Screening method for fuels in homogeneous charge compression

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Screening method for fuels in homogeneous charge compression ignition engines: application to valeric biofuels Francesco Contino, Philippe Dagaut, Fabien Halter, Jean-Baptiste Masurier, Guillaume Dayma, Christine Mounaim-Rousselle, and Fabrice Foucher Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b02300 • Publication Date (Web): 30 Nov 2016 Downloaded from http://pubs.acs.org on December 3, 2016

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Screening method for fuels in homogeneous charge compression ignition engines: application to valeric biofuels. Francesco Contino,∗,†,‡ Philippe Dagaut,¶ Fabien Halter,¶,§ Jean-Baptiste Masurier,¶,§ Guillaume Dayma,¶,k Christine Mouna¨ım-Rousselle,§ and Fabrice Foucher§ †Department of Mechanical Engineering, Vrije Universiteit Brussel, Brussels, Belgium ‡BURN Joint Research Group, Vrije Universiteit Brussel & Universit Libre de Bruxelles, Belgium ¶ICARE, CNRS-INSIS, Orléans, France §Université d’Orléans, Laboratoire Prisme, Orléans, France kUniversité d’Orléans, Orléans, France E-mail: [email protected] Abstract The successful implementation of innovative fuels relies on many factors and requires a precise description of the combustion characteristics. In practice, traditional indicators such as octane or cetane numbers are used. Obtaining these numbers however requires a specific setup. Moreover, they are not sufficient to predict the combustion timing for innovative concepts such as Homogeneous Charge Compression Ignition (HCCI). Evaluating and reporting the characteristics of new fuels is therefore challenging. Using a regular diesel engine converted to HCCI operation, we developed

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a screening methodology to easily characterize and present key features of new fuels. This paper describes the methodology and then applies it to new fuels produced from biomass: valeric biofuels. The methodology presented in this study intends to bridge the work on fundamental chemical kinetics with conventional engine experiments.

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1

Introduction

When investigating the potential and the engine combustion characteristics of new fuels, one of the features of the Homogeneous Charge Compression Ignition (HCCI) engine can be used as a screening tool: the heat release rate is governed by chemical kinetics. Given the right engine settings, HCCI can therefore be operated with any fuel. 1–3 Predicting or determining the right engine settings and the effects of fuel chemistry and properties such as octane number on ignition delay and burning rate are the key to find the envelope of HCCI operations for specific fuels. 4 Characterizing the conditions in the cylinder and the coupling effect appearing when changing inlet conditions are also very important. 5 The HCCI engine has already been used to characterize various fuels. 6–8 Kalghatgi and Head have presented the Octane Index (OI) methodology which predicts fairly well the combustion behavior according to the engine setup combined with the Research Octane Number (RON) and the Motor Octane Number (MON) of the fuel. 9 Jeuland et al. have presented a methodology based on operating region in HCCI mode, as this is one of the limitation compared to traditional engines. 10 Shibata and Urushihara have presented HCCI Indexes based on the fuel chemical composition. 11 Truedsson et al. have developed the LundChevron HCCI Number as a measure of the compression ratio of a Cooperative Fuel Research (CFR) engine to have a crank angle where 50% of the fuel heat is released (CA50) at 3 Crank Angle Degree (CAD) After Top Dead Center (ATDC). 12 Some studies focus primarily on the performance of the HCCI engine, trying to optimize the fuel formulation. Niemeyer et al. presented the potential fuel savings of Low Temperature Combustion (LTC), LTC fuel performance index calculated computationally for the FTP-75 light-duty driving cycle. 13 Some of these methods have limitations. For example, Liu et al. indicated that the OI was not adequate for oxygenated fuels 14 and Rapp et al. have demonstrated that more than one metric may be required for predicting auto-ignition. 15 In particular, low Temperature Heat Release (LTHR) should also be included in the analysis since it predicts auto-ignition order 3


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better than octane index. Lacey et al. have also shown that the original OI correlation is not able to capture the behavior of pump grade gasoline, especially those having high aromatic and high ethanol content. A modified OI model was proposed to include non-linear terms capturing the effect of main chemical constituents. 16 Additional test facilities could also complement the engine-type results.

