physical features of particles emitted from an automotive

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Chemical/physical features of particles emitted from an automotive modern dual-fuel methane-Diesel engine Chiara Guido, Michela Alfe', VALENTINA GARGIULO, Pierpaolo Napolitano, Carlo Beatrice, and Nicola Del Giacomo Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01011 • Publication Date (Web): 03 Jul 2018 Downloaded from http://pubs.acs.org on July 7, 2018

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Energy & Fuels

Chemical/physical features of particles emitted from an automotive modern dual-fuel methane-Diesel engine Chiara Guido1, Michela Alfe'2, Valentina Gargiulo2, Pierpaolo Napolitano1, Carlo Beatrice1, Nicola Del Giacomo1 1.

Istituto Motori - C.N.R., Napoli – Italy

2.

Istituto di Ricerche sulla Combustione - C.N.R., Napoli - Italy

ACS Paragon Plus Environment

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KEYWORDS dual-fuel engine, methane, soot, automotive Diesel-engine

ABSTRACT

The Dual Fuel (DF) Diesel Natural Gas (NG) combustion concept for automotive engine applications has received increased attention in the recent years. The reasons of such interest lie in the need to identify energy sources alternative to fossil oil fuels, valid and competitive from an environmental point of view. Gaseous fuels, like NG and methane, are very promising and highly attractive to this aim because of their easy availability, wide-spread distribution infrastructure, low-cost, and clean-burning qualities. With respect to conventional Diesel fuel, in fact, NG and methane permit a significant reduction of CO2 and are less prone to soot formation. Notwithstanding the significant soot reduction offered by the DF engine combustion mode is a consolidated result, the compliance with future stringent regulations on engine particulate emissions will still require the adoption of after-treatment devices, like the Diesel Particulate Filter (DPF). So, a complete characterization of the particles emissions in DF configuration appears of great interest for the design of future DPF equipping DF engines. To this aim, an experimental research activity was performed on a multi-cylinder automotive engine operated in DF mode, with a fixed methane substitution ratio value. The tests were carried out in transient conditions, typical of the engine homologation cycles, comparing the engine performance and emissions in conventional Diesel and DF combustion modes. The present paper is mainly focused on the results of the engine-out soot particles analysis, performed by means of on-line and off-line measurements techniques. The diagnostic systems led to the characterization of emitted particles, in terms of soot mass concentration, Particle Number (PN), Particle Size


2

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Energy & Fuels

Distribution Function (PSDF) and chemical-physical features. The use of methane in the DF engine showed relevant impact in terms of soot mass and PN reductions, but not on the particle average size. On the other side, the combustion mode proved to be not influential on the collected soot reactivity, as well as to show a negligible impact on the soot chemico-physical features.

Introduction The even more stringent worldwide emission legislation, the instability of the fuel price and the not very clear limit of fossil fuel reserves, push researchers to investigate on high efficiency and low emission technologies in the field of internal combustion engines (ICE). The use of alternative fuels together with the adoption of non-conventional combustion concepts are possible routes to reach these objectives. Fuel decarbonisation and gaseous fuels are two key points for emission lowering, including the CO2. On this way, the dual fuel (DF) engine for automotive applications based on the use of natural gas (NG) and Diesel has received increased attention in recent years [1-5]. In DF CI engine, the gaseous fuel is premixed with air in the intake manifold and subsequently compressed as in a conventional Diesel engine. A certain amount of high-cetane liquid fuel (conventionally Diesel fuel), usually called pilot, is then injected at the end of the compression stroke providing the energy to ignite the mixture and initiate the combustion. Within this framework, due to the higher-octane number (ON) of NG, DF combustion concept can be considered as reactivity controlled combustion (RCCI) combining direct injection (DI) of a high-reactivity fuel, with a premixed low reactivity fuel. In this mode, the combustion phasing and duration can be controlled, to some extent, through the reactivity gradient [2].


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The high ON of NG makes the fuel suitable for CI engines which usually operate with relatively high compression ratio (CR). Furthermore, NG is mainly constituted by methane (CH4) and has a favorable H/C ratio in terms of CO2 reduction compared to conventional fuels. However, at low engine loads conditions, the low reactivity premixed fuel tends to generate over lean NG-air regions inside the combustion chamber, with subsequent high emissions of CO and THCs (mainly CH4) [1,5]. Moreover, DF systems, based on port fuel injection of the gaseous fuel, make conversion of existing engines relatively simple, maintaining the full Diesel capability in case NG is unavailable. It is a consolidated opinion that DF combustion concept is an efficient way for controlling both NOx and PM emissions even on existing Diesel engines [6,7]. The PM reduction is an expected result when a high-sooting fuel (like Diesel) is replaced with a low-sooting fuel (like methane). Experiences reporting exhaust PM emissions in DF mode, higher than the corresponding Diesel ones, are very few. A slight soot increment was observed in [6], mainly at high loads, for moderate fuel replacement rate of Diesel with NG, whereas the charge temperature was inferior compared to the one under normal Diesel operation. Anyway, the majority of the trends reported in literature indicates a significant reduction of PM emissions for Diesel energy substitution rate (SRe) above 50% [8]. In particular, from previous experiences it was evidenced that in case of automotive DF engines operating with SRe ≥ 50%, the exhaust PM suppression at engine out is in the order of 60÷80% [6-8]. Therefore, also with the adoption of high SRe values, it is expected that the DF engine could not match the current Euro 6 PM emission target of 5 mg/km, as clearly reported in [8]. So, the use of a Diesel Particulate Filter (DPF) is required, even if the relevant soot load,


