Review and Performance Evaluation of Fifty Alternative Liquid Fuels

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Review and Performance Evaluation of Fifty Alternative Liquid Fuels for Spark Ignition Engines Dominik Gschwend, Patrik Soltic, Alexander Wokaun, and Frédéric Vogel Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b02910 • Publication Date (Web): 04 Feb 2019 Downloaded from http://pubs.acs.org on February 8, 2019

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Energy & Fuels

Review and Performance Evaluation of Fifty Alternative Liquid Fuels for Spark Ignition Engines Dominik Gschwend,‡ Patrik Soltic,¶ Alexander Wokaun,‡ and Frédéric Vogel∗,‡,§ ‡Paul Scherrer Institut (PSI), 5232 Villigen PSI, Switzerland ¶Swiss Federal Laboratories for Materials Science and Technology (Empa), 8600 Dübendorf, Switzerland §Fachhochschule Nordwestschweiz (FHNW), 5210 Windisch, Switzerland E-mail: [email protected] Abstract The currently discussed alternative fuels for spark ignition engines are numerous. 50 different liquid fuel compounds were identified from literature. Using a thermodynamic engine model, which adapts the engine to the fuel and thereby determines the performance potential of a fuel candidate, the different fuel candidates are investigated in terms of efficiency, tank-to-wheel CO2 emissions, and volumetric fuel consumption. Additionally, the Particulate Matter Index (PMI) of each compound is calculated to estimate the soot emissions. Furthermore possible negative impacts on health and the environment are taken into account. Thereby, the only compound leading to a lower volumetric fuel consumption than gasoline is found to be anisole (8.0 l/100 km), at the cost of increased CO2 emissions (225 g/km) and Particulate Matter Index (PMI) levels (2.27 bar−1 ). Minimum of tank-to-wheel CO2 emissions are achieved by isopropanol (175 g/km), but at the expense of increasing volumetric fuel consumption

1

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(10.2 l/100 km). CO2 reduction potential of 2,2,3-trimethylbutane (180 g/km) is not as significant, in return the increase in volumetric fuel consumption (8.5 l/100 km) is less pronounced. tert-butanol and isopropanol results in mininum PMI values (0.04 bar−1 ), while tert-butanol shows a slightly worse performance than isopropanol, with a full load efficiency of 36.8 % vs. 37.1 %, respectively. Although they contain oxygen, the levulinates are predicted to form a high amount of particulate matter.

Introduction The impact of global climate change to the environment and the ecosystem are potentially irreversible 1 . An upper limit to the global temperature rise of 2 ◦C and if possible 1.5 ◦C has been defined in the Paris Agreement (COP 21, 2015). To achieve this goal a significant reduction in CO2 emissions is required as soon as possible. The transportation sector is the second largest contributor to fossil CO2 emissions 2 . Furthermore it is almost exclusively based on fossil fuels, amounting to 95.8 % in 2015 3 . There are three different strategies to reduce CO2 emissions from transportation: alternative fuels, reducing consumption, and behavioral changes. Consumption reduction involves major changes to the vehicles, including new drivetrains and electrification. Fuel alternatives on the other hand potentially require co-optimization of fuel production, logistics, and engine design 4 . Finally, all three strategies will be required, to a different extent, in order to achieve the required CO2 reductions. The field of discussed fuel alternatives is wide and lacks a general comparison of the different compounds under similar conditions. Within this study we focus on compounds which have already been proposed in literature as alternative fuels and for which a possible way to produce them from biomass has been claimed. The first part of this paper consists of a literature review to identify the different compounds as well as to gather the relevant properties. To provide a meaningful comparison of the engine performance their performance in a Spark Ignition (SI) engine is simulated using a thermodynamic engine model. A "tank-to-wheel" comparison based on volumetric fuel consumption, specific CO2 emissions, 2


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Energy & Fuels

and particle matter formation is then performed using a vehicle model and the Worldwide harmonized Light vehicles Test Cycle (WLTC).

Alternative Fuels The discussion about fuels for Internal Combustion Engines (ICEs) is about as old as the engine itself. The famous Ford Model T launched in 1908, for example ran on bioethanol 5 . However, the availability of cheap petroleum displaced ethanol rapidly. Ever since, the discussion about fuel alternatives has frequently re-appeared, either due to reduced petroleum availability (e.g. oil crisis) or due to environmental considerations. The scientific community is still far from having reached a consensus on the fuel best-suited as gasoline replacement. One of the problems is that numerous aspects need to be considered (engine performance, environmental impact, health effects, production, costs, logistics, etc) and in the end an approach covering all of these will be required. Most studies cover only a very limited range of candidates and use different measures making comparisons between different studies difficult. Therefore, the aim of this publication is to compare the engine performance of the numerous fuel candidates applying a common methodology. In this section, a comprehensive overview of alternative fuels currently under discussion is given. A compound was included in this study when experimental values for either Research Octane Number (RON) or derived Cetane Number (dCN) were available. Compounds for which neither parameter has been reported are considered to lack the most basic experimental investigation and are thus not considered here. Only very few mixtures have so far been proposed as alternative to gasoline. For most of them, e.g. pyrolysis oil, neither a complete list of properties required as input for the ICE model nor the exact composition of the mixture has been published. Thus, these few mixtures are not covered within this study. A discussion of production pathways was not the aim of this study. Interested readers are referred to recent, comprehensive reviews on this subject 6–8 .

