Cetane-Improver Analysis and Impact of Activation Energy on the

Performance data are reported for 2-ethylhexyl nitrate (EHN) and tetraethylene glycol dinitrate (TEGDN). The data show that inconsistencies in reporte...
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Ind. Eng. Chem. Res. 1997, 36, 4397-4404

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Cetane-Improver Analysis and Impact of Activation Energy on the Relative Performance of 2-Ethylhexyl Nitrate and Tetraethylene Glycol Dinitrate G. J. Suppes,* Y. Rui, A. C. Rome, and Z. Chen Department of Chemical and Petroleum Engineering, 4006 Learned, Lawrence, Kansas 66045-2223

A testing procedure has been developed to evaluate the effectiveness of cetane improvers for improving the ignition properties of diesel fuels. Among the factors considered in developing this procedure were the availability, characterization, and toxicity of the base fuel, the ease of measurement of the data, and the sensitivity of the analysis. This particular method has several advantages over previously used methods and is suitable for obtaining fundamental information on performance trends. Performance data are reported for 2-ethylhexyl nitrate (EHN) and tetraethylene glycol dinitrate (TEGDN). The data show that inconsistencies in reported performances for these cetane improvers can be explained by different apparent activation energies for the respective ignition processes as well as the lower solubility of TEGDN. Introduction Cetane improvers are used to enhance the performance of diesel in a manner similar to the way ethanol improves the octane of gasoline. The higher cetane numbers of the diesel fuels lead to several improvements in diesel performance, including improved cold start, reduced hydrocarbon emissions, smoother engine operation, and reduced NOx formation (Robbins et al, 1951; Suppes et al., 1996a). These improvements in performance make cetane improvers valuable fuel additives that are expected to play an increasing role in the development of improved methods to reduce dieselengine emissions. Cetane improvers can also be added to alternative fuels such as ethanol, thereby allowing these fuels to be used in diesel engines at fuel economies which are 20-50% better than in spark-ignition engines. In the past, the economics of treating ethanol with the required, large amounts of cetane improvers (e.g., 4% TEGDN) have curtailed further research efforts in this area. Improved knowledge of how cetane improvers work and the trade-offs between cetane-improver addition and fuel properties may allow development obstacles to be overcome. Such advances are particularly important as new emphasis is being placed on compression-ignition direct-injection research as recommended by the auto industry and the National Research Council’s peer review of the Partnership for a New Generation of Vehiclessa current nationwide program to revitalize the American auto industry (Patil, 1997). A fuel’s cetane rating is directly related to the temperature where an atomized fuel spray will ignite at 2.4 ms. The lower this temperature, the higher the cetane rating. The time (e.g., 2.4 ms) between the fuel’s initial injection and its ignition is referred to as the ignition delay time. The ignition delay time is typically determined by following a pressure pulse; however, other methods such as the onset of a flame’s luminescence have also been used to determine related delay times (Hoskin et al., 1992). In practice, ASTM standard D613 defines a combination of engine speed and injection timing (corresponding to an ignition delay time of 2.4 ms) which is used to compare test fuels to reference fuels in a test engine. * Phone: (913) 864-3864. Fax: (913) 864-4967. Email: [email protected]. S0888-5885(97)00228-5 CCC: $14.00

