accelerations,rger is called particularly to operat ate from low speeds,zero boost where [1,2]. the turbochargerThese harsher lag accelerations is more pronounced are, in turn, as reflected the turbocharger into instantaneous is called higher to operate engine from loads zero (lower boost air– [1,2]. Thesefuel harsher ratios too accelerations that cannot are, be inpredicted turn, reflected by the intocurrent instantaneous investigation) higher and enginemuch bigger loads (loweramount air–fuel of ratiosemitted too that soot, cannot as the beupper predicted sub-diagram by the of current Figure investigation)10 illustrates. Closer and muchexamination bigger of amount the lower of emittedsub- soot,diagram as the upperof Figure sub-diagram 10 reveals ofanother Figure interesting 10 illustrates. feature. Closer The examination higher-loaded of vehicles the lower are sub-diagram actually of Figureunable 10to revealsfollow the another prescribed interesting driving feature. schedule The when higher-loaded it comes to the vehicles ‘highway’ are actuallysegment, unableas the to followengine the cannot prescribed provide driving the increased schedule torque when needed it comes (the to 8850-kg the ‘highway’ vehicle follows segment, the as driving the engine schedule cannot provideup to thethe increasedpoint it reaches torque a speed needed of 100 (the km/h, 8850-kg wher vehicleeas the follows10,350-kg the one driving up to 91 schedule km/h). Furthermore, up to the point it reachesthe vehicle a speed response of 100 is km/h,worse with whereas increasing the 10,350-kg mass at the one beginning up to 91 of km/h). the transient Furthermore, event, owing the vehicle to the 1st-gear mass factor MF being extremely high. response is worse with increasing mass at the beginning of the transient event, owing to the 1st-gear Following the above, the comparative emission data demonstrated in Figure 9, corresponds to mass factor MF being extremely high. the first 64 s of the transient schedule, during which all three vehicles were able to follow exactly the Following the above, the comparative emission data demonstrated in Figure9, corresponds to prescribed driving pattern, hence a direct comparison was feasible. the firstOverall, 64 s of the increase transient of the schedule, vehicle mass during from which 7350 all to three8850 kg vehicles (+20%) were resulted able in to 44% follow increase exactly in the prescribedsoot, ‘only’ driving 5.4% pattern,in NO and hence 15% in a directCO2/fueling. comparison Further was increase feasible. to 10,350 kg (+41% increase in mass) ledOverall, to 90% increaseincrease in of the the emitted vehicle soot, mass 9% from in NO 7350 and to 30% 8850 for kg CO (+20%)2/fueling. resulted Interestingly, in 44% increase for all three in soot, ‘only’examined 5.4% in emissions, NO and 15% their in correlation CO2/fueling. with Furthervehicle mass increase proves to 10,350highly statistical kg (+41% (R increase2 = 0.99). in mass) led to 90% increase in the emitted soot, 9% in NO and 30% for CO /fueling. Interestingly, for all three 2 examined emissions, their correlation with vehicle mass proves highly statistical (R2 = 0.99). Energies 2018, 11, 295 12 of 21 Energies 2018, 11, x FOR PEER REVIEW 12 of 21 Energies 2018, 11, x FOR PEER REVIEW 12 of 21