3


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 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 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Currently, the most commonly used alternative liquid fuel for SI engines is ethanol, one of the reasons being its well-established production process from carbohydrate fractions of biomass. Other compounds frequently discussed include methanol, n-butanol, and more recently 2-methylfuran (2-MF) and 2,5-dimethylfuran (DMF) 9–11 . An extensive review of the suitability of n-butanol as gasoline replacement concluded that the usage of n-butanol may overcome many of the drawbacks of ethanol 12 . However, empirically an excessive oil dilution of up to 24 % of the injected fuel was noticed. Thus pure n-butanol is not suited as fuel 13 , unless a dedicated motor oil is also developed. Numerous other compounds have been proposed based on different criteria (e.g. Computer Aided Molecular Design (CAMD), production pathways) 11,14–21 . An overview on the compounds considered within this study is given in Table 1. In the same table the published data on miscibility with both water and gasoline, emissions, toxicity and biodegradability is listed. The compounds are sorted according to the status criteria defined in Table 2, which is defined as a score ranging from 1 (proposed) to 5 (currently in use). Toxicity data is summarized into the classes I–U as defined by Globally Harmonized System of Classification and Labelling of Chemicals (GHS). The exact values on toxicity and miscibility are listed in the supporting information. Substances with a negative health impact (e.g. toxicity, carcinogenicity) require an appropriate handling, which will hinder their widespread adaption as fuel. The solubility in water is important with respect to different effects. Water dissolved within the fuel deteriorates the combustion properties and can lead to increased corrosion of the fuel system. Together with either a high toxicity or a low biodegradability, a high solubility in water might prove problematic. Especially during the transition phase from fossil to renewable fuels a good miscibility with gasoline is desirable, as this limits the number of simultaneously required changes and allows to use initially low productive capacities reasonably. The gasoline miscibility results reported within this work were obtained by mixing 10 ml of gasoline with 10 ml of the compound in question. After vigorously shaking, the samples were left standing for a week and finally visual inspected whether a phase separation took place. 4


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1 2 3 4 5 6 7 8 9 10 11 12 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 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

emissions


5

?

: reprotoxic,

+

furfuryl alcohol5

isobutanol

α-pinene

2-butanone

ethyl valerate

: addictive drug, •

↑

↓

0

: genetic toxic,

×

low

0

5

0

0

0

↑

↓

2

2

3

3

3

†

II

III

III

III

: tendency to polymerize,

↓

↓

0

0

U

3

methyl valerate

0

U

3

2-MthF†

↓

0

II

U

U

I

U

U

U

III

U

U

U

U

aquatic life

toxicity

×

×

×

×

×

×

×

×

×

×

×

×

carcinogenic

this work

easily

easily

easily

easily

easily

easily

biodegradable

: formation of explosive peroxides,

II/III

4 4

U

U

(5)

5

III

↑

%

↓

↓

↑

↓

3

sec-butanol?

%

%

↓

↓

↓

acute

II

×

DMF?5†

&

↓

&

PM

status

4

&

×

2-MF5† 0

↑

↓

methanol

n-butanol

&

gasoline water HC CO NOx

miscibility

ethanol

compound

Table 1: Overview on the properties and status of alternative fuel compounds. The legend to the symbols, status level (1–5) and the toxicity classes (Ia–U) are given in Table 2. Data taken from 6,9,11–59 .

Page 5 of 33 Energy & Fuels


6 •

5

II/III

1

isopentanol

?

: reprotoxic,

+

: addictive drug,

: genetic toxic,

1

ethyl levulinate

†

U

U

U

U

III

U

U

U

U

U

III

II

aquatic life

toxicity

×

×

×

propable

possibly

×

×

×

×

×

×

×

×

carcinogenic

this work

not easily

×

easily

easily

easily

easily

easily

×

biodegradable


II/III


1

1

methyl levulinate

butyl levulinate

1

GVL+

low

tetrahydrofuran

1

furan†

1

1

sec-pentanol

II/III

III

1

n-pentanol

low

III

1

tert-butanol

III/U

1

II/U

II

isopropanol

2

II

2

1

↑

III/U

acute 2

status

n-propanol

2-phenylethanol?

↑

low

4-methylanisole?5

PM ↑


emissions

anisole

compound miscibility

1 2 3 4 5 6 7 8 9 10 11 12 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 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table 1 - continued

Energy & Fuels Page 6 of 33


7 •

5

1

TMED

†

U

U

U

U

U

U

U

III

III

III

III

U

U

III

III

U

U

×

×

×

×

×

×

×

×

×

×

possibly

×

×

carcinogenic

this work

easily

easily

easily

easily

easily

easily

easily

×

easily

easily

biodegradable


II/III


1

isobutylacetate

: genetic toxic,

1

butyl acetate

+

1

isopropyl acetate

: reprotoxic,

1

propyl acetate

?

