Experimental and Numerical Study of Abnormal ... - ACS Publications

Article Cite This: Energy Fuels 2019, 33, 5230−5242

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Experimental and Numerical Study of Abnormal Combustion in Direct Injection Spark Ignition Engines Using Conventional and Alternative Fuels Tamara Ottenwälder,*,† Ultan Burke,‡ Fabian Hoppe,† Oguz Budak,† Sascha Brammertz,† Kevin Klintworth,† Gerd Grünefeld,§ Karl Alexander Heufer,‡ and Stefan Pischinger† †

Institute for Combustion Engines, RWTH Aachen University, Forckenbeckstr. 4, 52074 Aachen, Germany Physico-Chemical Fundamentals of Combustion, RWTH Aachen University, Templergraben 55, 52056 Aachen, Germany § Institute of Technical Thermodynamics, RWTH Aachen University, 52062 Aachen, Germany

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‡

ABSTRACT: In this paper, the potential of alternative fuels to reduce preignition in engines is primarily investigated. Two ketones (2-butanone and 3-methylbutanone), a furan (2-methylfuran), and five alcohols (ethanol, iso-propanol, 2-butanol, isobutanol, and 1-propanol) are compared to three conventional fuels (RON95E0, RON95E10, and iso-octane). If the alternative fuels can reduce preignition significantly, the efficiency of spark-ignition engines can be potentially increased. One major aim is to categorize the fuels in terms of preignition resistance. Additionally, the distributions of the initial preignition kernels are measured by high-speed luminescence imaging to analyze the reason for preignition. Furthermore, the occurrence of measured preignition is compared with computed ignition delay times at the pressures and temperatures of the engine load points used to find out whether the mixture is prone to autoignition in the bulk gas phase. This is most likely the case for the three conventional fuels because of thermodynamically critical gas-phase conditions. Seemingly, preignition is not induced by droplets, presumably because of an improved injector targeting. Moreover, the preignition resistance of the alternative fuels is significantly higher (critical intake pressure difference: ∼2 bar) compared to that of the three conventional fuels. It can also be concluded that 2-butanone is most resistant against preignition. Similarly, alcohols and 3-methylbutanone are highly beneficial compared to conventional fuels. Overall, the most important fuel properties for preignition appear to be research octane number and enthalpy of vaporization. Apparently, ignition is caused by glow ignition at a hot surface (spark plug) only for 2methylfuran, iso-butanol, and 1-propanol. (e.g., number of carbon atoms) developed by Dahmen et al.4 For gasoline-like fuels, the restrictions are as follows. A derived cetane number below 9, a heating value of >30 MJ/kg, a boiling point between 50 and 100 °C, and an enthalpy of vaporization of 99.0

C4H8O 66.63 11.18 22.19 799 1.073 23.9 80 10.8 46.10 10.52 31.45 117 107 >99.0

C5H6O 73.15 7.37 19.49 907.5 0.378 25.5 64 13.9 35.52 10.08 30.37 100.7 82.4 >99.8

C5H10O 68.13 13.72 18.15 803 0.42 24.8 93.85 0.27 42.24 11.21 33.40 115 104 >99.9

% % % kg/m3 mPa·s mN/m °C kPa kJ/kgair,λ=1 1 MJ/kg 1 1 %

λ = 1: stoichiometric conditions.

a

Table 4. Properties of Investigated Alcohols According to the Works by Hoppe et al., Yaws, Daubert and Danner, and the German “Institut für Arbeitsschutz”5,37−44 formula carbon mass fraction hydrogen mass fraction oxygen mass fraction density (25 °C) dynamic viscosity (25 °C) surface tension (25 °C) boiling temperature vapor pressure (20 °C) specific enthalpy of vaporization stoichiometric air requirement lower heating value RON MON purity