Figure 10. Development of vehicle acceleration, engine load and instantaneous soot emissions during Figure 10. Development of vehicle acceleration, engine load and instantaneous soot emissions during Figurethe first10. Development transient schedule of vehicle for three acceleration, vehicle masses. engine load and instantaneous soot emissions during the ﬁrst transient schedule for three vehicle masses. the first transient schedule for three vehicle masses. Figure 11 expands on the previous results by incorporating in the analysis the effect of the Figure 11 expands on the previous results by incorporating in the analysis the effect of the installed installedFigure 11gearbox. expands Four on cases the are previous examined results here, bytwo in withcorporating the nominal in the6-speed analysis gearbox the of effect Table of 2, the gearbox.and Four another cases two are with examined a 12-speed here, gearbox two with(for either the nominal the 73506-speed or the 10,350-kg gearbox total of Table vehicle2, and mass). another installed gearbox. Four cases are examined here, two with the nominal 6-speed gearbox of Table 2, two withThe results a 12-speed from Figure gearbox 11 indicate (for either the beneficial the 7350 effects or the of 10,350-kg multi-speed total gearboxes, vehicle as mass). these clearly The results and another two with a 12-speed gearbox (for either the 7350 or the 10,350-kg total vehicle mass). lead to lower engine-out pollutants and CO2/fueling. fromThe Figure results 11 from indicate Figure the 11 beneﬁcial indicate the effects beneficial of multi-speed effects of gearboxes,multi-speed as gearboxes, these clearly as these lead clearly to lower engine-out pollutants and CO /fueling. lead to lower engine-out pollutants2 and CO2/fueling.

Figure 11. Effect of gearbox on fueling and emissions (NO, soot, CO2) during the first transient schedule.

Figure 11. Effect of gearbox on fueling and emissions (NO, soot, CO2) during the first transient schedule. Figure 11. Effect of gearbox on fueling and emissions (NO, soot, CO2) during the first transient schedule.

Energies 2018, 11, 295 13 of 21

TheEnergies apparent 2018, 11 explanation, x FOR PEER REVIEW here is again located in the number and magnitude of the turbocharger13 of 21 lag occurrences. Namely, the more the available gears in the gearbox, the smaller the accelerations throughoutThe the apparent driving explanation schedule, here resulting is again located in smaller in the speed number changes and magnitude and milder of the turbocharger turbocharger lag lag occurrences. Namely, the more the available gears in the gearbox, the smaller the accelerations phases [17]. The benefit is expressed in terms of lower emitted pollutants, primarily soot which is throughout the driving schedule, resulting in smaller speed changes and milder turbocharger lag mostlyphases influenced [17]. The by steepbenefit transientsis expressed [ 1in,2 terms]; Figure of lower 12 supports emitted pollutants, these arguments. primarily soot In thiswhich figure, is it is also revealedmostly (upperinfluenced sub-diagram) by steep transients that the[1,2]; initial Figure acceleration 12 supports these from arguments. standstill In is morethis figure, demanding it is for the vehiclealso revealed equipped (upper with sub-diagram) the 12-speed that gearbox;the initial acceleration this is due from to its standstill much is higher more demanding MF value for (1st-gear ratio beingthe vehicle 14.94 equipped compared with to the 7.72 12-speed of the gearbox; 6-speed this gearbox).is due to its much Were higher it not MF for value this (1st-gear ‘discrepancy’, the 12-speed-gearboxratio being 14.94 vehiclescompared wouldto 7.72 of have the 6-speed exhibited gear evenbox). Were lower it not cumulative for this ‘discrepancy’, soot and NO the 12- emissions speed-gearbox vehicles would have exhibited even lower cumulative soot and NO emissions throughout the examined transient schedule. throughout the examined transient schedule.

