Experimental comparison of combustion and emission characteristics between a market gasoline and its surrogate

Combustion and Flame 214 (2020) 306–322

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Combustion and Flame journal homepage: www.elsevier.com/locate/combustflame

Experimental comparison of combustion and emission characteristics between a market gasoline and its surrogate Stefania Esposito a,∗, Liming Cai b, Marco Günther a, Heinz Pitsch b, Stefan Pischinger a a b

Institute for Combustion Engines, RWTH Aachen University, Aachen 52074, Germany Institute for Combustion Technology, RWTH Aachen University, Aachen 52056, Germany

a r t i c l e

i n f o

Article history: Received 3 August 2019 Revised 18 October 2019 Accepted 15 December 2019

Keywords: Internal combustion engine Gasoline surrogate Combustion Emission

a b s t r a c t Mixtures of few representative hydrocarbon species are often used in computational studies as surrogates of market petroleum fuels, which contain hundreds of components. While this simplification is imperative for computational costs of reaction kinetics, it introduces unavoidably uncertainties in simulations. Differences between the real and the surrogate fuels regarding mixture formation, oxidation chemistry, and fuel composition contribute to such uncertainties. An evaluation of the underlying concept of a surrogate fuel model in terms of engine performance is thus of high interest. This paper presents an experimental study with a spark-ignition (SI) single-cylinder engine (SCE) to compare the combustion and emission characteristics between a market gasoline fuel and its corresponding four-component surrogate (iso-octane, n-heptane, toluene, ethanol). The measurements cover a wide range of operating points in terms of engine load, speed, air-to-fuel ratio, and operating conditions. Together with standard performance and emission measurements, a non-standard hydrocarbons (HC) analysis has been performed with a fast flame ionization detector and an ion-molecule-reaction mass spectrometer. The comparison reveals very good agreement between the market gasoline and the surrogate fuel regarding combustion and global gaseous emission behaviors, with an average deviation for almost all of the analyzed quantities below 2%. The comparison of CO emissions in stoichiometric operation presents a higher scatter, due to the high sensitivity of the CO emissions on mixture formation and fuel volatility. The different compositions of the two fuels also lead to deviations of speciated-HC emissions, which is confirmed by mass spectrometry. Additionally, the sooting tendency of the surrogate fuel is found to be more than 10 times lower compared to the market gasoline fuel. © 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

1. Introduction Despite the increasing interest in bio-fuels and electro-mobility, internal combustion engines (ICE) with petroleum fuels are still dominant in the transportation sector. The continuous improvements regarding the ICE’s thermal efficiency with novel concepts keep this technology competitive for the future, especially in electric-hybrid configurations. For instance regarding SI engines, Luszcz et al. [1] showed the possibility to reach 46% of indicated efficiency with a lean combustion concept and Lee et al. [2] obtained more than 42% brake efficiency with an optimized multicylinder naturally-aspirated engine. Besides the efficiency improvement, the reduction of pollutant emissions is another major challenge. To tackle both aspects, the application of three dimensional

∗

Corresponding author. E-mail address: [email protected] (S. Esposito).

(3D) computational fluid dynamics (CFD) simulations is very helpful in developing novel engine concepts, since it allows for a joint optimization of the combustion and the emission behavior. Besides CFD engine simulations being very complex themselves, it is currently computationally unaffordable to couple the simulation with detailed reaction kinetics of real petroleum fuels. This would require consideration of the oxidation chemistry for all relevant components with thousands of chemical species and elementary reactions. Thus, it is necessary to define a surrogate fuel mixture that can emulate target quantities of interest of real fuels but is only composed of a small number of fuel components. Due to their importance for engine application, normally, the surrogate formulation process considers a number of important chemical kinetic fuel characteristics, such as ignition delay times, and burning velocities as well as global fuel properties, such as octane number, density, and the hydrogen to carbon atoms (H/C) ratio. The blending ratios between surrogate components are then optimized to minimize the differences of these targets between the surrogate

https://doi.org/10.1016/j.combustflame.2019.12.025 0010-2180/© 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

S. Esposito, L. Cai and M. Günther et al. / Combustion and Flame 214 (2020) 306–322

and the real fuels. Numerous studies are available in the literature regarding surrogate formulations for petroleum fuels. While for diesel [3–6] and jet petroleum fuels [7,8], the selection of surrogate components is diverse, the surrogates for gasoline fuels are mostly based on the so-called primary reference fuel (PRF) mixtures [9–11] of n-heptane and iso-octane, which are the two components used as reference fuels in octane number engine tests. To account for the content of aromatic compounds, toluene is often added (TRF – toluene reference fuel), as well as ethanol to consider its common presence in market gasoline fuels [11–25]. This four-component surrogate model with tailored blending ratios has been widely applied in numerical studies and shown to satisfactorily emulate the real gasoline fuel in CFD simulations for certain targets. On the other hand, surrogate mixtures of few compounds can be expected to fail in predicting all chemical and physical characteristics of real fuels. For example, in a recent work [26], the authors found that the exclusion of olefin and naphthene contents in surrogate mixtures may lead to the incorrect prediction of the first-stage ignition delay times, which are commonly not considered as targets in surrogate formulation and engine simulation. Other works [27–29] showed that the addition of olefins is particularly important to generate a proper surrogate for auto-ignition predictions, especially in engine applications which realize e.g. homogeneous charge compression ignition (HCCI). Also, concern has been raised regarding the heating and evaporation characteristics of gasoline surrogates [30]. Thus, various literature works have focused on the addition of further components to the aforementioned four-component surrogates to improve their performance in terms of particular properties [12,18,27–29,31–38]. The continuous incorporation of additional components can surely minimize the differences between real and surrogate fuels. However, it simultaneously enlarges the computational costs by extending the chemical mechanisms. An additional aspect is the question whether it is necessary for surrogate mixtures to reproduce all chemical and physical properties of real fuels. Certain properties may not be of interest for specific applications or barely affect the engine performance. In order to explore these aspects, a comprehensive comparison of the combustion and emission performance between real and surrogate fuels in engines is of very high interest, which is nevertheless scarce in the literature. Jiang et al. [39] employed a 4-cylinder engine to test the performance of a TRF and two four-component surrogates (TRF with cyclohexene and diisobutylene) in comparison to a market gasoline fuel. The surrogate formulation was determined to match only the octane number of the gasoline. The investigation was performed at one operating point with different injection pressures, finding that the TRF surrogate has a better agreement with real gasoline regarding the combustion characteristics. As far as emission characteristics are concerned, the surrogate fuels show lower nitrogen oxides (NOx ) and particle emissions, but higher HC and CO emissions than the market gasoline. In order to evaluate the differences regarding combustion and emissions of surrogate and market fuels, an experimental comparison between a market gasoline fuel (RON95E5) and a TRF surrogate mixture with ethanol is presented in this study. For a comprehensive assessment, the measurements have been conducted on a SI research SCE covering a very wide range of engine operating conditions. In total, 50 operating points were investigated and the data were evaluated by using statistical analyses. A specific focus of this study was posed on gaseous emission characteristics, more specifically, on HC emissions. By means of a fast flame ionization detector (FFID) and an ion-molecule-reaction mass spectrometer (IMR-MS), cycle-resolved total-HC (THC) and specific HC components have been measured.