Perez and

Boehman have shown that the use of an Ignition Quality Tester (IQT) could provide better information on the timing of ignition in HCCI than correlations based on MON and RON. 17 Most of these studies require the use of a specific engine (CFR) or an adequate diagnostic tool (IQT). Moreover, several are based on the prior knowledge of the fuel octane number or the precise chemical composition. Although these would certainly provide more accurate results and are steps towards normalisation, there is a need for a systematic yet simple screening method. Moreover, given the strong trends of development of fuel antiknock quality for new generation engines 18 , alternative indicators might also provide more insight. The methodology presented in this paper intends to rapidly screen fuels using any HCCI engine. It has similarities with the methodology presented by Yang et al. to study SI engine knock. 19 The new method is applied to innovative fuels produced from biomass. The production of fuels from biomass has been a topic of great interest for many years and in various research communities, from the production process to the engine compatibility. Although not the ultimate solution to fuelling the transportation sector on the long term, it has and will help shifting part of the fossil based fuels. The production of new generation biofuels focuses on converting non-food crops or waste to mitigate the tension on the food market. However, these production processes are more complex and require more upfront investment. As a consequence, their market penetration is weakened. The major constraint applied to the process is that the end product needs to be compatible with current internal combustion engines. As a consequence, the process needs to include many upgrading steps. In particular, for the conversion of lignocellulose to the compatible ethanol, the required process is very complex. Many alternatives exist but are not as well known or compatible. 20

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Among these alternatives, this paper focuses on the valeric biofuels introduced by Lange et al.. 21 Based on a cheap and efficient production of levulinic acid, valerate esters (methyl, ethyl, propyl, butyl, and pentyl valerate) can be produced, depending on the process settings 21 (more details on the fuels in section 2.1). Although many studies have focused on several production aspects of valeric biofuels 22–24 , only few studies focus on the modeling of the chemical kinetics 25,26 or report results on engine performances and emissions. 27,28 In these engine studies, we reported results in Compression Ignition (CI) and Spark Ignition (SI) engines. The complete range of valerate biofuels cannot be used in one type of traditional engines (SI or CI).The SI engine was operated with methyl and ethyl valerate, while butyl and pentyl were tested on the CI engine. In both cases, the performance and emissions were similar to the reference fuels. Since their combustion characteristics are not well known, the HCCI engine is used in this paper to establish a ranking and infer the applicability to other engines. The methodology, described in section 2.3, is applied to valeric biofuels (see results in section 4). The objective of this paper is to demonstrate the importance of the HCCI engine in the fuel characterisation as a more engine-oriented tool yet still governed by kinetics. As such, it fills the gap between fundamental studies aimed at determining the chemical kinetics of fuels ignition and the conventional engine experiments.

2

Experimental setup and methodology

This section first introduces the characteristics of the fuels. It then describes the details of the engine used in this work and finally discusses the experimental procedure.

2.1

Fuel characteristics

The Lower Heating Value (LHV) of all four fuels are significantly smaller than petroleumbased fuels due to their oxygenated nature. However, the stoichiometric air/fuel ratio is also

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smaller, resulting in stoichiometric air/fuel mixtures with similar energy content (see Table 1). We measured the derived cetane number for three of the fuels using a Herzog Cetane ID510 tester (ASTM D7668 method). However, only a small difference of Derived Cetane Number (DCN) among the fuels implies a dramatic change of ignition timing (as shown in section 4). The RON or MON of the valeric biofuels are not known. Only Blending Research Octane Number (BRON) were reported in the study of Lange et al. 21 Since these values are extrapolated from results with blends, they cannot be directly compared to RON but give trends. Ethyl to propyl valerate are rather close to gasoline properties for combustion characteristics, the main difference being the boiling temperature (140 to 170◦ C). Butyl and pentyl are closer to diesel fuel but with a lower DCN, 25-30 instead of the usual range 50-55.

Table 1: Main properties of the fuels. Valerate esters Methyl

Ethyl

Propyl

Density (15◦ C) [kg/l] LHV [MJ/kg] Stoich. A/F Tboiling [◦ C] ∆Hvap. [kJ/kg] Stoich. energy [MJ/m3cylinder ] Tflash [◦ C] DCN BRON RON MON

2.2

Butyl

Pentyl

i-Octane n-Heptane Ethanol Toluene C7 H16

C2 H 6 O

C7 H 8

0.875

0.874

0.870

0.868

0.874

0.692

0.684

0.789

0.867

28.8

30.3

31.5

32.6

33.5

44.6

44.9

26.8

40.6

9.5 137 371 3.90

10.1 142 361 3.87

10.6 167 277 3.86

10.9 187 335 3.85

11.2 206 257 3.85

15.1 99 309 3.81

15.2 98 352 3.82

9 78 837 3.86

13.4 111 402 3.92

22 n.a. 115 n.a. n.a.

34 17.1 100 n.a. n.a.

38 n.a. 90 n.a. n.a.

67 24.5 n.a. n.a. n.a.

76.5 30.0 10 n.a. n.a.

