Influence on Performance and Emissions of an Automotive Diesel

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Biofuels and Biomass

Influence on performance and emissions of an automotive diesel engine fueled with biodiesel and paraffinic fuels: GTL and bio-jet fuel Farnesane José Antonio Soriano, Reyes García-Contreras, David Leiva-Candia, and Felipe Soto Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03779 • Publication Date (Web): 20 Mar 2018 Downloaded from http://pubs.acs.org on March 21, 2018

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Data acquisition system Encoder signals

To atmosphere

In-cylinder pressure Injector energizing signal NanoScan SMPS Model 3910

Fuel system NO X,THC measurement system

Rotating disk diluter MD19-3E To atmosphere DOC

Exhaust gas system From atmosphere

Gravimetric fuel scale Fuel temperature control system

EGR system

Hot wire air flowmeter

Air filter

Fuel tank

Torque Engine speed Accelerator position Eddy current E90 dynamometer

Air inlet system Intercooler Air temperature control system

INCA PC software

ETAS interface

Opened engine ECU

INCA PC - ETAS - ECU communication system

2.2 liters 4 cylinders DI Diesel engine

Water temperature control system

Oil temperature control system

Auxiliary temperature control s ystem

Figure 1. Schematic of engine test bench

Figure 2. Relative position of the engine mode tested on the engine map.

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a) b) Figure 3. a) Specific fuel mass consumption and b) Brake thermal efficiency

a)

b)

Figure 4. a) Relative fuel-air ratio and b) EGR valve opening


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Figure 5. Injection (lower side) and combustion (upper side) timing parameters.

a)

b)

Figure 6. a) Specific THC emissions and b) Specific NOx emissions.


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a)

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c)

b)

d)

e)

Figure 7. Particle number distributions: a) Mode A, b) Mode C, c) Mode E, d) Mode G and e) Mode I.

a)

b)

Figure 8. a) Total particle number concentration and b) particle mean diameter.


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a)

b)

Figure 9. a) Particle mass concentration and b) Specific particle mass emission.


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Influence on performance and emissions of an

2

automotive diesel engine fueled with biodiesel and

3

paraffinic fuels: GTL and bio-jet fuel Farnesane

4

José Antonio SorianoϞ, Reyes García-ContrerasϞ*, David Leiva-Candiaϯ, Felipe SotoШ Ϟ

5

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Universidad de Castilla - La Mancha. Campus de Excelencia Internacional en Energía y

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Medioambiente, Escuela de Ingeniería Industrial, Real Fábrica de Armas, Edif. Sabatini, Av. Carlos III,

7

s/n, 45071 Toledo, Spain.

8

ϯ

9

Rabanales, Universidad de Cordoba, Campus de Excelencia Internacional Agroalimentario, ceiA3,

10

14071 Cordoba, Spain.

11

Ш

Dep. of Physical Chemistry and Applied Thermodynamics, Edificio Leonardo da Vinci, Campus de

Universidade Federal de São João del-Rei, Praça Frei Orlando 170, Centro, São João del-Rei, Minas

12

Gerais, Brazil.

13

*Corresponding author: Tel. (+34) 925 268 800. E-mail: [email protected]

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Keywords

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Paraffinic fuels, Farnesane, GTL, Fischer–Tropsch, pollutant emissions, diesel engine, bio–jet fuel,

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biodiesel.

17 18

Abstract

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The effects of a paraffinic biofuel (Farnesane) obtained from sugar cane, a Gas to Liquid (GTL)

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fuel and a biodiesel fuel on performance and emissions of a light duty engine were evaluated. Similar

21

engine performance is obtained with all fuels maintaining the default electronic control unit (ECU) 1 ACS Paragon Plus Environment

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configuration. Clear reductions in total hydrocarbons (THC) emissions were observed with all

23

alternative fuels and, comparing both paraffinic fuels, Farnesane was more beneficial. Reductions were

24

also obtained with these fuels in particle emissions, with the exception of the mode G, which is not an

25

operating mode included in the New European Driving Cycle (NEDC), where particle number

26

concentration of paraffinic fuels were higher compared with diesel and biodiesel fuels. This behavior

27

could be explained by the not optimization of ECU at this engine work condition. The benefits in terms

28

of pollutant emission obtained with Farnesane make it a potential biofuel for use in diesel engines.