% % % kg/m3 mPa·s mN/m °C kPa kJ/kgair,λ=1 1 MJ/kg 1 1 %

ethanol

1-propanol

iso-propanol

2-butanol

iso-butanol

C2H6O 52.14 13.13 34.73 787 1.073 22.1 78 5.8 101.6 8.98 26.84 109 89.7 >99.0

C3H8O 59.96 13.42 26.62 798 1.86 23.25 97 2.03 78.61 10.33 30.71 104

C3H8O 59.96 13.42 26.62 780 1.95 20.87 82 4.26 74.79 10.33 30.71 112 101.9 >99.0

C4H10O 64.82 13.60 21.58 804 2.95 22.86 99 1.7 66.12 11.16 33.03 105 90.9 >99.0

C4H10O 64.82 13.60 21.58 796 3.14 22.14 107 1.18 68.87 11.16 33.03 110 90.1 >99.0

>99.0

Turner et al., Cooney et al., Szybist et al., and He et al.6,8,29−36 Most beneficial is the high knock resistance and high heat of vaporization, which enable high efficiency improvements compared to conventional gasoline fuels, especially at high engine loads. Therefore, ethanol can be regarded as the benchmark biofuel for SI engines. 2-Butanone, 3-methylbutanone, and 2-methylfuran are renewable fuels as shown by Hechinger et al. and Hoppe et al.2,3 The autoignition tendency, mixture formation, combustion stability, and increased efficiency of 2-butanone and 2-methylfuran were also investigated in the studies by Hoppe et al., Thewes et al., Hülser et al., Pischinger et al., and Budak et al.5,7−10 2-Butanone provides high efficiencies in SI engines because of a low autoignition tendency, which is however higher compared to ethanol. As demonstrated in the work by Hoppe et al.,5 2-butanone leads to increased combustion stability at engine cold start, presumably because of reduced enthalpy of vaporization compared to ethanol (see Table 3). In Tables 3 and 4, the most relevant fuel properties are given. 2-Butanone and 3-methylbutanone contain an oxygen atom with a double bond (Figure 3), which could be a reason for the low autoignition tendency shown in Hoppe et al.5 Both species are promising fuel candidates for high- and low-load engine operation points. The RON and MON of 2-methylfuran, 2-butanone, and 3methylbutanone given in Table 3 were measured according to DIN 51756-7 because of the high oxygen contents, which was described by

transducers and sampled with a 0.1° CA resolution. In total, 307 consecutive cycles are measured for each fuel and pin-value. This is repeated four times. The number of cycles per measurement is limited by the data storage capacity of the optical system. In Table 2, a load range of the actual measurements is given because the load depends on pin. It should be noted that significantly higher load is expected for earlier spark timing. When the fuel is changed, a specific engine conditioning procedure described in the study by Pischinger et al.9 is conducted to avoid cross-effects of different fuel dilution levels and combustion chamber deposits on preignition. 2.2. Fuels. To classify the potential of the investigated alternative fuels, benchmark fuels are defined. Normally, for the use in SI engines, new fuels have to be compared to conventional gasoline. RON95E0 and RON95E10 gasoline fuels (supplier: Shell Global Solutions Germany GmbH, Shell Technology Center Hamburg) are chosen because they are the most widely used conventional pump fuels in the European gasoline market. They are multicomponent fuels with a RON value of 95. They have ethanol contents of 0 and 10%, respectively. Additionally, iso-octane is chosen as a typical gasoline surrogate because the composition of conventional pump fuels is not very well defined. Pure ethanol is also used as a fuel. Its influence on combustion and efficiency in SI engines has been investigated in studies by Thewes et al., Amer et al., Hülser et al., Brassat et al., Yoon and Lee, Karpov, 5233

DOI: 10.1021/acs.energyfuels.8b04040 Energy Fuels 2019, 33, 5230−5242

Article

Energy & Fuels

Figure 4. Hardware components within the field of view. corresponds to this time resolution. The image intensifier gain is set to 65% of the maximum possible gain to avoid the effects of a camera and intensifier noise. Image recording is started at 25° CA BTDC and is stopped at 1° CA before spark timing in each cycle (about −10° CA BTDC) in order to avoid the intensifier damage by the ignition spark. All presented preignition events are detected by the optical system, that is, pressure traces are not considered as in the work by Hülser et al.8 Some of the preignition peak pressures are below the pressure after compression because of retarded combustion so that corresponding preignition events may not always be detected unambiguously by only analyzing the pressure traces. Also with more sophisticated pressure trace analysis presented in the work by Pischinger et al.,9 some preignition events were not detected, which were observed by the optical measurements (not shown for brevity). A situation in which preignition is only observed by pressure trace analysis is never found. A preignition event is detected in the optical images as soon as light appears. As the recording is stopped before spark timing, all detected light results from preignition. To detect the locations of initial preignition kernels, a geometric centroid method is applied to the first detected luminescence distributions. This is needed because of the fast flame propagation of some preignition events, which is demonstrated below. Multiple preignition kernels within one cycle are never detected. 2.4. Simulation Approach. The boundary conditions for the performed simulations are extracted from a one-dimensional (1D) gas exchange engine model. For calculation, GT-Power is used47 and is matched with the corresponding experimental data of air mass flow, fuel mass flow, cylinder pressure, boost and back pressure, as well as charge air temperature and valve timings. From these 1D gas exchange simulations, p−T profiles are extracted to calculate effective volume− time histories. Out of these 1D simulations, Tin is also determined. The parameters used to match the model to the experimental data are given in the following. Intake air density is based on intake heat transfer (minor tuning parameter). Fuel evaporation is based on fuel properties from the literature by Daubert and Danner39 and threedimensional computational fluid dynamics (3D CFD) of air flow and fuel injection. For both the same method is used by Budak et al.10 Measured intake and cylinder pressure, air mass flow, and valve timing are also used. Note that throttle position is not used as a tuning parameter because it is always wide open. The overall exhaust-gas recirculation rate is estimated to be below 3% (simulation not shown for brevity). It results from the described 1D simulations, which are validated by 3D flow field simulations. Thus, the effect of trace species such as NO and formaldehyde on autoignition is neglected in the simulations. The engine process model includes 1D gas dynamics on intake and exhaust side and 0D fuel injection, vaporization, compression, and combustion. The 1D engine process model is set up as a threepressure analysis model. The enthalpy of vaporization is thereby taken into account for each fuel. The heat of vaporization is modeled based on the physical properties of liquid and gaseous phases adopted from the book by Daubert and Danner.39 The fraction of liquid fuel which evaporates from cylinder walls is modeled as follows. First, the