4

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Energy & Fuels

reduction related to the DF operation mode, could certainly increase the interval time between two consecutive filter regenerations. With this premise, it appears extremely important the evaluation of the chemico-physical characteristics of the emitted PM, as well as the characterization of the engine-out particles, in terms of total number and Particle Size Distribution Function (PSDF). Such information is of high relevance for the design of tailored DPFs for DF engines and their management during the regeneration events. They are usually available for conventional Diesel engine, but literature concerning the impact of the DF operating mode on the structural properties of the emitted particles is still not fully exhaustive. Moreover, the studies on this topic should be representative of the dynamic operating conditions typical of the automotive engines during urban and extra-urban runs. In the described scenario, a research activity was carried out on a multi-cylinder automotive engine installed on a test bench and operated in DF mode along the New European Driving Cycle (NEDC) [9]. The engine was properly calibrated to work in DF mode in almost whole the operating area, except at idle conditions. In this preliminary assessment, pure methane was employed as gaseous fuel in order to avoid possible effects on PM features from other components of the NG (e.g. CO, H2, ethane etc.) [10]. Several diagnostic devices were employed in order to monitor gaseous emissions and PSDF during the whole test cycle, and to collect particulate matter samples for chemico-physical analysis. The study outlined a sort of uninfluenced of the CH4 on the collected soot reactivity and indicated a relevant impact of CH4 on PM and PSDF reduction in the test cycle, but not on the average size, as well as, a negligible impact of DF mode on the soot chemico-physical features.


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Details on the material, method and the main outlined results are described in the following sections. Material and methods Laboratory engine system. The engine employed in the experimental activity was a Euro 5 four-cylinder 2L Diesel engine, properly modified for DF operations, equipped with a closed coupled DOC+DPF. A schema of the engine layout along with its main characteristics is reported in Figure 1.

Figure 1. Main characteristics of the experimental engine and schema of the whole engine layout. In its version series the engine is equipped with a pressure sensor glow plug (BERU). Therefore, the ECU is able to automatically adapt cylinder per cylinder and cycle per cycle the desired targets of 50% of the fuel mass burnt (MBF50%) and of the Indicated Mean Effective Pressure (IMEP), by the direct control of the Start Of main Injection (SOImain) and main injection quantity (ETmain) [11]. One additional pressure sensor (Kistler 6052) was placed on the second cylinder


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Energy & Fuels

and taken as reference for the BERU sensors data output. The engine was coupled with a variable frequency fast response dynamometer (AVL Dynodur) that permits the simulation of steady-state and transient operating conditions. The engine was fully instrumented for selected indicated measurements, stored through an AVL IndiSmart indicating system and for the whole thermal characterization both at the intake and exhaust lines (Figure S1, supporting information). The Dual Fuel methane-Diesel engine. The experimental engine has been properly adapted by means of an automotive methane PFI retrofit system. The port fuel injection has been realized through PFI gas injectors placed in the not-swirled runner just upstream the intake valve of each cylinder. The gas injectors are fed by a methane low-pressure line operating at a pressure of about 5 bar. Both Diesel and methane injectors were instrumented with current sensors in order to record the injection signals. Emerson Micro Motion ELITE Coriolis flow meter was employed for the gas flow measurement. The gas injection management was operated by an additional Electronic Control Unit (ECU), that was synchronized with the Diesel ECU. Diesel consumption was measured by means of both a second Emerson Micro Motion Elite Coriolis flow meter and an AVL 733 gravimetric balance. On-line emissions characterization. Exhaust gas was sampled at engine outlet downstream of the turbine and 0.5 m downstream the CDPF in order to fully characterize exhaust emissions through the whole exhaust system. In particular, gaseous emissions on-line measurements were performed by means of a Horiba Mexa 7100 gas analysis system. A transient high sensitive soot photo-acoustic sensor measured the soot mass emission (AVL, MicroSoot Sensor). The counting and sizing of particles were performed by means of a Differential Mobility Spectrometer (Cambustion DMS 500). The


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measurement principle of DMS500 is based on a deflection of electrically charged particles combined

with

electrical

counting.

