diesel RCCI engine enriched with reformer gas

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Full Length Article

A numerical study of the effects of reformer gas composition on the combustion and emission characteristics of a natural gas/diesel RCCI engine enriched with reformer gas Pourya Rahnamaa, Amin Paykania,b,⁎, Vahid Bordbarc, Rolf D. Reitzd a

Vehicle Powertrain System Research Laboratory, School of Automotive Engineering, Iran University of Science and Technology, Narmak, Tehran, Iran Aerothermochemistry and Combustion Systems Laboratory, Swiss Federal Institute of Technology, Zurich CH-8092, Switzerland c School of Mechanical Engineering, Iran University of Science and Technology, Narmak, Tehran, Iran d Engine Research Center, University of Wisconsin-Madison, Madison, WI, USA b

A R T I C L E I N F O

A B S T R A C T

Keywords: Reactivity controlled compression ignition (RCCI) Reformer gas Composition Efficiency Emissions Low load

In natural gas/diesel Reactivity Controlled Compression Ignition (RCCI) engines, the large reactivity gradient between the two fuels is beneficial in achieving lower pressure rise rate and peak pressure values at high loads. However, by using natural gas, combustion efficiency and engine performance suffer at low loads due to its lower reactivity and higher ignition delay compared to gasoline. The use of reformer gas (containing H2 and CO), which can be produced onboard by a catalytic fuel reformer integrated within the exhaust pipe, as an additive can improve the combustion process of the engine at low loads since it enhances burning rate and compensates the low reactivity of natural gas. The objective of the present study is to investigate the effect of reformer gas (syngas) composition on the performance and exhaust emissions properties of a natural gas/diesel RCCI engine at low loads numerically, when 3% of intake air is volumetrically replaced by reformer gas. Shortened ignition delay and combustion duration, advanced combustion phasing (CA50), and increased peak pressure rise rate, ringing intensity, and lower combustion efficiency were obtained by the mixture with higher CO content. The results indicated that reformer gas addition could enhance the combustion efficiency and decrease CO emission, however, the mixture with higher hydrogen content requires intake charge preheating more than that with lower hydrogen content and mixture with higher CO content is more sensitive to intake temperature.

1. Introduction In last decade the use of new and advanced combustion strategies along with alternative fuels in internal combustion engines is of global interest to achieve higher fuel economy and lower pollutant emissions. It has been stated that concurrent decrease in nitrogen oxides (NOx) and particulate matter or soot emissions can be enhanced via modern combustion approaches such as homogeneous charge compression ignition (HCCI) engines and recently patented RCCI combustion, which uses in-cylinder reactivity gradients to moderate the high-pressure rise rate which was seen in HCCI combustion [1–3]. In RCCI combustion by the direct in-cylinder blending of various reactivity fuels, the combustion mode can be effectively controlled [4]. The reactivity gradient in the cylinder is generated by the introduction of fuel with low reactivity, like natural gas (NG) or gasoline in the intake port, and a fuel injection with high reactivity, like diesel into the cylinder through an injector [5]. Lower NOx and soot emissions and higher gross indicated

⁎

efficiency (GIE) compared to conventional diesel and HCCI combustion [6], have led to considerable interest in the application of RCCI concept to internal combustion engines in America [7–10], Europe [11–15], and Asia [16–21]. There are comprehensive and thorough review papers in previous studies about RCCI combustion, that can be referred to for more detailed information regarding various aspects of RCCI combustion [22,23]. Regarding the higher reactivity gradient between diesel and natural gas, it can be considered to be a more suitable alternative port-injected fuel in RCCI [24] as the higher reactivity gradient results in longer combustion times and thus lower pressure rise rates and ringing intensity [25]. Since high pressure rise rate and ringing intensity are two inhibiting factor to achieve stable operation at high load, this could be beneficial for RCCI engines. There have been some studies conducted about natural gas usage in RCCI, but all the studies have shown that, despite the fact that a larger reactivity gradient aids in extending the combustion duration, and thus reducing the peak pressure rise rate at

Corresponding author at: Aerothermochemistry and Combustion Systems Laboratory, Swiss Federal Institute of Technology, Zurich CH-8092, Switzerland. E-mail addresses: [email protected], [email protected] (P. Rahnama), [email protected] (A. Paykani).

http://dx.doi.org/10.1016/j.fuel.2017.07.103 Received 25 February 2017; Received in revised form 1 July 2017; Accepted 26 July 2017 0016-2361/ © 2017 Elsevier Ltd. All rights reserved.

Please cite this article as: Rahnama, P., Fuel (2017), http://dx.doi.org/10.1016/j.fuel.2017.07.103

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Fig. 1. Flow chart of CONVERGE simulation for a single time-step.

Table 1 Caterpillar 3401E SCOTE engine geometry [25]. Displacement Bore × Stroke (cm) Connecting rod length (cm) Compression ratio Swirl ratio Bowl type Number of valves Intake valve closing Exhaust valve opening Common rail diesel fuel injector Number of holes Hole diameter (μm) Included spray angle (deg)

Table 2 The RCCI engine validation case summary [25]. 2.44 L 13.72 × 16.51 cm 26.16 cm 16.1:1 0.7 Mexican Hat 4 −1430 ATDC 1300 ATDC

Intake Pressure (bar) Intake Temperature (°C) Engine Speed (rpm) Fuel Mass (mg) Methane (% by mass) Diesel SOI1 (°ATDC) Diesel SOI2 (°ATDC) Diesel in 1st Inj. (%) EGR (%)

6 250 145

IMEP 4 bar

IMEP 9 bar

1.0 60 800 40 73 −52.9 −22.5 52 5%

1.75 60 1300 89 85 −87.3 −38.3 40 0

specific fuel consumption (BSFC) and total hydrocarbons (THC) and carbon monoxide (CO) emissions in the case of a 10%H2 blend compared to pure NG. Liu et al. [32] investigated the effect of hydrogen addition to a DME/CH4 dual-fuel RCCI engine. They demonstrated that with the addition of H2, the ignition time is advanced and the peak cylinder pressure is increased. Furthermore, CO emission is reduced, and NOx emission is increased. A recently published review article is available which provides a comprehensive study regarding effects of combined hydrogen and exhaust gas recirculation (EGR) addition on engine emissions in dual fuel engines [33]. It reported that most of the research works which have been done in this regard state that hydrogen addition alone can reduce CO, HC and soot emissions but increase NOx emission. However, the combined use of hydrogen and EGR can mitigate the adverse impact on NOx. Despite the favorable position of hydrogen among other alternative fuels, onboard hydrogen storage is deemed challenging due to the volume occupied in storage, safety issues, and weight increase. Onboard hydrogen production that can be adopted in internal combustion engines by injecting a hydrocarbon fuel in a catalytic fuel reformer