29 30

1. Introduction

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The need to reduce the dependence on fossil fuels and to fulfil emissions standards has promoted

32

the emergence of alternative fuels to replace, total or partially, the traditional fuels (diesel and gasoline).

33

These alternative fuels constitute a timely and relevant research field for fuel companies to produce

34

hydrocarbons that meet fuel quality standards as well as for automotive companies to optimize

35

management strategies implemented in electronic control unit (ECU). Alternative fuels can be classified

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depending on the fuel molecule in mainly three categories: biodiesel, alkanes/olefin mixtures and

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isoprenoids1.

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Biodiesel. Biodiesel fuel is a mixture of mono-alkyl esters (usually methyl esters but ethyl and

39

higher alcohol esters can be also included in this classification), obtained by trans-esterification process

40

of vegetable oils or animal fats. It is the most widely studied biofuel and reductions on THC and Particle

41

Matter (PM) emissions with respect to conventional diesel fuel have been widely reported in the

42

literature.

43

compounds in its composition, which favours a cleaner combustion process. Concerning nitrogen oxides

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(NOx) emissions, most of the surveyed literature (85% of works consulted in the review of Lapuerta et

2-4

Those benefits are mainly due to the presence of oxygen and the absence of aromatic

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al.2) reports an increase of NOX emissions from biodiesel combustion compared to diesel fuel

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combustion. The engine load,5 type of engine (heavy or light-duty)2 and the composition of biodiesel

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(percentage of saturated esters)6 are factors that have notable influence in NOx emissions obtained with

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biodiesel. Despite the advantages in the use of biodiesel in terms of the reduction of pollutant emissions,

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its lower heating value, its high tendency to fuel oxidation and its poor cold flow properties (particularly

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for highly saturated biodiesel) partially decrease the added value of this fuel. 7

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Alkanes/olefins mixtures. They are compounds obtained by the hydrotreatment process of

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vegetable oils. Although the raw materials may be similar to that used in the biodiesel fuel production,

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the properties of Hydrotreated Vegetable Oils (HVO) fuel are closer to those of diesel fuel. The main

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disadvantage of HVO is its high production costs. The integration of this process in hydroprocessing

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installations of conventional refineries may reduce the costs of production, making more attractive this

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fuel.8 HVO has been experimentally evaluated in compression ignition engines. Most of the authors

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obtain reductions in THC and PM emissions,

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composition and its paraffinic structure. Fischer-Tropsch is other process normally used to obtain

59

alkanes/olefins mixtures as XTL (X can be Coal, Gas or Biomass) fuels.

9-11

due to the absence of aromatic compounds in its

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Isoprenoids. They are a wide family of compounds derived from isoprene (C5H8), being

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Farnesane (2,6,10-trimethyldodecane) the most common fuel. Its production process starts with the

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fermentation of biomass derived sugars by genetically modified microorganisms, obtaining a compound

63

called Farnesene, which later is hydrogenated to generate the molecule known as Farnesane. This alkane

64

presents high heating value and cetane number. Amyris Biotechnology Inc. is the most known company

65

that carries out this process.12 The lower freezing point and the high cetane number of Farnesane makes

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this fuel a potential candidate as diesel-like or/and jet fuel.13 There are several research works which

67

report the thermal and combustion properties of Farnesane fuel. Most of these works are related to 3 ACS Paragon Plus Environment

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Energy & Fuels

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aviation,14-16 power generation17 and ship propulsion18-19 applications, while the literature is scarce

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regarding the use of Farnesane in light duty compression ignition engines. In this sense, Farnesane is

70

considered as a novel fuel for automotive engines. As example, Millo et al.20 evaluated a blend with

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30% of Farnesane and 70% diesel fuel (in volume) at full load conditions and at different partial load

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operation in a light duty engine (without modifications of ECU calibration). Authors observed

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reductions in THC emissions with the blend of Farnesane, mainly at low-medium load modes, and a

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decrease of the smoke level at all engine operating conditions.