Cetane improvers are fuel additives which are added to diesel at concentrations usually less than 1% (typically 0.1-0.25%) and decrease the ignition delay time by directly or indirectly effecting the free-radical ignition process. Treatment at 0.1-0.25% will typically increase the cetane number of No. 2 diesel fuels by an average of 6 cetane numbers. Reported or hypothesized mechanisms (Clothier et al., 1993a; Suppes et al., 1996a) include (1) the decomposition of cetane improvers into free radicals which react with the fuel-air mixture, (2) the decomposition of cetane improvers into gas-phase catalysts such as NO2, and (3) reactions between cetane improvers and free-radical inhibitors found in the fuel (i.e., reactions which inhibit free-radical scavangers found in the fuel). The most concise method for reporting cetaneimprover performance is the blending cetane number (BCN). The BCN can be approximated as the increase in the cetane number divided by the mass fraction of cetane improver added to achieve this increase. A disadvantage of the BCN is that it is concentration dependent. For example, the BCN can decrease from 4000 to less than 1000 as the treatment goes from 0.1% to 4% for applications with diesel and ethanol, respectively. The following highlights summarize the observed trends in cetane improver performance: (1) BCNs generally increase with increasing carbon number for many classes of compounds. (2) Nitrates and peroxides are the most effective cetane improvers. (3) The performance of alkyl nitrates as cetane improvers is directly related to their molecular architecture. The abilities to relate performance to molecular architecture are limited but useful. (4) Peroxides are primarily effective if natural ignition inhibitors exist in the base fuel, so nitrates are generally better cetane improvers than peroxides. (5) Trends of increasing BCNs with increasing carbon number are prevalent with ethylene glycol dinitrates, while the trends are essentially nonexistent with alkyl nitrates. Although this summary illustrates that industry has a working knowledge of cetane improvers, a lack of available data and inconsistencies in reported performances of some of the more powerful nitrate cetane © 1997 American Chemical Society

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improvers have limited advances in (1) producing lower cost cetane improvers, (2) reducing the impact of diminishing returns at high cetane-improver loadings, and (3) using combinations of cetane improvers to their best advantage. Many of the inconsistencies can be explained due to sulfur-based inhibitors present in fuels and the impact of loading on BCN. Others can be explained by concentration dependencies. However, even these observations are qualitative. To build a science behind the efficacy of cetane improvers, more and better data are needed. The purpose of the presently described testing procedure is to provide accurate performance data in a controlled environment. The data provide the opportunity for conventional cetane improvers (e.g., EHN) as well as alternative cetane improvers to be better understood and to be used more effectively. EHN (2ethylhexyl nitrate) was chosen for study since it is the most popular, cost-effective cetane improver for today’s No. 2 diesel fuel applications; TEGDN (tetraethylene glycol dinitrate) was chosen for study since it is one of the few cetane improvers which has been reported to be significantly more effective than EHN. Background History and Use of Cetane Improvers. Historically, diesel distillates (petroleum with a boiling range from 160 to 330 °C) have had paraffin contents approaching 40 wt % (Clothier et al., 1993a). The paraffins provide ignition at lower temperatures and an inherent, high cetane number for these distillates. Poorer crude oils as well as oils produced from shale and tar sands have lower paraffin contents which results in diesel distillates with lower cetane numbers. The preferred method to improve the cetane number for these later distillates is to add specialty chemicals known as cetane improvers. Engine responses from chemical cetane numbers (cetane number achieved with additives) are essentially the same as responses from natural cetane numbers (Olree and Lenane, 1984). The most comprehensive set of published, original data dates back to Robbins et al. (1951). Robbins et al. presents a summary of the effectiveness of nitrates, peroxides, amines, acetates, alkyl halides, ethers, and sulfides for improving the ignition of aircraft diesel fuels. This work was sponsored by the U.S. Naval Engineering Experiment Station in 1936 to develop technology to ensure smooth engine performance with the cold, rarefied air breathed by lighter-than-air craft. Cetane improvers allow compression ignition fuels to ignite at lower temperatures and facilitated the desired smooth engine operation. Since 1960, cetane improvers have found increasing utilization in diesel fuels due to both increasing demands for diesel relative to other petroleum products and increased processing of heavier crude. The neat cetane number (CN) of diesel has decreased from about 50 CN in 1962 to 45 CN in 1982 (Olree and Lenane, 1984). Today, much of the diesel cannot meet the 45 CN requirements and is piped at >40 CN specifications. Even with relatively low pipeline specifications of >40 and >45 CN, more and more cetane improver is being used prior to piping. Addition of cetane improvers to diesel out of the pipeline by local distributors is commonplace. To augment decreasing qualities of crude oil and increasing demand for diesel relative to gasoline (more distillates that would traditionally be converted to