1

ethyl acetate

low

1

methyl acetate

1

ethyl propanoate

low

1

1

isobutyraldehyde

: addictive drug,

ethyl furfuryl ether

1

diisopropyl ether?

III/U

III

1

diisopropyl ketone

III

1

methyl isobutyl ketone?

III/U

III

1

III

aquatic life

toxicity

U

acute

methyl isopropyl ketone

PM

status

1


emissions

ethylbutanoate


1 2 3 4 5 6 7 8 9 10 11 12 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 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table 1 - continued

Page 7 of 33 Energy & Fuels


8

?

: reprotoxic,

+

: addictive drug, •

: genetic toxic, 5

1

tert-pentanone


†

III

III

1

sec-pentanone

1

II

1

methyl-n-butyrate

tert-hexanone

U

1

U

aquatic life

toxicity

×

×

×

carcinogenic

this work

easily

biodegradable


acute

methyl isobutyrate

1

low

2,2,3-trimethylbutane

PM

status

1


emissions

ethylisobutyrate


1 2 3 4 5 6 7 8 9 10 11 12 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 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table 1 - continued

Energy & Fuels Page 8 of 33

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Energy & Fuels

Method The performance of each fuel candidate is evaluated using our previously published model of a four cylinder Direct Injection (DI) SI engine 60 . The model has been built to incorporate the main influences of a compound on its suitability as fuel for SI engines. It includes a turbocharger, an aftercooler, fuel delivery (including the fuel pump) and the engine itself. In order to provide a meaningful comparison of the different compounds, the compression ratio of the engine is optimized for each fuel. This is done by maximizing the efficiency at 2000 rpm and a boost pressure of 2 bar absolute, under the restriction of knock (based on Douaud and Eyzat 61 ) and a maximum allowable peak pressure of 25 MPa. While adapting the compression ratio, the total cylinder volume at Bottom Dead Center (BDC) (Vcyl = 535 cm3 ) as well as the bore-to-stroke ratio (0.97) are kept constant. This means that the displacement volume (VD ) can be determined, based on the definition of the compression ratio, as follows: VD = Vcyl

εCR − 1 εCR

(1)

Friction is modeled according to Chen and Flynn 62 in dependence of mean piston velocity and peak pressure. The following assumptions are made: • All gases are ideal. • Perfect gas exchange, no burnt gases remain within the cylinder. • Blow-by is neglected. • The engine is run under stoichiometric conditions at all times. • Except during combustion, the gases inside the cylinder are perfectly mixed. • During combustion the overall properties of the gas mixture inside the cylinder depend linearly on the progress of combustion. Besides the full load point defined as 2000 rpm and a boost pressure of 2 bar absolute, a part load point is defined as 2000 rpm and a power output of 6.6 kW. The power output is defined by a bmep of 2 bar in case the engine is running on gasoline. By including a car 9


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Table 2: Legend to Table 1. Status 5 (5) 4 3 2 1

currently in use abandoned after extensive road trials 24 well-established – numerous experimental studies experimentally tested pure experimentally tested mixed with gasoline proposed Emissions

↓ & 0 % ↑

reduction reported reduction and no change reported no change reported increase and no change reported increase reported Miscibility

yes low × no

>1 g/l 0.001 g/l to 1 g/l 100 mg/l > 100 mg/l

10


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Energy & Fuels

model it is possible to convert the velocity profiles defined by standardized driving cycles into profiles of required power. Thereby it is possible to simulate the performance of a fuel over a driving cycle. To estimate the soot emissions arising within a DISI engine, a Particulate Matter Index (PMI) has been proposed 63 : X DBEi + 1 wi PMI = p (443 K) vap,i i

(2)

based on the Double Bond Equivalent (DBE) (depending on the number of carbon atoms (x) and hydrogen atoms (y)), the vapor pressure at 443 K in bar and the mass fraction wi of each compound i in the fuel mixture.

DBE =

2x + 2 − y 2

(3)

Eq. (2) has been developed based on experimental measurements from SI engines running on defined fuel mixtures. These mixtures included different types of gasoline, hydrocarbons, and ethanol. Aromatic structures, and more generally unsaturated compounds are commonly known as soot precursors. This is reflected in the calculation of the PMI by the DBE. The second phenomenon responsible for soot is the incomplete combustion of non-evaporated fuel. This non-evaporated fuel is generally associated with a liquid fuel layer on the walls of the combustion chamber due to a low volatility. This is taken into account in Eq. (2) by the vapor pressure, with 443 K representing the cylinder wall temperature. The vapor pressure at 443 K is estimated using the Clausius – Clapeyron equation. The model requires: the elemental composition (Cx Hy Oz ), the liquid density, the liquid viscosity, the RON, the vapor heat capacity, the vapor pressure (at 298 K) and the enthalpy of vaporization as inputs. The properties of all compounds studied are listed in the Supporting Information. For five compounds a complete set of experimental data could not be found.