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Other alternative fuels, with paraffinic molecular structure, have been used in diesel engines,

76

especially that obtained by Fischer-Tropsch process. Several works have evaluated the effect of GTL

77

fuel on performance and emissions in compression ignition engines obtaining reductions in THC and

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PM emissions with respect to diesel fuel.9,21 Even though both Farnesane and GTL fuels have alkane

79

molecular structure, some properties are different (such as cold flow properties or density), apart from

80

the added value of the renewable origin of Farnesane.

81

In this work, the effects of a neat paraffinic fuel called Farnesane (normally used in jet

82

applications and produced by a biotechnology fermentation process) on performance and emissions in a

83

compression ignition engine are compared with respect to GTL, biodiesel and a conventional diesel fuel

84

(reference fuel). Five steady state engine operation modes are selected, three of them derived from the

85

NEDC and two modes more representative of engine conditions associated to current homologation

86

cycle (Worldwide harmonized Light-duty vehicles Test Cycle, WLTC).22 The ECU calibration was not

87

modified because the aim of this work is to evaluate the performance and pollutant emissions of these

88

neat alternative fuels are evaluated at the same engine power, using the stock ECU. Results obtained in

89

this work provide guidelines to optimise ECU configuration improving performance and reducing


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emissions with these alternative fuels, especially in the case of neat Farnesane fuel which has not been tested previously in light duty compression ignition engines.

2. Methodology and fuels 2.1. Engine test bench The experimental work was carried out in a 4 cylinder, 4 stroke, turbocharged, intercooled, 2.2 litres Nissan automotive compression ignition engine (Euro 3), equipped with diesel oxidation catalyst (DOC) and common-rail fuel injection system. This engine (whose main specifications are detailed in Table 1) is representative of typical diesel vehicles used in Europe. The engine was connected to an eddy current dynamometer brake Schenck E90. The brake control system allowed to measure and control the engine speed (n), throttle position (α) and torque (Me). For these tests, control mode α/n was used: the operator sets the engine speed value and the brake control varies the throttle position to reach the engine torque desired. Figure 1 shows a schematic view of the experimental installation used in this work. To reach the same effective torque, the throttle command was varied for each fuel. The rest of the engine parameters (injection pressure, injection timing, exhaust gas recirculation -EGR- valve opening, fuel quantity, etc.) were controlled by the internal mapping of the ECU to obtain the same brake engine power. The INCA PC software and the ETAS ES 591.1 hardware were used for the communication with the ECU. Despite the fact ECU was accessible, fuel effects were evaluated without modifying default ECU configuration. The start of pre-injection and main injection was defined from the fuel injection rates, which were obtained using the experimental methodology described in Armas et al.23 To acquire instantaneous in-cylinder pressure (necessary to perform the thermodynamic diagnosis of combustion) a piezoelectric


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pressure transducer (Kistler 6056A) coupled to a charge amplifier (Kistler 5018A) was used. A total of 20 in-cylinder pressure signals were registered (with 6 repetitions) to ensure the reliability of the combustion diagnosis results. Crankshaft rotational speed and instantaneous piston position were determined with an angle encoder (Kistler 2614CK) with a resolution of 720 pulses per revolution. The data was recorded with a digital scope (Yokowaga DL708E) and transferred to the computer via general purpose instrumentation bus (GPIB) card.

Data acquisition system Encoder signals

To atmosphere

In-cylinder pressure Injector energizing signal NanoScan SMPS Model 3910

Fuel system NO X,THC measurement system

Rotating disk diluter MD19-3E

Gravimetric fuel scale

To atmosphere

Fuel temperature control system

DOC EGR system

Exhaust gas system From atmosphere

Hot wire air flowmeter

Air filter

Fuel tank

Torque Engine speed Accelerator position Ed d y cu rren t E90 d yn amo meter

Air inlet system Intercooler 2.2 liters 4 cylinders DI Diesel engine

Air temperature control system

INCA PC software

ETAS interface

Water temperature control system

Oil temperature control system

Opened engine ECU

Auxiliary temperature control s ystem INCA PC - ETAS - ECU communication system

Figure 1. Schematic of engine test bench.