More

details

on

measurement

principle

and

characteristics/performance of the device can be found in [12]. Testing methodology. The fuels employed in the tests campaign were a EN590 compliant Diesel (hereinafter abbreviated as D) and methane, whose main characteristics are listed in Table 1. Table1. Fuel properties. Feature

EN590 Diesel

Methane

Density [kg/m3] STP

840

0.788

AutoignitionTemp [°C]

300

595

Cetane Number (CN)

53

-

OctaneNumber (ON)

-

≥120

LHV [MJ/kg]

42.95

49.5±0.2

AFRstoich [-]

14.7

17.2

H/C

~1.86

4

The percentage of the premixed methane was quantified on energy basis (SRe) according to the following equations: =

∙

∙ ∙

∙ 100 [%]

(1)

Where mCH4 and md are the mass flow rates of CH4 and Diesel fuel, while LHVCH4 and LHVd represent the lower heating values for methane and Diesel fuel, respectively. The experimental activity foresaw transient operating conditions, testing the engine during its homologation cycle, the New European Driving Cycle (NEDC), in D and DF modes. A first screening of the DF engine operating in transient mode showed very high gaseous unburned


8

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Energy & Fuels

emissions and combustion stability issues as the SRe exceed the 50% at partial loads. As consequence, a flat DF calibration map based on a constant gas substitution of 50% (on energetic basis) was adopted to define the amount of methane to be injected. The Euro 5 Diesel calibration was kept for all other control parameters (injection settings, EGR, boost pressure, etc.) and only the Diesel quantity of the main injection was reduced, accordingly with SRe target. To avoid excessive unburned methane emissions in DF configuration, the gas injection was excluded at idle condition. Such DF calibration gave an average substitution ratio over the whole homologation NEDC test of about 35%. About the SOI of PFI methane injectors, it was settled constant at 340° Crank Angle (CA) Before Top Dead Center (BTDC) in the whole engine operating map. The tests were repeated three times in D and DF mode in order to validate the engine test-to-test repeatability. A complete engine characterization, in terms of pollutant emissions and performance, was carried out, as it will be described in the results section. Off-line measurement: soot sampling, soot pre- treatment and characterization methods. The total particulate was collected from the exhaust pipeline by isokinetic sampling (Figure S1, supporting information) for a total sampling time of about 60 min (corresponding to three NEDC tests). The solid particulate, collected on a Teflon filter (Millipore, pore diameter 0.45 m) heated at 100 °C, was extracted with dichloromethane (DCM) in order to remove condensable species (soluble organic fraction, SOF) and fuel residuals. The carbonaceous solid after DCM extraction (dry soot) was dried, weighted and characterized. SOF accounts for 15-20 wt. % of the total particulate. Soot reactivity was evaluated by thermogravimetric analysis (TGA) performed on a Perkin– Elmer Pyris 1 thermogravimetric analyzer (oxidative environment: air, 30 mL/min from 50 °C


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up to 750 °C at a rate of 10 °C/min). Fourier Transform Infrared (FTIR) spectra were acquired through a Nicolet iS10 spectrometer in the transmittance mode (650-4000 cm-1 range) on soot/KBr pellets (1-2 wt.%). The hydrodynamic diameter and the polydispersity index (PI, a dimensionless number extrapolated from the autocorrelation function accounting for the size distribution broadness) were measured by Dynamic Light Scattering (DLS) by a Malvern Zetasizer Nano ZS instrument. Measurements were performed on soot suspended in N-methyl pyrrolidinone (NMP) at a concentration of 0.01 mg/mL. UV–Vis spectra of soot, suspended in NMP (0.01 mg/mL) were acquired on an HP 8453 Diode Array spectrophotometer. Being the soot molecular mass unknown, the absorption coefficients were expressed on a mass basis (m2/g).

Results and discussion Main engine performances and gaseous emissions overview. The transient conditions were investigated, as above introduced, performing NEDC tests and comparing the engine performance in D and DF modes. The traces of engine torque, speed and substitution ratio (SRe) measured during a typical NEDC test are reported in supporting information (Figure S2); depending on the dynamic response of the whole hardware, the instantaneous SRe was variable in the range 40-60%. The engine behavior in both D and DF configurations is summarized in Table 2 in which engineout emissions (Total Hydrocarbons (THCs), methane (CH4), carbon monoxide (CO), NOx, soot and CO2) are reported. Energy Consumption (EC) values are also specified and calculated as follows: = !