11


Energy & Fuels

The missing data has therefore been estimated using Group Contribution Methods (GCMs). In particular these are the vapor heat capacity of ethyl furfuryl ether, methyl valerate, ethyl valerate, methyl levulinate and butyl levulinate 64 , the enthalpy of vaporization of ethyl furfuryl ether and ethyl valerate 65 as well as the viscosity of ethyl furfuryl ether, methyl valerate, ethyl valerate and methyl levulinate 66 . The GCM were chosen based on the recommendations by Nieto-Draghi et al. 67 . Experimental data for the RON is not readily available, especially for those compounds not well-established within the scientific discussion. However, an inverse correlation between dCN and RON exists. As dCN is easier to measure, a correlation between RON and dCN has been established using Eq. (4) fitted to data from literature 18,68–76 . The correlation is shown in Fig. 1 and the statistical quantities are given in Table 3. 140 120 Research Octane Number

1 2 3 4 5 6 7 8 9 10 11 12 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 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 33

100 80 60 40 20 0 10

20

30

40

50

derived Cetane Number

Figure 1: Correlation between RON and dCN. Blue dots: data points, red: regression line.

RON = 126.72 − 1.945dCN

12


(4)

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Energy & Fuels

Table 3: Statistical quantities of the fit. Number of observations: 45 Error degrees of freedom: 43 Root Mean Squared Error: 9.99 2 R 0.878 2 Radj 0.875 F-statistic vs. constant model: 310 p-value 2.78 × 10−21

Results & Discussion Properties Analyzing the properties of the different compounds listed in Table 1 it can be seen that with few exceptions all compounds fulfill the basic requirements. γ-valerolactone (GVL) is one of these exceptions as it is an addictive drug, which renders its widespread use as a pure fuel highly unlikely. Thus it has been suggested to upgrade GVL to form alkenes 56 . Furan, tetrahydrofuran and methyl isobutyl ketone are suspected to be carcinogenic, which most certainly lower their acceptance as a fuel. The same is true for DMF, sec-butanol, 4-methylanisole, 2-phenylethanol, methyl isopropyl ketone and diisopropyl ether due to their reprotoxic impact. For the well-established alternatives a relatively high acute toxicity (class II) is reported, which could represent a hindrance for their widespread use. The storage problem arising in conjunction with the tendency to polymerize and/or the formation of peroxides is deemed less problematic as these effects can be neutralized using appropriate additives. From an environmental point of view the biodegradability of a fuel compound is important. However, all of the compounds reported as not biodegradable have already been mentioned above as being either reprotoxic or carcinogenic (4-methylanisole, furan, tetrahydrofuran and diisopropyl ether). Furthermore, α-pinene is highly toxic to aquatic life, and thus not a valid option.

13


Energy & Fuels

Engine Performance In Table 4, efficiency, power output, compression ratio, and specific CO2 emissions of the studied fuels are listed for different load points and for the WLTC. The PMI of gasoline is calculated as an average of values published in literature 49,63,77 . Table 4: Overview of the performance of all fuels. ηFL efficiency at full load, ηPL efficiency at 2000 rpm and Pout = 6.6 kW, PFL power output at full load, eFL specific CO2 emissions, εCR compression ratio, Particulate Matter Index calculated according to Eq. (2), ηcyc cycle efficiency, eD CO2 emissions per distance and c consumption over the WLTC.

gasoline (RON = 98) isooctane

iO

ηFL

ηPL

PFL

eFL

εCR

PMI

ηcyc

eD

[%]

[%] [kW]

[1/bar]

[%]

c

[-]

[l/100km]

label

[g/km]

fuel

[kg/kWh]

1 2 3 4 5 6 7 8 9 10 11 12 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 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 33

33.7 20.8

70.8 0.77

7.7

1.5 ± 0.7 21.1 193

8.2

34.5 21.2

74.2 0.72

8.0

0.14 21.5 182

8.6

n-heptane

knocking

1-hexene

Hx

30.1 19.1

63.2 0.86

5.7

0.13 19.4 210 10.0

diisobutylene

diB

35.0 21.4

73.5 0.74

8.5

0.30 21.7 187

8.4

cyclohexane

ch

32.3 20.1

69.4 0.80

7.3

0.20 20.3 201

8.3

mch

29.7 19.0

63.8 0.87

5.6

0.30 19.2 212

8.8

33.1 20.3

72.0 0.93

8.7

0.45 20.6 234

7.9

34.1 20.8

74.1 0.88

9.1

0.87 21.1 223

7.7

MTBE

35.7 21.6

75.6 0.72

9.4

0.06 22.1 183

9.9

ETBE

36.0 21.9

76.2 0.71

9.6

0.08 22.3 181

9.5

methanol

30.3 18.3

69.4 0.82

9.6

0.04 18.6 209 19.3

ethanol

36.0 22.0

80.1 0.70 18.3

0.05 22.5 177 11.8

methylcyclohexane benzene toluene

T

n-propanol

n3

36.6 22.0

80.3 0.70 13.8

0.06 22.6 178 10.1

isopropanol

i3

37.1 22.3

81.5 0.69 14.7

0.04 22.9 175 10.2

14


Page 15 of 33

Table 4 - continued ηFL

ηPL

PFL

eFL

εCR

PMI

ηcyc

eD

[%]

[%] [kW]

[-]

[1/bar]

[%]

c [l/100km]

label

[g/km]

fuel

[kg/kWh]