Table 1. Engine characteristics. Engine characteristics Type

Nissan YD22, 2.2L, turbocharged + intercooler

Fuel injection

Common rail, pilot injection

Maximum power

82kW at 4000min-1

Maximum torque

248Nm at 2000min-1


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Cylinder arrangement

4 cylinders, in line

Bore/stroke

86.5/94mm

Compression ratio

16.7:1

The uncertainties in measured and calculated engine parameters were estimated according to the methodology proposed by Bielaczyc and Szczotka24 and are shown in Table 2. Table 2. Accuracy and uncertainty of measured and calculated parameters. Parameter

Units

Accuracy

Engine speed

min [0 – 12000]

±1

Torque

Nm [0 - 200]

±2

Mass of fuel

g [0 - 10000]

±0.1

Volumetric flow air

kg/h [0 - 583]

±1.86

-1

Uncertainty of calculated parameters Engine power Brake specific fuel consumption Fuel-air ratio

kW

± 0.26

g/kW·h

± 4.47

-

± 0.0058

Brake thermal efficiency

%

± 0.57

2.2. Tested fuels Four neat fuels were tested in this work: i) an ultra-low sulfur diesel fuel without biodiesel supplied by REPSOL company which is used as reference fuel (denoted as Diesel), ii) a gas to liquid fuel obtained from natural gas by a Low Temperature Fischer-Tropsch process supplied by Sasol company and denoted as GTL, iii) a blend of 72% soybean and 28% palm biodiesel (in volume) supplied by Repsol company and iv) a renewable iso-paraffinic fuel obtained from sugar or its biomass by genetically modified micro-organisms12 fermentation and later hydrotreated to obtain a fuel denoted as Farnesane, which is provided by Amyris Biotechnology Inc. Main physical-chemical properties of all tested fuels are detailed in Table 3.


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The biodiesel blend used in this work has been selected because its combination between chain length and unsaturation degree optimizes five of the most important fuel physical properties (heating value, cetane number, viscosity, flash point and cold filter plugging point (CFPP)),25 without penalizing pollutant emissions.26 In a previous work, a similar biodiesel blend has been used to study lubricity properties of biodiesel-ethanol-diesel fuel blends.27 GTL and Farnesane are both paraffinic fuels which provides them with similar physical-chemical properties, while some differences are observed regarding the ignitability, distillation temperatures and cold flow properties. The very low values of CFPP of Farnesane fuel, much lower than that corresponding to GTL, is considered as a key property in order to use this fuel in aviation (up to 10% is allowed according to the D7566 ASTM standard). The differences in CFPP between both paraffinic fuels are justified as follows. Farnesane is a pure compound (2,6,10-trimethyldodecane), while GTL fuel is a mixture of normal and iso-alkanes. This is confirmed in their different volatility (distillation curves) resulting in a higher final boiling point (T90 and/or T95) for GTL, which has been reported leading to worse cold flow properties.28 The potential presence of olefins and/or aromatics in Farnesane composition, as it has been reported in other work,13 also could lead to its lower CFPP with respect to GTL. Table 3. Main properties of tested fuels. Properties

Diesel

Molecular formula

C15.18H29.13

Molecular weight (g/mol)

Farnesane a

GTL

Biodiesel a

C18.52H34.52O2b

C15H32

C16.89H35.77

211.4c

212.41

238.6c

289.25

H/C Ratio

1.92

2.13

2.12

1.86

Stoichiometric fuel/air ratio

1/14.64

1/14.92

1/14.95

1/12.46

C (% w/w)

86.13

84.91

84.82

76.91

H (% w/w)

13.87

15.09

15.18

12.03

O (% w/w)