"#

+,

& ∗ ()* ! "# & $%%"

(2)


10

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Energy & Fuels

The calculation, for DF case, takes into account the Fuel Consumption and LHV values of both fuels, Diesel and methane, so the energy consumption is the sum of the contributions from the both fuels. Global warming potential (GWP) values (Table 2) are expressed as mass of CO2 equivalent and were calculated considering CO2 and CH4 engine out emissions as the major contributions to GWP. Methane was estimated to have a GWP of 25 (over 100 years). The COV values were calculated, for each species, as the ratio of the standard deviation values to the mean values, referred to three repetition tests. The COV takes into account the test-to-test variability and the analyzers measurement accuracy, so it represents the range of significance of the detected differences between D and DF modes. The significant increase of THC (mainly CH4) and CO emissions, respectively about ten and two times higher in DF mode with respect to D case (Table 2), highlights a well-known issue related to the DF application. This result is a consequence of the very high THC emissions measured at low/partial engine load conditions, as evidenced by the same authors in previous works [8,13]. The low in-cylinder charge temperature and the too lean local gas-air mixture in DF mode, as widely reported in literature [7,14,15], explain such results, together with the contribution of methane escaping combustion in the combustion chamber crevice volume and/or for flame quenching, both wall and bulk [16]. The CH4 emission represents the main problem, because the methane, differently from the CO, cannot be oxidized by a standard Diesel oxidation catalyst device. Table2. Engine out emissions, Energy consumption, GWP in D and DF modes and COV values.

Total HC [g/km]

D

DF

COV [%]

0.252

3.176

8


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CH4 [g/km]

-

2.494

8

CO [g/km]

0.978

1.84

10

NOx [mg/km]

181

179

4

Soot [mg/km]

32.33

19.10

12

CO2 [g/km]

153

135

1.5

Energy Consumption [MJ/100 km]

211

219

1.5

GWP [g CO2eq/km]

153

197

-

As known, the EGR is the main driver for NOx control. Therefore, since the same EGR calibration was adopted for both D and DF modes, no significant difference in NOx emissions was evidenced, changing the combustion mode, in line with other literature results [8]. As expected, DF operating mode produced a significant advantage in terms of CO2 emission. Tests revealed a CO2 emission saving of about 12% in DF mode, at about 35% of SRe, with respect to the D case (135 mg/km of CO2 in DF case versus 153 mg/km in D). Anyway, at the state of art of DF technology, despite the CO2 saving, the increase in CH4 still represents a crucial issue in the evaluation of the DF impact on greenhouses gasses (GHG) emissions. The lower combustion efficiency of the DF configuration (overall value: 93% in DF mode with respect to 99% of D mode) produces a penalty of EC over the whole NEDC of 4%, as evidenced by the results of Table 2. This is consistent with the high THCs and CO emissions typical of DF conditions, as before commented.

Physical characterization of soot emission: mass concentration, PN and PSDF. The soot emissions traces, obtained by MicroSoot Sensor, are reported in Figure 2, for D and DF modes, over the ECE15 and EUDC parts of the NEDC, on the left and right side, respectively.


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Energy & Fuels

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Figure 2. Instantaneous engine out soot emission and torque traces for D and DF modes over the ECE15 (left) and EUDC (right) driving tests. For both ECE15 and EUDC driving tests, the engine out soot traces are generally lower in DF with respect to D mode, but some events with higher emission can be observed. In particular, in the ECE15 mode, in quasi steady-state condition, DF soot mass emission is about 30% higher than D mode (in the brief time window between the 60th and 80th second). In EUDC part, moreover, higher soot spikes for DF with respect to D mode occurred. Such unexpected highsooting events were a result of the adopted calibration. In particular, in the ECE15 part of the cycle, the high-sooting event is characterized by low torque with very low fuel delivering for both methane and Diesel injectors, while in EUDC they occurred at the end of acceleration phases. In such conditions, it could be possible that critical conditions of methane and particularly of Diesel fuel metering occurred. The described issue can be overcome by means of a further tuning of DF calibration that should shift in D mode in such operating conditions. Anyway, as evidenced in Table 2, the global soot emissions are drastically lower in DF case, confirming a well-known advantage offered by the methane use. The impact of DF configuration on Particle Number (PN) emissions was evaluated by means of DMS that classifies the total PN in nucleation mode (particles with diameter from about 5 nm to about 50 nm) and accumulation mode (particles with diameter from about 50 nm to about


13

Energy & Fuels

1000 nm). Figure 3 reports, for D and DF modes, the instantaneous engine out emission of PN owing to nucleation (upper part) and accumulation modes (bottom part), together with the engine torque traces. The left panels (a and c) refer to ECE15 part of the cycle, while the right panels (b