high-load, it may pose a challenge at light-load (i.e., the combustion efficiency could suffer because of the low in-cylinder temperature level at low loads compared with high load operation). Also, natural gas has a longer ignition delay compared with other high octane fuels which makes the combustion achievement harder at low loads [26–28]. Hydrogen seems to be a promising candidate for application as an alternative fuel in internal combustion engines. The wide flammability limit property of hydrogen makes it an ideal fuel to combine with natural gas to compensate for the demerits of the limited lean-burn ability and slow burning velocity of the natural gas [29]. It allows an engine to operate at leaner equivalence ratios and has a burning velocity that is several times higher than that of methane. Better combustion, higher efficiency, lower carbon dioxide (CO2) production and emissions (apart from NOx) have been shown with the addition of hydrogen to natural gas [30]. Lounici et al. [31] performed experiments on natural gas enrichment with various H2 blends for improving dual fuel combustion at low engine loads. They found a reduction in brake

2

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1400

14 12

1200

800

30 20 10

6 -30

-25

600

0 -15

-20

Intake Pressure (bar) Intake Temperature (°C) Engine Speed (rpm) Diesel Fuel Mass Methane Fuel Mass Diesel SOI1 (°ATDC) Diesel SOI2 (°ATDC) Diesel in 1st Inj. (%) EGR (%)

1000 40

8

IMEP 4 bar

50 HRR (J/Deg)

Pressure (MPa)

10

Table 4 Base case.

4

400

2

200

HRR (J/Deg)

IMEP 4 Nieman et al. IMEP 9 Nieman et al. IMEP 9 Present Work IMEP 4 Present Work Inj Prof IMEP9 Inj Prof IMEP4

of the longer ignition delay and higher octane number of natural gas compared with other fuels. Yamasaki and Kaneko [39] applied syngas as a fuel in an HCCI engine. They concluded that the combustion rate is mainly determined by the hydrogen and CO. Furthermore, they showed that syngas composition does not have an effect on ignition timing and the in-cylinder temperature governs it. With the benefits of the RCCI concept, Chuahy and Kokjohn [40] found that the fuel reforming can increase engine net indicated efficiencies of a conventional diesel engine by as much as 9%. They studied the combined effect of the reformer performance and engine performance for three different reforming processes (Partial oxidation, steam reforming, and autothermal reforming). They used a CFD solver coupled with an equilibrium solver for the reformer process. Since the syngas composition is varied significantly as a result of the operating condition and the way it is produced [41,42], some studies have investigated the effect of syngas composition on emissions and performance of an engine. Sahoo et al. [43] experimentally studied different volumetric compositions of syngas in a diesel engine under dual fuel action. Improved engine performance was shown with the gaseous fuel with 100% hydrogen content at the expense of NOx, compared to syngas that involved CO. Also, HC and CO emissions were increased as the CO rate in the gas combination raised. Mahgoub et al. [44] experimentally analyzed the syngas composition effect on the emissions of a diesel-syngas dual fuel engine at various engine speeds. It was found that as the hydrogen content in the syngas is increased, the emissions of CO and HC are reduced. Bhaduri et al. [45] found that richer CO mixture has a wider burn duration and more delayed CA50 (the crank angle at 50% burn) than an H2 richer mixture in an HCCI engine fueled by impure syngas. It was also shown that combustion efficiency was improved by elevating the H2/CO ratio. Bika et al. [46] reported that the intake temperature had to be increased with increasing CO fraction in the mixture to achieve stable combustion. However, increasing CO fraction had an adverse effect on combustion efficiency. Azimove et al. [47] and Sahoo et al. [48] concluded that increased hydrogen content in the mixture led to higher combustion temperatures and NOx emissions but had a positive effect on thermal efficiency and CO-HC emissions in a diesel/syngas dual fuel engine. Longer ignition delay with higher H2 content was also mentioned by Sahoo et al. [48]. As can be seen from the above, natural gas/diesel RCCI engines can suffer from the lack of high-quality combustion and high HC-CO emissions at low loads. The addition of reformer gas which can be

0

0

-80

-60

-40

-20

0

20

40

Crank Angle ( OATDC)

10-4

1600 50 40 30 20

10-5

10

-6

10 -24

-22

-20

-18

-16

1200

HRR (J/deg.)

Mass (Kg)

10

HRR Temperature C7H16 OH CH4 CO CH2O

800

0

400

10-7 -25

Temperature (K) and HRR (J/Deg)

Fig. 2. The validation cases heat release rate and pressure traces.

-3

0 -20

-15

-10

-5

0

5

10

15

1.0 60 800 8 32 −80 −40 50 0

20

O

Crank Angle ( ATDC) Fig. 3. The mass change of some key species at 9 bar IMEP (validation case).

combined over the exhaust pipe can be a solution and is widely under research [34]. Also, the syngas generated, enables the thermochemical recovery of exhaust energy. Apart from hydrogen, the reformate (known as reformer gas or syngas or producer gas, depending on the production technique [35]) also contains a significant amount of CO [36]. These two species are the main contributing species to the release of energy from the reformer gas. There have been recent papers that mention the use of reformer gas or syngas as a good option either in natural gas spark ignition engines [37] or in compression ignition engines [38]. Both of them showed that use of syngas yields stable and robust combustion at low load because

Table 3 The validation cases emissions and performance.

Nieman et al. IMEP Present Work IMEP Absolute errors Nieman et al. IMEP Present Work IMEP Absolute errors

9 bar 9 bar 4 bar 4 bar

GIE (%)

RI (MW/m2)

NOx (g/KW.h)

Soot (g/KW.h)

UHC (g/KW.h)

CO (g/KW.h)

50.4 50.7 0.3 45.1 47.1 2.0

1.2 0.9 0.3 0.2 0.19 0.01

0.02 0.016 0.004 0.24 0.158 0.082

0.003 0.0012 0.0018 0.004 0.0032 0.0008

2.5 2.72 0.22 10.8 12.6 1.8

1.8 1.22 0.58 10.5 4.2 6.3

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Table 5 The best results of RCCI with a different combination of fuels at medium load.