gasoline are converted to diesel), the industry is also leaning toward increasing CN requirements. Several researchers (Beatrice et al., 1996; Ladommatos et al., 1996; Bertoli et al., 1993) have established correlations between increased CN and reduced hydrocarbon emissions. Based on conversations at the SAE meeting in San Antonio (October 1996), the consensus is that engine manufacturers are pushing for increased CN requirements to assist them in manufacturing engines which can be cleaner burning and meet Clean Air Act requirements. For these reasons, the demand for cetane improvers is expected to increase more than other chemicals of the fuel industry. Hardenberg (Hardenberg and Ehnert, 1981; Schaefer and Hardenberg, 1981) evaluated the use of cetane improvers with ethanol and methanol as a means to promote the use of these alternative fuels in diesel engines. This was driven primarily by applications in Brazil, South Africa, and New Zealand to replace petroleum products that had increased markedly in cost. Since compression-ignition engine efficiencies are typically 20-50% better than spark-ignition engine efficiencies, this technology provided an opportunity to improve the economic viability of alcohol fuels. At the time of this study, high cetane-improver treatment rates increased the cost of the alcohol fuels by greater than 20% and did not offer economic advantages over sparkignition applications. In the early 1990s, ICI marketed AVOCET for use with methanol in the U.S. (Betton, 1991; Unnasch et al., 1993; Dunlap et al, 1993; Wuebben et al., 1990) and with ethanol in Sweden (Proceedings of the Tenth International Symposium on Alcohol Fuels, 1993). If lower cost cetane improvers were available, the use of ethanol in diesel engines would increase and eventually be preferred over the use of ethanol in gasoline engines. Although the composition of AVOCET is not published, its performance patterns are similar to TEGDN. Pritchard (Clothier et al., 1993a,b, 1995) has performed more fundamental work on how cetane improvers improve diesel ignition and combustion. This and other works (Hinkamp and Hanlon, 1984; Poirier, 1996) looked at synergistic combinations of cetane-improvers to reduce the overall costs of the cetane improver packages. This work appears to have been motivated by a continued urgency to compensate for low paraffin contents in cracked gas oil from lower quality crudes and from fuels produced from shale and tar sands. Factors Impacting the Accuracy of Analytical Techniques. Although cetane improvers are in common use, inconsistencies in reported performances have hindered efforts to advance the science. A significant portion of the inconsistencies can be attributed to the multitude of factors that can impact experimental results as well as the multitude of experimental methods. The performance of cetane improvers is commonly evaluated by using the ASTM D613 procedure to determine the CN of a fuel with (fuel blend) and without (neat fuel) the addition of the cetane improver. One method of reporting the performance is to report the volume fraction of cetane improver which is required to achieve a specified CN (e.g., the volume fraction in ethanol required to produce a CN of 50) (Betton, 1991; Schaefer and Hardenberg, 1981). Alternatively and preferred is to report a blending cetane number (BCN) as implicitly defined by (Pecci et al., 1991; Stournas et al., 1995)

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CNfuel blend ) (1 - x)CNneat fuel + xBCN

(1)