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Figure 3. Instantaneous engine out PN emissions and torque traces for D and DF modes over the ECE15 (a and c) and EUDC (b and d) driving tests. Nucleation mode particles (a and b), Accumulation mode particles (c and d). As well as the soot mass emission traces, the PN is remarkably reduced switching from D to DF mode. However, it can be noted that in some events also the PN traces present emission spikes in DF at the same level of the Diesel ones. In particular, in the range from 60 to 80 s of the ECE15 part (diagram a), the PN in the accumulation mode, that is the major contributor to the mass of emitted soot, is quite similar for both D and DF modes. Also for this aspect, further


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Energy & Fuels

improvement of the engine calibration can be beneficial. Anyway, looking at the traces of the accumulation mode in the EUDC part of the cycle (Figure 3, diagrams b and d), it is evident that the largest PN reduction, offered by the DF configuration, occurred when the engine runs at high speed and load conditions. Downstream the CDPF, the PN values were always at the limit of threshold detection of the measurement chain, so any significant comparison was possible between the two combustion configurations. Figure 4 reports the diameters traces of the engine-out emitted particles owning to nucleation mode, in the upper part (a and b) and accumulation mode, in the lower part (c and d). The left panels (a and c) are relative to the ECE15 cycle, while the EUDC results are illustrated in the right panels (b and d). More in details, the characteristic particle diameter is here assumed to be the median diameter of all, nucleation and accumulation mode particles distribution functions. The particles diameter traces show that no systematic differences between D and DF modes were detected: during some phases of the tested cycle, the DF diameter was lower than the D one, while the opposite trend occurred in other segments of the cycle, depending from the engine speed/torque evolution. Besides, in some phases, the particle size was characterized by a monomodal distribution, so it was not easy to distinguish the nucleation/accumulation relative contribution. This occurrence determined the impossibility to define the count mean diameter for both modes, as happened for example, at about 1000 s (see the spike in Figure 4 b). Also, the motoring phases were characterized, as expected, by emissions of only particles owning to the nucleation mode and so the mean diameter of accumulation was not calculated (see Figure 4 d, about 900 and 1100 s).


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Torque [Nm]

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880

980 Time [s]

1080

1180

Figure 4. Instantaneous particles diameter and torque traces for D and DF modes over the ECE15 (a and c) and EUDC (b and d) driving tests. Nucleation mode particles (a and b), Accumulation mode particles (c and d). For sake of completeness the results disclosed in Figures 3 and 4 are summarized in Figure 5 where total, nucleation and accumulation PN during the whole NEDC, for D and DF combustion modes are reported. A 40% reduction of total PN (All) emission was achieved moving from D to DF mode. The main reduction regarded the accumulation mode particles (-50%), but a significant reduction (-30%) is evident also in terms of smaller particles (nucleation mode). This last result, is a very interesting aspect of DF application, taking into account the growing concern on nanoparticles emissions from internal combustion engines in the urban area, both from environmental and health points of view.


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The results of the median diameter reported in the right diagram of Figure 5 confirm what claimed in the above discussion on particle diameters. The graph reveals, in fact, that the median diameters of nucleation and accumulation particles were quite the same, with a slight decrease in the DF case that led to a reduction of 8% of diameters calculated on the all particles.

All

All

-8%

-40%

Diesel Nucleation

Dual Fuel

Nucleation

-30%

Diesel

-50%

Accumulation

0.0E+00

1.0E+14

2.0E+14

Dual Fuel

3.0E+14

Accumulation

4.0E+14

0

20

40

60

80

Count Median Diameter [nm]

PN [#/km]

Figure 5. Total, nucleation and accumulation PN during NEDC, for D and DF combustion modes. Soot chemico-physical characterization. The chemico-physical characterization of Diesel and DF soot particles was carried out combining spectroscopic, optical and thermogravimetric techniques. The data reported in this section are related to dry soot. The FTIR spectra of both soot samples, acquired in the transmittance mode and reported in Figure 6 (left panel) are baseline corrected, height normalized and shifted for clarity. The shape of the both FTIR spectra is representative of the presence of complex carbonaceous networks [17,18].


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Figure 6. Left panel: FTIR spectra of Diesel soot and DF soot; right panel: UV-Vis spectra of Diesel soot and DF soot.