-12 15.2 100 100 100

-4 55.7 0 0 0

12 n.a. n.a. 107 89

4 n.a. n.a. 120 103.5

C6 H12 O2 C7 H14 O2 C8 H16 O2

C9 H18 O2

C10 H20 O2 C8 H18

Test rig setup and experimental procedure

The engine is based on the PSA DW10 model and has been described in details in previous studies 27,29 (see Figure 1). It is converted to single-cylinder and driven by an electric motor that can maintain a constant revolution speed (1500 rpm in this study). The specifications of the engine are listed in Table 2. 6


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Table 2: Engine details Bore [mm] Stroke [mm] Displacement [cm3 /cyl.] Connecting rod length [mm] Geometric Compression ratio Effective Compression ratio Number of valves

85 88 499 145 15.35 14.15 4

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Figure 1: The engine (based on a PSA DW10) is converted to single-cylinder and driven by an electric motor. The compressed air flow rate is controlled together with an electric heater to adjust inlet pressure and temperature. The fuel is vaporized before being mixed with the inlet air in a large intake plenum allowing a good mixture. The intake air is supplied by an air compressor and is electrically heated to the desired set point. It is metered and controlled by a Brooks 5853S model to obtain an intake pressure up to 1.6 bar. The fuel flow from a pressurized tank is metered and controlled by a Bronkhorst M13 Coriolis flow sensor. Fuel is fully premixed with the intake air by supplying a fuel/air mixture from the heated fuel-vaporization chamber upstream of the intake plenum. This plenum is designed to minimize the pressure oscillations and to improve the mixture homogeneity. The equivalence ratio was set at 0.3. The intake conditions are measured in the intake manifold by a Kistler 4075A piezoresis7


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tive absolute pressure sensor (±3 kPa) and by two K thermocouples (±2 K) in both intake ducts just above the inlet valves. Cylinder pressure measurements are made with a Kistler 6043A piezoelectric transducer (±2% of the measured value) at 0.1 CAD increments. The piezoelectric transducer offset is determined by the mean value of the absolute pressure in the intake manifold at the end of the intake stroke when the pressure is stabilized. The temperatures of the cooling water and the lubricating oil were held constant during all the experiments at a value of 366 K and 363 K respectively. An exhaust outlet pressure controller is mounted after the exhaust manifold to emulate the back-pressure of a turbocharge when the pressure is above 1 bar. Unlike many other studies, the test bench is operated in an open loop configuration. The inlet parameters are kept constant without any feedback from the combustion. While this is questionable for the successful operation of HCCI engines in practice, it brings more control on the region being investigated in the context of this study. It also provides more stable boundary conditions for later numerical simulation studies.

2.3

Experimental procedure

Pressure data from 100 consecutive cycles were stored and analyzed by the data acquisition system when combustion was stabilized. To determine stabilization and operating limits, the mean values and standard deviations of these parameters based on 100 cycles were also calculated online. Each stabilized point was repeated three times. The reported temperatures are mass-averaged and computed using the ideal gas law in combination with the measured pressure, the known cylinder volume and the temperature at Bottom Dead Center (BDC):

T =

pV TBDC RBDC pBDC VBDC R

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(1)

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where the mass is considered constant, T is the mass-averaged temperature, p is the pressure, V is the volume, R is the ideal gas constant divided by the molar mass of the mixture. The temperature at BDC is determined using the method of Sjöberg and Dec, which considers the main effects affecting the temperature from the measured value in the inlet port: mixing with residuals, heat-transfer, gas dynamic effects and vaporization of directly injected liquid fuel 30 . Since, the air/fuel mixture was prepared externally without any vaporization in the cylinder, only the three first effects were considered. Following this method, and since the engine speed and equivalence ratio were held constant, we have established that the temperature at BDC mainly varies with the measured intake temperature according to the following equation: TBDC = 0.652Tin + 138 [K],

(2)

where Tin is the measured intake temperature. This evaluation entails uncertainties related to simplifications in the method (around 5% 30 ) and to the precision on the pressure, the dynamic effect on this engine geometry and the temperature measurement at the exhaust. We estimate the overall error to around 10%. Using TBDC from eq. (2) instead of Tin significantly improves the computation of the average in-cylinder temperature. Moreover, it also reduces the dependance of the reporting of the ignition timing to the engine specific design. The difference between the measured inlet port temperature and the computed TBDC is up to 30 K for the minimum and maximum inlet temperatures (313K and 473 K). The Heat Release Rate (HRR) was computed from the cylinder-pressure data and the cylinder volume: dQ Cp dV CV dp dQwall = p + V + , dθ R dθ R dθ dθ