Figure 3. Molecular structure of the used single-component fuels (red: oxygen, gray: carbon, white: hydrogen). Anderson et al. and in the DIN.23,45 For all investigated alternative fuels, the RON values are higher than for the conventional pump fuels. The ketones feature even higher RON values than the alcohols. For 2-butanone, the highest RON value of all investigated fuels was measured. It is ∼20 units higher than that of the conventional gasoline fuel and still 8 units higher than that of ethanol. For the investigated alcohols, the RON values are also higher than that of iso-octane. The alcohols also feature high enthalpy of vaporization, which results in significantly higher charge cooling than for conventional gasoline fuels. Another important fuel property is the boiling temperature. It is kept below the engine operation temperature of 90−120 °C to avoid significant oil dilution as shown by Hoppe et al.2 As indicated by red dots in Figure 3, the alternative fuels contain oxygen atoms as noted previously. One might think that this leads to reduced soot tendencies. Indeed, this was observed in a prior engine cold start investigation by Hoppe et al.5 2.3. Optical Setup. Because optical measurements are conducted at high load, an endoscopic detection technique is required. The used imaging endoscope (LaVision GmbH) provides about 1.3 times more light than a conventional UV lens (f = 4.5, Nikon) at the same image magnification and at 313 nm as shown by Gessenhardt et al.46 The endoscope consists of two parts. The first part contains field lenses with an effective aperture of 8 mm and is mounted in the cylinder head by using a water-cooled SW shown in Figure 1. The second part contains hybrid relay optics, which is connected to the camera. Both main optical parts are not connected so that vibrations of the engine do not affect the camera system. The lens coating is optimized between 230 and 450 nm. The field of view is about 30 × 30 mm2 at a working distance of 35−42 mm. The current field of view and observed hardware components are depicted in Figure 4. As the endoscope is not resistant to cylinder pressure, an optical sealing tube is used, which is illustrated in Figure 1b. To avoid overheating (over 200 °C) of the field lenses, the optical sealing tube is water cooled. A digital high-speed camera (Photron SA1.1) is used. It provides a high frame rate with an acceptable spatial resolution. To enhance the sensitivity at the relevant wavelengths (∼308 nm), the camera is combined with a high-speed intensifier (LaVision GmbH, HS-IRO). This is necessary because the luminescence of the preignition early in the cycle is generally weak and at ultraviolet wavelengths as shown by Dahnz et al.26 The optical data are recorded at a frame rate of 18 kHz, which corresponds to a resolution of 0.5° CA at an engine speed of 1500 1/ min. The integration time of every single image essentially 5234