0

0

0

11.06

Density at 15ºC (kg/m3)

843

770

771

883


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Viscosity at 40ºC (cSt)

2.97

2.32

2.57

4.2

Lower Heating Value (MJ/kg)

41.37

43.39

42.56

37.14

Cetane number

54.2

56.7 29

>73

53.3

CFPP (ºC)

-17

-40

-7

0

Distillation (vol.) 10% 50% 90%

207.6 278.2 345.0

243.5 243.8 244.0

213.9 269.3 340.7

279.5 282.7 302.2

a

Calculated by value of molecular weight and speciation of fuels (in the case of GTL, paraffinic structure is considered).

b

Calculated from ester composition.

c

Calculated by AspenTech HYSYS software from CHNS analysis and density value.

2.3. Engine test plan The test plan was designed to evaluate the impact of the type of fuel (a conventional fossil fuel – diesel-, a biodiesel fuel, and two paraffinic fuels -GTL and Farnesane-) on a typical automotive diesel engine, using the default ECU configuration. The engine was tested under five steady state operating modes which cover most of the zone (torque vs. engine speed) characterized by NEDC, in light-duty vehicles in both urban and extraurban conditions.30 Applying longitudinal dynamics equations, the velocity profile imposed by the New European Driving Cycle (NEDC) used for light-duty vehicle certification was translated into engine operating conditions (torque and engine speed), shown as black dots in Figure 2. Points plotted in colour dots indicate the five modes selected in this work, limited by the full load curve (maximum torque at each engine speed). The characteristics of engine modes tested are shown in Table 4. The modes outside of the NEDC zone (C and G) were selected because they are within the engine operation area established in the WLTC22 and, in these engine work conditions the ECU is not optimized. Table 4. Characteristics of steady state operating modes selected. Mode

Engine speed (min-1)

A

1000

C

2400

Engine Torque (Nm) 10

Effective power (kW) 1.05 2.51


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Energy & Fuels

E

1700

G

1000

I

2400

60 110

10.68 11.52 27.64

Figure 2. Relative position of the engine mode tested on the engine map.

2.4.

Measurement emission equipment Gaseous pollutant emissions (THC and NOx) and particle emissions (number and mass) were

studied, being measured downstream of the DOC. THC emissions were measured with a flame ionization detector (Graphite 52M), and a chemioluminiscence analyzer (Topaze 32M) was used to measure NOx emissions. Particle size distribution was measured with a Scanning Mobility Particle Sizer (SMPS) NanoScan Model 3910 coupled to a rotating disk diluter model MD19-3E. The dilution air ratio used in this work was 80:1. Particle mass emissions were estimated from the Particle Number Concentration (PNC) and the particle density correlation, following the methodology proposed by Gómez et al.31 Mean value results (corresponding to the mean and standard deviation values) are calculated from three repetitions for THC, NOx and particle size distribution. 10 ACS Paragon Plus Environment

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Results and discussion

3.1 Engine performance Specific fuel mass consumption and brake thermal efficiency of all fuels tested at the five steady state mode selected are shown in Figure 3a and 3b, respectively. The low engine power of modes A and C (see Table 4) produces the higher specific fuel mass consumption in these modes compared with medium-high engine load modes. Comparing the effect of fuels, biodiesel specific fuel mass consumption is the highest, which is coherent with its lower heating value and the other fuels show similar results, except in the case of Farnesane in modes A and C due to the slightly lower power reached with this fuel. The heating value of the fuels is the main responsible of the differences in brake specific fuel consumption as it is reflected on similar values of brake thermal efficiency for all fuels. The uncertainty in torque measurement has a notable influence in the values of power and thus specific fuel consumption and thermal efficiency.

1000 800 600 400 200 0

A

C

E

G

I

a) b) Figure 3. a) Specific fuel mass consumption and b) Brake thermal efficiency.