where x is the mass fraction of cetane improver added to the fuel blend and BCN is the blending cetane number of the cetane improver. Typical cetane-improver applications result in CN increases from 3 to 6 by adding 0.05-0.25 wt % cetane improver. The recommended reporting interval for CN is to the nearest integer, a relatively low accuracy but one which is supported by Russell (1989), who reports a CN reproducibility varying from 0.6 to 0.9. Accordingly, a reported increase in a cetane number by 3 may actually be an increase of as low as 1.22 or as high as 5.78 in the extreme case. Even in well-performed studies including several measurements for both fuels, a reported CN increase of 4 may actually be as low as 3.5 or as high as 4.5. Compounding the error in the actual method itself, the chemical composition of the fuel can also impact the performance of the cetane improver. For example, Clothier et al. (1993a) report that certain sulfurcontaining compounds can interfere with the ignition process and nitrate-based cetane improvers may not be as effective when in the presence of these compounds. However, hydroperoxides are primarily effective as cetane improvers via neutralizing these sulfur-containing inhibitors. If the fuel does not contain these inhibitors, the hydoperoxides can be ineffective. Both a lack of sufficient accuracy of the method as well as inconsistencies in randomly selected base fuels can lead to errors in estimating the blending cetane numbers. Suppes et al. (1996a) compared CN correlations with ignition delay times as derived by different researchers and found significant variations in these correlations. In some cases, the results were of different magnitudes and delay time data ranges did not even overlap when compared at the same temperatures. Based on these results, a relation between the cetane number and ignition delay times which would apply to the spectrum of currently used equipment seems unreasonable; however, good results have been demonstrated by comparing a test fuel’s performance to the performance of reference fuels on the same experimental apparatus. Reference fuels such as U-13, T-20, and their mixtures (Phillips 66 Company) are available over a range of cetane numbers. Possible reasons for the range of ignition delay times reported from different constant-volume combustors include variances due to (1) different air or simulated air compositions, (2) temperature gradients in the combustors, (3) different empirical relations to convert ignition delay times to cetane numbers, (4) different fuel:air stoichiometries, and (5) different pressures used by the combustors. For the present method, air is used to minimize the errors that may be associated with varying simulated air compositions. Temperature gradients are minimized by the location of the heating elements. To develop the most sensitive procedure, correlations are performed based on temperatures with ignition delay times of 2.5 ms. This typically requires extrapolation of data; however, based on the ASTM D-613 standard, it is fundamentally correct. A calibration curve of CN vs temperature (with an ignition delay time of 2.5 ms) would be used to convert measured ignition delay times to cetane numbers. Correlations based on constant ignition delay times would be preferred over

correlations based on measurements at one temperature since comparison at a constant ignition delay time is fundamentally similar to the ASTM standard. The method development discussed in this paper evaluated the impact of fuel:air stoichiometries and air pressure on ignition delay times. One purpose of these studies was to identify the impact of normal variances in air pressure and fuel:air stoichiometries on the repeatability of the data. In addition, a test fuel was identified which is readily available, well-characterized (free of potential inhibitors), nontoxic, and a liquid at typical handling conditions. Constant-Volume Combustor Analysis. An alternative to using test engines, ignition delay times can be measured and used to estimate CNs (Allard et al., 1996; Ryan and Callahan, 1988; Ryan and Stapper, 1987). Based on ASTM D613, the fuel must ignite in 13° at 900 rpm. This translates to ignition at 2.41 ms. During this 13° rotation, the cylinder volume is reduced by about 2.5%sconstant volume and constant temperature in practical terms. Several researchers have used constant-volume combustors to simulate diesel engines during the ignition process (Ryan and Callahan, 1988; Ryan and Stapper, 1987; Siebers and Dyer, 1986; Hoskin et al., 1992). Ryan et al. typically refer to their combustor as an IQT (ignition-quality tester), while others have referred to the combustor as a CVA (constant-volume apparatus). The primary differences between constant-volume combustor and diesel engine environments are (1) different turbulence, (2) potentially different air compositions, and (3) the absence of active radicals which may be present in engine environments (Pucher et al., 1996). The gas’s turbulence may impact combustion, but it has minimal impact on ignition since the turbulence generated by the charge front is much more apparent to the igniting fuel, so differences in turbulence are not a significant concern for diesel fuels injected with typical diesel-fuel injectors. The varying air compositions result from methods using precombustion to provide the high temperatures for combustion. Simulated air compositions can vary from system to system, but this problem can be avoided by using constant-volume combustors that do not use a hydrogen preignition. For systems not using hydrogen preignition, air of uniform composition can be used as the ignition environment. The impact of active radicals may be a variance between combustors and engines; however, this is a phenomenon which also varies considerably from engine to engine. As such, the influence of active radicals on constant-volume combustors is neither an attribute nor a deficit of testing methods using constant-volume combustors. In addition to the differences between test engines and constant-volume combustors, ignition delay time results may also vary from combustor to combustor; therefore, calibration of a constant-volume combustor with several reference fuels is reocommended. Ryan and co-workers (Allard et al., 1996) have developed a constant-volume combustor for measuring CNs from 33 to 58 with a repeatability of 0.30. In general, constantvolume combustors are valuable and effective for estimating cetane numbers; they can provide more information and insight into fuel performance than cetane engines. Figure 1 shows a typical pressure pulse measured by a constant-volume combustor. As indicated, the ignition delay time is the time from the start of injection to the point where the pressure has increased to at least 10

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Figure 3. Constant-volume combustor without hydrogen pilot ignition. The internal dimensions of the combustor section are 2.5cm i.d. by 10-cm length. Three ports allow temperature and pressure measurement as well as an inlet and exhaust port. Figure 1. Pressure pulse generated by igniting fuel in constantvolume combustor.