Figure 12. Development of selected gear, engine load and instantaneous soot emissions during the Figure 12.firstDevelopment transient schedule of selectedfor the two gear, examined engine gearboxes. load and instantaneous soot emissions during the first transient schedule for the two examined gearboxes. 4.2. Effect of Driving Attributes (Driver ‘Aggressiveness’ and Gear-Shift Schedule) 4.2. Effect ofFurther Driving interesting Attributes comparisons (Driver ‘Aggressiveness’ can be made on andthe basis Gear-Shift of the driver’s Schedule) driving habits. In this section, the effect of the accelerating and the gear-shift profile will be studied. Figure 13 focuses on Further interesting comparisons can be made on the basis of the driver’s driving habits. In this the accelerating profile throughout the transient event, investigating one more aggressive (average section,acceleration the effect of+20% the compared accelerating to the and nominal) the gear-shift and one profilemilder will(−20% be compared studied. to Figure the nominal) 13 focuses on the acceleratingapproach. profileIt is noted throughout that the acceleration the transient remains event, identical investigating when the 1st one gear more is selected aggressive (owing to (average accelerationalready +20% very high compared MF value), to whereas the nominal) from the and2nd gear one onwards, milder (the−20% differ comparedent accelerating to profile the nominal) approach.is applied. It is noted that the acceleration remains identical when the 1st gear is selected (owing to already verySince high the vehicle MF value), speed whereasprofile is fromdifferent the for 2nd each gear case, onwards, the comparison the different is now acceleratingmade on the profile basis of the covered distance, namely 895 m—see Figure 14 (middle sub-diagram). For soot and is applied. CO2/fueling, the results presented in Figure 13 were rather expected. More aggressive driving results Sincein harsher the vehicle accelerations, speed hence profile more is differentpronounced for tu eachrbocharger case, lag the that comparison enhances the is production now made of on the basis ofsoot. the The covered increase distance, in engine namely loading is 895 then m—see reflected Figure into increased 14 (middle fuel consumption. sub-diagram). A different For soot and CO2/fueling,picture theis, however, results presented drawn when in FigureNO emissions 13 were are rather investigated. expected. As observed More aggressive in Figure driving13, and results in harsherfurther accelerations, expanded in hencethe upper more sub-diagram pronounced of Figu turbochargerre 14, NO emissions lag that increase enhances during the both production the of milder and more aggressive acceleration. For the latter case, the explanation is obvious. For the soot. The increase in engine loading is then reflected into increased fuel consumption. A different former (−20% acceleration rate), this result was perhaps not anticipated. The instantaneous absolute picture is,values however, of emitted drawn NO whenwhen NOdriving emissions conservatively are investigated., are, of course, As lower. observed However, in Figure owing 13 to, and the further expandedmuch in smaller the upper accelera sub-diagramtion (compared of to Figure the other 14 cases),, NO the emissions time needed increase to reach during the distance both of the895 milder and more aggressive acceleration. For the latter case, the explanation is obvious. For the former (−20% acceleration rate), this result was perhaps not anticipated. The instantaneous absolute values of emitted NO when driving conservatively, are, of course, lower. However, owing to the much smaller acceleration (compared to the other cases), the time needed to reach the distance of 895 m is longer, Energies 2018, 11, 295 14 of 21

Energies 2018, 11, x FOR PEER REVIEW 14 of 21 hence more NO is cumulatively emitted, as the upper sub-diagram of Figure 14 depicts (recall that m is longer, hence more NO is cumulatively emitted, as the upper sub-diagram of Figure 14 depicts NO is also emitted when cruising, much more than soot). The same result would be reached if the (recall that NO is also emitted when cruising, much more than soot). The same result would be comparison was made on the basis of the driving time for all three scenarios. In that case, the emitted reachedEnergies if 2018 the, 11 comparison, x FOR PEER REVIEW was made on the basis of the driving time for all three scenarios.14 In of 21that NO for the lower accelerating case would be lower (owing to milder accelerations) but the covered case, the emitted NO for the lower accelerating case would be lower (owing to milder accelerations) distancem is evenlonger, shorter; hence more distance NO is being cumulatively the denominator emitted, as in the the upper g/km sub-diagram values of Figureof Figure 14 .14 depicts but(recall the covered that NO distance is also evenemitted shorter; when distance cruising, being much the more denominator than soot). in theThe g/km same values result ofwould Figure be 14. reached if the comparison was made on the basis of the driving time for all three scenarios. In that case, the emitted NO for the lower accelerating case would be lower (owing to milder accelerations) but the covered distance even shorter; distance being the denominator in the g/km values of Figure 14.