The relative fuel-air ratio, defined as the quotient between absolute and stoichiometric fuel-air ratios, and the percentage of EGR valve opening (recorded with INCA PC) are shown in Figure 4a and 11 ACS Paragon Plus Environment

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Energy & Fuels

4b, respectively. Results associated to mode G are remarkable because it presents the highest fuel-air ratio and the EGR valve remains close. It is thought that this operation condition is in the proximity to the smoke limitation zone (very high relative fuel-air ratio, see Figure 4a) forcing the closure of EGR valve. Regarding to the effect of fuels, biodiesel shows relative fuel-air ratios slightly lower than the other fuels. Results of both paraffinic fuels were similar to those of diesel fuel. With respect to EGR valve opening, no important differences were observed between fuels for all operating modes.

80 70 60 50 40 30 20 10 0

a)

A

C

E

G

I

b)

Figure 4. a) Relative fuel-air ratio and b) percentage of EGR valve opening.

3.2 Thermodynamic diagnosis Figure 5 shows parameters related to experimental injection (lower side) and combustion processes (upper side). The crank angle position of Start of Energizing (SoI) for both injections (pre and main) are plotted (lower side of Figure 5). Related to combustion process, the Start of combustion (SoC) is defined as the crank angle obtained by the crossing of the line adjusted between 0.8% and 2% of the heat release and the crank angle degree axis. The end of combustion (EoC) is defined as 90% of the heat release. Both parameters are shown in the upper side of Figure 5. Ignition delay (ID) is defined as the angle interval between SoI and SoC and combustion duration (CD) is defined as the angle interval


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Page 18 of 32

between SoC and EoC. As example, the combustion duration of all fuels for the five operating modes is plotted using dashed lines in Figure 5. The injection strategy for this engine is divided in two steps: an early injection (denoted as preinjection) and a main injection. The combustion of pre-injection reduces the ignition delay of main injection and, consequently, attenuates the phase of premixed combustion. This multiple injection strategy reduces the noise of combustion32 and usually tends to reduce NOx emissions, although this effect depends on the engine configuration and fuel used.33 While the main injection takes place around top dead centre (TDC) for all engine modes, timings of pre-injections depend on the engine load. The ignition delay is notably longer in modes A and C (low load) compared to the rest of engine modes. The fact that the SoC at low load modes takes place after the main injection indicates the combustion of preinjection occurs at the same time as the main injection, mainly due to the low temperature in the cylinder that hinders the fuel evaporation and its mixing with air. However, in modes E, G and I, the combustion process takes place shortly after the pre-injection. Biodiesel shows the lower combustion duration and, consequently, the fastest combustion velocity. The presence of oxygen in the fuel composition is the main justification for this effect.5,34


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Energy & Fuels

Figure 5. Injection (lower side) and combustion (upper side) timing parameters.

3.3 Gaseous emissions Specific THC and NOx emissions obtained with the four fuels in all tested modes are shown in Figure 6a and 6b, respectively. At the lowest engine load modes (A and C), THC emissions are notably higher justified by the low combustion temperatures that hinder fuel atomization, evaporation, air mixing and combustion reactions as well as the oxidation reactions in DOC and, consistently, reduces its pollutant conversion efficiency. Comparing the effect of fuels, biodiesel shows the lowest THC emissions, followed by Farnesane and GTL. The absence of aromatic compounds in the alternative fuels and the additional effect of oxygen in the case of biodiesel are the main factors that favour the reduction of THC emissions with respect to diesel fuel, both in the combustion chamber and DOC in agreement with literature.35-38 Comparing the two paraffinic fuels, Farnesane is more effective reducing THC 14 ACS Paragon Plus Environment

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emissions than GTL, particularly in mode C. Farnesane is a pure compound and its evaporation process is faster and more homogeneous than the corresponding of the GTL fuel (this is a mixture of normal and iso-paraffins).39-40 Therefore, in spite of the start of evaporation (T10) for GTL occurs at lower temperature than for Farnesane, GTL fuel keeps a wide distillation temperature range and its values for T50 and T90 are notably higher than those corresponding to Farnesane (see Table 3). In summary, GTL fuel will take longer time to fully evaporate than Farnesane fuel, this effect being more critical in modes A and C, where the combustion temperature is relatively cold. Regarding NOx emissions, the higher specific emissions in low load are mainly due to the low effective power in these engine modes. Results obtained with biodiesel were slightly higher than those observed with the other fuels. This is usually justified by the presence of oxygen in its composition.41-43 The higher cetane number (particularly in the case of GTL fuel) and the higher H/C ratio of paraffinic fuels implies lower adiabatic flame temperatures17,44 and may explain the slight reductions obtained with these fuels in modes C and E.

a)

b)

Figure 6. a) Specific THC emissions and b) Specific NOx emissions.