307

Table 1 SCE hardware specifications. Engine parameter / unit

Value

Displacement / cm3 Bore / mm Stroke / mm Compression ratio Valves per cylinder Maximum peak firing pressure / bar Intake valve angle / ◦ Exhaust valve angle / ◦ Intake event length (at 1 mm) / ◦ CA Exhaust event length (at 1 mm) / ◦ CA

400 75.0 90.5 11.8 4 170 17.5 17.5 186 186

CA = Crank angle.

2. Experimental methodology The experimental setup for these particular investigations has already been described in detail by the authors in [40]. Therefore, the basic information is shortly summarized, while the relevant aspects for this study are presented in more detail. 2.1. Engine The measurements were conducted on an SI direct injection (DI) research SCE already used for other research studies [41–44]. The main specifications of the SCE are summarized in Table 1. The DI injector and the spark plug are both centrally located in the combustion chamber. The spark plug position is between the exhaust valves, while the fuel injector is positioned between the intake valves. The intake ports are symmetrical high tumble ports. 2.2. Test-bench instrumentation 2.2.1. Mechanical and thermodynamic measurements Regarding the thermodynamic measurements, the cylinder pressure was measured with two Kistler 6045B pressure transducers, flush-mounted in the combustion chamber roof each between the intake and the exhaust valve seat rings. The cylinder pressure signal sampling was performed via Kistler 5064 charge amplifiers and an FEV indication system (FEVIS) with a resolution of 0.1◦ CA. The dynamic intake and exhaust pressures were measured with Kistler 4045 A5 pressure transducers and sampled via Kistler 4665 and Kistler 4603 charge amplifiers also with 0.1◦ CA resolution. In total, 10 0 0 consecutive cycles were measured for each operating point. The measurements of static pressures and temperatures were taken with standard pressure transducers and thermocouples, averaged over an interval of 30 s. The oil and coolant conditioning systems allowed steady-state operation. The intake air was conditioned to the selected temperature (normally 25 ◦ C) upstream of the electronically controlled throttle flap. The pressure upstream of the throttle flap and in the exhaust manifold was controlled to 1.013 bar during throttled operation. For boosted operation, same pressures upstream of throttle flap and in the exhaust manifold were imposed to simulate turbocharging. The engine was coupled to an eddy-current brake and an electric dynamometer to maintain the selected engine speed with an accuracy of ± 1 1/min, independently from the engine load. The intake air mass flow was measured with an ultrasonic air mass meter, while the fuel consumption was measured via Coriolis-type mass flow sensor. 2.2.2. Emission measurements In this experimental campaign, major focus was given to the gaseous emission measurements in different sampling positions in the exhaust system and with different devices. For this reason,

308

S. Esposito, L. Cai and M. Günther et al. / Combustion and Flame 214 (2020) 306–322 Table 2 Standard exhaust gas analyzers specifications. Chracteristic

FEVER device

AVL415SE

CLD

NDIR

Sensed species

NOx

CO

Measuring range Accuracy Linearity Response (T10-90%)

0–10,000 ppm 0–5,000 ppm 1% of measured value 2% of measured value <2s

low

PMD

FID

CO2

O2

THC

CH4

PM

0–20%

0–25%

10–10,000 ppmC3

10–3000 ppmC1

0–10 FSN 0.001 FSN – –

high 0–10%

<4s

CLD = chemiluminescence detector, NDR = non-dispersive infrared detector, PMD = para-magnetic detector, FID = flame-ionization detector, THC = total-HC referred to propane (C3 ).

standard exhaust gaseous emissions and smoke emissions measurements were coupled with non-standard devices such as a FFID and an IMR-MS. Standard emission measurements. The standard exhaust emission measurements are important to define the global behavior of the engine regarding regulated emissions. The gaseous emissions measurements were performed with a FEV emission rate (FEVER) measurement system. This device is a combination of different analyzers that can measure cycle-averaged concentrations of particular species in the exhaust gases. The exhaust gas samples were taken in two different positions downstream of the cylinder head flange. The sampling lines were heated to 193 ◦ C and they could be connected to the FEVER system alternatively by means of a switch. In this way, of the total 10 0 0 measured cycles for each operating point, the gaseous emissions were sampled for 500 cycles in each position. The particulate matter (PM) emissions were quantified by means of an AVL smoke meter (AVL-SM) 415SE. This device uses the filter paper method to determine the filter smoke number (FSN) defined according to ISO standards [45]. The PM sampling position was further away from the cylinder head in comparison to the gaseous emission sampling position. The sampling line was heated at 70 ◦ C. In Table 2, the technical data of the standard emission analyzers are reported. FFID The FFID was adopted with the objective to investigate the cycle-resolved trends of THC in different positions to collect indications about HC-formation mechanisms, as described in [40], and to compare these measurements between the two fuels. The FFID device used is a Cambustion HFR500 with two lines and equipped with two probes of 210 mm, with an internal diameter of 1.07 mm (0.042 in). The sampling lines were heated to 200 ◦ C and had a length of approximately 45 cm up to the FID measuring head. The estimated response time (T10-90%) from the manufacturer is in the range of 1.8–2 ms. The FFID signal output (0–10 V) was connected to the FEVIS system and recorded with the same resolution of 0.1◦ CA. The accuracy and the drift pro hour are ± 1% of the selected full-scale. In comparison to the FEVER FID, the accuracy is lower and an underestimation of around 20% of the average THC level was observed. The FFID design achieves a fast response. However, the FFID raw signal differs from the real concentration in the measuring point due to the transit time and the filtering effect in the probe, as explained in [46,47]. The FFID raw signal needs to be reconstructed with the estimation of the transit and response times. This is performed by means of a tool provided by the manufacturer [46]. The signal reconstruction procedure is explained more in detail in [40]. IMR-MS. The IMR-MS used in these investigations is a V&F TwinSense, the same used also in other works [48–50]. This device belongs to the soft-ionization mass spectrometer category, which ionizes the sample with a lower ionization energy (IE) in comparison to the traditional electron-ionization mass spectrometry

Fig. 1. Comparison of the n-heptane mass spectra with EI-MS [59] and IMR-MS.