RI (MW/m^2) GIE (%) NOx (g/kW-hr) Soot (g/kW-hr) UHC (g/kW-hr) CO (g/kW-hr)

RCCI Natural gas/Diesel

RCCI Natural gas/Hydrogen/Diesel

RCCI Natural gas/Syngas

RCCI gasoline/Diesel

Conventional Diesel Combustion

0.5 50.4 0.02 0.003 2.5 1.8

2.72 51.02 0.1 0.00083 1.82552 0.95542

7.81 51.44 0.05975 0.00102 2.22976 1.43474

3.7 52.2 0.01 0.014 4 3.6

1.4 48.7 0.81 0.23 – –

Fig. 4. Ternary plot contour of ignition delay time of methane/syngas mixtures (ϕ = 0.5 T = 1000 K P = 40 bar ).

2. Proposed Patterns and validation

produced onboard could ensure complete combustion [49] and compensate for the demerits of natural gas usage at low loads. However, the composition of reformer gas is varied significantly based on the operating condition and the production techniques. The objective of the present work is to study the effects of syngas composition on the combustion and emission characteristics of natural gas/diesel RCCI engine enriched with syngas. Furthermore, increasing the quantity of directly injected diesel fuel at low loads to ensure combustion and control the start of combustion timing would result in more soot emission, which is mainly affected by local diesel-rich zones. In this study, intake temperature variation is investigated as an alternative to increased diesel fuel quantity with the different composition for the syngas since onboard generating syngas enables the thermochemical recovery of exhaust energy.

2.1. Computational pattern In the current research RCCI engine simulations were carried out using the CONVERGE CFD tool [50]. CONVERGE provides automatic mesh generation and refinement and the mesh is constructed over the run time of the simulation. Traditional CFD method requires a grid to be made before the calculation which is very time-consuming and the user has to guess where the mesh should be fine. Also, once the mesh is created, the mesh should be tuned to achieve grid independent results. CONVERGE proposes adaptive mesh refinement (AMR) which makes the mesh fine where it is needed based on gradients of field variables during run time. Port fuel injected hydrogen and natural gas was considered to be homogeneously combined at IVC and the diesel injection procedure was simulated by the standard Discrete Droplet Model 4

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1920

6

1440

4

960

2

480

0 -40

-30

-20

KH and RT instabilities are responsible beyond the breakup length [52]. The No Time Counter (NTC) collision approach [53] was used for collision of fuel droplets which has been shown to be faster and more accurate than O’Rourke’s model, and droplet drag was described using the dynamic drag model [54] which accounts for variations in the drop shape through a drop distortion parameter. The RNG (re-Normalization Group) k − ɛ model with wall functions was employed to model turbulent flow [55]. Detailed chemical kinetic was utilized for combustion modeling, and it is solved by SAGE solver in CONVERGE. SAGE determines the reaction rates for each elementary reaction and the reaction rate is used to update the species concentration at each time step. Tetradecane (C14H30) was used as a fuel surrogate to model physical properties of diesel fuel due to the similarity of their for combustion modeling. The simulations were conducted utilizing the multi-zone chemistry solver to decrease the runtime [56]. In the multi-zone chemistry solver, based on the thermodynamic state of the cells, the cells are grouped in zones, and the detailed chemical kinetics is solved on each zone to decrease the runtime. A 76 species and 464 reactions reduced mechanism was utilized [57]. A phenomenological soot model was employed to predict soot [58] according to the method of Hiroyasu [59] which uses acetylene as an inception species, allowing coupling of the soot model and the detailed chemical kinetics solver. The NOx formation was modeled utilizing an extended Zeldovich procedure [58]. A crevice model based on the model of Namazian and Heywood [60] was used which is a substitute to analyzing the crevice regions in the CFD grid directly. The model includes two rings and five crevice regions and calculates the relevant parameters needed to determine the amount of

-10

0

10

20

30

40

0

O

Crank Angle ( ATDC) Fig. 5. Effect of H2 fraction in the reformate on heat release rate and cylinder pressure at 4 bar IMEP (3% syngas enrichment of intake air).

(DDM) which utilize a “nearest node” method to exchange mass, momentum, energy terms of a parcel with the fluid-phase values of the computational node that it is adjacent to [51]. The primary and secondary breakups of the injected fuel were modeled employing the hybrid KH-RT model which considers only KH mechanism are accountable for drop breakup within the characteristic breakup distance while both

20

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12

24

16

98

12

96

8

94

92

9

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12

3

6

4

0

0

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0

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80

100

100 Comb.Duration(deg) Comb.Efficiency(%)

Comb.Duration(deg)

RI(MW/m^2) CA50(deg.ATDC) PPRR(bar/deg)

PPRR(bar/deg)

RI(MW/m^2), CA50(deg.ATDC)

15

0

20

40

60

80

100

Comb.Efficiency(%)

8

Pressure (MPa)

2400

H2 100% H2 75% H2 50% H2 25% H2 0%

HRR (J/Deg)

10

90

H2(%vol)

H2(%vol) 38 IgnitionDelay(deg)

IgnitionDelay(deg)

37

36

35

34

33

0

20

40 60 H2(%vol)

80

100

Fig. 6. Effect of H2 fraction in the reformate on CA50, peak pressure rise rate, ringing intensity, combustion duration, combustion efficiency and ignition delay at 4 bar IMEP (3% syngas enrichment of intake air).

5

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Base case without using syngas

25 % CO (3% enrichment of intake air)

75 % CO (3% enrichment of intake air)

Fig. 7. Cylinder temperature cut planes at CA10, CA50, CA90 for base case, 25% CO and 75% CO syngas in the 3% enrichment.

CA10

CA50

CA90

Base case CA10 CA50 CA90

-2.54 8.68 20.72

25 % CO (3% enrichment of intake air) -1.42 5.12 7.10

75 % CO (3% enrichment of intake air) -3.12 1.82 2.90

10

10

0.2 NOx(g/Kw.h)

8

8

6

6

4

4

2

2

0.04

0

0

0

0

20

40

60

80

100

H2(%vol)

NOx(g/Kw.h)

0.16

CO(g/Kw.h)

HC(g/Kw.h)

HC(g/Kw.h) CO(g/Kw.h)

0.12

0.08

0

20

40

60

80

100

H2(%vol)

Fig. 8. Effect of H2 fraction in the reformate on HC, CO and NOx emissions at 4 bar IMEP (3% syngas enrichment of intake air).

mass entering and leaving the cylinder. A flowchart representing the overview of the solvers and models was depicted in Fig. 1.

