Sasol IPK Research Articles

Experimental characterization of jet fuels under engine relevant conditions – Part 1: Effect of chemical composition on autoignition of conventional and alternative jet fuels

Conventional and synthetic alternative jet fuels possess distinctive chemical compositions and physical/chemical properties since they are produced through different production processes. In this paper, the autoignition characteristics of four conventional aviation jet fuels (Jet-A POSF-4658, Jet-A POSF-10325, JP-5 POSF-10289, JP-8 POSF-6169), five alternative jet fuels (Syntroleum S-8 (S8), Shell synthetic paraffinic kerosene (Shell SPK), Sasol iso-paraffinic kerosene (Sasol IPK), hydro-processed renewable jet (HRJ8), alcohol to jet (ATJ), and 50/50 vol% blends of JP-8/alternative jet fuels were explored. The objective of the current work is to understand how the chemical composition of the fuels affects the gas phase chemical kinetic behavior and the ignition process of the liquid spray in an engine-relevant environment, thereby ultimately enabling their use in military ground vehicles with compression ignition engines. A modified Cooperative Fuel Research (CFR) motored engine was utilized and the gas phase ignition behavior was observed over a wide range of temperatures and pressures, while varying the compression ratio. In addition, an optically accessible constant volume spray combustion chamber was employed to observe significant differences in the global heat release characteristics and physical/chemical ignition delay times of synthetic alternative jet fuels in comparison to the conventional jet fuels. The results of this study show that synthetic alternative jet fuels with predominantly linear and lightly-branched alkane content (S8, Shell SPK and HRJ8) provide stronger low-temperature ignition characteristics, while other types of synthetic alternative jet fuels with high content of highly-branched alkanes (Sasol IPK and ATJ) exhibit weaker low-temperature ignition characteristics, when compared to conventional jet fuels (JP-8, Jet-A and JP-5). Their unique fundamental ignition behaviors, including the percentage of low-temperature heat release (% LTHR), the critical compression ratio (CCR), and the critical equivalence ratio (ϕcrit), are strongly related to the dominant chemical compositions of the fuel in a motored engine.

Experimental characterization of jet fuels under engine relevant conditions – Part 2: Insights on optimization approach for surrogate formulation

Computational combustion modeling is an essential complementary tool to engine experiments and the combination of computational fluid dynamics and detailed chemical kinetics provides the promise for optimizing engine performance. However, predicting the effects of chemical and physical properties of fuels on engine performance is a great challenge since transportation fuels are composed of several hundreds to thousands of chemical species. Surrogates are simpler representations of real fuels and are comprised of selected species of known concentrations that exhibit combustion characteristics similar to those of the real fuels. In this paper, drawing from our previous work on surrogate formulations, we investigate the effects of surrogate components on the autoignition characteristics of conventional and alternative jet fuels, using a combination of experimental and modeling approaches. As target fuels, we analyzed a conventional jet fuel (Jet-A) and two alternative jet fuels (coal-derived IPK, natural-gas-derived S8). Experimental data on the properties of surrogate mixtures, such as liquid density and threshold sooting index, their combustion behaviors, together with those from their corresponding target real fuels are compared and analyzed using predictions obtained from the surrogate model. Results from a modified CFR engine show that the ignition reactivity of Jet-A and Sasol IPK surrogates are stronger than their target fuels, while the S8 surrogate displayed an ignition behavior very similar to the target S8. Results from a constant volume spray combustion chamber provided reasonable agreements between the surrogates and their target jet fuels in terms of physical and chemical ignition delay times and apparent heat release trends, with the S8 surrogate showing the best agreement. In addition, surrogate mixtures that were modified from the original Jet-A and IPK surrogates to achieve a better agreement with CFR experiments performed poorly when tested in a spray chamber. These results imply that the agreement between the behaviors of surrogates and real fuels in one device or in one condition does not guarantee similarity in other devices or in other conditions. This study also highlights the need for improvements in the current surrogate formulation methodologies to provide a more universal emulation of the autoignition behaviors of target transportation fuels.