(EI-MS) [51–54]. Instead of ionizing directly the sample with electrons as in EI-MS, a primary ion gas is ionized and then used to ionize the sample [53,54]. For this device, the possible primary source gases are Hg++, Xe+ and Kr+ that have IEs of respectively 10, 12 and 14 eV. Compared to traditional EI-MS with IE of 70 eV, the lower IE of IMR-MS allows to reduce the fragmentation of the molecules content in the sample. However, hydrocarbons have relatively low IE, especially the heavier ones (around EI = 10 eV [54]) and are anyway subject to fragmentation [54–58]. In Fig. 1, a comparison of the n-heptane mass spectra between traditional EI-MS [59] and IMR-MS (measured by the authors with Hg++) is shown. Significantly less fragmentation is observed. This is the reason why this kind of mass spectrometry is more suitable for HC measurements in engine exhaust. Even if reduced with IMR-MS, the inevitable HC fragmentation makes the application of this measurement technique on the complex exhaust gas composition of an internal combustion engine quite challenging. The molecules fragmentation patterns and the heterogeneous composition of the sample lead to cross-sensitivities and inaccuracies in determination of certain species. As a consequence, not all of the interesting HC species can be measured and a selection of some of them was made. In Table 3, the HC species measured with IMR-MS and relevant for this study are listed. This device has two channels available and, like the FFID, can measure in two different positions simultaneously. The approximatively 1 m long sampling lines were insulated metallic capillaries with 0.5 mm diameter, heated to 100–120 ◦ C. The device measures the single species for a user-defined time interval (set to 100 ms after consultation with the manufacturer), one after the other. Since more than 10 species have been selected for the investigations, the total measuring time was longer than an engine

S. Esposito, L. Cai and M. Günther et al. / Combustion and Flame 214 (2020) 306–322

309

Table 3 IMR-MS measured species relevant for the comparison between RON95E5 and the surrogate fuel. Mreal is the real molar mass of the compound while (Mmeas. ) is the one used for the measurements (if different from the real one). Formula

Calibration gas

Mreal (Mmeas. )a

Ion. gas

CH4 C2 H2 C6 H6 C2 H5 OH C4 H6 C4 H10 C7 H8 C7 H16

Methane Acetylene Benzene Ethanol 1,3-Butadiene n-Butane Toluene n-Heptane

16 26 78 (79) 46 (45) 54 58 92 100

Xe+ Xe+ Xe+ Hg++ Hg++ Hg++ Hg++ Hg++

a

in amu (Atomic Mass Unit).

Pos. 1 d1

I

Intake

I Intake Ports

Fig. 3. FFID measurements for the operating point: engine speed nE = 1500 1/min, IMEP (Indicated Mean Effective Pressure) = 8 bar, λ = 1, in Pos. 1 and Pos. 2. The scatter-bands represent 500 cycles, while the lines are the reconstructed average signal performed as explained in [40].

Pos. 3

Pos. 2

d2

d3

E Exh. Comp. Volume

E Exhaust Ports

Exhaust Runner

FFID IMR-MS FEVER AVL-SM

Fig. 2. Emission measurement positions: d1 =12 mm, d2 =120 mm, d3 =400 mm.

Pos. SM Exhaust the

distances

are

approximately

cycle. For this reason, only cycle-averaged results were determined by means of an average of the total measuring time (corresponding to a number of cycles ≥ 10 0 0). Measurement positions. Since all the gaseous emission measurement devices had two channels, all of them were used to measure in two positions. Overall the gaseous emissions were measured in three different positions in the exhaust path, as depicted in Fig. 2. The FFID and the IMR-MS were used in Pos. 1 (exhaust port) and Pos. 2 (exhaust runner) for all measurements. The FEVER standard exhaust analysis channels were positioned in Pos. 2 and Pos. 3. The smoke measurements were taken further downstream in the exhaust system (Pos. SM), after the exhaust compensation volume, as shown in Fig. 2. Overall, the gaseous emission trends show differences in the various measurement positions. Regarding HC emissions, the average levels and the cycle-resolved trends are clearly different between Pos. 1 and Pos. 2. In Fig. 3, the FFID measurements of an exemplary operating point are shown. In particular, the scatter-bands of the raw signals belonging to 500 cycles and the reconstructed average signal are displayed over crank angle (referenced after topdead-center firing = aTDCF ). The exhaust valve opening (EVO) and closing (EVC) angles are indicated too. In general and on average, the concentrations of THC and HCspecies in Pos. 1 are higher than in Pos. 2. The reason for the high value with exhaust valve closed results from the pistonring-crevice HC-mechanism. While, for the majority of the exhaust stroke, the HC-level in Pos. 1 is relatively low, at the end of the exhaust stroke, the HCs from the piston-ring crevice are exhausted. These HCs remain near the closed exhaust valve in a high concentration, up to the arrival of the exhaust massflow at the next EVO. In Pos. 2, this local effect is not visible because in that location the

exhaust flow is already well mixed. An almost constant value occurs with only a peak due to the passage of the HCs stored near Pos. 1 shortly after EVO. Regarding the gaseous emissions in Pos. 3 measured with FEVER, the trends observed are comparable to what is measured in Pos. 2, even if in some operating points differences in absolute values are observed. As pointed out in [40], these deviations are due to gas-dynamics and mixing effects that can influence the time averaged measured value up to 15–20%. Indeed, in Pos. 3 with exhaust valves closed, there are pressure and mass-flow oscillations that can result in a back flow from the exhaust compensation volume, with consequent alteration of the average emission concentration. For this reason, in the following, only Pos. 2 is taken into account for the analysis of the results. 2.3. Test plan In this experimental study, a systematic investigation was performed, which evaluated single- and cross-dependencies of engine operating parameters on emission levels and particularly on HC trends and composition in different operating points. The investigations involved many variations of engine operating parameters, as listed in [40]. The following variations are relevant for the comparison between the market RON95E5 and the surrogate fuel: • Engine indicated mean effective pressure (IMEP): 3–16 bar; • Engine speed (nE ): 150 0–40 0 0 1/min; • Valve timing: intake valve opening (IVO) at 1 mm lift from 340–380◦ CA aTDCF , exhaust valve closing (EVC) at 1 mm lift from 340–360◦ CA aTDCF ; • Relative air-to-fuel ratio (λ) from 0.9 to 1.5; • Intake air temperature: 25–45 ◦ C; • Coolant and oil temperature (Tcool = Toil ): 30–90 ◦ C. • Catalyst heating operation: spark timing (ST) variation at nE = 1200 1/min, IMEP = 3 bar and Tcool = Toil = 30 ◦ C; In total, 50 different operating points that belong to the abovementioned variations are available for comparison. In Fig. 4, the operating points available for the comparison are shown in a IMEPnE diagram. 3. Gasoline fuel and its surrogate A market gasoline fuel RON95E5 was investigated in this work. The results of its PIONA analysis, which determines the content of n-paraffins, iso-paraffins, olefins, naphthenes, and aromatics, are shown in Table 4.

310

S. Esposito, L. Cai and M. Günther et al. / Combustion and Flame 214 (2020) 306–322 Table 5 Comparison of main fuel properties.

Fig. 4. Operating points for comparison in a IMEP-nE diagram. Table 4 PIONA analysis of the RON95E5 fuel. The values are given in mass fractions (%).

Component / property

RON95E5a

Surrog. calc.b

Surrog. realc

C2 H5 OH / wt% i-C8 H18 / wt% n-C7 H16 / wt% C7 H8 / wt% Alkanes / wt% Alkenes / wt% Aromatics / wt% Aver. C-atoms x Aver. H-atoms y Aver. O-atoms z H/C ratio RON/MON ρ fuel @15 ◦ C / kg/m3 Mfuel / kg/kmol AFRst e

5.4 6.9 1.0 7.6 52.7 9.2 32.1 6.7 13.0 0.12 1.94 96.2/85.4 729.9 95.5 14.29

5.4 52 13.3 29.3 65.3 – 29.3 6.9 13.3 0.11 1.93 94.0/87.2 744.1 97.8 14.30

6.1 51.8 13.3 28.8 65.1 – 28.8 6.8 13.2 0.13 1.94 95.0/88.1 742.3 (744.1d ) 97.0 14.26

a b

C.A.