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Energy & Fuels engine’s volumetric efficiency is sensitively coupled to the internal cooling effect of liquid fuel vaporization during suction stroke. Thus, measurements of air mass flow, temperature, and pressure are used as a validation reference for the 1D gas exchange model. Second, the distribution of liquid fuel in the gas phase and on cylinder walls is analyzed based on 3D CFD simulations of gas exchange and compression, coupled with liquid fuel injection. Heat-transfer assumptions were made in the GT-Power model according to the model of Woschni, which was also investigated by Ž ák et al.48 These volume profiles are then incorporated into homogeneous batch reactor simulations as boundary condition. This procedure is analogous to a conventional approach to simulate variable pressure and temperature in RCM experiments by Sung and Curran49 and has been used in a similar way by Westbrook et al.50 to simulate knocking in an engine. Consequently, only a brief description will be given here. The procedure is based on the assumption of an adiabatic compression of a part of the unburnt fuel air mixture. Heat losses and other effects are accounted for by deducing the effective volume profiles from the measured pressure traces instead of using geometrical volume profiles. In essence, this procedure describes the compression of an unburnt part of the fuel/air mixture induced by the piston movement including heat losses and other effects in a quasi 0D simulation. The simulations are initialized at the time of intake valve closing assuming a stoichiometric mixture. The pressure and temperature evolution depends on the conditions at initialization, the prescribed effective volume profile, and the chemical energy release that is simulated. The autoignition event is defined as the time of steepest pressure increase because of chemical energy release, which correlates well with the maximum rise in temperature or OH*. A kinetic mechanism from the work by Cai and Pitsch51 is used for iso-octane, RON95E0, RON95E10, and ethanol. The mechanism for 2-butanone from the study by Burke et al.,52 for iso-butanol from Sarathy et al.,53 and for 2-methylfuran from Somers et al.54 is employed. The homogeneous batch reactor simulations are performed using Chemkin-PRO.55 The other investigated fuels are not simulated because of the lack of adequate kinetic mechanisms.

3. RESULTS AND DISCUSSION 3.1. Experimental Results. Raw data are discussed first. Typical single-cycle image sequences are shown in Figure 5 for engine operation with iso-octane (a) and 2-methylfuran (b). The sequences start shortly (0.5° CA corresponding to one frame) before the detected onset of preignition. Note that the first luminescence distribution found in the second frame of Figure 5a is substantially larger and more rugged than in the corresponding frame of sequence (b) and in the works by Hülser et al. and Pischinger et al.8,9 Thus, the location of the very first preignition kernel can only be estimated as described in Section 2.3 in the case of Figure 5a. Basically, this is attributed to the high velocity for the spread of the luminescence, which is observed in the following frames of panel (a). It is about 150 m/s, which is comparable to the ones previously measured in HCCI engines (∼100 m/s) by Dec et al., Hultqvist et al., and Thirouard et al.56−58 Seemingly, the mixture is prone to autoignition in the bulk gas phase in the case of Figure 5a. This also holds for the corresponding measurements with the conventional gasoline fuels (images are not shown for brevity). By contrast, the apparent flame propagation is substantially slower (∼7 m/s) in the case of 2methylfuran, that is, in Figure 5b. This is also observed for all the other alternative fuels (not shown) and similar to the flame speed of ∼5 m/s reported by Hülser et al.8 Thus, it seems to be rather conventional preignition for these fuels and conditions. Actually, preignition apparently commences at the spark plug in Figure 5b, so it is attributed to glow ignition in this example.

Figure 5. Two single-cycle luminescence image sequences starting shortly before the onset of preignition for iso-octane (a) and 2methylfuran (b). Measurement CAs given in the frames. Values of pin are 1.76 (a) and 2.98 bar (b).

By contrast, preignition is seemingly not triggered by any hot hardware component for the three rather conventional fuels (RON95E0, RON95E10, and iso-octane), as demonstrated more clearly below. Note that the striking difference in the appearance of the preignition for the two fuels in Figure 5 cannot be explained by charge-motion effects. Note that the same low-tumble engine configuration is used for all fuels. The resulting flow effect on preignition is weak as observed in Figure 5b and in the work by Hülser et al.8 In the following, optically detected preignition is discussed for all fuels. The preignition probability is inferred from the number of measured preignition events divided by the number of recorded engine cycles. Note that preignition is initiated by running the engine at critical operating conditions given in Table 2. For each fuel, pin is raised until the preignition probability is above 5%, unless the limit of the external boosting device is reached. The latter is the case for 2-butanol, 1-propanol, ethanol, and 2-butanone. The boosting device is limited to 3.65 bar, which is at a relatively high level, because it cannot be achieved by a turbocharger in a conventional gasoline engine. For certain other fuels (2-methylfuran, 3-methylbutanone, iso-propanol, and iso-butanol), the 5% preignition probability 5235


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Figure 6. Preignition probability for conventional fuels, furan, ketones, and ethanol (a) and for alcohols (b). Note the different scales of the subfigures. Arrows below the plots indicate CIP values.

Figure 7. Preignition ranking. Alternative fuels on the right side of the red line.