The most important initiation reaction tends to be Eq. (2) because it has lower endothermicity compared to the other one. If compression heating (temperature) conditions exist, the other reaction can also contribute to initial stages. With the production of H radical, a chain reaction is initiated leading to further radical formation and finally H2O production:

2.2. Kinetic mechanism for reformer gas oxidation As discussed in previous sections, the main contributing species to the release of chemical energy from the syngas are H2 and CO. To better understand the combustion procedure of H2 and CO mixtures with air, chemical kinetic basics regarding hydrogen and carbon monoxide oxidation will be considered. Basically there are two main initiation reactions; dissociation of H2, and a H2–O2 reaction.

H2 + M → H + H + M (1)

(1)

H2 + O2 → HO2 + H

(2)

H + O2 → O + OH

(3)

O + H2 → H + OH

(4)

OH + H2 → H + H2 O

(5)

H2O is assumed to be a terminating for its unreactivity. When pressure increases Eq. (6) (which is a three body reaction) occurs and replaces Eq. (3). 6

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IMEP4/ SOI1 -80/ SOI2 -40/fuel mass 40 mg Diesel Fraction 0.5 Diesel Fraction 0.3

-75 BTDC

-35 BTDC

-17 ATDC

High local Eq. ratio (before SOC)

Diesel Fraction 0.5

High temperature (after SOC)

Fig. 9. contours of local equivalence ratio during diesel fraction sweep.

H + O2 + M → HO2 + M

(6)

HO2 + O → OH + O2

(11)

CO oxidation needs a small amount of OH radical (Eq. (13)), even at high temperature, since the direct reaction between CO and O2 (Eq. (12)) has high activation energy. In addition, the O atom produced does not lead to any rapid chain-branching reactions [62].

When the HO2 molecule is produced it is considered relatively inactive, therefore can be considered terminating [61]. As pressure increases the reactions Eqs. (7) and 8 overtake the stability of the generally unreactive HO2 molecules.

HO2 + H2 → H2 O2 + H

(7)

CO + O2 → CO2 + O

(12)

H2 O2 + M → OH + OH + M

(8)

CO + OH → CO2 + H

(13)

The H atom produced in Eq. (13) facilitates the chain branching reactions (Eqs. (3)(5)), and thereby accelerate the CO oxidation rate. Water can also accelerate CO oxidation through

Moreover, active species, H, O, and OH are produced. As temperature again elevates, more radicals are generated, and reactions between them become more important.

HO2 + HO2 → H2 O2 + O2 HO2 + H → OH + OH

(9)

O2 + M → O + O + M

(14)

(10)

O + H2 O → OH + OH

(15)

7

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10

2000 Tin40H250% Tin50H250% Tin60H250%

6

1200

4

800

2

400

0 -40

-30

-20

To improve computational efficiency, closed-cycle calculations on sector grids via periodic boundaries were conducted. As the direct injector has six evenly classified holes, a sixty-degree sector geometry was created. The CFD code utilizes a structured Cartesian grid with base cell 1.4 mm dimension, a fixed embed scale of four at the piston and cylinder head boundaries and near the injector nozzle, and an AMR embed scale of 4 according to fixed thresholds for the spatial gradients in velocity and temperature. To support the modeling results, developed models of IC engines should be validated against experimental or previous developed model results [63] in the literature. The model results presented here have been validated against the numerical results reported by University of Wisconsin-Madison in the paper published in SAE International Journal of Engines [25]. After validation, one possibility is to use the model to study the effect of the addition of some gasses such as nitrogen, EGR, hydrogen, and syngas numerically [64–67]. Specifications of single-cylinder Caterpillar 3401E SCOTE test engine are provided in Table 1. The 4 and 9 bars indicated mean effective pressure (IMEP) are used for validation. Table 2 presents the operating conditions for simulation results validation. Fig. 2 shows cylinder pressure variation over various loads compared to [25]. As can be seen, the predictions of combustion phasing and pressure traces are good, and the model also captures the lowtemperature heat release region at −24 and −23 for the two loads which is the result of the energy release of the decomposition of early injected pilot fuel (low-temperature reactions). Fig. 3 demonstrates the mass change of some key species for the medium load case. The region coincides with the consumption of n-heptane and the appearance of formaldehyde (CH2O). Formaldehyde consumption and OH accumulation appear to track with methane consumption. Then, CO oxidation begins, energy is released, cylinder temperatures rise, and reactions of methane accelerate. The combustion procedure is similar to the gasoline/diesel RCCI case [68]. However, the heat release duration is wider which is due to increase in reactivity gradient between natural gas and diesel compared to gasoline/diesel case [25]. The absolute errors in the peak in-cylinder pressure are 0.3956 and 0.0842 MPa, and the absolute errors in the relevant crank angle at the peak in-cylinder pressure are 1.2448 and 1.15 CA for the IMEP 9 and 4 bars respectively. The errors for other outputs were added in Table 3. Furthermore, as presented in Table 3, the results match reasonably the emissions and performance parameters. Operating conditions for the base case used to report the intake air enrichment effects via reformer are summarized in Table 4.

1600

-10 0 10 20 O Crank Angle ( ATDC)

30

40

HRR (J/Deg)

Pressure (MPa)

8

2.3. Validation of model

0

Fig. 10. Effect of intake temperature on heat release rate and cylinder pressure (50% hydrogen in 3% syngas enrichment of intake air).

2000

Tin40H2100% Tin50H2100% Tin60H2100%

8

1600

6

1200

4

800

2

400

0 -40

-30

-20

-10 0 10 20 Crank Angle (OATDC)

30

40

HRR (J/Deg)

Pressure (Mpa)

10

0

Fig. 11. Effect of intake temperature on heat release rate and cylinder pressure (100% hydrogen in 3% syngas enrichment of intake air).

High pressure which makes the concentration of the HO2 higher provides another route of CO to CO2 conversion. (16)

CO + HO2 → CO2 + OH

7 50% H2 100% H2

6

80

Fig. 12. Effect of intake temperature on combustion efficiency and ringing intensity at different mixture composition (3% syngas enrichment of intake air).

50% H2 100% H2

5

RI (MW/m2)

Comb. Efficiency (%)

100

60 40

4 3 2

20 0

1 Tin 40

Tin 50

Tin 60

0

Tin 40

8

Tin 50

Tin 60

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60

0.08 0.07

50% H2 100% H2

296

50% H2 100% H2

50

HC (g/KW.h)

0.06

NOx (g/KW.h)

79

0.05 0.04 0.03 0.02

40 30 20 10

0.01 0

0

Tin 40

Tin 50

60

Tin 60

96

Tin 50

Tin 60

50% H2 100% H2

50 CO (g/KW.h)

Tin 40

40 30 20 10 0

Tin 40

Tin 50

Tin 60

Fig. 13. Effect of intake temperature on HC, NOx and CO emissions at different mixture composition (3% syngas enrichment of intake air).