In both the FTIR spectra the signals were mainly located in three regions: a region between 3100 and 3600 cm-1 containing signals of exchangeable protons from alcohol, phenol, carboxylic acid groups and adsorbed water, a region between 2850 and 3050 cm-1 containing signals due to the stretchings of aromatic and aliphatic C-H groups, mainly -CH2-, (attributable to residuals of unburned or partially burned fuel, not completely removed by DCM washing), a region between 1800 and 900 cm-1 containing overlapped signals of stretching and bending absorptions of many different functional groups (C=O of carbonylic and carboxylic groups, C-OH, C-H, C=C, C-C) [18,19]. Bending absorptions of "out of plane" aromatic C-H groups in the region between 900 and 700 cm-1 are not easily discernable, possibly submerged by the background [20]. No significant differences were observed by comparing the shape and the relative intensities of the signals of the FTIR spectra of both soot samples, indicating that the surface chemistry of the soot particles is not influenced by the DF configuration at the explored substitution rate. This result


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agrees with previous studies on the effect on soot structure related to ethanol and methane fumigation [17, 21]. The UV-Vis spectra of both soot samples in NMP suspension are reported in Figure 6 (right panel). Specific absorbance is expressed on a mass basis (m2/g). As typical of carbonaceous materials produced in combustion environment [18], a broad shape extending in the visible region and ascribable to a large condensation degree of the aromatic moieties was found for both soot samples. The specific absorption (sensitive to the sp2/sp3 ratio) of the DF soot, both in the UV (300 nm) and in the visible (500 nm) regions, is slightly lower compared to Diesel soot (Figure 6 right panel). This finding indicates a possible influence of the methane in the soot formation process [22]. It is worth to note that overall both soot samples exhibited specific absorptions higher than those of carbons with a high graphitization degree and a good level of structuration (furnace carbon blacks, mature soot from benzene laminar flame [18]). Despite the differences between the specific absorptions, the shape of the spectra appears quite similar (height normalized spectra Figure 6, right, inset), thus indicating no detectable variation in the graphitic arrangement of the particles. Dynamic Light Scattering (DLS) measurements, performed on NMP soot suspensions, allowed the estimation of the hydrodynamic diameter of the soot particles. The evaluation of this parameter is of keen interest since the particles dimensions, together with the surface chemistry characteristics, are strictly connected to the penetration degree into the human respiratory apparatus and subsequent channeling by biological fluids after sticking on the respiratory apparatus mucosa. The mean hydrodynamic diameter of soot isolated from engine operated in Diesel mode was 121.3 nm while that of soot isolated from engine operated in dual fuel configuration is larger (138.7 nm). This trend on hydrodynamic diameter agreed with previous


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work on the soot collected from an engine operated in dual fuel configuration with ethanol [17]. The two soot appeared quite monodispersed, being the PI lower than 0.1. Homogeneous dispersions are typically characterized by PI values < 0.1, whereas highly heterogeneous dispersions by PI > 0.3 [23]. The evaluated hydrodynamic diameters are larger with respect to the geometric mean diameters (Figures 4 and 5) because they are derived by measures of translational diffusion coefficient of soot particles surrounded by a solvent cage (NMP). For this reason, the hydrodynamic diameters trends are quite flattened with respect to the GMS. Possible particle sticking must be also taken into account because the hydrodynamic diameter has been measured on soot after particles impact on the collection filter.

Figure 7. TG curves of Diesel soot and DF soot. Soot reactivity was evaluated by thermogravimetric analysis (TGA) in oxidative environment (air) and the corresponding profiles are reported in Figure 7 (derivative TG curves are reported in Fig. S3, supporting information). Soot oxidation took place in the 500-550 °C range for both soot samples and it occurred in a temperature range typical of Diesel soot oxidation [17] but at a lower temperature with respect to standard carbon black (690 °C) [18]. A progressive weight


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decreasing up to 10-20% ascribable to the loss of oxygen functional groups and/or physisorbed molecules strongly adsorbed on soot surface and not completely removed by the DCM washing was detected before the complete oxidation of the soot. A very low amount of ashes was detected (between 2-4 wt%) for both soot samples and ascribable to engine wear. Overall the TG profiles and the bulk oxidation temperatures are comparable (577 °C for pure Diesel soot and 588 °C for methane DF soot) indicating that the DF configuration did not significantly affected the soot combustion behavior. This result is not surprising at all since the invariance of the overall chemico-physical and morphological features of soot was recently observed by Nithyanandan et al. [21] in soot emitted from prototype single-cylinder diesel engine operated in comparable substitution rate (40% methane energy-based substitution rate). Conclusions The present paper describes the results of an experimental campaign aimed at the characterization of particles emission from an automotive modern engine, working in DF Dieselmethane configuration with 50% of methane substitution ratio, during a real driving homologation cycle (NEDC). The Diesel engine was properly modified to easily switch from the original Diesel to DF working operation. The response of the engine to the DF application was in line with literature trends, showing a significant increase of THC (mainly CH4) and CO emissions. In DF mode, the CO2 emission was reduced of about 12% with respect to Diesel mode. Anyway, at the state of art of DF technology, despite the CO2 saving, the increase in CH4 still represents a crucial issue in the evaluation of the DF impact on GHG emissions. The experimental activity highlighted the strong benefit of DF application on engine-out soot mass and PN emissions. In particular, the PN main reduction regarded the accumulation mode particles, but a significant cut (30%) was evident also in terms of smaller particles (nucleation).