Figure 13. Effect of vehicle acceleration profile on fueling and emissions (NO, soot, CO2) during the Figure 13. Effect of vehicle acceleration proﬁle on fueling and emissions (NO, soot, CO2) during the first transient schedule. ﬁrst transient schedule. Figure 13. Effect of vehicle acceleration profile on fueling and emissions (NO, soot, CO2) during the first transient 10schedule. 8

6 10

4 8

2 6

0 4 1.6 2 1600

0 1.2 1200 1.6 1600 0.8 800 1.2 1200

0.4 400 0.8 800

00.4 4000 120 100 0 0 80120 60100 40 80 60 20 40 0 200 5 10 15 20 25 30 35 40 45 50 55 60 65 70 0 Time (s) 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 Time (s) Figure 14. Development of vehicle speed, acceleration, distance covered and instantaneous NO Figure 14. Development of vehicle speed, acceleration, distance covered and instantaneous NO Figureemissions 14. Developmentduring the first oftransi vehicleent schedule speed, acceleration,for the two examined distance vehicle covered acceleration and instantaneous profiles. NO emissions during the first transient schedule for the two examined vehicle acceleration profiles. emissions during the ﬁrst transient schedule for the two examined vehicle acceleration proﬁles.

Energies 2018, 11, 295 15 of 21 Energies 2018, 11, x FOR PEER REVIEW 15 of 21

FigureFigure 1515 illustrates the the effects effects of of the gear-c gear-changehange timing on the cu cumulativemulative soot, soot, NO NO and and fueling/COfueling/CO2 2duringduring the the studied studied transient transient schedule. schedule. Two Two cases cases are are examined examined here here apart apart from from the the nominal,nominal, namelynamely oneone with with earlier earlier and and one withone laterwith gear-changelater gear-change strategy. strategy. A ﬁrst important A first conclusionimportant conclusionfrom the analysis from the is analysis that when is that an earlier when gear-shiftan earlier scenariogear-shift was scenario selected, was the selected, vehicle the was vehicle unable was to unablefollow to the follow prescribed the prescribed transient transient schedule. schedule. This was Th dueis was to thedue fact to the that fact the that engine the engine was called was called upon uponto operate to operate with higher with loadinghigher loading at lower at engine lower speeds, engine whichspeeds, was which not always was not feasible. always In feasible. general, Inthe general, engine operatedthe engine at higheroperated loading at higher levels load throughouting levels the throughout whole transient the whole (upper transient sub-diagram (upper of sub-diagramFigure 16) when of Figure earlier 16) gear when changes earlier were gear selected, changes a factwere that selected, inﬂuenced a fact decisively that influenced the amount decisively of the theemitted amount pollutants. of the emitted As expected pollutants. also, theAs expected late-change also, scenario the late-change led to higher scenario engine led speeds to higher during engine the speedstransient during event the (middle transient sub-diagram event (middle of Figure sub-diagram 16). of Figure 16).

FigureFigure 15. EffectEffect of of gear gear change change profile proﬁle on on fueling and emissions (NO, soot, CO 22)) during during the the first ﬁrst transienttransient schedule. schedule.

FromFrom the the vehicle vehicle acceleration acceleration profil proﬁle,e, it itis is obvious obvious that that the the earlie earlierr gear-change gear-change timing timing led led also also to moreto more abrupt abrupt accelerations, accelerations, a fact a factthat thatwas wasresponsi responsibleble for the for increased the increased soot emissions. soot emissions. On the On other the hand,other hand,NO increased NO increased more when more the when gears the where gears wherechanged changed later than later normal, than normal, as owing as to owing the higher to the enginehigher speeds engine speedsinvolved, involved, the quasi-steady the quasi-steady NO emission NO emissionss were higher were too. higher Interestingly, too. Interestingly, fueling fuelingseems toseems be independent to be independent of the gear-shift of the gear-shift strategy. strategy. This comes This as comes a result as aof result the combined of the combined effect of effect higher of enginehigher speeds engine coupled speeds coupled with lower with accelerations lower accelerations (late-change (late-change scenario) scenario) or vice versa or vice (for versa the (forearlier- the changeearlier-change case). case).