(3)

where θ is the crank angle, Cp and CV are the specific heats at constant pressure and constant volume respectively, R the ideal gas constant, p the pressure, V the volume of the

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combustion chamber and Qwall the wall heat transfer. The wall heat transfer is modelled by the Hohenberg correlation. 31 The specific heats are computed from the JANAF tables and include the change of composition due to the combustion with a two steps algorithm. First, they are arbitrarily modified at Top Dead Center (TDC) from the composition of the air/fuel mixture to the composition of the complete combustion products. The cumulative heat release is then computed from the HRR obtained with eq. (3). As this change of specific heats is arbitrary, a second step use the cumulative heat release normalized by its maximum as a conversion factor between reactants and products and a better approximation of the heat release rate is obtained. To characterize the combustion, three parameters were calculated for each cycle: CA10, CA50 and CA90 (crank angles where 10%, 50% and 90% of the cumulative heat release is reached, respectively). The uncertainties on these parameters come from the uncertainty on the heat release rate which is due to the precision of the pressure sensor, the error on the effective compression ratio and the assumptions of the heat transfer model. Uncertainty combining precision in the measurements and reproducibility is ±0.3 CAD when CA50 is around 0 CAD ATDC or below, and is ±1 CAD, when CA50 is around 5 and above. 32,33 When characterising the heat release in the region of LTHR (see section 4.2), the uncertainty becomes significant and depends on the low temperature flame intensity. The uncertainty on the heat release in that region goes from 30% up to 100%. One aspect that has a direct impact on the ignition timing but is only indirectly controlled is the temperature inhomogeneity before combustion. 34–36 This inhomogeneity results from the heat transfer with the engine wall. Measuring the spread of temperature was not possible with the current engine setup. Therefore, to estimate it, we have used Computational Fluid Dynamics (CFD) simulations of the same geometry. The setup of these simulations is beyond the scope of this paper and is described in details in previous articles. 32,37 The inhomogeneity of the temperature has been evaluated with different intake conditions leading to a range of average in-cylinder temperature at 25 CAD Before Top Dead Center

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3

Methodology

The methodology consists of exploring many pressure/temperature combinations leading to autoignition in a HCCI engine. In this study, the intake pressure was varied from 0.5 to 1.6 bar and the intake temperature from 303 to 493 K which represents a large screening region. The temperature and pressure steps at which points were captured are 5 K and 0.05 bar. Since the purpose of this method is to report results independently of the engine design, the results below will be presented as a function of the temperature and pressure near TDC. 9 This has the additional advantage of removing the effect of the different specific heat ratios resulting from the large span of stoichiometric air fuel ratios, especially in the case of oxygenated molecules. Unlike other studies where this point is 15 CAD BTDC, an earlier point is selected here (25 CAD BTDC) to avoid the pressure and temperature increase due to LTHR. As can be seen in Figure 3 for selected conditions and fuels so that CA50 is around 0, some of the fuels have a small increase of pressure (and therefore temperature) due to LTHR around 20 CAD BTDC. For high reactivity fuels, this earlier point might still be affected by early heat release, however this is unlikely when considering the region of interest for the CA50. More details on that aspect are provided in section 4.2. Pressure and temperature are two parameters that are easily modified and directly affect the kinetics. Other parameters such as engine speed or equivalence ratio also have an impact. Engine speed is important for the applicability of HCCI but combines several effects 39 , and equivalence ratio is limited due to the maximum pressure rise rate. Of course, the two selected parameters have an impact on other aspects influencing the engine performance, e.g. it changes charge density, hence Indicated Mean Effective Pressure (IMEP). However, the focus of this paper is not to analyse engine performance but rather to use the engine as a combustion analysis tool.

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4

Results

This section presents the main results of this study. It first introduces the innovative way to present the data of the screening process. Then, it provides more details on the sensitivity of the fuels to pressure and temperature. Finally, it describes the importance of LTHR.