1 2 3 4 5 6 7 8 9 10 11 12 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 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

n-butanol

n4

36.5 22.0

79.8 0.70 11.6

0.10 22.5 179

9.3

isobutanol

i4

36.9 22.3

80.7 0.69 12.4

0.07 22.8 177

9.3

sec-butanol

s4

37.0 22.3

80.7 0.69 12.3

0.06 22.8 177

9.3

tert-butanol

t4

36.8 22.2

80.4 0.70 11.8

0.04 22.7 177

9.5

n-pentanol

n5

34.3 20.9

74.7 0.75

8.5

0.16 21.3 189

9.3

isopentanol

i5

36.1 21.8

78.5 0.71 10.3

0.14 22.3 181

9.0

36.0 21.8

77.6 0.71 10.6

0.09 22.4 180

9.0

37.6 22.6

85.7 0.90 15.2

2.78

9.0

furan

32.5 19.8

70.5 1.03

0.13 20.1 261 10.8

2-MF

34.6 21.0

76.8 0.91 10.4

0.45 21.4 232

9.5

DMF

35.9 21.7

79.2 0.85 10.0

0.83 22.1 216

8.8

27.2 17.7

59.7 1.02

5.0

0.14 20.2 216 10.0

2-MthF

31.5 19.6

68.9 0.87

6.7

0.19 19.9 217 10.0

GVL

37.9 22.8

85.5 0.83 14.1

5.36 23.4 210

9.1

sec-pentanol furfuryl alcohol

tetrahydrofuran

FFA

thF

9.0

23.3 228

methyl valerate

MV

36.2 22.1

80.5 0.79 10.0

0.67 22.9 195

9.6

ethyl valerate

EV

35.3 21.5

77.9 0.80

0.51 22.0 201

9.6

methyl levulinate

ML

38.3 23.0

87.4 0.83 13.8

3.83 23.7 212 10.2

ethyl levulinate

EL

38.6 23.2

87.2 0.80 14.3

5.09 23.9 203

9.4

butyl levulinate

BL

36.8 22.2

82.2 0.81 10.0

39.58 22.8 204

9.1

B

35.3 21.3

79.0 0.79

9.7

0.17 21.8 200 10.3

EB

36.0 21.9

79.1 0.79 10.0

0.33 22.3 200 10.1

2-butanone ethylbutanoate

15


8.7

Energy & Fuels

Table 4 - continued ηFL

ηPL

PFL

eFL

εCR

PMI

ηcyc

eD

[%]

[%] [kW]

[1/bar]

[%]

c

[-]

[l/100km]

label

[g/km]

fuel

[kg/kWh]

1 2 3 4 5 6 7 8 9 10 11 12 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 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 33

35.4 21.4

78.0 0.77

9.9

0.24 21.9 197

9.6

MibK 32.9 20.5

71.9 0.82

7.1

0.35 20.6 207

9.8

diisopropyl ketone

dipK

33.8 20.8

72.8 0.80

8.2

0.49 21.0 201

9.3

diisopropyl ether

ipE

34.9 21.3

73.8 0.74

8.5

0.08 21.7 186 10.0

ethyl furfuryl ether

EFE

35.0 21.3

78.3 0.89

9.2

0.59 21.7 225

isobutyraldehyde

IBA

30.6 19.3

67.4 0.91

6.2

0.11 19.5 224 11.7

ethyl propanoate

EP

33.4 20.6

74.6 0.87

7.5

0.22 20.8 219 11.5

34.3 20.7

75.9 0.92 10.4

0.10 21.3 233 14.1

methyl-isopropyl ke-

MIK

tone methyl isobutyl ketone

methyl acetate

9.3

ethyl acetate

EA

33.8 20.7

77.3 0.89

8.1

0.15 21.0 224 12.5

propyl acetate

PA

33.5 20.7

74.9 0.87

7.6

0.24 20.8 219 11.5

isopropyl acetate

IA

35.0 21.3

78.4 0.83

8.9

0.20 21.7 210 11.2

butyl acetate

BA

35.2 21.4

77.4 0.81

9.4

0.40 22.0 204 10.2

isobutylacetate

iA

35.3 21.5

77.6 0.81

9.4

0.36 22.0 203 10.3

TMED

35.2 21.5

77.5 0.79

9.0

0.23 21.8 200

9.4

α-pinene

33.5 20.6

72.3 0.83

7.8

1.72 20.9 207

7.5

A

35.5 21.4

78.4 0.88 11.2

2.27 21.9 225

8.0

4-methylanisole

MA

36.3 22.0

80.0 0.84 11.6

5.76 22.5 213

7.6

2-phenylethanol

PE

36.9 22.2

81.1 0.83 12.9

13.28 22.7 212

7.2

ethylisobutyrate

EiB

35.3 21.4

77.6 0.81

anisole

16


9.5

0.34 21.9 204 10.4

Page 17 of 33

Table 4 - continued

2,2,3-

ηFL

ηPL

PFL

eFL

εCR

PMI

ηcyc

eD

[%]

[%] [kW]

[1/bar]

[%]

c

[-]

[l/100km]

label

[g/km]

fuel

[kg/kWh]

1 2 3 4 5 6 7 8 9 10 11 12 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 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