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3.4 Particle emissions Particle size distributions for the five engine tested conditions are shown in Figure 7. The highest particle number concentrations were obtained in mode G for all the tested fuels, as it is shown in Figure 7d (particle size distribution) and, more clearly, in Figure 8a (total particle number concentration). Additionally, the particle mean diameter is notably higher in this mode (Figure 8b). This is the engine operating condition with the highest value of relative fuel-air ratio, but it has not the highest load and the EGR valve remains closed (the latest would favour the reduction of particle emissions). The high PNC for all fuels in mode G are commented in a previous work45 and it may be justified by the ECU not been optimized in these engine zones (close to the smoke limitation zone of this engine and different to those established NEDC).46 Regarding the effect of fuels on particle emissions, biodiesel is the most beneficial fuel in all the tested engine operating modes. The absence of aromatic compounds and the presence of oxygen in its composition reduce the formation of soot nuclei and favour the oxidation of formed particles.46-47 Besides, the slightly lower relative fuel-air ratios of biodiesel contribute to these results. The effect of paraffinic fuels on PNC, compared with reference one, depends on the engine load: at low load GTL and Farnesane show lower particle number but in modes G and I their emissions are slightly higher than those of reference fuel. These trends are consistent with relative fuel-air ratio (Figure 4a) and are also influenced by shorter premixed combustion (longer diffusion combustion) stage with paraffinic fuel (higher cetane number, mainly in the case of GTL) which favours the increment of accumulation mode and reduction of nuclei mode with these fuels, especially at high engine loads.48 Comparing the two paraffinic fuels, values of particle number concentration from Farnesane were slightly lower in modes A and, particularly, in mode C (low engine torque but high engine speed) that those emitted from GTL combustion. This trend is consistent with results shown in THC emissions


Energy & Fuels

(lower emission with Farnesane fuel). The lower level of THC emissions decreases the probability of particle formation and growth through hydrocarbon adsorption/condensation onto the surface of soot particles.49 Additionally to the effect of lower mean volatility of GTL (wide range of temperature distillation) the larger carbon chain of this fuel favours its higher PNC compared with Farnesane.50

a)

c)

b)

d)

e)

Figure 7. Particle size distributions: a) Mode A, b) Mode C, c) Mode E, d) Mode G and e) Mode I.

160

Total particle number conc. (#/cm3)

1 2 3 286 4 5 6 287 7 8 288 9 10289 11 12 290 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40291 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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140 120 100 80 60 40 20 0

a)

A

C

E

G

I

b) 17 ACS Paragon Plus Environment

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Figure 8. a) Total particle number concentration and b) particle mean diameter.

From particle number concentrations and the particle density correlation (in function of particle diameter, Figure 8b) published in the work of Gómez et al.,31 particle mass concentration and specific particle mass emissions were estimated (Figure 9a and 9b, respectively). In all modes, alternative fuels showed lower particle mass emissions than those corresponding to diesel fuel, in agreement with previous works.51-54 An exception is found with GTL (mode G) because its value of particle mass concentration was similar to that obtained with diesel fuel and higher than that observed with Farnesane. These results are justified by two reasons: i) GTL mean diameter was similar to diesel fuel and higher than Farnesane (Figure 8b), ii) the long ignition delay of Farnesane in this mode (see Figure 5) which implies a larger premixed combustion stage and, consistently, a shorter diffusive or mixed controlled phase leading to lower PM formation. Benefits with the use of biodiesel are justified by the presence of oxygen and the lack of aromatic in its composition and, in the case of paraffinic fuels, by the absence of aromatics and the higher hydrogen content in the fuel molecule.49-55 The reduction of particle mass with biodiesel is even more notable than in terms of particle number, justified by the smaller mean particle size.49 The change of trend between particle number and particle mass concentrations for GTL and Farnesane fuels (with respect to the reference one) is explained by the smaller mean size of particles with these alternative fuels, as observed by these authors previously.56