Due to these same reasons, lower standard deviations in data with ignition delay times less than 1.5 ms do not translate to an increased utility of these data since changes in the ignition delay times become considerably less sensitive to increases in temperature. Ideally, highly accurate ignition delay times between 1.5 and 10 ms would provide the best insight into a fuel’s performance. In laboratory practice, ignition delay times between 1.5 and 100 ms provide significant insight into the fuels performance and potential and are generally sufficient. Experimental System and Procedure

Figure 2. Ignition delay times vs temperature for a 25:75 (mass: mass) mixture of ethanol to hexanes with a constant initial pressure of 200 psig. Each point is an average of six data points. Indicated error bars represent 1 standard deviation as estimated by at least six data points at each temperature.

psia above the baseline pressure. The time of injection is taken as 0.8 ms before the decrease in the pressure due to evaporation of the fuel, which is consistent with reported physical delays associated with the physical processes required for ignition (Hoskin et al., 1992). By evaluating the ignition delay time at several temperatures (Figure 2), a plot of ignition delay time vs temperature can be used to characterize ignition over a range of possible engine conditions. As illustrated by Figure 2, the log of ignition delay time is approximately linear with reciprocal temperature, thereby allowing extrapolation of the data to delay times of 2.41 ms when estimating cetane numbers. The slope of the line is the apparent activation energy for the ignition process (Ryan and Callahan, 1988). Test engines are not able to provide apparent activation energies. The data’s standard deviation decreases considerably (both in magnitude and as a fraction of the value) as the ignition delay times decrease to about 1.5 ms. This could be due to nonhomogenaities in the system (e.g., hot spots, deposits generating active radicals, or metal catalysis) which tend to have negligible effects at times less that 10 msschaotic variances in ignition at longer delay times are significant. With the exception of gasphase fuels (e.g., methane), liquid-phase fuels tend not to ignite at delay times less than about 1.2 ms due to the physical delay associated with fuel evaporation (Fraser et al., 1991). Consequently, ignition delay time data cannot be accurately extrapolated below 1.5 ms.

Figure 3 provides a schematic diagram of the constantvolume combustor used for the present investigation. The combustor does not use a hydrogen pilot combustion; rather, a feedback PID control system heats the combustor section and contents to temperatures > 800 K via two cartridge heaters (400 W each) and three band heaters (250 W each) around the combustor section (not indicated by Figure 3). The constant-volume combustor is centered around a MUI 5C60 Diesel Technology injector which is commonly used in Detroit Diesel 453T engines. In place of an engine’s rotating cam, a hydraulically activated push rod is used to activate the injector (Suppes et al., 1995). As indicated by Figure 3, the portion of the injector closest to the tip is cooled by circulating cooling water. This cooling water in addition to fuel circulated in the injector maintains the test fuel temperature below 80 °C. Special tips were manufactured for the injector to provide an axial fuel spray as compared to the standard radial spray. The tips contain a single orefice with an i.d. of 0.5-0.7 mm. Atomization through this relatively large orifice is assisted by a needle that controls the start of injection. A Kistler 601B1 piezoelectric pressure transducer, Kistler Type 5010 amplifier, and National Instruments high-speed data acquisition provide data acquisition up to 10 000 Hz. A 228P water jacket is used to maintain transducer temperatures below 260 °C, while the diaphragm is contacting gases at temperatures in excess of 500 °C. Compressed air is purchased and introduced into the system via a gas regulator set up to >400 psig. Approximately 3 min is allowed from the time air is introduced into the combustor until the injector is activated. During injection, a high-speed data-acquisition board mounted in a 486 computer activates a high-speed solenoid valve that routes 1800 psig of hydraulic oil behind a piston which contacts the push rod used to