However, 2-butanone is most resistant against preignition. Its overall measured preignition probability is 0.017% while that of ethanol is 0.052% (equal pin ranges from 2.83 to 3.65 bar scanned for both fuels). Thus, 2-butanone is in the first place in the preignition ranking of the fuels illustrated in Figure 6. Similarly, ethanol and 2-butanol are distinguished, although their CIP values are almost equal based on significantly different preignition probabilities. For all of the other fuels, this ranking is based solely on CIP values, which are marked by vertical arrows below the plots in Figure 6. As shown in Figure 7, the apparent preignition resistances of 1-propanol and 2-butanol are essentially equal. This also holds for 3-methylbutanone and iso-propanol. Because of the low preignition probabilities found for the alternative fuels and the limited number of recorded engine cycles, the resulting uncertainty in CIP is relatively large. It is estimated to be generally about ±0.05 bar for these fuels (more fuel-specific values are given below). Thus, the preignition resistances of some of the candidates cannot be distinguished as noted above. However, for all of the other fuels, different preignition resistances are measured as suggested in Figure 7. For two of the rather conventional fuels (RON95E10 and iso-octane), the difference in CIP is smaller than ±0.05 bar. However, the preignition resistances of these two fuels can be distinguished because the uncertainty in CIP is significantly smaller (about ±0.005 bar) for the rather conventional fuels compared to that for the alternative ones. This is caused by the differences in the preignition probability response to pin variation for these two groups of fuels discussed above. Note that preignition probabilities increase significantly more strongly with increasing pin for the conventional fuels. Accordingly, the curves of the rather conventional fuels are generally less noisy than those of the alternative fuels as shown in Figure 6. Limited numbers of investigated engine cycles and low preignition probabilities lead to relatively poor repeatability of the data in Figure 6 for the alternative fuels. The repeatability

cannot be achieved because the peak pressures of three consecutive cycles exceed the engine shut-off limit of 200 bar. Thus, preignition probabilities above 5% are established only for the rather conventional fuels. Figure 6 shows the preignition probability of all investigated fuels as a function of pin. For clarity, the results of the alcohols are presented in a separate plot (panel b), and only nonvanishing probabilities are depicted. The latter implies that the scanned pin-ranges (not shown) are significantly larger than those covered by the depicted curves. Obviously, the preignition probability response to pin variation is different for the rather conventional fuels compared to that for the alternative candidates. The probability increases significantly more strongly with increasing pin with the rather conventional fuels, indicating engine operation very close to thermodynamically critical gas-phase conditions. This seems to confirm that autoignition in the bulk gas phase occurs for these fuels in contrast to the alternative ones. The fuels are categorized in terms of preignition resistance as follows. For each fuel, a specific pin-value is determined, which corresponds to 0.1% preignition probability, so that it reflects the reliably observed onset of preignition. It is denoted critical intake pressure (CIP). Because of the threshold of 0.1%, measurements with only one preignition event in a complete ensemble of 1228 engine cycles for one pin-value are neglected. This is justified by the finding that in these rare events, preignition generally commences very late in the cycle, that is, significantly ATDC, and the peak pressure of these preignition events is very low (not shown for brevity). Note that the CA50 used is at 45° CA for all fuels because of the knocking limitation of the conventional gasoline fuels. With an earlier spark timing (before 10° CA ATDC), it is highly expected that these preignition events can be avoided. Note that ethanol and 2-butanone are found to be essentially resistant against preignition up to the current pin limit of 3.65 bar. For these fuels, CIP is therefore assigned to 3.65 bar. 5236


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Figure 8. Locations of initial flame kernels of all detected preignition events for RON95E0 (a), RON95E10 (b), iso-octane (c), 2-methylfuran (d), 3-methylbutanone (e), iso-propanol (f), iso-butanol (g), 1-propanol (h), 2-butanol (i), ethanol (j), and 2-butanone (k). Order of panels corresponds to preignition ranking in Figure 7. Used pin ranges depicted in Figure 6.

and pin-values smaller than their CIP values are only about 0.01 bar. Importantly, Figure 7 shows that there is a large difference of at least ∼1 bar in CIP for the rather conventional fuels compared to that for the alternative ones. The latter have therefore the potential to improve engine efficiency in the future. 3.2. Optical Results. The locations of initial preignition kernels, which are detected by optical imaging, are illustrated in Figure 8 for all preignition events shown in Figure 6. The displayed hardware components are described in Figure 4. Each preignition kernel is represented by a colored spot in Figure 8. The number of shown preignition events is not equal for all fuels basically because of the limitations described above. Primarily, optical preignition site imaging yields information on the question whether ignition is caused by a hot surface (glow ignition). In this case, concentrations of preignition spots are expected at specific hardware components. In Figure 8, this is essentially only observed at the spark plug in panels (d) with 2-methylfuran, (g) with iso-butanol, and (h) with 1propanol, that is, for some of the alternative fuels. Basically, because of the higher pin-values used for the alternative fuels, the hardware components in the combustion chamber are likely hotter compared to the operating conditions with the rather conventional fuels. Indeed, the spark plug is likely the hottest hardware component (simulation not shown for brevity). In contrast to the work by Pischinger et al.,9 glow ignition at the spark plug is apparently avoided with the RON95E0 fuel by using a different spark plug as noted in the Research Engine section. However, more preignition sites are observed at the exhaust side (on the right side) for the rather conventional