Energies 2018, 11, 295 16 of 21 Energies 2018, 11, x FOR PEER REVIEW 16 of 21 Engine Load (%) Engine Speed (rpm) ) 2 (m/s Vehicle Acceleration

Figure 16. Development of vehicle acceleration, engine speed and load during the first transient Figure 16. Development of vehicle acceleration, engine speed and load during the first transient schedule for the two examined gear-shift strategies. schedule for the two examined gear-shift strategies. 4.3. Effect of Road Characteristics (Road-Type and Gradient) 4.3. Effect of Road Characteristics (Road-Type and Gradient) The last category of examined variables deals with road parameters, namely road-type and Thegradient. last categoryFigure 17, ofin examinedparticular, variablescompares dealsfour combinations with road parameters, of road-types/vehicle namely road-typetires. It is and gradient.reminded Figure here 17 that, in the particular, road-type and compares tire influence four the combinations friction coefficient of road-types/vehicle between vehicle and tires. road. It is remindedThus, herethey determine that the road-type the rolling andresistance tire influence force Fr through the friction Equation coefficient (3) mentioned between earlier vehicle in the andtext. road. The investigated parameter here is the friction coefficient cfr in Equation (3), which is influenced by Thus, they determine the rolling resistance force Fr through Equation (3) mentioned earlier in the text. tire deformation (biggest contribution), tire penetration and slippage. In general, cfr increases with The investigated parameter here is the friction coefficient c in Equation (3), which is influenced by increasing loading, vehicle speed and decreasing tire pressure.fr tire deformationTwo road-types (biggest are contribution), studied, cold and tire hot penetration blacktop, andeach slippage.one with radial In general, or bias cplyfr increases tires. The with increasingpresented loading, results vehicle in Figure speed 17 andseem decreasing rather self-exp tirelanatory pressure. in this case. Increasing the resistance Twocoefficient road-types between are tires studied, and road, cold leads and to an hot increa blacktop,se in the respective each one force, with hence radial engine or bias load plyand tires. The presentedemitted pollutants results inas Figurewell as fuel17 seem consumption. rather self-explanatory in this case. Increasing the resistance coefficientOne between important tires remark and road, is that leads the tire to aneffect increase is greater in thethan respective the road one, force, as moving hence engine from radial load and emittedto bias pollutants ply has asa larger well as impact fuel consumption. on the resistance coefficient than when moving from cold to hot Oneblacktop. important Further, remark increasing is that the theresistance tire effect coefficient is greater results than in thethe vehicle road one, being as unable moving to fromfollow radial the prescribed transient schedule at high speeds. For the fourth, most demanding, examined case (hot to bias ply has a larger impact on the resistance coefficient than when moving from cold to hot blacktop/bias-ply tires), the vehicle follows the schedule up to the 73rd second. Consequently, the blacktop. Further, increasing the resistance coefficient results in the vehicle being unable to follow results presented in Figure 17 are limited to the 73 first seconds of the transient schedule (recall that the prescribedsimilar remarks transient were schedulemade earlier at highin the speeds. text, in ForSection the 4.1, fourth, when most discussing demanding, another examinedimportant case (hot blacktop/bias-plycomponent of the rolling tires), resistance the vehicle force, follows namely thevehicle schedule mass). up to the 73rd second. Consequently, the resultsA presentedthird important in Figure conclusion 17 are is limitedthat although to the th 73e examined first seconds resistance of the coefficients transient schedulespan over (recalla that similarrather wide remarks range, were their made effect earlieron emissions in the is text, much in smaller, Section as4.1 the, when aerodynamic discussing term another prevails important over componentits rolling of theresistance rolling counterpart resistance force,at higher namely vehicl vehiclee speeds, mass). hence the rolling resistance effects are Anarrowed third important down; this conclusion is especially is true that for although NO. The the relation examined between resistance soot or CO coefficients2/fueling and span road over a 2 ratherfriction wide range,was found their highly effect statistical, on emissions with R is values much smaller,0.975 and as 0.972 the respectively. aerodynamic term prevails over its Finally, the effect of road grade θ is investigated. Grade is defined as the ‘rise’ over the ‘run’ rolling resistance counterpart at higher vehicle speeds, hence the rolling resistance effects are narrowed (vertical distance over horizontal), and is the tangent of the grade angle θ. Frequently it is expressed down; this is especially true for NO. The relation between soot or CO2/fueling and road friction was as a percentage after multiplying by 100. The corresponding road grade resistance term Fgr in 2 foundEquation highly statistical,(1) is given withby [30]: R values 0.975 and 0.972 respectively. Finally, the effect of road grade θ is investigated. Grade is defined as the ‘rise’ over the ‘run’ FWsinθ mgsinθ (vertical distance over horizontal), and isgr the tangent of the V grade angle θ. Frequently it is expressed(4) as a percentage after multiplying by 100. The corresponding road grade resistance term Fgr in Equation (1) is given by [30]: Fgr = W sin θ = mVg sin θ (4) Energies 2018, 11, x FOR PEER REVIEW 17 of 21

