ASTM D7826 Standard Guide for Evaluation of New Aviation Gasolines and New Aviation Gasoline Additives
6. Procedure
6.1 Special Considerations for Additives - The following procedure is applicable to both aviation gasolines and aviation gasoline additives. Therefore, the terms "aviation gasoline", "gasoline", and "fuel" will only be used to describe the test product unless special considerations exist for additives. When these special considerations for additives exist, they will be specified in the appropriate section of the procedure.
6.1.1 The additive's final chemistry, carrier solvent, recommended treatment level, location in the production or supply chain for treatment, and conditions for retreatment should be identified.
6.1.2 Complete information on the base fuel into which the additive is to be added should be provided. If the base fuel is fuel approved to current major aviation gasoline specifications such as Specifications D910, D7547, or D6227, this shall be noted and all further testing shall be done on fuel samples blended with locally available fuel.
6.1.3 All testing of additives, unless otherwise noted, should be conducted with base fuel containing 400 % (4x) of the maximum additive dosage.
6.2 Phase 1: Draft Specification and Preliminary Data Package - The tests and analysis in Table 1 should be conducted and the resulting data should be compiled for review by the task force and subcommittee.
6.2.1 Pilot Production Report - A report describing the simulated production, pilot plant ramp up, and/or production capability to confirm that adequate production capacity is available to support the test and analyses of this procedure. Ideally, several batches of fuel should be produced to reflect a range of specification properties to support "worst-case" testing of fuel for the below requirements.
6.2.2 Basic Specification Properties - These should be based on, but not be limited to Specification D910 Table 1 or Specification D7547 Table 1 properties. The basic specification property results for evaluation of additives should be compared to the corresponding data for the base fuel. Special focus should be provided for the following properties:
6.2.2.1 Octane - This requirement should consider the variations in the correlation between the motor octane test (Test Method D2700) and the rich rating test (Test Method D909) with actual engine anti-knock capability for unleaded fuels.
6.2.2.2 Freezing Point - Critical for flight safety (Test Method D2386).
6.2.2.3 Distillation Curve - Critical for adequate engine performance throughout the entire operability range, including engine starting. Provide distillation points of Specification D910, Table 1 or D7547, Table 1 (Test Method D86).
6.2.2.4 Vapor Pressure - Important for vapor lock and engine starting. Test at 38 °C (Test Method D5191).
6.2.2.5 Net Heat of Combustion - Determines aircraft range (Test Method D4809).
6.2.2.6 Density - Determines aircraft range, possible impact on structure, weight, and balance and its impact under different flight attitudes. Note that traditional density/range relationships are based on traditional hydrocarbon fuels. The relationships may change with different base compositions. Test at 15 °C (Test Method D4052).
6.2.2.7 Water Reaction and Separation - Important for control of water in fuel and confirm the absence of significant quantities of alcohol (Test Method D1094).
6.2.2.8 Electrical Conductivity - Fire safety (Test Methods D2624).
6.2.3 Fuel Composition - Detailed chemical analysis of hydrocarbons and trace materials. The composition of additives should be defined to the extent necessary to establish conformance of the products used for testing (GC X GC).
6.2.4 Fit-For-Purpose Properties, Part 1 (FFP-1) - The following FFP-1 tests should be performed to evaluate the fuel properties. The test results should be compared to the corresponding data for Specification D910 100LL or D7547 unleaded fuels. The FFP-1 results for evaluation of additives should be compared to the corresponding data for the base fuel.
6.2.4.1 Distillation Characteristics - A complete boiling point distribution and comparison of the distillation curve, residue, and loss with Specification D910 100LL per Test Method D86. Include simulated distillation to Test Method D7096.
6.2.4.2 Fuel Vaporization Properties - Liquid/vapor ratio per Test Methods D5188 and vapor pressure per Test Methods D323 or D5191. Temperature for a vapor-liquid ratio of 20 should be reported. Temperatures of other vapor-liquid ratios may be requested.
6.2.4.3 Viscosity - Measure from freezing point to room temperature per Test Method D445 or D7042. Because of a lack of a precision statement with aviation gasoline and a lack of information regarding correspondence between test results from the D445 and D7042, the test method used must be reported. No statement overt or implied of equivalence between method results should be made. Offerors may choose to use either method; however, both the test fuel and a baseline reference should be tested using the same method.
6.2.4.4 Density - Measure over operating temperature range per Test Methods D1298 or D4052.
6.2.4.5 Water Solubility - Measure over operating temperature range per Test Method D6304.
6.2.4.6 Low-Temperature Fuel Characterization - Phase transition to freezing as compared to Specification D910 100LL freeze point per Test Method D2386/IP 16 or Test Method D5972/IP 435, cloud point per Test Method D2500 (IP 219), and pour point per Test Method D97 (IP 15).