is indicated by error bars in Figure 6. It is determined by splitting up each ensemble of 1228 cycles into four groups of 307 cycles and computation of the standard deviation of the preignition probabilities. For some of the data points in Figure 6, error bars cannot be seen because they are very small, indicating high repeatability. However, other error bars are relatively large, for example, for iso-butanol in Figure 6b. Thus, it is described in more detail in the following how the preignition resistance presented in Figure 7 can be inferred from Figure 6, in particular for iso-butanol. For this fuel, Figure 6b shows that the repeatability is poor at pin = 3.50 bar, but no preignition is consistently found for slightly lower pin-values starting at 3.45 bar (data not shown for clarity as noted above). Consequently, the CIP uncertainty is estimated to be about ±0.05 bar for iso-butanol. Figure 7 indicates that 3methylbutanone and iso-propanol have lower preignition resistances. This is justified by the sufficiently small CIP uncertainties of 3-methylbutanone and iso-propanol described in the following. For 3-methylbutanone, five measurements (with 1228 cycles each) are conducted with pin-values very close to its CIP (pin range from 3.27 to 3.29 bar). Thus, the CIP uncertainty is low for this fuel, that is, it is only about ±0.01 bar. For iso-propanol, Figure 6b shows that highly repeatable preignition events are found for pin at its CIP value, while this is not the case for lower pin-values (starting at 3.20 bar). Hence, the CIP uncertainty is estimated to be smaller than ±0.05 bar. Figure 7 also suggests that the preignition resistances of 1propanol and 2-butanol are higher compared to that of isobutanol. This is justified by the finding that no preignition events are consistently observed for pin < 3.6 bar for 1propanol and 2-butanol, while this is clearly different for isobutanol. The CIP uncertainty for 1-propanol and 2-butanol 5237


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Figure 9. CIP vs RON (a), laminar flame speed61−71 (b), specific enthalpy of vaporization (c), boiling temperature (d), and OI (e). Alcohols with apparent glow ignition are marked by red bordered diamonds. Data in (b) are for constant temperature (393 K), pressure (1 bar), and equivalence ratio (1). Correlation coefficients as a function of K in (f).

To the authors’ knowledge, the only prior study in which the preignition resistance of ethanol was higher than those of conventional fuels is the study by Pischinger et al.9 In the concerning part of that work, the effects of glow ignitions and fuel/oil droplets were reduced as described in the Research Engine section. This is further optimized in the present work as also noted in that section. Apparently, this leads to an even lower tendency of (unwanted) glow ignition and a significantly increased difference in CIP for ethanol compared to that for the conventional fuels, as shown in Figures 6−8. The current engine was previously also used to study preignition for a larger ensemble of fuels [ethanol, 2-butanone, 1-propanol, iso-butanol, PRF95 (primary reference fuel with a RON of 95), 2-methylfuran, iso-octane, 2-butanol, isopropanol, and RON95E10], which is similar to the current one in the work by Budak et al.10 However, because glow ignition was intentionally investigated in that prior work, a significantly different fuel ranking was found. One striking similarity is, however, that 2-butanone revealed the lowest glow-ignition tendency of the investigated fuels. Thus, the high preignition resistance of 2-butanone, which is currently observed as noted above, can be partly explained by its low glow-ignition tendency. 3.3. Effects of Fuel Properties. For all alternative fuels, there are preignition sites which are not concentrated at any specific location, as shown in Figure 8d−k. The origin of these preignition events is further investigated in the following. In Figure 9, possible correlations between CIP and RON in (a), laminar flame speed in (b), specific enthalpy of vaporization in (c), and boiling temperature in (d), respectively, are