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activate the injector. The electrical signal to the solenoid valve is monitored to determine the time when the injector is activated. Prior to introducing the fuel into the fuel tank, lines, and injector, methanol is first circulated and than drained from the fuel-handling system. Air is then used to purge the system for about 30 s. Approximately 125 g of fuel is loaded into a fuel tank that is connected to the injector through a gear pump and via 0.25-in. 316 stainless steel and brass tubing. After loading the new fuel, at least 10 injections are performed and disregarded to purge any fuel that may be remaining from previous tests between the plunger and tip of the injector. Typically, six or more injections are performed near temperatures of 700, 750, and 800 K. Some earlier work used temperatures of 670, 710, and 740 K due to temperature limitations. The data points near each of these temperatures are then averaged to produce three data points that are plotted on a graph such as Figure 2. During this investigation, the ignition delay times of several fuel blends containing ethanol, hexanes, and cetane improvers were evaluated. The ethanol was >99.9% ethanol. The HPLC-grade hexanes (98.5+%) and 2-ethylhexyl nitrate (EHN, 97%) were purchased from Aldrich. Tetraethylene glycol dinitrate (TEGDN) was synthesized by nitrating tetraehylene glycol (99%), which was purchased from Aldrich. The purity of TEGDN was >95% as determined by NMR spectroscopy.

Figure 4. Comparison of time delays for 100:0, 75:25, 50:50, and 25:75 mass ratios of ethanol and hexanes.

Experimental Results Experiments were designed to identify a nontoxic base fuel in which cetane improvers could be tested. EHN was then tested in the base fuel at several compositions. Finally, experimental parameters such as system pressure and fuel-to-air stoichiometry were evaluated to determine the potential impact of day-to-day variances of these parameters on the sensitivity of the method. The performances of two known cetane improvers (EHN and TEGDN) were then compared. The primary constraint on the base fuel was that it be a liquid with a cetane number between 35 and 45 to allow the impact of the cetane improver to be easily detected. Saturated hydrocarbons such as pentane and butane have the desired cetane number but were considered too volatile. 1-Octene and 1-heptene were ruled out since they are irritants and possibly toxic. Suitable properties were ultimately identified with mixtures of hexanes (CN ) 44.8 for n-hexane (Ryan and Stapper, 1987)) and ethanol (CN ) 10). Figure 4 shows the ignition delay times vs ignition temperatures that were obtained for 100:0, 75:25, 50: 50, 25:75, and 0:100 mass ratios of ethanol to hexane. Neither the cetane number nor the ignition delay times are expected to be linear with composition; however, the ignition delay times should continuously decrease with increase concentrations of hexanes. Figure 4 shows this anticipated behavior as well as a good consistency of the data. A relatively small decrease in activation energy is noted for the mixtures with higher hexane contents. Based on the results of Figure 4, the 50:50 and 25:75 mixtures were further evaluated with cetane-improver additives. The sensitivity of these two base fuels to fuel additives was evaluated with 0.5-10% EHN as the cetane improver. Figures 5 and 6 show the results of

Figure 5. Impact of EHN on ignition of a 50:50 wt % mixture of ethanol and hexanes.

Figure 6. Effect of EHN on ignition of a 25:75 wt % mixture of ethanol and hexanes. The symbols correspond to different data sets. The stars are at 0.5% EHN, and the × is a repeat of the 0% EHN data.

adding EHN to 50:50 and 25:75 mixtures. The preferred method would compare fuels at ignition delay times between 1.5 and 100 ms and most preferably between 1.5 and 10 ms since standard deviation is lower in these ranges and since ASTM standard D613 is defined in terms of a 2.41-ms ignition delay. As illustrated by Figure 6, the 25:75 mixture clearly meets this criterion better than the 50:50 mixture. The results of Figure 6 illustrate how the 25:75 base fuel is sensitive to the concentration of EHN.

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Figure 7. Comparison of EHN and TEGDN cetane improvers in a 25:75 mass ratio mixture of ethanol and hexanes.