34.9 21.3

73.8 0.71

8.8

0.11 21.7 180

8.5

trimethylbutane methyl isobutyrate

mib

34.2 20.9

75.9 0.85

8.6

0.21 21.4 214 11.2

methyl-n-butyrate

mnB

34.9 21.2

77.3 0.83

9.1

0.25 21.7 210 10.9

sec-pentanone

sP

34.2 20.8

74.2 0.80

8.7

0.27 21.3 202

9.9

tert-pentanone

tP

34.5 21.1

75.1 0.79

8.7

0.25 21.5 200

9.7

tert-hexanone

tH

34.5 21.1

74.4 0.79

8.4

0.41 21.4 199

9.3

Compression ratios are mostly between about 7 to 14, which lies in the range of today‘s SI engines. The engine can be operated at a significantly higher compression ratio with ethanol than with any of the other fuels. Such a compression ratio is rather in the range of a Diesel engine than in the range of an SI engine. These findings are in line with experimental studies that report engines to be operated at significantly higher compression ratios with ethanol 13,29,78–80 . First tests to exploit this potential have already been conducted by converting a Diesel engine to SI operation 81 . Minimum PMI is achieved by methanol, isopropanol and tert-butanol (0.04 bar−1 ). The compounds that show both increased efficiency as well as lower specific CO2 emissions, compared to gasoline, also lead to decreased PMI levels. Analyzing Fig. 4 the following correlation between PMI, volumetric fuel consumption and specific CO2 emissions can be observed: The compounds with decreasing volumetric fuel consumption lead to significantly increased PMI levels, whereas the compounds minimizing specific CO2 emissions also lead to decreased values of the PMI. Increasing PMI for decreasing volumetric fuel consumption

17


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 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 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 33

can be explained by the fact that minimum consumption is achieved by aromatics. These compounds show a high carbon to hydrogen ratio and thus a high DBE and a high Lower Heating Value (LHV). Additionally, they also possess a low vapor pressure. Interestingly, the aromatic compounds incorporating an oxygen atom (anisole, 4-methylanisole, 2-phenylethanol) show very high PMIs, whereas the values for benzene and toluene are significantly lower. Another class of compounds with remarkably high PMI are the levulinates (3.8 bar−1 to 39.6 bar−1 ) mainly due to their low vapor pressures. GVL (5.4 bar−1 ) and furfuryl alcohol (2.8 bar−1 ) are two other compounds displaying the same phenomenon. Comparison of the experimental results from literature show good agreement with the predictions based on the PMI. For ethanol (PMI = 0.05 bar−1 ), n-butanol (PMI = 0.10 bar−1 ) as well as 2-MF (PMI = 0.45 bar−1 ) reductions in Particulate Matter (PM) emissions (compared to gasoline (PMI = 1.50 bar−1 )) have been reported 9,32–36,40,41 . For DMF (PMI = 0.83 bar−1 ) no significant change could be detected 34,49 . This might be attributed to the wide spread in PMI values of gasoline (PMI = 1.50 bar−1 ± 0.69). Increased PM emissions are reported for 4-methylanisole (PMI = 5.76 bar−1 ) and 2-phenylethanol (PMI = 13.28 bar−1 ) both having a PMI significantly higher than gasoline 49 . PM emissions of mixtures consisting of gasoline and methanol (PMI = 0.04 bar−1 ) also show increased PM emissions 23 . This cannot be explained by the PMI. A possible explanation might be found in the high enthalpy of vaporization of methanol, leading to a slower temperature rise during compression. When mixed with gasoline this could result in a incomplete evaporation of the higher boiling fraction of gasoline and thereby in higher PM emissions. The PMI only indicates the tendency for PM emissions of a specific fuel. Results might change if PM emissions per km or per kWh are considered because compounds with higher efficiency or higher LHV result in lower amounts of fuel burnt. Another fact to consider is that DISI engines are increasingly equipped with particle filters. Therefore the engine out emissions are of lower importance for vehicle emissions. However, they influence the sizing of the filter systems and/or the regeneration frequency. 18


Page 19 of 33

Compounds with a low volatility are a major source of PM emissions as these compounds do not fully evaporate before ignition 63 . In the correlation for the PMI this is reflected by the introduction of the vapor pressure at 443 K. When adapting the compression ratio of the engine the question arises whether the use of a fixed value for temperature to calculate the PMI is still valid. It could be argued that for engines with higher compression ratios the overall temperature levels are higher thus leading to improved evaporation of low volatile compounds and vice versa. Thus it could be more meaningful to use the vapor pressure at a temperature defined by the temperature curve during compression. The exact choice of temperature needs to be supported by experimental studies on soot formation. 2.0 thF

0.28−1

1.4 IBA

s5

0.36−1

n3 t4 i3

0.38−1

s4

n4 i4

n5 diB

MTBE ETBE i5

furan

EP α-pinene PA

dipK TMED

iO

MibK

gasoline

ch

ipE

ethanol 2,2,3-trimethylbutane

0.34−1

1.2 MthF

tH

B

MIK

sP tP EV BA

mib mnB

EiB IA EB

MV BL

DMF MA PE

GVL

1.0

benzene

0.8

EA methyl acetate

T EFE A

PMI [1/bar]

mch

Hx

0.32−1

1.8 1.6

methanol

0.30−1

−1 ηFL [−]