fuels, as depicted in Figure 8a−c, presumably because of higher wall temperature and heat transfer into the concerning mixture. This could result in a ∼20 °C higher temperature of the mixture (simulation not shown for brevity). Overall, the smooth widespread distributions of preignition sites for rather conventional fuels seem to be consistent with the suggestion that autoignition occurs basically in the bulk gas phase. In a similar previous investigation by Hülser et al.,8 a much higher probability of preignition was detected with ethanol even at a lower level of pin (∼1.2 bar) most probably because of the injector used as explained in the Research Engine section. Accordingly, a wide distribution of preignition kernels throughout the whole combustion chamber was observed in that previous work. In contrast to the current distributions for conventional fuels discussed in the prior paragraph, preignition commenced predominantly on the intake side, which is expected to be relatively cold, in the previous work. This was presumably a result of fuel/oil droplets investigated by Palaveev et al.24 Similarly, in other prior studies, ethanol resulted in increased preignition probability compared to conventional gasoline fuels shown in the work by Haenel et al., Kocsis et al., Hamilton et al., and Budak et al.10,15,59,60 In the study by Haenel et al.,59 the reason for this behavior was also assumed to be fuel/oil droplets. By contrast, in the studies by Hamilton et al. and Budak et al.,10,15 the discussed finding was explained by a low glow-ignition resistance of ethanol because preignition was intentionally initiated by glow ignition in these works. This seems to be not relevant in the present work because glow ignition does apparently not occur for ethanol as noted above. 5238


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Figure 10. Simulated autoignition temperatures at intake valve closing (red) and temperatures at start of compression from 1D simulation (gray).

preignition is not always induced by a heated hardware component as noted above. Obviously, the three fuels with apparent glow ignitions given above do not lead to a significant correlation. This seems to confirm that glow ignition plays a minor role in the present study. However, the avoidance of glow ignition presumably contributes to very high observed CIP values for some of the fuels, as suggested for 2-butanone above. The avoidance of glow ignition is presumably also one reason why the alcohol fuels have similar CIP values, although they have different glow-ignition tendencies as observed in this work by optical imaging and in the study by Budak et al.10 The RON and enthalpy of vaporization values of the neat alcohol fuels are rather similar. According to the study by Thewes et al.,7 with higher boiling temperature an increased fuel wall impingement is expected. This can result in lower preignition resistance because of fuel/ oil droplet effects as shown by Zahdeh et al.16 On the basis of Figure 9d, it may therefore be assumed that this mixture formation issue is currently less important as suggested above. The octane index (OI) was defined, for instance, in the article by Kalghatgi72 as follows

considered to understand the effect of fuel properties. In panels (a,c), black and blue lines show linear regression for all of the fuels excluding and including, respectively, the alcohols. Strong correlations are observed in Figure 8a,c, only when the alcohols are not considered, as indicated by the given R2-values which are larger than 0.9. If the alcohols are included, weaker correlations are found (R2 > 0.5). However, in both cases, the current correlations between RON and preignition resistance are significantly stronger than those between RON and preignition tendency observed previously by Kalghatgi.22 Presumably, this can be explained by the fact that only glow ignition was considered in these prior studies, whereas it is largely avoided in the present work. The current finding that CIP increases generally with increasing RON seems to confirm that autoignition in the bulk gas phase plays an important role in contrast to prior work. For instance, the currently observed high preignition resistance of 2-butanone can be presumably partly explained by its very high RON value. In Figure 9c, the specific enthalpy of vaporization is considered because charge cooling expectedly leads to reduced reactivity of the mixture. This seems to be indeed an important effect because CIP increases generally with increasing enthalpy of vaporization, as shown in that figure. Therefore, the effects of RON and specific enthalpy of vaporization on preignition are superposed. This may be the reason why correlations are poorer when the alcohols are also considered in Figure 9a−c as noted above. For instance, the high CIP of ethanol is likely partly caused by the high enthalpy of vaporization (see Table 3). This may also be a reason for the higher preignition resistance of the conventional fuel with the nonvanishing ethanol content (RON95E10) compared to the other type of gasoline (RON95E0), which is observed in Figures 6 and 7. Note that the RON method incorporates enthalpy of vaporization because the test fixes the air temperature not the mixture temperature as shown by Bauer et al. and in the DIN.23,42 However, this effect is expected to be weaker than in the current study. Note that direct fuel injection is currently used in contrast to the RON method. Overall, CIP seems to be not solely determined by enthalpy of vaporization. For instance, 2-butanone has the highest CIP value, although its enthalpy of vaporization is considerably smaller than those of the investigated alcohols. Also note that the enthalpy of vaporization can have a negative effect on CIP if it leads to cylinder wall wetting and droplet formation as shown by Dahnz and Spicher.11 Essentially, no correlations are found in Figure 9b,d. In contrast, a strong correlation between preignition resistance and laminar flame speed was reported in the work by Kalghatgi,22 in which glow-ignition effects were investigated. In Figure 9b, no correlation appears, presumably because