1 2 3 4 5 6 7 8 9 10 11+ Poly Sum

Alkanes

Alkenes

Aro.

P.

N.

O.

C.O.

< 0.1 < 0.1 < 0.1 4.8 14.8 13.1 6.0 6.9 1.2 0.4 0.8 < 0.1 48.0

– – < 0.1 < 0.1 0.5 1.6 1.4 0.8 0.3 0.1 < 0.1 < 0.1 4.7

– < 0.1 < 0.1 0.7 3.3 2.4 1.0 0.4 0.2 < 0.1 < 0.1 < 0.1 8.0

– – – – 0.2 0.4 0.3 0.2 0.1 < 0.1 < 0.1 < 0.1 1.2

Oxy.

Sum

c d e

– – – – – 0.9 7.6 10.5 8.3 4.4 0.4 < 0.1 32.1

< 0.1 5.4 < 0.1 < 0.1 0.2 0.4 < 0.1 < 0.1 < 0.1 < 0.1 < 0.1 < 0.1 6.0

< 0.1 5.4 < 0.1 5.5 19.0 18.8 16.3 18.8 10.1 4.9 1.2 < 0.1 100

Market gasoline, data from fuel analysis. Numerically determined surrogate formulation. Real blended surrogate fuel, data from fuel analysis. Calculated on the basis of measured composition. Calculated according to Eq. (1).

C.A. = Carbon Atoms, P. = Paraffins, N. = Naphthenes, O. = Olefins, C.O. = Cyclic Olefins, Aro. = Aromatics, Oxy. = Oxygenates. Fig. 5. Measured distillation curves of RON95E5 and blended surrogate fuels.

The focus of the investigation was on the behavior of a surrogate fuel suitable for CFD regarding gaseous emissions in a wide engine operating range. In general, the SI operation mode is expected to be less sensitive than other modes (e.g HCCI) to surrogate formulation, since it does not rely on auto-ignition. Indeed, the matching of chemical indicators (e.g. H/C ratio, RON, MON) normally ensures a good agreement in terms of laminar burning velocity [10] and high temperature heat release for PRF/TRF surrogates [60]. As previously mentioned, literature studies [27– 29,60] identified the addition of olefins as more relevant for the matching auto-ignition characteristics. However, their inclusion inevitably introduces an additional effort in determining the blending behavior with TRF surrogates with ethanol. Since no specific investigations on knock, pre-ignition, or auto-ignition mode were considered, even if the target market RON95E5 contains 9.2% of olefins, it was decided to not introduce an olefin-representative component and to select the most widely used surrogate mixture of TRF with ethanol for the investigations. The surrogate fuel is formulated numerically by minimizing the differences between the properties of real and surrogate fuels [26]. H/C ratio, liquid density, research (RON) and motor octane numbers (MON) are taken into account as property targets. Linear mole and volume fraction-based blending rules are used to calculate the H/C ratio and the liquid density of the surrogate fuel, respectively. The non-linear volumetric blending approach [61] is applied to determine the octane numbers of the base gasoline fuels, which are emulated with the mixtures of n-heptane, isooctane, and toluene. The mole fraction-based blending rule [62] is then employed to determine RON and MON for the mixtures of the base gasoline fuels with ethanol. Detailed description about the formulation method can be found in [26].

The surrogate mixture used in the experiments is prepared according to the determined composition. However, because of inaccuracies in the practical mixing, the composition of the real surrogate mixture used in the experiments slightly differs from the determined one. Table 5 compares major properties of the RON95E5 fuel with the measured compositions of the prepared mixture (c ) and those numerically determined (b ). The RON and MON values of the prepared surrogate mixture are also experimentally determined and listed in Table 5. The calculation of the stoichiometric air-to-fuel ratio AFRst is performed according to the following formula:

AFRst =

1

ξO2,air

·



MO2 z y · x+ − Mfuel 4 2



(1)

where x, y and z are respectively the average numbers of carbon, hydrogen and oxygen atoms in the fuel that define the average fuel formula Cx Hy Oz ; ξO2,air is the mass fraction of oxygen in air assumed as 0.232; Mfuel is the molar mass of the fuel and MO2 the one of molecular oxygen assumed equal to 31.998 kg/kmol. Figure 5 shows the comparison of the measured distillation curves between the RON95E5 fuel and the real surrogate mixture used in the measurements. 3.1. Surrogate sooting tendency analysis The threshold sooting index (TSI), derived from the smoke point (SP), has been widely considered in the formulation of diesel fuel surrogates [63] to ensure that the formed surrogate fuel is capable of correctly predicting soot formation. However, it is impossible to consider this property in the formulation of gasoline surrogates,

S. Esposito, L. Cai and M. Günther et al. / Combustion and Flame 214 (2020) 306–322

as experimental data of TSI or SP for gasolines are very scarce in the literature. To characterize the particulate matter emissions of gasoline fuels, Aikawa et al. [64] proposed the particulate matter index (PMI) based on empirical modeling. It is defined as:

PMI =

n  i=1

 DBE + 1 i pi



+ Yi .

(2)

Here, pi and Yi are the vapor pressure at 443 K and the weight fraction of the component i, respectively. DBEi is the double bond equivalent value of the component i. It is estimated as (2xi +2-yi )/2, where xi and yi are the numbers of carbon and hydrogen atoms in the molecule, respectively. The PMI has been adopted later in several studies [65,66] and shown to predict successfully the particulate matter emissions of gasoline engines. In the work of Barrientos et al. [66], the PMI was found to be equally accurate as SP and TSI for the prediction of particle number emissions. Using the equation above, the PMI of the present surrogate fuel is estimated as 0.49. The PMI is not taken into account in the surrogate formulation of this study, as it is difficult to determine the PMI for the considered RON95E5. Aikawa et al. [64] have calculated the PMI for a set of commercially available gasoline fuels and found that their PMI values vary between 1 and 4. Thus, the present surrogate fuel is expected to have a much lower sooting tendency than the market gasoline fuel. Therefore, even though soot formation is a complex process strongly affected by e.g. wall– fuel interactions and mixture formation in a real engine, it is expected that differences occur in the soot emissions of the surrogate fuel and the market gasoline.

311

Table 6 Base operating point for catalyst heating measurement. Variable

Value

Engine speed / 1/min IMEP / bar Air-to-fuel ratio λ SOI /◦ CA aTDCF IVO (at 1 mm) /◦ CA aTDCF EVC (at 1 mm) /◦ CA aTDCF Intake air temperature /◦ C Tcool = Toil /◦ C

1200 3 1.01 440 380 356 25 30

SOI = Start of injection.

Fig. 6. Average in-cylinder pressure traces comparison between RON95E5 and the surrogate fuel at the catalyst heating operation.