Figure 9. Efect of the fuel-to-air ratio in a constant-volume combustion chamber on the time delay of the system. Temperature was held constant at 701 K for a 25:75 mixture of ethanol to hexanes.

Figure 10. Effect of initial pressure in the combustor on the ignition time delay of the system. Temperature was held constant at 701 K for a 25:75 mixture of ethanol to hexanes. Figure 8. Comparison of EHN and TEGDN cetane improvers in No. 2 diesel fuel.

The performance of EHN was than compared to TEGDN as illustrated by Figure 7. These data show that TEGDN has a lower ignition delay time than EHN at similar application rates. This comparison was repeated in diesel fuel to determine if the results of the 25:75 base fuel as developed specifically for cetaneimprover evaluation could be extrapolated to actual performance in No. 2 diesel fuel. Figure 8 illustrates the impact of these cetane improvers in a diesel fuel with an initial cetane number of approximately 42. Finally, to complete the analysis and development of this procedure, the sensitivity of the results to select experimental procedures was evaluated. The initial combustor pressure and the volume of fuel injection are two parameters that can be easily adjusted during analysis. The pressure is adjusted by the pressure regulator that is used to direct compressed air into the combustor at the onset of the procedure. The volume of fuel injection is changed by adjusting the rack on the Diesel Technology 5C60 injector. Both parameters have direct impacts on the fuel-to-air ratio. Prior to measuring the ignition delay times, the volume of fuel injected was compared to the rack position by accumulating and weighing the volume of fuel injected/collected during approximately 10 injections. A calibration curve was prepared and used to preset volumes of fuel for injection. The influence of the fuel-to-air ratio is shown in Figure 9. The ignition

delay times tend to be insensitive to the setting of the rack and corresponding volume of fuel injected. To confirm that the volume of fuel was changing, the pressure pulses were evaluated to detect trends which would indicate that different volumes of fuel were actually being injected and ignited. The pressure pulses showed significant increases in maximum pressure increases as the injection volume increased. The ignition delay times are relatively insensitive to the rack setting and the fuel-to-air ratio. Figure 10 illustrates the impact of initial air pressure on the ignition delay time. This is the pressure of the air entering the combustor (the valve is open about 3 s). The valve is then closed, and about 3 min is allowed for temperature equilibrium to be established. The sensitivity to initial air pressure is evident even on a logarithmic scale for ignition delay time. The data offer insight into the impact of air density on the ignition process and suggest care should be taken to make sure the initial air pressure is repeatably loaded to the desired pressure. A pressure of 200 psig was used for all procedures except those used to obtain Figure 10. Discussion The 25:75 mass ratio mixture of ethanol and hexanes was tested as a base fuel to evaluate cetane improvers and was superior to other blends due to sensitivity. The blend was superior to other base fuels due to its suitable volatility and low toxicity. The 25:75 mixture allowed