4.1

Ignition characteristics

Figure 4 presents the regions of CA50 between 1 and 3 CAD ATDC in the plane of temperature and pressure at 25 CAD BTDC for the five valerate esters. These regions were obtained by interpolating data points through the explored range of temperature and pressure. They are compared to a scale of Primary Reference Fuel (PRF) fuels, to ethanol, and to toluene. For clarity, the scale only starts at PRF60 in Figure 4, whereas the region for n-heptane lies in the region (700-800K; 8-10 bar). Two lines are also added to this figure showing the pressure and temperature obtained in a CFR engine. 40 This lines were obtained through a 0D model taking into account the RON or MON condition sets and changing the compression ratio from 9 to 18. The explored region with the engine in HCCI goes far beyond what is explored in these traditional conditions. Methyl valerate is more resistant to auto-ignition than iso-octane but lower than toluene. It also presents a similar angle as iso-octane. Ethyl valerate is nearly exactly aligned with iso-octane, which indicates similar sensitivities to pressure and temperature change. Propyl valerate is between PRF90 and iso-octane while butyl and pentyl valerate are more reactive and lies in the region of PRF80, and between PRF60 and PRF80, respectively. The regions for butyl and pentyl valerate are not similar to the PRFs in the lower temperature domain. This might be explained by the difference in the LTHR region (see section 4.2). Sensitive fuels, such as ethanol with a sensitivity (RON – MON) of 18, become more resistant to auto-ignition when intake pressure is increased. 14,41 However, temperature has a strong impact on the resistance to auto-ignition. Therefore, the isolines of CA50 as a

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describes the sensitivity of the fuels to pressure and temperature; it also predicts their overall position in the pressure/temperature plane through the constant of the regression. Finally, it can also emphasize the role of the LTHR in the overall combustion process. Using this methodology, we could clearly see that valeric esters have a large span of autoignition reactivity, while they have sensitivity to pressure and temperature between PRFs and Toluene. We have shown in previous studies that trace species, such as NO, can affect ignition delays and combustion development 32,37 , future studies will analyze this effect in more details.

Acknowledgement The authors would like to thank the technical staff and trainee from Laboratoire PrismeUniversité d’Orléans (B. Moreau, J. Lemaire) for their technical help in this study. This work was supported by the ERC Advanced Researcher Grant n 291049 - 2G-CSafe.

References (1) Christensen, M.; Hultqvist, A.; Johansson, B. Demonstrating the Multi Fuel Capability of a Homogeneous Charge Compression Ignition Engine with Variable Compression Ratio. SAE Technical Paper 1999, 1999-01-3679 . (2) Tanaka, S.; Ayala, F.; Keck, J. C.; Heywood, J. B. Two-stage ignition in HCCI combustion and HCCI control by fuels and additives. Combust. Flame 2003, 132, 219–239. (3) Mack, J. H.; Aceves, S. M.; Dibble, R. W. Demonstrating direct use of wet ethanol in a homogeneous charge compression ignition (HCCI) engine. Energy 2009, 34, 782–787. (4) Silke, E. J.; Curran, H. J.; Simmie, J. M. The influence of fuel structure on combustion as demonstrated by the isomers of heptane: a rapid compression machine study. Proceedings of the Combustion Institute 2005, 30, 2639–2647. 21


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(5) Janecek, D.; Rothamer, D.; Ghandhi, J. Fuel-Substitution Method for Investigating the Kinetics of Low-Volatility Fuels under Enginelike Operating Conditions. Energy & Fuels 2016, 30, 1400–1406. (6) Sjoberg, M.; Dec, J. E. Ethanol Autoignition Characteristics and HCCI Performance for Wide Ranges of Engine Speed, Load and Boost. SAE Int. J. Engines 2010, 3, 84–106. (7) Yang, Y.; Dec, J. E.; Dronniou, N.; Simmons, B. Characteristics of Isopentanol as a Fuel for HCCI Engines. SAE Int. J. Fuels Lubr. 2010, 3, 725–741. (8) Yang, Y.; Dec, J. E. Bio-Ketones: Autoignition Characteristics and Their Potential as Fuels for HCCI Engines. SAE Int. J. Fuels Lubr. 2013, 6, 713–728. (9) Kalghatgi, G. T.; Head, R. A. Combustion Limits and Efficiency in a Homogeneous Charge Compression Ignition Engine. Int. J. Engine Res. 2006, 7, 215–236. (10) Jeuland, N.; Montagne, X.; Duret, P. New Hcci/Cai Combustion Process Development: Methodology for Determination. Oil Gas Sci. Technol. 2004, 59, 571–579. (11) Shibata, G.; Urushihara, T. Auto-Ignition Characteristics of Hydrocarbons and Development of HCCI Fuel Index. SAE Technical Paper 2007, 2007-01-0220 . (12) Truedsson, I.; Cannella, W.; Johansson, B.; Tuner, M. Development of New Test Method for Evaluating HCCI Fuel Performance. SAE Technical Paper 2014, 201401-2667 . (13) Niemeyer, K. E.; Daly, S. R.; Cannella, W. J.; Hagen, C. L. A Novel Fuel Performance Index for Low-Temperature Combustion Engines Based on Operating Envelopes in Light-Duty Driving Cycle Simulations. Journal of Engineering for Gas Turbines and Power 2015, 137, 101601–101601.

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Screening method for fuels in homogeneous charge compression

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