4. Results and discussions In this section, results at selected operating parameter variations are shown first. Afterwards, a correlation analysis is reported in which all the 50 operating points available for comparison (listed in Section 2.3) are included. 4.1. Selected operating parameter variations For the sake of brevity, only some specific parameter variations are analyzed in this section. These are selected in order to show in detail the capability of the surrogate fuel to reproduce the RON95E5 behavior at very different boundary conditions. One aspect of particular interest is the reproduction of the combustion in less stable operating conditions as in catalyst heating operation or with lean air-to-fuel-ratios. Regarding gaseous emissions, the complex phenomena of the emission formation are inevitably bounded to the fuel properties: CO and NOx mainly dependent on mixture formation and combustion temperature while HC emissions are influenced by laminar burning velocity (for flame quenching) and oxidation chemistry. For this reason, it is very interesting to analyze the effect of enleanment together with low coolant temperature as a good test case especially for HC emission characteristics. 4.1.1. Catalyst heating operation In this section, selected results of the catalyst heating measurements for the RON95E5 and the surrogate fuel are presented. These measurements are performed to determine the combustion retardation capability of the engine with a certain fuel at low load, low speed, and cold engine conditions. This procedure emulates a typical calibration of the catalyst heating mode at cold start. Thereby a high exhaust temperature is achieved by means of late combustion through spark timing retardation in combination with a slightly lean air-to-fuel ratio at constant load (IMEP), up to combustion stability limits. The stability limit criteria used is the IMEP coefficient of variation (CoV) defined as the ratio between the IMEP

standard deviation and the IMEP mean value. This measurement is performed starting from a stable base point with stepwise retarding the spark timing up to CoV of 10%. In Table 6, the boundary conditions and the operating parameters for the catalyst heating measurements are reported. In order to keep the load constant with later spark timing, the intake pressure and the amount of injected fuel must be increased, to compensate for the lower indicated efficiency. Figure 6 shows the measured in-cylinder pressure traces with the two fuels for different spark timings. With spark timing retardation, the intake pressure and the incylinder pressure at the end of compression increase. At very late spark timings, the combustion is strongly retarded and the combustion peak pressure is lower than the pressure at the end of the compression stroke. For the two latest spark timings, the term “peak pressure” refers to the peak relative to combustion, not the absolute one that accurs at TDC. The agreement in the in-cylinder pressure traces is overall very good, even if the surrogate fuel has a slightly higher peak pressure than the RON95E5. However, a higher pressure is also observable in the compression phase, highlighting a difference in intake pressure and also in IMEP. This results from an imperfect load control with small differences in IMEP between the two fuels and variations among the operating points. Figure 7 depicts the comparison regarding IMEP (a), peak pressure value and position (b), combustion stability (c), exhaust temperature and air mass flow (d). As already noted for the pressure curves (s. Fig. 6), the IMEP (Fig. 7a) along the variations is not exactly constant and higher in the case of the surrogate fuel. This is the cause for slightly higher peak pressure values. With spark retardation, the combustion peak pressure (Fig. 7b) is reduced and the position of peak pressure moves from optimal values of approximately 12◦ CA aTDCF to about 90◦ CA. The trends concerning peak pressure positions and values are almost identical between the fuels. Regarding combustion stability (Fig. 7c), the IMEP CoV is near to the stability limit

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Fig. 7. Measured combustion and performance average quantities for RON95E5 and the surrogate fuel at the catalyst heating operation.

for the latest spark timing. The trend of the surrogate IMEP CoV is the same, despite an offset in the absolute value. However, in the most interesting point that is the most unstable one, the relative deviation is only of approximately 7.5%. Regarding the exhaust temperature (Fig. 7d), one of the most important aspects in catalyst heating operation, a perfect match is observable. The trend of the exhaust temperature is the same of the air mass flow (Fig. 7d), which then also corresponds to that of fuel mass flow at fixed λ. For the catalyst heating operation, no combustion progress postprocessing on the basis of the measured cylinder pressure (combustion center, burn durations) is shown, due to inaccuracies in the calculation. Indeed, the heat release analyses on the basis of pressure indication can be affected by high inaccuracies in case of very retarded and incomplete combustion, due to some assumptions necessary to perform the energy balance in the combustion chamber (heat release, amount of fuel burned, etc.). In the catalyst heating operation at cold start, the engine-out emissions are more critical than in normal operation due to low catalyst conversion efficiency. Figure 8 shows the average global pollutant emissions for the RON95E5 and the surrogate fuel for the catalyst heating measurements in Pos. 2. While the THC emissions (Fig. 8b) monotonically decrease with spark retardation due to increasingly higher exhaust temperatures, the other pollutant emissions show a minimum for spark timing of 0–10◦ CA aTDCF . The CO emissions (Fig. 8a) decrease first with later spark timing due to lower peak pressures that lead to less CO production. The increase in CO at late spark timings is bounded with more incomplete and late oxidation of the HCs not only in the combustion chamber, but also in the exhaust port due to the high exhaust temperature. The NOx emissions (Fig. 8c) decrease at first with spark retardation due to lower peak pressures and peak temperatures. With further spark retardation, the peak pressure continues to decrease and at the same time the indicated

Fig. 8. Measured pollutant emission in Pos. 2 for RON95E5 and the surrogate fuel at the catalyst heating operation.

efficiency decreases very strongly. Thus, to maintain a constant air-to-fuel ratio, the air and fuel mass flow must therefore be increased to compensate for the reduction in efficiency. In Fig. 7d, the air mass flow is shown: the strong increase after spark timing of 0◦ CA aTDCF is visible. The higher fuel mass injected for the latest spark timings corresponds to higher energy introduced in the system and overall in higher energy release that causes higher NOx emissions. These fundamental trends are visible for both fuels. It is worth mentioning that emission deviations of 5–10% can also be expected with the same fuel on different days, due to small differences in boundary conditions, ambient air conditions (humidity), oil dilution level, deposits, etc. The surrogate fuel measurements have been performed approximately one week after the experiments with RON95E5 and the deviations in gaseous emissions is within 5–10% in many points. The overall higher level of NOx emissions (Fig. 8c) can be attributed to the higher in-cylinder pressure and then higher maximum temperature. As far as CO and THC are concerned (Fig. 8a and b), an inversion of the relative trend between the two fuels can be observed. The RON95E5 shows higher CO emissions and lower THC emissions than the surrogate fuel for earlier spark timings. However, for spark timings later than 0◦ CA aTDCF , the trends are reversed, with the RON95E5 having lower CO and higher THC emissions than the surrogate. This inversion implies that the late oxidation of the surrogate fuel due to high temperatures, most likely occurring partly in the exhaust ports too, is faster than the one with RON95E5. Regarding the smoke emissions (Fig. 8d), a much lower level is observed for the surrogate fuel, as to be expected from the PMI calculation of Section 3.1. In Fig. 9, the FFID measurements in Pos. 1 and Pos. 2 for the earliest and the latest spark timings for the catalyst heating variation are reported.

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Fig. 10. Measured mean cycle-averaged FFID THC for RON95E5 and the surrogate fuel at the catalyst heating operation. Table 7 Base operating point for the λ variations.

Fig. 9. Average FFID traces comparison between RON95E5 and the surrogate fuel for two different spark timings of the catalyst heating operation for Pos. 1 (top) and Pos. 2 (bottom).