1 2 3 4 5 6 7 8 9 10 11 12 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 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

0.6

MF

0.4 FFA

0.2

ML EL

0.40−1 0.65

0.70

0.75

0.0

0.80 0.85 eCO2 ,FL [kg/kWh]

0.90

0.95

1.00

1.05

Figure 2: Comparison of the different fuels with respect to specific CO2 emissions and efficiency at full load. Circles: alternative fuels, squares: gasoline components, diamond: gasoline. The gray scale of the marker indicates the PMI level. As shown in Fig. 2 peak efficiency is reached by ethyl levulinate with 38.6 %, followed by methyl levulinate (38.3 %), GVL (37.9 %) and furfuryl alcohol (FFA) (37.6 %). However, the increase in efficiency comes at the costs of increased specific CO2 emissions ranging from 4 % to 17 % compared to gasoline. The other alcohols with the exception of methanol show 19


Energy & Fuels

increased efficiency while also decreasing specific CO2 emissions. Diisopropyl ether (ipE) and 2,2,3-trimethylbutane outperform gasoline to a lower extent. As the results for the alcohol isomers are difficult to distinguish they are plotted separately in Fig. 3. With the exception of PMI there is little difference between the iso and sec isomer. Peak efficiency is reached by isopropanol (37.1 %) which also correlates with minimum CO2 emissions (175 g/km). Consumption and PMI are directly linked to carbon content. 210 eCO2 ,D [g/km]

38

ηFL [%]

36 34 gasoline n-x-OH iso-x-OH sec-x-OH

32 30

1

2 3 4 number of carbon atoms

5

(b) CO2 emissions vs. chain length

0.18 PMI [1/bar]

n-x-OH iso-x-OH sec-x-OH

14 12


0.14 0.10 0.06

10 8

gasoline

190

1

20

16

200

170

2 3 4 5 number of carbon atoms

18


180

(a) Efficiency vs. chain length

consumption [l/100 km]

1 2 3 4 5 6 7 8 9 10 11 12 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 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 33

gasoline 1

2 3 4 number of carbon atoms

(c) volumetric fuel consumption vs. length

0.02

5 chain

1

2 3 4 5 number of carbon atoms (d) PMI vs. chain length

Figure 3: Influence of carbon chain length on the performance of alcohols. In Fig. 4, the results for the WLTC are summarized regarding CO2 emissions per distance and volumetric fuel consumption. No compound is superior to gasoline for both metrics. 2-phenylethanol, 4-methylanisole, anisole and α-pinene are the only fuels with lower 20


Page 21 of 33

consumption than gasoline. Thereof only anisole is considered a valid option, due to serious negative impacts on both health and the environment of the other compounds. Lower CO2 emissions at the cost of higher consumption are achieved by alcohols (except methanol and FFA), diisopropyl ether, and 2,2,3-trimethylbutane. The other fuels perform worse than gasoline with respect to both criteria, with a an outstandingly high consumption of almost 20 l/100 km for methanol. This is in good agreement with literature, where a volumetric fuel consumption twice that of gasoline has been reported 82 . 2.0

20 methanol

1.8

18 1.6 16

1.2

methyl acetate

14

i3 t4 s4 i4

8

ipE n5

n4

ETBE i5 iO diB

s5

MV

2,2

,3-

tri

me

ylb

PA IA

190

0.8

IBA furan

mib EP

EiB BA

ML

0.6

thF EFE

sP Hx MthF EL MibK GVL tHdipK BL ch

mch

α-pinene

6 180

B

EA

MF

0.4

FFA

DMF MA

th

ut

170

mnB iA

TMED

n3

MIK

10

MTBE

12

1.0 tP EV EB

PMI [1/bar]

1.4

ethanol

consumption [l/100 km]

1 2 3 4 5 6 7 8 9 10 11 12 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 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

anisole

benzene

0.2

toluene

PE

an

e

0.0 200

210

220

230

240

250

260

270

eCO2 ,D [g/km]

Figure 4: Comparison of the different fuels with respect to volumetric fuel consumption and specific CO2 emissions over the WLTC. Circles: alternative fuels, squares: gasoline components, diamond: gasoline. The gray scale of the marker indicates the PMI level. In Fig. 5 all energy fractions are investigated with respect to compression ratio. The three main parts are the mechanical power output, the wall heat losses and the enthalpy of the exhaust gas. Together they account for about 95 % of the total input power. Generally, the wall heat losses as well as the output power increase with increasing compression ratio. Both can be attributed to the higher temperatures and pressures associated with higher 21


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 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 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

compression ratios. An increase in compression ratio leads to further expansion of the gases within the cylinder and thereby to a decrease in exhaust enthalpy. Considering the minor energy flows it becomes apparent that both the after cooler and the fuel pump are independent of compression ratio. As could be expected, friction is strongly correlated with compression ratio. It almost doubles over the εCR range from 5 to 20. This can be explained by the fact that higher preak pressures, associated with higher compression ratios, require, amongst others, more robust bearings. A broad optimum for Pout around a compression ration of 14 to 15 can be identified in Fig. 5a. Furthermore, methanol and butyl levulinate (BL) are identified as the only two exceptions to the mentioned trends. For butyl levulinate the main reason is that its vapor pressure is very low and therefore a portion of the fuel does not evaporated before ignition. As many of methanol’s properties are extreme it is difficult to pin point one of them. However, the high enthalpy of vaporization will lead to lower temperatures during evaporation, thereby slowing down further evaporation of the fuel.