6.2.5 Preliminary Materials Compatibility - Perform soak testing of two metallic materials from Table A2.1 (5052-0 aluminum, tube and AMS 4505 brass) and three nonmetallic materials from Table A2.2 (Buna - N (nitrile), fluorosilicone, and SAE AMS 7276 Viton) in accordance with the procedures described in Annex A2 to measure property changes such as percent volume change, hardness, tensile strength, and so forth.
6.2.6 Engine Testing - The basic specification property data and FFP-1 data should be compared to similar data for known aviation gasolines such as Specification D910 100LL. This analysis should be used to determine the engine model to be used for these tests.
6.2.6.1 New production or newly overhauled engines that have not been operated on any fuel other than the test fuel should be used for this testing. The engine should be broken in and exclusively operated on only the test fuel. Limited operation with other fuels may be permitted under controlled conditions if accompanied by purging run periods of acceptable duration.
6.2.6.2 Performance and Operability Testing - Engine-rated power, steady-state performance, transient performance, and starting should be evaluated on a dynamometer-equipped engine test stand that meets industry standards for facility configuration and instrumentation calibration. Performance data should be compared to the engine manufacturer's published performance data.
6.2.6.3 Detonation Testing - Detonation testing should be conducted in accordance with the procedures described in ASTM International Specifications D6424 and D6812. A detonation measurement system that uses piezoelectric sensors that are flush mounted in the combustion chamber, or a system found to be equivalent, should be used for this testing. Detonation threshold levels and measurement accuracy and sensitivity should be correlated to known systems.
6.2.7 The preliminary data package should provide data and summarize results of above fuel and engine testing. It should include the draft specification properties that are sufficient to control the performance of the fuel for testing in the next phase of this procedure. Both the draft specification and preliminary data package should be submitted as an initial ballot for determination of the additional testing required to support the eventual balloting of the new specification.
6.3 Phase 2 - Upon completion of Phase 1, the following tests and analysis should be conducted and the resulting data should be compiled in an ASTM Research Report (see Table 2).
6.3.1 Production Report - Areport describing the pilot plant or, preferably, a refinery/chemical plant production process. The fuel used in the following testing should be produced from representative production processes, including the fuel's blending components. Fuel produced for this phase should be derived from an integrated process from feedstock to finished fuel. Chemical facsimiles of production fuel or fuel produced in a manner not representative of finished production routes are not acceptable for development of an ASTM production specification.
6.3.2 Fit-For-Purpose Properties, Part 2 (FFP-2) - FFP-2 includes additional properties relating to engine and aircraft operability and performance and also includes properties relating to fuel handling and distribution. The data generated during this testing should be compared to corresponding data for Specification D910 100LLfuel properties. Differences from Specification D910 FFPs should be reconciled in the research report. The FFP-2 results for evaluation of additives should be compared to the corresponding data for the base fuel.
6.3.2.1 Carburetor Icing - A simulated or actual flight test evaluation of carburetor icing propensity of the candidate fuel. An example of carburetor testing may be seen in Coordinating Report No. AV-17-13.
6.3.2.2 Fuel Gauging and Capacitance - Comparative analysis to 100LL per Test Methods D2624 or D4308.
6.3.2.3 Conductivity and Static Charge Dissipation - Comparative analysis to 100LL over the operating temperature range per Guide D4865.
6.3.2.4 Surface tension versus temperature compared to baseline test fluid per Test Method D1331.
6.3.2.5 Thermal conductivity versus temperature compared to baseline test fluid per Test Method D2717.
6.3.2.6 Dielectric constant versus density compared to baseline test fluid per Test Method D924. Users should be aware this test method includes the capacitance of the air which may contribute to variability in resulting test values.
6.3.2.7 Hot surface ignition temperature compared to baseline test fluid using FED-STD-791, Test Method D6053 Manifold Ignition Test, or ISO 20823.
6.3.2.8 Gum formation per Test Method D873.
6.3.2.9 Potential Gums - Test Method D873.
6.3.2.10 Water reaction per Test Method D1094.
6.3.2.11 Microbial Contamination Susceptibility per Guide D6469 and Test Method E1259 - Since alkyl lead compounds are biocides, microbial growth has not generally been an issue in aviation gasoline containing lead compounds. However, microbial growth in lead-free aviation gasoline could become a concern. Microbial growth should be compared with Specification D910 fuels or suitably identified test fluid over ambient operating range and fuel compositional range.
6.3.2.12 Electrical conductivity per Test Methods D2624.
6.3.2.13 Fuel Stability Over Time (Weathering) - Evaluate for impact on fuel performance over long-duration storage, include anti-knock capability, cold starting, and so forth. The method for weathering the fuel and selection of data collected is user defined. The offeror is encouraged to obtain industry input on the weathering plan prior to execution.