Three cases are examined in Figure 18, apart from the nominal one of zero grade, namely 2%, 4% and 6%. Owing to the considerable burden the road grade imposes on the vehicle [27], for this investigation, the second transient schedule (from Figure 7) will be examined, i.e., the low-speed urban scenario; the first transient was impossible for the vehicle to run in the prescribed manner (particularly for vehicle speeds up to and exceeding 100 km/h). On the other hand, even the 6% road Energiesgrade2018 was, 11 manageable, 295 by the engine during the second, milder in terms of vehicle speeds, transient17 of 21 schedule.

Figure 17. Effect of road-tire type on fueling and emissions (NO, soot, CO2) during the first 73 s of the Figure 17. Effect of road-tire type on fueling and emissions (NO, soot, CO2) during the ﬁrst 73 s of the first transient schedule. ﬁrst transient schedule.

ThreeThe results cases arepresented examined in Figure in Figure 18 seem 18 ,rather apart obvious from the when nominal fueling/CO one2 of are zero concerned; grade, increasing namely 2%, the road gradient increases accordingly the total force the vehicle has to overcome, hence higher 4% and 6%. Owing to the considerable burden the road grade imposes on the vehicle [27], for this engine loading is established (Figure 19), leading to elevated fuel consumption and CO2. The correlation investigation, the second transient schedule (from Figure7) will be examined, i.e., the low-speed urban between road gradient and fuel consumption is ‘adequately’ statistical (R2 = 0.88). What is quite scenario; the first transient was impossible for the vehicle to run in the prescribed manner (particularly interesting, however, is the effect the road grade has on the emitted pollutants. for vehicle speeds up to and exceeding 100 km/h). On the other hand, even the 6% road grade was From Figure 18 it is obvious that the biggest amount of emitted soot and NO is noticed when the manageablevehicle operates by the on engine a flat duringroad. On the the second, contrary, milder as the in grade terms increases, of vehicle soot speeds, and NO transient are reduced. schedule. In orderThe to results interpret presented this, rather in unorthodox, Figure 18 seem findin ratherg, the middle obvious and when upper fueling/CO sub-diagram2 are of Figure concerned; 19 increasingare helpful. the Notice road gradient in Figure increases 19, that for accordingly the most part the of total the forcespecific the driving vehicle schedule, has to overcome, soot and NO hence higheremissions engine are loading rather low is established(very close to (Figure zero), owing19), leading to cruising to elevated at low speed fuel consumptionand deceleration. and It COis 2. The correlation between road gradient and fuel consumption is ‘adequately’ statistical (R2 = 0.88). What is quite interesting, however, is the effect the road grade has on the emitted pollutants. From Figure 18 it is obvious that the biggest amount of emitted soot and NO is noticed when the vehicle operates on a flat road. On the contrary, as the grade increases, soot and NO are reduced. In order to interpret this, rather unorthodox, finding, the middle and upper sub-diagram of Figure 19 are helpful. Notice in Figure 19, that for the most part of the specific driving schedule, soot and NO emissions are rather low (very close to zero), owing to cruising at low speed and deceleration. It is practically the three accelerations during the beginning of the transient schedule that determine the Energies 2018, 11, 295 18 of 21