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At the same time, the particles showed quite the same size in the two combustion modes. Downstream the DPF, the PN measures were at the limit of threshold detection of the measurement chain, so any significant comparison was possible between the two combustion configurations. The chemical characterization of soot sampled upstream the DPF filter outlined a sort of irrelevance of the CH4 on the reactivity of collected soot as well as a negligible impact on the soot chemico-physical features. This circumstance offers interesting information in view of the designing of after-treatment soot trap systems tailored for DF engine. The results here disclosed indicate that the DF configuration offers remarkable advantages in the management and durability of the DPF systems, thanks to the lower filter regeneration frequency resulting from very low soot emissions levels. Furthermore, the PN reduction in the whole particle size range represents an interesting feature of the DF system, taking into account the growing concern on nanoparticles emissions from internal combustion engines in the urban area, both from an environmental and health points of view. Further investigations are still ongoing, in order to evaluate the potentiality offered by a proper engine Dual Fuel calibration to reduce its impact on GHG emissions, also considering the WLTP (Worldwide harmonized Light vehicles Test Procedures) that is recently in force in Europe. Taking into account the more dynamicity and heaviness of the WLTP, the Dual Fuel performances are expected to be improved as the Dual Fuel engine is operated on the new cycle. Acknowledgments Authors would like to thank Mr. Alessio Schiavone and Mr. Roberto Maniscalco for their technical assistance in the engine testing. Authors gratefully acknowledge the COST Action SMARTCATs (CM1404, www.smartcats.eu), supported by COST (European Cooperation in Science and Technology, www.cost.eu) to allow the dissemination of their work.


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References (1) Liu, J.; Yang, F.; Wang, H.; Ouyang, M.; Hao, S. Effects of pilot fuel quantity on the emissions characteristics of a CNG/Diesel dual fuel engine with optimized pilot injection timing. Applied Energy, 2013, 110, 201-206. (2) Reitz, R. D.; Duraisamy, G. Review of high efficiency and clean reactivity controlled compression ignition (RCCI) combustion in internal combustion engines, Progress in Energy and Combustion Science 2015, 46, 12-71. (3) Yang, B.; Xi, C.; Wei, X.; Ming-Chia Lai, K. Parametric investigation of natural gas port injection and Diesel pilot injection on the combustion and emissions of a turbocharged common rail dual-fuel engine at low load. Applied Energy 2015, 143, 130-137. (4) Papagiannakis, R.G.; Rakopoulos, C.D.; Hountalas, D.T.; Rakopoulos, D.C. Emission characteristics of high speed, dual fuel, compression ignition engine operating in a wide range of natural gas/Diesel fuel proportions. Fuel 2010, 89, 1397-1406. (5) Taniguchi, S.; Masubuchi, M.; Kitano, K.; Mogi, K. Feasibility Study of Exhaust Emissions in a Natural Gas Diesel Dual Fuel (DDF) Engine. SAE Technical Paper 2012-011649, 2012, doi:10.4271/2012-01-1649. (6) Papagiannakis, R. G.; Hountalas, D. T. Experimental investigation concerning the effect of natural gas percentage on performance and emissions of a DI dual fuel Diesel engine. Appl. Therm. Eng. 2003, 23 (3), 353−365.


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(7) Sahoo, B.B.; Sahoo, N.; Saha, U.K. Effect of engine parameters and type of gaseous fuel on the performance of dual-fuel gas Diesel engines-A critical review. Renewable and Sustainable Energy Reviews 2009,13, 1151-1184. (8) Di Blasio, G.; Belgiorno, G.; Beatrice, C.; Fraioli, V.; Migliaccio, M. Experimental Evaluation of Compression Ratio Influence on the Performance of a Dual-Fuel Methane-Diesel Light-Duty Engine. SAE Int. J. Engines 2015, 8(5), doi:10.4271/2015-24-2460. (9) New European Driving Cycle (NEDC) https://www.dieselnet.com/standards/cycles/ece_eudc.php. (10) Selim, M. Y. E. Sensitivity of dual fuel engine combustion and knocking limits to gaseous fuel composition. Energy Conversion and Management, 2004, 45(3), 411-425. (11) Guido, C.; Beatrice, C.; Napolitano, P. Application of Bioethanol/RME/Diesel blend in a Euro 5 automotive Diesel engine: potentiality of Closed Loop Combustion Control technology. Applied Energy 2013, 102, 13-23. (12) Jonathan P.R. Symonds, Kingsley St.J. Reavell, Jason S. Olfert, Bruce W. Campbell, Stuart J. Swift, Diesel soot mass calculation in real-time with a differential mobility spectrometer, Journal of Aerosol Science, Volume 38, Issue 1, January 2007, Pages 52-68, ISSN 0021-8502. (13) Napolitano, P.; Guido, C.; Beatrice, C.; Del Giacomo, N. Application of a Dual Fuel Diesel-CNG Configuration in a Euro 5 Automotive Diesel Engine.SAE Technical Paper 201701-0769, 2017, doi:10.4271/2017-01-0769.