3. Results and discussion

reformer gas as an additive produced by a catalytic fuel reformer, which results in a richer premixed equivalence ratio. In addition, Christodoulou [70] reported that when a mixture of hydrogen and CO is utilized, a smaller quantity of diesel is needed to enhance a specified load and speed, compared to the use of hydrogen alone. In this investigation, the amount of diesel and methane fuel was kept constant during intake air enrichment with reformer gas. Therefore, the peak pressure gain in the case of higher CO content can be attributed to the higher total equivalence ratio and energy density of the mixture (although the LHV of hydrogen is higher than that of CO, the molar mass of carbon monoxide is much higher than that of hydrogen and this leads to an increase in total fuels mass, energy density and equivalence ratio for the CO rich mixture) and advanced CA50 (Crank angle a 50% burn). Advanced CA50 is due to a shorter ignition delay period with higher CO percentage (see Fig. 6), which is the result of the lower specific heat of carbon monoxide compared to hydrogen (specific heat of carbon monoxide is relatively similar to that of air). Moreover, CO is more reactive than hydrogen (Research Octane Number of hydrogen and carbon monoxide are 140 and 106, respectively [71,72]) which can lead to energy release during compression. Since the intake air enrichment with syngas leads to the formation of the premixed CH4/ syngas, this fact can further be justified by the representing variation of the ignition delay time in different compositions (Fig. 4). The method for reading the value of ignition delay corresponds to a specific combination of the three gases is represented by three arrows. For example, the point in the region represents the mixture with 40% volume fraction of CH4, 40% H2, and 20% CO. When only CO exists in the mixture, ignition delay time is very high which means that the CO oxidation is too slow and combustion does not occur in combustion engines because the resident time is limited. As we discussed in Section 2.2, the direct reaction between CO and O2 (Eq. (12)) has high activation energy, and CO oxidation needs a small amount of OH radical. With changing the

3.1. Reformer gas composition It has been shown that intake air enrichment with hydrogen and syngas can reduce UHC and CO emissions, improve the thermal efficiency of the engine, and make the combustion more stable at the expense of higher RI at low load operation [49]. Table 5 shows the best results of RCCI with a different combination of fuels at relatively similar operating conditions on Caterpillar 3401E SCOTE engine for medium load [25,49,69]. It can be seen that adding syngas and hydrogen is favorable for decreasing UHC and CO emissions. However, it adversely affects RI and NOx. In this section, the influence of reformer gas composition with 3% (by volume) syngas enrichment of intake air is investigated. In all cases, the composition of the syngas was selected hydrogen and carbon monoxide which resembles that of a typical diesel reformer product gas. Therefore, for example, 25% H2 indicates that 75% CO occupies the total volume of the syngas. The engine operating conditions follow the base case in Table 4 (a low load case). In the introduction, it was stated that natural gas-diesel RCCI engines suffer from achieving good combustion, with low combustion efficiency and high CO and HC emissions, which is caused by longer ignition delay durations and lower reactivity associated with natural gas. This is the reason for choosing the low load case for investigating syngas addition into the intake port with different compositions. Fig. 5 illustrates the heat release rate and cylinder pressure over an H2 volume fraction in syngas sweep (0%–100%). As the percentage of H2 is increased, the peak pressure is decreased. This result is compatible with the experimental study of Bika et al. [46] whose work studied the impact of syngas composition on an HCCI engine. In their investigation equivalence ratio was kept constant. In this work, the intake air was partly replaced by syngas to resemble the use of a

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scale of ignition delay in the figure, it can be seen that in the presence of small amount of hydrogen in the initial mixture, OH radical is generated (through Eqs. (1)(5)) and the oxidation of CO is facilitated by Eq. (13). (The brown region corresponds to the region with out of range colors). These factors tend to increase the charge temperature at the end of compression which shortens ignition delay period. Furthermore, these results are compatible with the work of Garnier et al. [45] and Lieuwen et al. [73,74] who conducted an experimental and modeling study on ignition delay in a diesel-syngas dual fuel engine. It should be noted that the ignition delay is defined as the difference between CA5 and SOI [75]. The previous study showed that SOI1 does not have an effect on the start of combustion timing [5]. Thus, in this work, the difference between CA5 and SOI2 is used to define the ignition delay. The combustion efficiency is calculated by [49,76,77]:

clear impact on SOC determination. This is due to increased local equivalence ratio and reactivity which advance start of combustion. Fig. 9 depicts the contours of local equivalence ratio during diesel fraction sweep. With increasing diesel fraction, local equivalence ratio significantly changes. Also, The combustion is started in the region with higher equivalence ratio and reactivity. However, local equivalence ratio plays a prominent role in soot formation [81]. Higher equivalence ratio results in accumulation of carbon which subsequently leads to the formation of the soot emission. It is hypothesized that change in intake temperature could be beneficial to control over SOC. Figs. 10 and 11 depict the cylinder pressure and heat release rates over intake temperature sweep for two different compositions. As can be seen, as the intake temperature increases, the start of combustion advances and ignition delay is shortened for both mixtures. Increasing intake temperature increases reaction rates, which is responsible for making the combustion duration shorter and releasing more energy in shorter time [82]. It can be inferred that the mixture with higher CO content is more sensitive to the intake temperature of the charge. Fig. 10 shows a sharp rise in the rate of pressure rise with an increase of 20 K in intake temperature, which makes the ringing intensity higher than the acceptable level. Fig. 12 illustrates the corresponding combustion efficiency variation with the intake temperature sweep. It is evident that the H2 rich mixture needs higher intake charge preheating in order to accomplish efficient combustion. This could be the result of the higher reactivity and the lower specific heat of carbon monoxide compared to that of hydrogen. The effect of charge preheating on emissions is demonstrated in Fig. 13. Increasing intake temperature increases flame temperature which plays a dominant role in thermal NOx formation, hence increases the NOx emissions, but again the mixture with higher CO content is more sensitive, and care should be taken to ensure low NOx emissions as the intake temperature rises. HC-CO emissions decline with increasing intake temperature which is the result of enhanced combustion efficiency. However, the mixture with a higher percentage of carbon monoxide needs lower intake charge preheating and increasing to a higher intake temperature does not boost the performance of the engine. This could be attributed to the resulting lower volumetric efficiency with higher intake temperatures, which reduces the oxygen required to oxidize CO into CO2. It should be noted that the CO richer mixture results in an increase in the total fuel mass and premixed equivalence ratio, which needs more oxygen to have more complete combustion and lower carbon emissions.