In Pos. 1, an increase in the FFID average level measurements is observed with spark retardation. This is due to more incomplete combustion and higher pressure at EVO that results in higher amounts of HC stored in crevices. In Pos. 2, as already observed in Fig. 8, the average THC level decreases with more retarded spark timing due to higher exhaust temperatures and late HC oxidation. A difference in the gas dynamics for the two different spark timings is also observable. In Pos. 1, a faster blowdown at EVO is visible for the more retarded spark timing due to a higher cylinder pressure at EVO and higher mass flow. For the same operating point, a shorter peak is observable in Pos. 2 that results from a faster transit of the HC stored in the exhaust port from the previous cycle. These qualitative trends over crank angle are very similar between the two fuels. This proves that the HC-formation mechanisms are comparable and thus no fundamental differences can be observed. The offset in the absolute values can be explained with FFID accuracy, the calibration status in addition to the abovementioned engine deviations on different days due to boundary conditions and engine state. Since the FID response is HC-species dependent, this could be an additional cause for deviation between the fuels. The fuels are expected to generate different HC-species due to different chemical compositions and therefore the FFID signal will be affected by this difference. This applies in particular to Pos. 1 where radical species from combustion can be present. Figure 10 depicts the cycle-averaged FFID THC measurements for both fuels in the two measurement positions. As already noticed in the cycle-resolved measurements, an offset is visible, but the trends are almost identical. In Pos. 1, the THCs are increasing with spark retardation, due to increasingly incomplete fuel oxidation in the combustion chamber and higher amounts of HCs from crevices expelled at the end of the exhaust stroke. In Pos. 2, as observed in Fig. 8b, also the FFID THCs are constantly decreasing with spark retardation, even if lower values are observed due to FFID inaccuracy and underestimation tendency (as pointed out in Section 2.2.2). These THC opposite trends in the two measuring positions again are an indicator for oxidation processes in the exhaust ports. The relative trend inversion between RON95E5 and the surrogate fuel is also visible in the FFID THC at Pos. 2, but not in Pos. 1. This further supports the hypothesis of

Variable

Value

Engine speed / 1/min IMEP / bar SOI / ◦ CA aTDCF IVO (at 1 mm) / ◦ CA aTDCF EVC (at 1 mm) / ◦ CA aTDCF Intake air temperature / ◦ C

1500 8 440 380 356 25

a faster late oxidation process of the surrogate fuel in the exhaust system in comparison to RON95E5. The IMR-MS HC-species measurements for the catalyst heating operation are reported for completeness in Appendix B. 4.1.2. Air-to-fuel ratio (λ) and coolant temperature variation In this section, selected results of λ-variations for the RON95E5 and the surrogate fuel are presented. In particular, two variations per fuel are reported, one at warm engine conditions (Tcool = Toil = 90 °C) and one at cold engine conditions (Tcool = Toil = 30 °C). In Table 7, the boundary conditions and the operating parameters for the λ-variations are reported. The spark timing is set for each operating point in order to keep the combustion center (50% of mass fuel burned, MFB50%) in an optimal position (7–8◦ CA aTDCF ), because there is no knock detected in these operating points. In order to keep the load (IMEP) constant with higher λ, the intake pressure is increased, while the amount of injected fuel is reduced to compensate for higher indicated efficiency. In Fig. 11, the comparison regarding combustion average quantities for the two λ-variations between the two fuels is depicted. In particular, the spark timing (a), the burn durations 0–5% and 10– 90% of mass fuel burned, BD0-5% (b) and BD10-90% (c), and the IMEP CoV (d) are analyzed. Due to slower combustion resulting for enleanment, an advance in spark timing (Fig. 11a) is needed to keep the combustion phasing optimal. This is observable for both warm and cold cases. The combustion values (Fig. 11b and c) are very similar between the two cases, with the exception of the leanest operating point, in which for the cold operation a stronger increase in combustion duration and IMEP CoV (Fig. 11d) is observable. The overall agreement between the two fuels is very good. Regarding combustion durations (Fig. 11b and c), the maximum deviation is of 1◦ CA. The agreement regarding combustion stability (Fig. 11d) is also excellent. This is very important considering that the leanest operating point has an IMEP CoV of 2.5%, near to the maximum allowed in normal engine operation, which is about 3% (for normal engine operation, not catalyst heating). Figure 12 depicts the comparison regarding pollutant emissions between the two fuels for the two λ-variations measured in Pos. 2.

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Fig. 13. Average FFID traces comparison between RON95E5 and the surrogate fuel at warm and cold operation with λ = 1.5 for Pos. 1 (top) and Pos. 2 (bottom).

Fig. 11. Measured combustion average quantities for RON95E5 and the surrogate fuel at warm and cold λ-variations.

Fig. 14. Measured mean cycle-averaged FFID THC values for RON95E5 and the surrogate fuel at warm and cold λ-variations.

Fig. 12. Measured pollutant emission in Pos. 2 for RON95E5 and the surrogate fuel at warm and cold λ-variations.

As to be expected from basis knowledge on emission dependency as a function of λ, the CO emissions (Fig. 12a) strongly increase in rich conditions and are reduced in lean operation. The trends are similar between the two fuels, with higher deviation for λ equal to 1.0 and 0.9. This is probably due to differences in mixture formation which are a result of different fuel volatilities. With lean average λ, CO production is not strongly dependent on mixture formation because less fuel rich zones exist. The CO in lean operation is expected to come mainly from the oxidation in of the HCs from the piston-ring crevices late in the expansion phase. A slight increase with λ from 1.1 to 1.5 is probably due to earlier quenching of the crevices-HC oxidation in the expansion phase due to low cylinder temperatures. As far as THC emissions are concerned (Fig. 12b), the λ trends show a minimum for λ = 1.1. At richer operation, the incomplete oxidation increases and at leaner operation flame quenching occurs more extensively. At cold operation, almost twice the THC emissions measured with warm engine are observed. The increase arises from the following aspects: increased wall wetting, increased quenching, not only of flame quenching at the walls but also of HC-crevices oxidation in late expansion, increased pistonliner gap, and increased adsorption/desorption mechanism of the

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Fig. 15. Scatter plots of combustion and global quantities.

fuel in the oil layer. Indeed, the solubility of fuel in oil is increased at low temperatures and results in more desorbed fuel HC in the expansion stroke. In particular, the oil layer mechanism is supposed to increase the most at cold operation together with fuel film, in comparison to the other effects. While there is almost a match between the two fuels regarding the THC emissions for warm operation (Fig. 12b), slightly lower values are observed for the surrogate fuel at cold operation. Even if the absolute deviation is still within the abovementioned 5–10% range, the relative change in THC emissions for the surrogate fuel between cold and warm operation (measured one after the other) is lower than for the RON95E5. The adsorption/desorption mechanisms are actually quite complex, dependent on the single fuel HC components. In general, heavier aromatics have a higher solubility in oil. Since the RON95E5 has not only a higher total aromatic fraction, but also heavier aromatics than toluene, the RON95E5 is supposed to have a higher solubility in the oil which results in a stronger HC increase at cold operation in comparison to the surrogate fuel.