[OI] = (1 − K ) ·RON + K ·MON

(1)

where K is a constant depending only on pressure and temperature variation in the engine. In the present data set, a K value of −0.6 gives the best correlation between CIP and OI. This is illustrated in Figure 9e,f. In panel (e), the correlation between CIP and OI for K = −0.6 is presented, while panel (f) shows correlation coefficients (R2) as a function of K. Negative K values have been previously found in most cases for modern engines as reviewed by Kalghatgi.72 3.4. Simulation Results. In order to evaluate whether the origin of preignition results from pure gas-phase kinetics, simplified engine simulations are performed. The results are depicted in Figure 10. The predicted mean temperature in the engine and the temperature which is needed for autoignition in the gas phase are displayed in gray and red colors, respectively. Obviously, both temperatures are very similar for RON95E0, RON95E10, and iso-octane. For these fuels, ignition may take place in the bulk gas phase as suggested above. For the other simulated fuels, namely, 2-methylfuran, isobutanol, ethanol, and 2-butanone, the predicted temperatures deviate significantly, that is, higher temperature levels are required for gas-phase ignition than present during engine experiments. This indicates that autoignition of the bulk fuel/ air mixture in the gas phase does not occur for these alternative fuels. Indeed, glow ignition is apparently detected for 2methylfuran and iso-butanol as stated above. 5239


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Energy & Fuels The root cause for this behavior might be in the different ignition behavior of the fuels. The conventional fuels all show a so-called negative temperature coefficient behavior shown by Cai and Pitsch.51 Thus, they show a relatively high reactivity over a wide temperature regime. In contrast, the alternative fuels only show Arrhenius-type ignition behavior as shown by Burke et al., Sarathy et al., and Somers et al.,52−54 resulting in very low reactivity at engine relevant conditions. Consequently, a gas-phase autoignition is unlikely for these fuels. Note that another reason for preignition can appear for all of the simulated alternative fuels, namely, fuel/oil droplets. This also seems to be the case for the other alternative fuels which are not simulated because of their high RON values.

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4. Indeed, optical preignition site imaging indicates that glow ignition at the spark plug occurs for 2-methylfuran, iso-butanol, and 1-propanol. This is most likely one reason for the finding that 2-methylfuran reveals the lowest preignition resistance of the alternative fuels.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Tamara Ottenwälder: 0000-0002-3299-966X Notes

The authors declare no competing financial interest.

■

4. SUMMARY AND CONCLUSIONS The probability and origin of preignition are investigated by endoscopic high-speed imaging in a boosted DI SI engine at high load and low speed. Three rather conventional fuels, namely, RON95E0, RON95E10, and iso-octane, as well as eight biofuels, namely, 2-methylfuran, 2-butanone, 3-methylbutanone, ethanol, iso-propanol, 1-propanol, 2-butanol, and iso-butanol, are employed. Major conclusions are given in the following. 1. With rather conventional fuels, autoignition in the bulk gas phase appears to be the dominant reason for preignition, as indicated by both optical imaging and kinetic simulations. Apparently, preignition is not always induced by particles or glow ignition in this engine in contrast to many previous works. Presumably, droplet effects are avoided by an optimized very narrow injector targeting. 2. All used alternative fuels have significantly increased resistance to preignition compared to the rather conventional fuels. Thus, future DI SI engines (with higher compression ratio) may be more efficient with the alternative fuels. Presumably, this benefit would not be negated by the knock resistance because it is expected to be higher for the alternative fuels compared to that for the conventional ones according to the corresponding RON values. In particular, preignition can be avoided almost completely even with very high pin-values (up to 3.65 bar) with 2-butanone. Similarly, the current alcohols and 3-methylbutanone are highly preignition resistant. It should be noted that increasing RON leads to enhanced preignition resistance only if classical reasons for preignition such as droplets and glow ignitions are less important. 3. RON and specific enthalpy of vaporization appear to be the most important fuel properties with regard to preignition resistance. The effect of the enthalpy of vaporization indicates that charge cooling by DI is important. By contrast, laminar flame speed and boiling temperature appear to be rather unimportant properties, presumably because effects of glow ignitions and liner wetting are reduced. However, simulations indicate that autoignition of the bulk mixture in the gas phase cannot occur for the alternative fuels because of their low reactivity. Thus, preignition seems to be initiated by classical reasons for preignition in this case, such as particles (including droplets) and glow ignitions, in contrast to the rather conventional fuels. This is also consistent with optical preignition imaging results.

ACKNOWLEDGMENTS This work was performed as part of the Cluster of Excellence “Tailor-Made Fuels from Biomass”, which is funded by the Excellence Initiative by the German federal and state governments to promote science and research at German universities. It was also funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany’s Excellence StrategyExzellenzcluster 2186 “The Fuel Science Center” ID: 390919832. The authors thank technicians R. Herwig and M. Rombach for their assistance.

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