n

ηcomb . =

Efuel−Ecomb . loss Efuel

∑ =

EVO EVO mfi LHVi −mUHC LHVfuel−mCO LHVCO

i=1 n

∑

mfi LHVi

i=1

(17) Ringing Intensity (RI) is an in-cylinder pressure-based metrics which have been used for assessing the combustion noise and determining the upper limit of operation in compression ignition engines. RI can be calculated as:

( ) dP

RI =

2 1 (0.05 dt max ) 2γ Pmax

γRTmax

(18)

Higher CO content has an adverse effect on RI and peak pressure rise rate and increases them significantly, which finally results in knocking combustion, as shown in Fig. 6 (RI < 5 MW/m2 is an acceptable level for a heavy duty engine [78,79]). Combustion duration (defined as the 10–90% of the burn duration) lengthens with the richer H2 mixture, which can be caused by higher premixed equivalence ratio and combustion temperature obtained with a higher CO percentage. Fig. 7 shows the cylinder temperature in cut planes at CA10, CA50, CA90 for the base case (without enrichment of intake air), 25% CO and 75% CO syngas in the 3% enrichment of intake air (by volume). The high burning rate of hydrogen leads to more complete combustion and a higher flame temperature. The temperature is slightly higher in the case of the richer CO mixtures. This is because of the increased equivalence ratio and higher energy density obtained by higher CO content. The CO emission levels are sensitive to the fraction of CO in the syngas and increase with the use of high CO content mixture, which is the result of incomplete combustion of CO in the mixture. The HC emission remains quite unchanged as can be seen in Fig. 8. Although the carbon content of reformer gas declines as the hydrogen percentage elevates in the reformer, the CO-rich mixture provides higher combustion temperature and peak pressure, as discussed above. These factors have equal importance in the HC emission levels. Generally the higher combustion temperature leads to more complete combustion and lower HC emission, but higher carbon content in the mixture has a detrimental effect on the HC emission levels. The NOx emission reduces as the hydrogen percentage increases which stems from the lower combustion temperature. (The soot emission is not studied in the present since a recent research has shown that the Hiroyasu soot model is oversimplified for accurate predictions because it contains no dependence on the type, composition or structure of the fuel [80]. In this case, the combustion of four different fuels, namely hydrogen, carbon monoxide, natural gas, and diesel has been simulated.)

4. Conclusions A numerical analysis has been conducted to analyze the impact of syngas (reformer gas) composition and intake charge preheating on the performance and exhaust emissions characteristics of a natural gasdiesel RCCI engine at low loads. The major conclusions are as follows: 1. Peak pressure, ringing intensity and pressure rise rate increases significantly with increasing CO fraction in the syngas, and a hydrogen-rich mixture was favorable for boosting combustion efficiency. 2. Shortened ignition delay and combustion duration and advanced CA50 were obtained by the mixture with higher CO content. 3. CO and NOx emissions declined as the H2 percentage in the syngas increased. On the other hand, HC emissions remained relatively constant. 4. Intake charge preheating was found to be a worthy alternative to increasing the diesel fuel quantity as it enhanced the combustion efficiency and decreased HC-CO emissions. 5. The results also indicated that a mixture with higher hydrogen content requires intake charge preheating more than that with lower hydrogen content.

3.2. Intake temperature In the previous study, it was shown that the diesel percentage has a 10

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[29] Shahsavan M, Mack JH. The effect of heavy working fluids on hydrogen combustion. 2017. [30] Fayaz H, Saidur R, Razali N, Anuar F, Saleman A, Islam M. An overview of hydrogen as a vehicle fuel. Renew Sustain Energy Rev 2012;16:5511–28. [31] Lounici MS, Boussadi A, Loubar K, Tazerout M. Experimental investigation on NG dual fuel engine improvement by hydrogen enrichment. Int J Hydrogen Energy 2014;39:21297–306. [32] Liu J, Yang F, Wang H, Ouyang M. Numerical study of hydrogen addition to DME/ CH4 dual fuel RCCI engine. Int J Hydrogen Energy 2012;37:8688–97. [33] Banerjee R, Roy S, Bose PK. Hydrogen-EGR synergy as a promising pathway to meet the PM–NO x–BSFC trade-off contingencies of the diesel engine: a comprehensive review. Int J Hydrogen Energy 2015;40:12824–47. [34] Bogarra M, Herreros J, Tsolakis A, York A, Millington P. Study of particulate matter and gaseous emissions in gasoline direct injection engine using on-board exhaust gas fuel reforming. Appl Energy 2016;180:245–55. [35] Bika AS. Synthesis gas use in internal combustion engines. University of Minnesota; 2010. [36] Mujeebu MA. Hydrogen and syngas production by superadiabatic combustion–a review. Appl Energy 2016;173:210–24. [37] Lim EG, Dames EE, Acocella AJ, Needham TR, Arce A, Cohn D, et al. The engine reformer: syngas production in an engine for compact gas-to-liquids synthesis. Can J Chem Eng 2016. [38] Pourfallah M, Ranjbar AA. Studying the effect of reformer gas and exhaust gas recirculation on homogeneous charge compression ignition engine operation. Int J Eng Trans B 2015;28:1239–46. [39] Yamasaki Y, Kaneko S. Prediction of Ignition and Combustion Development in an HCCI Engine Fueled by Syngas. SAE Technical Paper; 2014. [40] Chuahy FD, Kokjohn SL. High efficiency dual-fuel combustion through thermochemical recovery and diesel reforming. Appl Energy 2017;195:503–22. [41] Palumbo AW, Sorli JC, Weimer AW. High temperature thermochemical processing of biomass and methane for high conversion and selectivity to H 2-enriched syngas. Appl Energy 2015;157:13–24. [42] Neshat E, Saray RK, Hosseini V. Effect of reformer gas blending on homogeneous charge compression ignition combustion of primary reference fuels using multi zone model and semi detailed chemical-kinetic mechanism. Appl Energy 2016;179:463–78. [43] Sahoo BB, Sahoo N, Saha UK. Effect of H 2: CO ratio in syngas on the performance of a dual fuel diesel engine operation. Appl Therm Eng 2012;49:139–46. [44] Mahgoub B, Sulaiman S, Karim Z, Hagos F. Experimental study on the effect of varying syngas composition on the emissions of dual fuel CI engine operating at various engine speeds. IOP conference series: materials science and engineering. IOP Publishing; 2015. p. 012006. [45] Bhaduri S, Contino F, Jeanmart H, Breuer E. The effects of biomass syngas composition, moisture, tar loading and operating conditions on the combustion of a tartolerant HCCI (Homogeneous Charge Compression Ignition) engine. Energy 2015;87:289–302. [46] Bika AS, Franklin L, Kittelson DB. Homogeneous charge compression ignition engine operating on synthesis gas. Int J Hydrogen Energy 2012;37:9402–11. [47] Azimov U, Tomita E, Kawahara N, Harada Y. Effect of syngas composition on combustion and exhaust emission characteristics in a pilot-ignited dual-fuel engine operated in PREMIER combustion mode. Int J Hydrogen Energy 2011;36:11985–96. [48] Sahoo B, Saha U, Sahoo N. Effect of load level on the performance of a dual fuel compression ignition engine operating on syngas fuels with varying h2/co content. J Eng Gas Turbines Power 2011;133:122802. [49] Rahnama P, Paykani A, Reitz RD. A numerical study of the effects of using hydrogen, reformer gas and nitrogen on combustion, emissions and load limits of a heavy duty natural gas/diesel RCCI engine. Appl Energy 2017;193:182–98. [50] Richards K, Senecal P, Pomraning E. CONVERGE (v2. 2.0). Madison, WI: Convergent Science. Inc; 2014. [51] Dukowicz JK. A particle-fluid numerical model for liquid sprays. J Comput Phys 1980;35:229–53. [52] Beale JC, Reitz RD. Modeling spray atomization with the Kelvin-Helmholtz/ Rayleigh-Taylor hybrid model. Atomization Sprays 1999;9:623–50. [53] Schmidt DP, Rutland C. A new droplet collision algorithm. J Comput Phys 2000;164:62–80. [54] Liu AB, Mather D, Reitz RD. Modeling the effects of drop drag and breakup on fuel sprays. DTIC Document; 1993. [55] Han Z, Reitz RD. Turbulence modeling of internal combustion engines using RNG κε models. Combust Sci Technol 1995;106:267–95. [56] Raju M, Wang M, Dai M, Piggott W, Flowers D. Acceleration of detailed chemical kinetics using multi-zone modeling for CFD in internal combustion engine simulations. SAE Technical Paper; 2012. [57] Smith G, Golden D, Frenklach M, Moriarty N, Eiteneer B, Goldenberg M, et al. GRI 3.0 mechanism, version 3.0. Berkeley, CA: Gas Research Institute, University of California; 1999. http://www me berkeley edu/gri_mech. [58] Heywood JB. Internal combustion engine fundamentals. New York: Mcgraw-Hill; 1988. [59] Hiroyasu H, Kadota T. Models for combustion and formation of nitric oxide and soot in direct injection diesel engines. SAE Technical Paper; 1976. [60] Namazian M, Heywood JB. Flow in the piston-cylinder-ring crevices of a sparkignition engine: effect on hydrocarbon emissions, efficiency and power. SAE Technical Paper; 1982. [61] Law CK. Combustion physics. Cambridge University Press; 2010. [62] Lewis B, Von Elbe G. Combustion, flames and explosions of gases. Elsevier; 2012. [63] Gamiño B, Aguillón J. Numerical simulation of syngas combustion with a multi-