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(14) Korakianitis, T.; Namasivayam, A.M.; Crookes, R.J. Natural-gas fueled spark-ignition (SI) and compression-ignition (CI) engine performance and emissions. Progess in Energy and Combustion Science 2011, 37, 89-112. (15) Selim, M. Y. E. Effect of exhaust gas recirculation on some combustion characteristics of dual fuel engine. Energy Conversion and Management March 2003, 44(5), 707-721. (16) Olsen, J.; Crookes R. J.; Bob-Manuel, K. D. H. Experiments in dual fuelling a compression ignition engine by injecting di-methyl ether as a pilot fuel to ignite varying quantities of natural gas. SAE paper 2007-01-3624. (17) Gargiulo, V.; Alfè, M.; Di Blasio, G.; Beatrice C. Chemico-physical features of soot emitted from a Dual-Fuel ethanol-Diesel system. Fuel 2015, 150, 154-161 (18) Arnal, C.; Alfè, M.; Gargiulo, V.; Ciajolo, A.; Alzueta, M. U.; Millera, A.; Bilbao R. "Characterization of Soot" Cap. 13 (pp. 333-362)- in Cleaner Combustion- Developing Detailed Chemical Kinetic Models, Editors: F. Battin-Leclerc, J.M. Simmie, E. Blurock; 2013, XIII, p. 659, ISBN 978-1-4471-5306-1-SpringR.M. (19) Silverstein, M.; Webster, F.X.; Kiemle, D. Spectrometric Identification of Organic Compounds, 4th ed. Wiley, 2008. (20) Centrone, A.; Brambilla, L.; Renouard, T.; Gherghel, L.; Mathis, C.; Müllen, K.;Zerbi, G. Structure of new carbonaceous materials: the role of vibrational spectroscopy. Carbon 2005, 43, 1593-1609. (21) Nithyanandan, K.; Lin, Y.; Donahue, R.; Meng, X.; Zhang, J.; Lee, C. F. Characterization of soot from diesel-CNG dual-fuel combustion in a CI engine. Fuel 2016, 184, 145-152.


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(22) Alfè M, Apicella B, Barbella R, Rouzaud JN, Tregrossi A, Ciajolo A. Structure property relationship in nanostructures of young and mature soot in premixed flames. Proc Combust Inst 2009; 32, 697–704. (23) Carella, E.; Ghiazza, M.; Alfè M.; Gazzano E.; Ghigo D.; Gargiulo V.; Ciajolo, A.; Fubini, B.; Fenoglio, I. Graphenic nanoparticles from combustion sources scavenge hydroxyl radicals depending upon their structure. BioNano Sci. 2013, 3, 112–122

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge: scheme of the engine and the on-line measurement devices, Energetic Substitution Rate, speed and Torque trace during the NEDC profiles, DTG curves.

AUTHOR INFORMATION Corresponding Author Dr. Michela Alfè [email protected] Istituto di Ricerche sulla Combustione IRC-CNR Piazzale V. Tecchio 80, 80125 Napoli, Italy


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Phone: +39 081 7682247 - +39 081 7682230 Fax: +39 081 5936936

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ABBREVIATIONS BTDC, before top dead center; BMEP, brake mean effective pressure; CA, crank angle; CDPF catalyzed Diesel particulate filter; COV coefficient of variability; CR compression ratio; ECU electronic control unit; EUDC extra urban driving cycle; DCM dichloromethane; DF dual fuel; DI direct injection; DLS dynamic light scattering; DPF Diesel particulate filter; DMS differential mobility spectrometer; DOC Diesel oxy catalyst; EC energy consumption; SRe energy substitution rate; EGR exhaust recirculation gases; FTIR fourier transform infrared; MBF fuel mass burnt; GHG greenhouse gases; GMD geometric mean diameter; GWP global warming potential; IMEP indicated mean effective pressure; ICE internal combustion engines; LHV lower heating values; ETmain main injection quantity; NG natural gas; NEDC new European driving cycle; NMP N-methyl pyrrolidinone; ON octane number; PFI port fuel injection; PI polydispersity index; PM particulate matter; PSDF particle size distribution function; PN particle number; RCCI reactivity controlled compression ignition; SOImain start of main injection; SOF soluble organic fraction; TGA thermogravimetric analysis; THC total hydrocarbons; WLTP Worldwide harmonized Light vehicles Test Procedures.


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physical features of particles emitted from an automotive

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