Conclusion A previously presented model has been used to review the performance of different alternative fuels in pure form in relation to gasoline. Further information (toxicity, environmental impact) have been collected from literature. None of the fifty fuels investigated performs better than gasoline for both volumetric fuel consumption and specific CO2 emissions. Depending on the relevance of certain parameters, different fuels can be recommended. If reduction of tank-to-wheel CO2 emissions is most important, 2,2,3-trimethylbutane and butanol and propanol isomers are most promising. The use of isopropanol leads to CO2 emissions of only 175 g/km (−9 %), while increasing the consumption by 24 % from 8.2 l/100 km (gasoline) to 10.2 l/100 km. Minimum volumetric fuel consumption is achieved by 2-phenylethanol (7.2 l/100 km). However, 2-phenylethanol is reported to be reprotoxic making its widespread

22


Page 22 of 33

Page 23 of 33

1.71 1.79

displacement volume VD [cm3 ] 1.87 1.91 1.95 1.99

2.03

fraction of total energy input [%]

45

40 gasoline 35 BL methanol

Pout Q˙ wall

30

H˙ ex 25

20 6

8

10 12 14 compression ratio εCR [-]

16

18

20

(a) The three main energy fractions, wall heat losses (Q˙ wall ), exhaust enthalpy (H˙ ex ) and power output (Pout ) with respect to compression ration (εCR ).

1.71 1.79

displacement volume VD [l] 1.87 1.91 1.95

1.99

2.03

3.5 fraction of total energy input [%]

1 2 3 4 5 6 7 8 9 10 11 12 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 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

3.0 2.5 Q˙ ac

2.0

Pfric

1.5

Pfp 1.0 0.5 0 6

8

10 12 14 compression ratio εCR [-]

16

18

20

(b) The minor energy fractions, friction losses (Pfric ), fuel pump power (Pfp ) and after cooler (Q˙ ac ) with respect to compression ration (εCR ).

Figure 5: Energy fractions with respect to compression ratio. 23


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 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 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

application as fuel questionable. Anisole is the only compound studied resulting in lower volumetric fuel consumption than gasoline (−4 %) and no threat to either health or the environment. However, the CO2 emissions increase by 16 % to 225 g/km compared to gasoline. As soot is both an important emission in DISI engines as well as difficult to model, the PMI has been applied to estimate the PM levels of the different compounds. Both tertbutanol and isopropanol minimize the PMI (= 0.04 bar−1 ) , compared to gasoline with a PMI of 1.5 bar−1 . Although they contain oxygen, the levulinates are predicted to form a high amount of particulate matter. Suitability criteria not linked to performance, such as oil dilution, cold start behavior or emissions (other than CO2 ), have not been assessed and need further investigation. Sustainability is generally investigated using life cycle assessments, combining engine performance, production and environmental impact. Therefore, the final decision on an alternative fuel will require further studies covering these issues. The methodology and results presented within this contribution may prove useful in both the life cycle assessments as well as the selection of compounds for further analysis.

24


Page 24 of 33

Page 25 of 33 1 2 3 4 5 6 7 8 9 10 11 12 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 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Glossary

ipE diisopropyl ether.

BDC Bottom Dead Center.

LHV Lower Heating Value.

CAMD Computer Aided Molecular Design. 2-MF 2-methylfuran. ML methyl levulinate.

DBE Double Bond Equivalent.

MTBE methyl tert-butyl ether.

dCN derived Cetane Number.

2-MthF 2-methyltetrahydrofuran.

DI Direct Injection.

3-MthF 3-methyltetrahydrofuran.

diB diisobutylene.

PM Particulate Matter.

DMF 2,5-dimethylfuran.

PMI Particulate Matter Index.

EL ethyl levulinate. ETBE ethyl tert-butyl ether.

RON Research Octane Number.

FFA furfuryl alcohol.

SI Spark Ignition.

GCM Group Contribution Method.

TMED 2-ethyl-2,4,5-trimethyl-1,3-dioxolane.

GVL γ-valerolactone.

WLTC Worldwide harmonized Light vehi-

ICE Internal Combustion Engine.

cles Test Cycle.

Acknowledgement We would like to thank G. Ackermann (FHNW) measuring the miscibilities with gasoline. D. Wüthrich and K. Boulouchos (ETHZ) have contributed to this work by sharing their knowledge on internal combustion engines as well as in many hours of fruitful discussions on the model. Furthermore we would like to thank L. Bärtsch for his support in programming. This research project was supported by the Swiss Innovation Agency Innosuisse and is part of the Swiss Competence Center for Energy Research SCCER BIOSWEET.

25


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Review and Performance Evaluation of Fifty Alternative Liquid Fuels

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