References [1] Li Y, Jia M, Chang Y, Kokjohn SL, Reitz RD. Thermodynamic energy and exergy analysis of three different engine combustion regimes. Appl Energy 2016;180:849–58. [2] Reitz RD. Directions in internal combustion engine research. Combust Flame 2013;1:1–8. [3] Wang Y, Yao M, Li T, Zhang W, Zheng Z. A parametric study for enabling reactivity controlled compression ignition (RCCI) operation in diesel engines at various engine loads. Appl Energy 2016;175:389–402. [4] Benajes J, Molina S, García A, Belarte E, Vanvolsem M. An investigation on RCCI combustion in a heavy duty diesel engine using in-cylinder blending of diesel and gasoline fuels. Appl Therm Eng 2014;63:66–76. [5] Paykani A, Kakaee A-H, Rahnama P, Reitz RD. Effects of diesel injection strategy on natural gas/diesel reactivity controlled compression ignition combustion. Energy 2015;90:814–26. [6] Dempsey AB, Walker NR, Gingrich E, Reitz RD. Comparison of low temperature combustion strategies for advanced compression ignition engines with a focus on controllability. Combust Sci Technol 2014;186:210–41. [7] DelVescovo D, Kokjohn S, Reitz R. The Effects of charge preparation, fuel stratification, and premixed fuel chemistry on Reactivity Controlled Compression Ignition (RCCI) combustion. SAE Int J Engines 2017;10. [8] Kavuri C, Kokjohn SL, Klos DT, Hou D. Blending the benefits of Reactivity Controlled Compression Ignition and Gasoline Compression Ignition combustion using an adaptive fuel injection system. Int J Engine Res 2016;17:811–24. [9] Kavuri C, Paz J, Kokjohn SL. A comparison of Reactivity Controlled Compression Ignition (RCCI) and Gasoline Compression Ignition (GCI) strategies at high load, low speed conditions. Energy Convers Manage 2016;127:324–41. [10] DelVescovo D, Wang H, Wissink M, Reitz RD. Isobutanol as both low reactivity and high reactivity fuels with addition of di-tert butyl peroxide (DTBP) in RCCI combustion. SAE Int J Fuels Lubricants 2015;8:329–43. [11] Benajes J, García A, Monsalve-Serrano J, Boronat V. An investigation on the particulate number and size distributions over the whole engine map from an optimized combustion strategy combining RCCI and dual-fuel diesel-gasoline. Energy Convers Manage 2017;140:98–108. [12] Benajes J, Pastor JV, García A, Boronat V. A RCCI operational limits assessment in a medium duty compression ignition engine using an adapted compression ratio. Energy Convers Manage 2016;126:497–508. [13] Benajes J, Pastor JV, García A, Monsalve-Serrano J. The potential of RCCI concept to meet EURO VI NOx limitation and ultra-low soot emissions in a heavy-duty engine over the whole engine map. Fuel 2015;159:952–61. [14] Benajes J, García A, Monsalve-Serrano J, Boronat V. Achieving clean and efficient engine operation up to full load by combining optimized RCCI and dual-fuel dieselgasoline combustion strategies. Energy Convers Manage 2017;136:142–51. [15] Benajes J, Pastor JV, García A, Monsalve-Serrano J. An experimental investigation on the influence of piston bowl geometry on RCCI performance and emissions in a heavy-duty engine. Energy Convers Manage 2015;103:1019–30. [16] Li Y, Jia M, Chang Y, Fan W, Xie M, Wang T. Evaluation of the necessity of exhaust gas recirculation employment for a methanol/diesel Reactivity Controlled Compression Ignition engine operated at medium loads. Energy Convers Manage 2015;101:40–51. [17] Kakaee A-H, Rahnama P, Paykani A. CFD study of Reactivity Controlled Compression Ignition (RCCI) combustion in a heavy-duty diesel engine. Periodica Polytech Transp Eng 2015;43:177. [18] Kakaee A, Rahnama P, Paykani A. Numerical study of Reactivity Controlled Compression Ignition (RCCI) combustion in a heavy-duty diesel engine using 3DCFD coupled with chemical kinetics. Int J Auto Eng 2014. [19] Salahi MM, Esfahanian V, Gharehghani A, Mirsalim M. Investigating the Reactivity Controlled Compression Ignition (RCCI) combustion strategy in a natural gas/diesel fueled engine with a pre-chamber. Energy Convers Manage 2017;132:40–53. [20] Poorghasemi K, Saray RK, Ansari E, Irdmousa BK, Shahbakhti M, Naber JD. Effect of diesel injection strategies on natural gas/diesel RCCI combustion characteristics in a light duty diesel engine. Appl Energy 2017;199:430–46. [21] Kakaee A-H, Nasiri-Toosi A, Partovi B, Paykani A. Effects of piston bowl geometry on combustion and emissions characteristics of a natural gas/diesel RCCI engine. Appl Therm Eng 2016;102:1462–72. [22] Paykani A, Kakaee A-H, Rahnama P, Reitz RD. Progress and recent trends in reactivity-controlled compression ignition engines. Int J Engine Res 2016;17:481–524. [23] Reitz RD, Duraisamy G. Review of high efficiency and clean Reactivity Controlled Compression Ignition (RCCI) combustion in internal combustion engines. Prog Energy Combust Sci 2015;46:12–71. [24] Kakaee A-H, Paykani A, Ghajar M. The influence of fuel composition on the combustion and emission characteristics of natural gas fueled engines. Renew Sustain Energy Rev 2014;38:64–78. [25] Nieman DE, Dempsey AB, Reitz RD. Heavy-duty RCCI operation using natural gas and diesel. SAE Int J Eng 2012;5:270–85. [26] Dahodwala M, Joshi S, Koehler E, Franke M, Tomazic D. Experimental and computational analysis of diesel-natural gas RCCI combustion in heavy-duty engines. SAE Technical Paper; 2015. [27] Doosje E, Willems F, Baert R. Experimental demonstration of RCCI in heavy-duty engines using diesel and natural gas. SAE Technical Paper; 2014. [28] Zoldak P, Sobiesiak A, Wickman D, Bergin M. Combustion simulation of dual fuel CNG engine using direct injection of natural gas and diesel. SAE Int J Eng 2015;8:846–58.