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0.2

0.15

0.1

0.05

0

a)

A

C

E

G

I

b)

Figure 9. a) Particle mass concentration and b) Specific particle mass emission.

4. Conclusions

A comparative study of the effect on engine performance and emissions of two paraffinic fuels (a renewable fuel produced by sugar fermentation using a biotechnological process -called Farnesane- and a GTL fuel), a biodiesel (produced from a blend of soybean and palm biodiesel) and a diesel fuel is developed in this work. Tests were carried out under five steady state modes, three derived from the vehicle NEDC and the other two (C and G) included in the actual driving cycle (WLTC).

Based on the similarity in engine performance observed with all the fuels tested, it is concluded the adequate combustion behaviour of Farnesane (jet fuel) in compression ignition engines without penalizing pollutant emissions working under the default ECU configuration under realistic driving conditions. All alternative fuels reduced THC emissions with respect to diesel fuel combustion. Higher benefits in THC emissions are observed with biodiesel and Farnesane, both renewable fuels. The single fuel molecule of Farnesane leading to favourable vaporisation and reactivity and the presence of oxygen in biodiesel composition are the main justifications for these differences. Higher values of NOx were 19 ACS Paragon Plus Environment

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Energy & Fuels

obtained with biodiesel (mainly due to the presence of oxygen) and slight benefits were observed with paraffinic fuels, explained by the higher H/C ratio and higher cetane number of GTL and Farnesane. There is a considerable reduction in particle emissions with all alternative fuels at all tested modes, being biodiesel the fuel showing the highest benefit. The two paraffinic fuels showed similar PNC except in mode C, where Farnesane is more beneficial due to its single composition (pure compound). It is worth noting the high particle number and mass emissions observed in mode G for all fuels, justified by proximity of this mode to the area of smoke limit of this engine.

Benefits obtained with Farnesane make it a feasible alternative fuel not only for its favourable cold flow properties, but also for their lower pollutant emissions. Although the ECU is not optimized for the use of alternative fuels, emissions obtained with these fuels indicate that they are suitable candidates for using in compression ignition engines. Further engine management strategies optimization and ECU implementation for these fuels in actual work areas of the engine (WLTC) will further improve their performance and reduce their pollutant emission levels.

Acknowledgements This study was carried out under the framework of the POWER Ref. ENE2014-57043-R project financed by the Spanish Ministry of Economy and Competitiveness. Authors gratefully acknowledge the companies Repsol (Spain), Sasol (South Africa) and Amyris (Brasil) for providing the test fuels. Authors wish to thank the technical support provided by CMT - Motores Térmicos at Universidad Politécnica de Valencia and Álava Ingenieros S.A.

ABBREVIATIONS


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CD, Combustion Duration; CFPP, Cold Filter Plugging Point; DOC, Diesel Oxidation Catalyst; ECU, Electronic Control Unit; EGR; Exhaust Gas Recirculation; EoC, End of Combustion; GPIB, General Purpose Instrumentation Bus; GTL, Gas to Liquid; HVO, Hydrotreated Vegetable Oils; ID, Ignition Delay; NEDC, New European Driving Cycle; NOx, Nitrogen Oxides; PNC, Particle number concentration; PM, Particulate matter; SMPS, Scanning Mobility Particle Sizer; SoC, Start of Combustion; SoI, Start of injection; TDC, Top Dead Centre; THC, Total hydrocarbons; WLTC, Worldwide harmonized Light duty vehicles Test Cycle.

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Influence on Performance and Emissions of an Automotive Diesel

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