Energies 2018, 11, x FOR PEER REVIEW 18 of 21 total amount of the emitted pollutants, and particularly the second acceleration, which, as was depicted in Figurepractically8, is the the most three demanding accelerations from during the the ones beginning studied of in the this transient work (corresponding schedule that determine to 5.22 km/h/s). the As istotal made amount obvious of the from emitted the engine pollutants, loading and graphparticularly in Figure the second 19, the acceleration, higher the which, grade, as the was higher the instantaneousdepicted in Figure (and 8, average)is the most engine demanding loading. from This,the ones in turn,studied leaves in this very work small (corresponding room for to abrupt 5.22 km/h/s). As is made obvious from the engine loading graph in Figure 19, the higher the grade, accelerations, hence the turbocharger lag occurrences are considerably milder. Since the engine loading the higher the instantaneous (and average) engine loading. This, in turn, leaves very small room for is high,abrupt the turbochargeraccelerations, hence is called the uponturbocharger to operate lag fromoccurrences higher are initial considerably points, hence milder. the Since turbocharger the lag isengine minimized loading [1 is,2 ,high,5,6]. the Similar turbocharger results hadis called been upon reached to operate in a previous from higher publication initial points, [9] applyinghence a ® zero-dimensionalthe turbocharger commercial lag is minimized code(GT-Power [1,2,5,6]. Similarsoftware, results had version been 7.2reached from in Gamma a previous Technologies publication LLC, Chicago,[9] applying IL, USA). a zero-dimensional Following this commercial logic, the 2% code grade (GT-Power is the® one software, with theversion second 7.2 from highest Gamma emission overshootTechnologies after the LLC, ﬂat Chicago, road case. IL, USA). As mentioned Following above,this logic, it isthe the 2% second grade is acceleration the one with inthe the second transient (betweenhighest 10 emission and 14 s),overshoot i.e., the after most the abrupt flat road one, case. that As ismentioned mostly contributing above, it is the to second soot emissions.acceleration Since duringin thisthe transient acceleration, (between the emission10 and 14 overshoot s), i.e., the ismost very abrupt pronounced one, that on is the mostly ﬂat road,contributing it affects todecisively soot the cumulativeemissions. Since results during too. this On acceleration, the other hand, the emissi duringon overshoot the acceleration is very pron betweenounced 15on andthe flat 18 road, s, as well it affects decisively the cumulative results too. On the other hand, during the acceleration between as during the cruising period, the trend is reversed, and the higher road grades lead also to (slightly) 15 and 18 s, as well as during the cruising period, the trend is reversed, and the higher road grades higherlead emissions, also to (slightly) as expected. higher However, emissions, in as these expected. cases, However, the soot andin these NO cases, absolute the valuessoot and are NO overall veryabsolute small, hencevalues theare overall emission very overshoot small, hence during the emission the second overshoot acceleration during the proves second decisiveacceleration for the cumulativeproves decisive emissions. for the cumulative emissions.

Figure 18. Effect of road gradient on fueling and emissions (NO, soot, CO2) during the second Figure 18. Effect of road gradient on fueling and emissions (NO, soot, CO ) during the second transient schedule. 2 transient schedule.

Energies 2018, 11, 295 19 of 21 Energies 2018, 11, x FOR PEER REVIEW 19 of 21

Figure 19. Development of vehicle speed, engine load and instantaneous soot during the second Figure 19. Development of vehicle speed, engine load and instantaneous soot during the second transient schedule for the four examined road-grade cases. transient schedule for the four examined road-grade cases.