Predicting the global combustion behaviors of petroleum-derived and alternative jet fuels by simple fuel property measurements

Pre-vaporized global combustion behaviors of a petroleum-derived jet fuel (JP-8), five alternative jet fuels (Shell SPK, Sasol IPK, HRJ Camelina, HRJ Tallow, and Gevo ATJ), and five 50/50 (liquid volume) blends of JP-8/alternative fuels are experimentally examined and compared. Three experiments are performed to investigate the gas-phase combustion behaviors of the tested fuel samples: (1) global oxidative species profiles in a variable pressure flow reactor, (2) diffusion flame extinction in a counterflow burner, and (3) premixed flame initiation in a heated spherical combustion chamber. Multivariate linear regression methods have been applied to investigate the sensitivities of pre-vaporized global combustion behaviors to individual combustion property targets of the fuels, including Derived Cetane Number (DCN), H/C ratio, mean molecular weight, and smoke point. As a proof of concept for fuel screening tool based on the standardized fuel property measurements, a “combustion property target (CPT) index” based upon this regression analysis is found to show promise as a rapid means to evaluate the global pre-vaporized combustion behaviors of the tested fuel samples against each other as well as the spectrum of JP-8 fuels found in use. The present work suggests the applicability of such a methodology not only as an expeditious fuel screening tool for assessing the fully pre-vaporized, kinetically coupled behaviors of emerging alternative jet fuel candidates, but further supports the use of combustion property targets in developing kinetic models that are specific to each real fuel.

Effect of Cetane Improver on Autoignition Characteristics of Low Cetane Sasol IPK Using Ignition Quality Tester1

This paper investigates the effect of a cetane improver on the autoignition characteristics of Sasol IPK in the combustion chamber of the ignition quality tester (IQT). The fuel tested was Sasol IPK with a derived cetane number (DCN) of 31, treated with different percentages of Lubrizol 8090 cetane improver ranging from 0.1 to 0.4%. Tests were conducted under steady state conditions at a constant charging pressure of 21 bar. The charge air temperature before fuel injection varied from 778 to 848 K. Accordingly, all the tests were conducted under a constant charge density. The rate of heat release was calculated and analyzed in detail, particularly during the autoignition period. In addition, the physical and chemical delay periods were determined by comparing the results of two tests. The first was conducted with fuel injection into air according to ASTM standards where combustion occurred. In the second test, the fuel was injected into the chamber charged with nitrogen. The physical delay is defined as the period of time from start of injection (SOI) to point of inflection (POI), and the chemical delay is defined as the period of time from POI to start of combustion (SOC). Both the physical and chemical delay periods were determined under different charge temperatures. The cetane improver was found to have an effect only on the chemical ID period. In addition, the effect of the cetane improver on the apparent activation energy of the global combustion reactions was determined. The results showed a linear drop in the apparent activation energy with the increase in the percentage of the cetane improver. Moreover, the low temperature (LT) regimes were investigated and found to be presented in base fuel, as well as cetane improver treated fuels.

Investigation of Physical and Chemical Delay Periods of Different Fuels in the Ignition Quality Tester