Regarding NOx emissions (Fig. 12c), a peak is observed for λ = 1.1 due to the combination of oxygen availability and high combustion temperature, while for leaner conditions, the combustion temperature decreases strongly due to excess air mass. The cold variation shows approximately 10% less NOx emissions in stoichiometric and lean operation in comparison to the warm operation. This is caused by the increased heat losses that reduce the maximum process temperature. For the rich operating condition, the warmer engine condition seems to have no influence on the NOx emissions. This is probably due to the limited oxygen availability that already results in the maximum possible production of NOx at cold conditions. As for the catalyst operation, a disagreement between the fuels regarding smoke emissions is observed (Fig. 12d). The RON95E5 fuel shows a strong increase with cold engine temperature, while for the surrogate fuel, the smoke measurements are always about zero. In Fig. 13, the FFID THC comparison between the two fuels for the operating points with λ = 1.5 in Pos. 1 and Pos. 2 is shown.

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CO2, dry average / %

15.0

Regression (y=kregr·x)

R2 = 0.998 kregr= 0.991

Surrogate

0.45 Surrogate

13.5

λ=1 Cat. heating (CH) λ 1

Filter Smoke Number / 0.60

12.0 10.5

no λ 1 R2 = 0.893

Regression (y=kregr·x)

R2 = 0.124 kregr= 0.073

λ=1 Cat. heating (CH) λ 1

0.30 0.15

9.0

0.00 9.0

10.5 12.0 13.5 RON95E5

15.0

(a)

0.00

0.15 0.30 0.45 RON95E5

0.60

Fig. 18. Scatter plots of AVL-SM smoke measurements.

O2, dry average / %

10.0

5.0 2.5

4.2. Statistical analysis

2 1

no λ 1 R2 = 0.945

0.0

no λ 1 R2 = 0.563

0 0.0

2.5 5.0 7.5 RON95E5

10.0

0

(b)

Surrogate

2200

THCwet average / ppmC

3

2 3 RON95E5

4

NOx, wet average / ppm

6000

R2 = 0.969 kregr= 0.982

1600

1

(c)

R2 = 0.992 kregr= 0.984

4500 Surrogate

2800

The IMR-MS HC-species measurements for the two λ variations are reported for completeness in Appendix C.

R2 = 0.997 kregr= 0.999

3 Surrogate

Surrogate

4

R2 = 0.999 kregr= 0.993

7.5

COdry average / %

1000

3000 1500

400

In the following, a correlation analysis for all the 50 measured points is performed. The correlation between the RON95E5 and the surrogate fuel is shown by means of scatter plots. In these plots, the values measured for the surrogate fuel are plotted against the corresponding RON95E5 measurement data. The points in the scatter plots are shown in three groups: • λ = 1 operating points in black; • Catalyst heating (CH) operating points in grey; • λ = 1 points in white. Additionally, a quantification of the agreement is included in the plots by means of the calculation of the coefficient of determination R2 and the regression coefficient kregr of the fitting curve y = kregr · x (theoretical requirement for the perfect surrogate fuel is kregr = 1). The regression line y = kregr · x is then also added to the plot.

0 400 1000 1600 2200 2800 RON95E5

(d)

0

1500 3000 4500 6000 RON95E5

(e)

Fig. 16. Scatter plots of FEVER emission measurements in Pos. 2.

Fig. 17. Scatter plots of FEVER CO measurements in Pos. 2 without the λ = 1 operating points.

Also in this case, very similar trends over crank angle are observed, even if an offset in absolute values is visible. Figure 14 depicts the cycle-averaged FFID THC measurements for the fuels at the two temperatures and in the two measurement positions. As already noticed in the cycle-resolved measurements, an offset is visible, but the trends are very similar in both positions.

4.2.1. Combustion characteristics and global quantities In Fig. 15, the scatter plots of the combustion related and global quantities are shown. A good overall agreement in all the quantities is observable. The combustion progress quantities (e.g. MFB50%, BDx%) are not included for the catalyst heating due to the inaccuracy in the heat release analysis, as mentioned in Section 4.1.1. The correlation of the R2 is repeated for some quantities with the exclusion of the catalyst heating (CH) operating points, if the CH values lie outside of the range of the other operating points. The correlation coefficient is above 0.92 for all quantities, with exception of the IMEP CoV (Fig. 15g) that without CH shows a R2 of 0.824. The kregr has a deviation to 1 that, for the majority of the quantities, is below 2% and reaches maximum 5.4% for the IMEP CoV (Fig. 15g). The very good correlation of the air mass flow (Fig. 15k) underlies also the very good agreement in indicated efficiency too. To obtain the same IMEP (controlled) with the same λ (controlled), the same air mass flow is required. 4.2.2. Emission characteristics In Fig. 16, the scatter plots of the FEVER emission measurements in Pos. 2 are presented. The correlation coefficient is above 0.99 for all the emissions, with exception of the HC emissions (Fig. 16d) that show more scatter for high values (corresponding to the measurements at cold engine conditions) with a R2 of 0.969. However, the correlation for CO, CO2 and O2 (Fig. 16a–c) is improved by the presence of the λ sweeps with rich and lean engine operating conditions. In the

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Fig. 19. Scatter plots of mean cycle-averaged FFID THC measurements in Pos. 1 and Pos. 2.

cases of CO2 and O2 (Fig. 16a and b), the R2 is reduced respectively of approximately 10% and 5% without the λ = 1 operating points. Regarding CO (Fig. 16c), the reduction is significant, from 0.997 to 0.563. To analyze more in detail this strong difference, the CO emission plot without the λ = 1 operating points is shown in Fig. 17. Due to the high sensitivity of CO emissions to mixture formation, the scatter regarding CO is assumed to derive from the difference in air–fuel distribution in the combustion chamber as consequence of the different fuel distillation curves, as shown in Fig. 5. Regarding kregr , its deviation from the ideal value of 1 is of maximum 2% for all the gaseous emissions. In Fig. 18, the AVL-SM smoke measurements scatter plot is shown. As observed in the selected variations shown before, strong differences between the fuels are obvious. The surrogate fuel shows

317

more than 10 times less sooting tendency as the RON95E5 fuel. This result represents the major discrepancy between the two fuels and also one of the major observations from this experimental analysis. Also in [39], the tested surrogate fuels (all based on TRFs) showed by a factor 10 lower particle number emissions compared to the market gasoline. In Fig. 19, the scatter plots of the cycle-averaged FFID values in Pos. 1 (a) and Pos. 2 (b) are shown. The correlation in both cases is quite good, with more scatter for the measurements in Pos. 1 (Fig. 19a) due to device inaccuracies (in combination with high scales), the calibration status and the FID HC-species sensitivity. The correlation coefficient is rather high, especially in Pos. 2 (Fig. 19b) with a R2 value of 0.95. The kregr has a deviation of only 1% to 1 in Pos. 1 and of 7.5% in Pos. 2. In Figs. 20 and 21, the scatter plots of the IMR-MS measurements in respectively Pos. 1 and Pos. 2 are presented. Overall correlation is visible for all the species, even if the correspondence is not always 1 to 1, due to the different fuel chemical compositions. In Pos. 1 (Fig. 20), a higher scatter than in Pos. 2 (Fig. 21) is observable because of higher deviations in the local concentrations of the HC originating from the piston-ring crevices. Worth mentioning is that for C2 H2 (Fig. 20b), C6 H6 (Fig. 20d), and C4 H6 (Fig. 20e) (all partially oxidated species, not present in the surrogate fuel and very low concentrated in the RON95E5), the highest values are achieved during the catalyst heating operation for both fuels. In Pos. 2 (Fig. 21), overall higher correlation is observed with all the R2 above 0.75 with exception of C4 H6 (Fig. 21e) that shows a lower value. The kregr for CH4 is very similar to 1 in both measuring positions (Fig. 20b, Fig. 21b). The C2 H2 and C4 H6 emissions from the surrogate fuel are between Pos. 1 (Fig. 20b and e) and Pos. 2 (Fig. 21b and e) 70–80% of the values from RON95E5. The C6 H6 emissions of the surrogate fuel between Pos. 1 (Fig. 20d) and Pos. 2 (Fig. 21d) are approximately 50% and 45% of the RON95E5 values, respectively. This can be linked to

Fig. 20. Scatter plots of IMR-MS measurements in Pos. 1.