5. Summary and Conclusions 5. Summary and Conclusions A detailed parametric study was conducted on a turbocharged truck running on two different A detailed parametric study was conducted on a turbocharged truck running on two different transient schedules, a 100-s continuous up-gear shift one, typical when entering a highway, and a 52-s transient schedules, a 100-s continuous up-gear shift one, typical when entering a highway, and a 52-s urban route, typical when driving between two stop lights. The mean-value modeling analysis was urban route, typical when driving between two stop lights. The mean-value modeling analysis was based on a previously conducted transient engine testing, applying a detailed vehicle simulation model. based on a previously conducted transient engine testing, applying a detailed vehicle simulation model. The effect of six important parameters was studied, namely vehicle mass and gearbox, driver The effect of six important parameters was studied, namely vehicle mass and gearbox, driver ‘aggressiveness’ and gear-shift strategy, and road-type and gradient. The results from the analysis ‘aggressiveness’ and gear-shift strategy, and road-type and gradient. The results from the analysis regarding engine-out emissions as well as fuel consumption/CO2 can be summarized as follows for regarding engine-out emissions as well as fuel consumption/CO can be summarized as follows for the engine in hand: 2 the engine in hand: • Vehicle speed was identified as the most critical property influencing performance and practically • allVehicle vehicle speed parameters. was identified In asthis the respect, most critical low- propertyspeed (‘urban’) influencing and performance high-speed and(‘motorway’) practically segmentsall vehicle were parameters. found to In thisaffect respect, vehicle low-speed operation (‘urban’) in a different and high-speed way. For (‘motorway’)the engine, it segments was the werespeed foundfluctuations to affect (due vehicle to gear operation changes) inthat a affected different mostly way. the For emissions the engine, and it performance. was the speed • Atfluctuations each step (duein the to transient gear changes) schedule that where affected a steep mostly vehicle the emissions velocity andincrease performance. was experienced, • aAt peak each in step engine-out in the transient emissions schedule (particularly where soot) a steep was vehiclenoticed velocity as well owing increase to turbocharger was experienced, lag. Thisa peak was in more engine-out pronounced emissions during (particularly the first examined soot) was transient noticed as schedule well owing owing to turbochargerto its continuous lag. Thisup-gear was shift more profile. pronounced In fact, during one abrupt the first acceleration examined produced transient schedulemore engine-out owingto soot its continuousthan many minutesup-gear shiftof constant-speed profile. In fact, operation. one abrupt acceleration produced more engine-out soot than many • Increasingminutes of the constant-speed vehicle mass operation. resulted in deterioration of its performance at high velocities, hence the vehicle failed to follow the prescribed driving schedule; elevated amount of pollutants was also observed. Similar results were reached when studying the effect of road-tire friction and road grade; all the above being parameters directly affecting the vehicle resistance forces.

Energies 2018, 11, 295 20 of 21

• Increasing the vehicle mass resulted in deterioration of its performance at high velocities, hence the vehicle failed to follow the prescribed driving schedule; elevated amount of pollutants was also observed. Similar results were reached when studying the effect of road-tire friction and road grade; all the above being parameters directly affecting the vehicle resistance forces. • The selected gearbox was a critical parameter affecting driving behavior as well as emissions and performance. More specifically, for gearboxes with high number of gears, the accelerations were in general milder (hence the amount of soot lower), although the vehicle drivability is expected to be adversely affected. • Predictably, driving in a more aggressive way, as a result of higher average accelerations, had significant impact on all emissions and the vehicle performance in general. • A noteworthy statistical correlation was established between pollutant emissions (NO, soot) and vehicle mass or road-tire friction, as well as between fueling/CO2 and vehicle mass, road-tire friction and road grade. • From the range of values investigated in this paper, and for the current engine/vehicle combination, it seems that the most critical parameters for the emission of NO and soot are the vehicle mass, the driving ‘aggressiveness’ and the exact gear-change profile (the latter only for soot). Vehicle mass, driving ‘aggressiveness’ and road-grade were identified as the most influential parameters for the emission of CO2.

Acknowledgments: The authors would like to thank Cambustion Ltd. and particularly M. Peckham for the loan of the CLD500 NO analyzer and his continuous support during the experimental investigation of the engine. The significant contribution of A. Dimaratos in setting up the experimental installation and carrying out the measurements is greatly acknowledged. Author Contributions: George Triantafillou developed the vehicle model subroutines and was responsible for the program runs and analysis of the results. Evangelos G. Giakoumis supervised the study, authored the paper and formulated many of the final graphs. Conflicts of Interest: The authors declare no conflict of interest.

References

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