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ignition improvers to be added with resulting ignition delay times in the desired 1.5-10-ms range. Extrapolation of the data suggests that analysis of straight hexanes would also work well for some applications. The 25:75 mixture also exhibits solubility with a wide range of cetane improvers. As shown by Figures 5-8, the additives decrease the ignition delay times and result in increased cetane numbers. These Arrhenius-type plots allow apparent activation energies for the fuel mixtures to be readily determined. TEGDN-treated fuels have a significantly higher apparent activation energy than EHN. This feature describes the inconsistencies of performence which have been reported in the literature. For example, Robbins et al. (1951) report that TEGDN is about 50% more effective than EHN, while others (Russell, 1989; Schaefer and Hardenberg, 1981) report TEGDN to be 300% more effective than EHN. Based on the data of Figure 7, the relative performance of TEGDN and EHN depends upon the temperature at which they are evaluated. In practice, engines with different compression ratios would result in different temperatures for ignition and thus different relative performances of cetane improvers. These types of trends in performance also illustrate the inadequacy of ASTM standard D613 for characterizing diesel fuels and cetane improvers. Researchers also observed that EHN went into solution quicker and with less agitation than TEDGN, especially in solutions containing higher hexane concentrations. For all systems presently reported, the mixtures formed clear, homogeneous phases; however, higher TEGDN concentrations in diesel would have a propensity to form a second liquid phase. Solubility problems could also account for some of the discrepancies in the reported performances of TEGDN. Ryan and Stapper (1987) report apparent activation energies for 42 fuels and fuel blends. The reported activation energies vary from 131 to 901 J/mol. No definitive trends between molecular structure and activation energy have been reported for diesel fuels; however, the present data as well as other data which are not presently reported suggest that cetane improvers have increasing activation energies with increasing carbon number and increasing ether group content. Apparent activation energies also appear to increase with increasing concentrations of cetane improver. The data of Figure 9 suggest that ignition delay time is independent of the amount of fuel that is injected. This contradicts what is calculated using adiabatic and ideally mixed assumptions that predict a distinct minimum in ignition delay times between equivalence ratios of 0.5 and 1.0. This contradiction illustrates that the stratified fuel charge is not well represented by a model assuming ideal mixing. The actual performance of the injector can be explained by recognizing that an adjustment of the injector’s rack decreases the duration of the fuel charge; however, the rate of fuel injection is essentially independent of the rack setting or total fuel charge. From the perspective of the moving front of the stratified fuel charge (where ignition takes place), the duration of the fuel charge is of minimal significance. In hindsight, injector designs that allow the fuel charge to be varied without changing the ignition delay times are desirable. This may have been an important criteria in the injector design. Although good experimental procedures should be followed to assure similar fuel charges during a series of experiments, the experimental method is relatively

insensitive to the rack setting and total fuel charge. Conversely, lack of attention on the initial air pressure in the combustor can lead to deviations similar in magnitude to the standard deviations of the data, and attention to system pressure is necessary for the collection of consistent data. Conclusions A base fuel consisting of a 25:75 mass ratio of ethanol and hexanes exhibited exceptional properties as a base fuel for evaluating cetane improvers. The components of this blend are readily available and well characterizedsignition inhibitors should not be present. Desirable characteristics such low toxicity and a high sensitivity to cetane improvers at typical diesel engine temperatures of 750-850 K make this mixture particularly useful as a base fuel. This base fuel can be used to compare cetane improvers without adverse impacts from inhibitors. In addition, it provides a matrix to which inhibitors can be added and their impact on cetane improvers evaluated. Performance trends exhibited by the cetane improvers in the 25:75 mixture were followed in No.2 diesel fuel; however, this may not always be the case. One of the primary reasons for using a synthetic fuel as a testing environment is to eliminate variances in performance that can be caused by the changing compositions encountered with No. 2 diesel fuel. The performances of EHN and TEGDN were evaluated. The results indicated significantly different activation energies, which suggests that TEGDN has increasing performance advantages with increasing engine compression ratios. These different activation energies as well as low solubilities of TEGDN in diesel explain some of the inconsistencies of the relative performance of EHN and TEGDN reported in the literature. The impact of cetane improvers on the apparent activation energies of fuels is an important criterion for evaluating a cetane improver and designing fuels to work with a given engine. The described base fuel and methodology were effective for identifying the activation energies and quantifying the performance of cetane improvers; however, increased accuracy is needed to evaluate performance at treatment rates less than 0.5 wt % cetane improver. Improved accuracy should be possible and is an active area of research. Acknowledgment Funds for development of the constant-volume combustor used in these studies were provided by the Kansas Soybean Commission and the Kansas Value Added Center. Diesel Technology donated several of the injectors used in these studies. Conoco provided assistance through its donations to KU’s undergraduate Research Program. Funding from The University of Kansas was also provided through the New Faculty Award, Department of Chemical and Petroleum Engineering, and the Energy Research Center. Mike Hiskey of Los Alamos National Laboratory provided extremely helpful consulting on nitration chemistry. Professor Joe Heppert provided facilities for synthesizing tetraethylene glycol dinitrate. The experimental contributions of Matthew Bryan, Adam Hinton, and Ed Atchison are also greatly appreciated.

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Received for review March 17, 1997 Revised manuscript received June 23, 1997 Accepted July 1, 1997X IE9702284

X Abstract published in Advance ACS Abstracts, September 1, 1997.