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Fig. 21. Scatter plots of IMR-MS measurements in Pos. 2.

the little amount of C6 H6 in the RON95E5, around 1% as reported in Table 4, but also to the different composition of the aromatics fraction that in the surrogate fuel is represented only by toluene. Regarding C2 H5 OH, the surrogate fuel emissions are approximately 64% of the RON95E5 in Pos. 1 (Fig. 20c). In Pos. 2 (Fig. 21c), they are almost the same with a kregr of 0.925. Also regarding C4 H10 , the agreement between the surrogate fuel and RON95E5 increases from Pos. 1 (Fig. 20f) to Pos. 2 (Fig. 21f), from approximately 55% to almost 83%. Lastly, regarding C7 H8 and C7 H16 , both are much higher for the surrogate fuel, due to the higher fraction of these components in the surrogate in comparison to the RON95E5. In particular, the C7 H8 emissions of the surrogate are between Pos. 1 (Fig. 20g) and Pos. 2 (Fig. 21g) respectively approximately 1.65 and 1.5 times the RON95E5 emissions, even if the concentration of this HC-species in the original fuel is almost 4 times higher than in the surrogate fuel. Regarding C7 H16 , though it is more than 13 times more present in the surrogate fuel composition than in the RON95E5, the factors between the surrogate fuel and the RON95E5 C7 H16 emissions are approximately 2.7 and 3.3 times in Pos. 1 (Fig. 20h) and Pos. 2 (Fig. 21h) respectively. These two HC species (C7 H8 and C7 H16 ) likely originate from the crevices-HC that are expelled at the end of the exhaust stroke, while they are not likely present earlier in the exhaust stroke. For this reason, since the value measured with the IMR-MS are averaged, the average ratio of these HC-emission between the fuels is lower than the ratio of these components in the original fuels. 5. Conclusions In this work, an experimental comparison of combustion and emission characteristics between a market RON95E5 gasoline and its corresponding surrogate fuel was presented. To gain a comprehensive overview, the correlation analysis considered a strong vari-

ation of operating conditions with respect to engine load, speed, relative air-to-fuel ratio, valve timing, spark timing, and operation at cold start. The selected surrogate fuel was a toluene reference fuel (TRF) with ethanol addition. With its composition optimized to match the H/C ratio, the density, and the octane numbers of real fuel, the surrogate fuel very accurately reproduced the combustion and global gaseous emission characteristics of the market gasoline, with a coefficient of determination R2 mainly above 0.92 and a deviation from the 1 to 1 ideal correspondence mainly below 2%. During stoichiometric operation, notable differences regarding CO emissions were observed between the surrogate and the market fuel. This is possibly due to differences in mixture formation, which was strongly influenced by the distillation curves of the fuels. It was also found that the exclusion of the soot formationrelated properties, such as the Particulate Matter Index (PMI), in the surrogate fuel formulation led to a strong under-prediction of the smoke values. Even though the total HC emissions were quite accurately reproduced with the surrogate fuel, a deviation in the single HC-species measurements was observed. This applies with the exception of the CH4 concentrations, which showed a 1 to 1 correspondence in all the measurement positions. In summary, major differences are observed for the aspects that are either bound to phenomena not taken into account in the surrogate formulation or that are overall difficult to match with only few surrogate components on the basis of primarily chemical reference indicators, like evaporation curve, sooting tendency, and intermediate oxidation products. In conclusion, the present work demonstrated that using the experimentally tested surrogate fuel for 3D-CFD simulations is expected to provide reliable predictions of global gaseous emission concentrations, despite the different composition and the deviation in the distillation curves.

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Declaration of Competing Interest The authors declare that they do not have any financial or nonfinancial conflict of interests.

Acknowledgments This work was funded by the DFG (Deutsche Forschungsgemeinschaft, German Research Foundation), within the framework of the research training group “mobilEM” (grant number GRK 1856). The authors wish to thank the DFG also for enabling this work by funding the IMR-MS used for the measurements.

Appendix A. Abbreviation list

Abbreviation 3D AVL-SM AFR aTDCF BD CA CFD CLD CoV DI DBE EI-MS EVC EVO FEVER FEVIS FFID FID FSN HC H/C HCCI ICE IE IMEP IMR-MS IVO MON NDIR NOx PIONA Pos. 1 Pos. 2 Pos. 3 Pos. SM PM PMD PMI PRF RON RON95E5 SCE SI SOI SP ST TDC TSI THC TRF

Description Three-dimensional AVL smoke meter Air-to-fuel ratio After top dead center firing Burn duration Crank angle Computational fluid dynamic Chemiluminesce detector Coefficient of variation Direct injection Double bond equivalent Electron ionization mass spectrometer Exhaust valve closing Exhaust valve opening FEV emission rate FEV indication system Fast FID Flame ionization detector Filter smoke number Hydrocarbon Hydrogen to carbon atoms ratio Homogeneous charge compression ignition Internal combustion engine Ionization energy Indicated mean effective pressure Ion molecule reaction mass spectrometer Intake valve opening Motor octane number Non-dispersive infrared detector Generic nitrogen oxides Paraffins, i-paraffins, olefins, naphthenes, aromatics Emission measurement position 1 Emission measurement position 2 Emission measurement position 3 Emission measurement position of AVL-SM Particle matter Para-magnetic detector Particulate matter index Primary reference fuel Research octane number Market gasoline RON=95 with 5% ethanol content Single cylinder engine Spark-ignition Start of injection Smoke point Spark timing Top dead center Threshold sooting index Total HC (measured with FID principle) Toluene reference fuel

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Appendix B. IMR-MS HC-species measurements for catalyst heating operation Fig. B1

Fig. B1. Measured HC species emissions in Pos. 1 and Pos. 2 for RON95E5 and surrogate fuel in the catalyst heating operation.

Appendix C. IMR-MS HC-species measurements for λ-variations Fig. C1 and Fig. C2

Fig. C1. Measured HC species emissions in Pos. 1 for RON95E5 and surrogate fuel in warm and cold λ-variations.

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Fig. C2. Measured HC species emissions in Pos. 2 for RON95E5 and surrogate fuel in warm and cold λ-variations.

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