This paper investigates the physical and chemical ignition delay (ID) periods in the constant volume combustion chamber of the Ignition Quality Tester (IQT). IQT was used to determine the derived cetane number (DCN) according to ASTM D6890-10a standards. The fuels tested were ultra low sulfur diesel (ULSD), jet propellant-8 (JP-8), and two synthetic fuels of Sasol IPK and F-T SPK (S-8). A comparison was made between the DCN and cetane number (CN) determined according to ASTM-D613 standards. Tests were conducted under steady state conditions at a constant pressure of 21 bars and various air temperatures ranging from 778 K to 848 K. The rate of heat release (RHR) was calculated from the measured pressure trace, and a detailed analysis of the RHR trace was made particularly for the auto-ignition process. Tests were conducted to determine the physical and chemical delay periods by comparing results obtained from two tests. In the first test, the fuel was injected into air according to ASTM standards. In the second test, the fuel was injected into nitrogen. The point at which the two resultant pressure traces separated was considered to be the end of the physical delay period. The effects of the charge temperature on the total ID as defined in ASTM D6890-10a standards, as well as on the physical and chemical delays, were determined. It was noticed that the physical delay represented a significant part of the total ID over all the air temperatures covered in this investigation. Arrhenius plots were developed to determine the apparent activation energy for each fuel using different IDs. The first was based on the total ID measured according to ASTM standards. The second was the chemical delay determined in this investigation. The activation energy calculated from the total ID showed higher values for lower CN fuels except Sasol IPK. The activation energy calculated from the chemical delay period showed consistency in the increase of the activation energy with the drop in CN including Sasol IPK. The difference between the two findings could be explained by examining the sensitivity of the physical delay period of different fuels to the change in air temperature.

Systematic study of ignition delay for jet fuels and diesel fuel in a heavy-duty diesel engine

Measurements of ignition delay (ID) for five fuels: jet-A, three blends of Sasol IPK and jet-A, and a #2 diesel fuel, were performed in a 2.44L heavy-duty single-cylinder diesel engine. A single injection of fuel was used at 5 crank angle degrees before top-dead center (TDC). The in-cylinder conditions at the start of injection were systematically varied over a range of in-cylinder densities from 15 to 40kg/m3 in 5kg/m3 increments. For each density, average in-cylinder temperature was varied over as large of a range as possible by heating the intake air. The range of obtainable temperatures was density dependent, but for each density the temperature was varied from approximately 900–1100K. In-cylinder pressure measurements were performed allowing for ID determination. Methods for in-cylinder temperature determination and ID measurement criteria were tested and compared. The results indicate that the fuel volatility and properties influencing spray breakup have minimal impact on the ignition process. Ignition delay results show good agreement with previous measurements by other investigators performed in a constant volume combustion chamber.

Autoignition studies of conventional and Fischer–Tropsch jet fuels

Here we report autoignition studies for five compositionally-distinct jet aviation fuels: Jet A; Jet A blended with the JP-8 additive package; two Fischer–Tropsch (F–T) jet fuels synthesized in gas-to-liquid processes, S-8 and Shell GTL; and a coal-to-liquid F–T fuel, Sasol IPK. Ignition delay time measurements were made in a heated shock tube at pressures from 8 to 39atm and at a wide range of temperatures, from 651 to 1381K, for gaseous jet fuel/air mixtures at equivalence ratios from 0.25 to 1.5. Ignition delay times were determined by monitoring pressure and electronically-excited OH chemiluminescence behind reflected shock waves. Experiments reveal three regions of reactivity: a high-temperature regime with positive apparent activation energy, for temperatures greater than approximately 900K at 20atm; a negative-temperature-coefficient (NTC) regime exhibiting slightly decreasing ignition delay with decreasing temperature, for approximately 750–900K at 20atm; and a low temperature regime where ignition delay returns to positive apparent activation energy, for temperatures less than approximately 750K at 20atm. Measurements for Jet A/air mixtures at a wide range of conditions quantify the ignition delay dependence on temperature, pressure, and equivalence ratio; where appropriate, scaling laws are reported. Comparison of ignition delay times for Jet A and the three F–T fuels studied shows that at high temperatures (T>1000K) and common conditions (fuel/air at ϕ=1.0 and 20atm) these four fuels have indistinguishable reactivity. However, in the NTC and low-temperature regimes the ignition delay times for these four fuels diverge, in accordance with their derived cetane numbers (DCNs), by up to a factor of three. Comparisons are made with predictions from literature jet fuel surrogate kinetic models for Jet A, S-8, and Shell GTL.