11

Fuel xxx (xxxx) xxx–xxx

P. Rahnama et al.

[64]

[65] [66]

[67]

[68]

[69]

[70]

[71] Speight J. Synthetic fuels handbook: properties, process and performance. 2008; 2008. [72] Topinka JA, Gerty MD, Heywood JB, Keck JC. Knock behavior of a lean-burn, H2 and CO enhanced. SAE Technical Paper: SI gasoline engine concept; 2004. [73] Garnier C, Bilcan A, Le Corre O, Rahmouni C. Characterisation of a syngas-diesel fuelled CI engine. SAE Technical Paper; 2005. [74] Lieuwen T, Yang V, Yetter R. Synthesis gas combustion: fundamentals and applications. CRC Press 2009. [75] Xu G-L, Yao Chun-De, Rutland Christopher J. Simulations of diesel-methanol dualfuel engine combustion with large eddy simulation and Reynolds-averaged NavierStokes model. Int J Engine Res 2014. [76] Dempsey AB. Dual-fuel Reactivity Controlled Compression Ignition (RCCI) with alternative fuels. Madison: The University of Wisconsin; 2013. [77] Splitter DA. High efficiency RCCI combustion. Madison: The University of Wisconsin; 2012. [78] Dec JE, Yang Y. Boosted HCCI for high power without engine knock and with ultralow NOx emissions-using conventional gasoline. SAE Int J Eng 2010;3:750–67. [79] Eng J. Characterization of pressure waves in HCCI combustion. SAE Technical Paper; 2002. [80] Omidvarborna H, Kumar A, Kim D-S. Recent studies on soot modeling for diesel combustion. Renew Sustain Energy Rev 2015;48:635–47. [81] Neely GD, Sasaki S, Huang Y, Leet JA, Stewart DW. New diesel emission control strategy to meet US Tier 2 emissions regulations. SAE Technical Paper; 2005. [82] Nobakht AY, Saray RK, Rahimi A. A parametric study on natural gas fueled HCCI combustion engine using a multi-zone combustion model. Fuel 2011;90:1508–14.

spark ignition system in a diesel engine adapted to work at the Otto cycle. Fuel 2010;89:581–91. An H, Yang W, Maghbouli A, Li J, Chou S, Chua K, et al. Numerical investigation on the combustion and emission characteristics of a hydrogen assisted biodiesel combustion in a diesel engine. Fuel 2014;120:186–94. Gong C, Li D, Li Z, Liu F. Numerical study on combustion and emission in a DISI methanol engine with hydrogen addition. Int J Hydrogen Energy 2016;41:647–55. Khairallah HA, Koylu UO. A Computational Study of In-Cylinder NOx Reduction Strategies for a Compression-Ignition Engine Fueled With Diesel/Hydrogen Mixtures. In: ASME 2015 9th International Conference on Energy Sustainability collocated with the ASME 2015 Power Conference, the ASME 2015 13th International Conference on Fuel Cell Science, Engineering and Technology, and the ASME 2015 Nuclear Forum: American Society of Mechanical Engineers; 2015. p. V001T08A-VT08A. Mansor MRA, Abbood MM, Mohamad TI. The influence of varying hydrogen-methane-diesel mixture ratio on the combustion characteristics and emissions of a direct injection diesel engine. Fuel 2017;190:281–91. Kokjohn SL, Hanson RM, Splitter DA, Reitz RD. Experiments and modeling of dualfuel HCCI and PCCI combustion using in-cylinder fuel blending. SAE Int J Eng 2009;2:24–39. Kokjohn S, Hanson R, Splitter D, Reitz R. Fuel Reactivity Controlled Compression Ignition (RCCI): a pathway to controlled high-efficiency clean combustion. Int J Engine Res 2011;12:209–26. Christodoulou F. Hydrogen, nitrogen and syngas enriched diesel combustion. Brunel University; 2014.

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