6.3.2.14 Test Method Validation - Test methods and associated criteria are based on Specification D910. They need to be validated for applicability and accuracy with the new fuel. Additional and or replacement methods should be provided.
6.3.2.15 Additive Response and Compatibility - The new fuel should respond to currently approved additives in the same manner as existing fuels, such as Specification D910 100LL fuel. Typical additives are antioxidants, fuel system icing inhibitor (FSII), electrical conductivity, and corrosion inhibitor. Refer to Annex A1.
(1) New additives should be evaluated for compatibility with additives approved for the base fuel in accordance with Annex A1 and Annex A2.
6.3.2.16 100LL Fuel Compatibility - Data indicates that MON and other fuel properties may not vary linearly when mixing 100LL with other liquid fuels. Fuel blends need to be prepared representing the range of blend ratios with 100LL of 20:80, 30:70, 50:50, 70:30, and 80:20. Table 1 properties from Specifications D910, D7547, or D6227 as appropriate shall be confirmed at each blend ratio.
6.3.2.17 Lubricating Oil Compatibility - Assessment of the fuel's compatibility with lubricating oils approved for use with aviation piston engines. It is recommended the assessment to be accomplished by evaluating the oil from the engine Durability and Operability test (see 6.3.2(1)).
(1) Data from engine durability testing may be used to support this analysis. Sample the oil before (virgin sample for baseline), every 25 h during testing, and after the engine test using industry standard practices (Practice D4057). Execute a standard oil analysis including a spectrometric oil analysis program (SOAP) test for wear materials, physical properties changes including acid number (Test Methods D664 or D3339), base number (Test Method D2896), viscosity (Test Method D445), density (Test Methods D1298 or D4052), oil dilution with fuel (Test Method D3525), nitrogen (Test Method D5762), foaming (Test Method D892), oxidation (Test Method D943), and moisture content (Test Method D6304).
6.3.2.18 Fuel Coloration - Dyes need to be specified to produce fuel color. Variations in color need to be evaluated. If, by choice or availability, none of the current approved Specification D910 dyes are to be used, any proposed new dye should be evaluated in both Phase 1 and Phase 2.
6.3.2.19 Health, Safety, and Environmental - The fuel should meet health, safety, and environmental (HSE) criteria for sale to the general public. Examples include vapor exposure, skin exposure cancer risk, and spill/water table contamination (that is, methyl tertiary butyl ether (MTBE) issue). Additional information may be available from the Environmental Protection Agency (EPA). Other HSE requirements include the following:
(1) Flammability - Acceptable flammability range. Products with very broad range (for example, hydrogen) represent additional handling risk/explosive atmosphere. Visible flame on combustion, for example, alcohol fire may not be visible.
(2) Ignition Energy - Appropriate ignition energy. Very low ignition energy represents additional hazard from friction and so forth, for example, hydrogen.
(3) Autoignition Temperature - Suitable autoignition temperature and not unstable in storage (for example, peroxide risk). Compare to Specification D910 fuel, Test Method E659.
(4) Firefighting Media - Current firefighting media need to be effective.
(5) Static/Conductivity - Similar/better than current product. Risk of excessive static being generated/charge dissipation rate - hazard for over-wing refueling.
6.3.2.20 Toxicity - Does the fuel possesses mutagenic properties, is it classified as an irritant, and so forth? See MIL-HNDBK-510-1.
(1) Combustion Products - Should be analyzed and compared to existing fuels such as Specification D910 100LL.
6.3.2.21 Long-Term Fuel Storage Stability:
(1) Long-term storage stability is covered by Test Method D873 tested as part of Specification D910 Table 1 and D7547 Table 1 properties.
(2) Dirt/Water Dropout - Allows dirt/water to partition from fuel at a similar rate to current product.
(3) Density - Density appropriate for storage/manual handling (drums).
6.3.2.22 Laboratory Tests - The fuel properties and quality should be controlled by laboratory tests, which are readily available. Currently available personal protective equipment (for example, flame-resistant meta-aramid material coats, gloves) should be appropriate for conducting specified laboratory tests.
6.3.2.23 Fuel Distribution System Component Compatibility - In addition to the fuel-wetted components in the aircraft (6.2.5), it is recommended that the fuel sponsor also consider the compatibility of the fuel with the fuel-wetted components present in the aviation gasoline fuel supply distribution system. The new fuel should not negatively impact the fuel-wetted materials. Negative effects include, but are not limited to, unacceptable swelling/shrinkage, unacceptable hardening or softening of components, corrosion or unacceptable impact on fuel delivery rates, and filter media or storage.
(1) A typical tank lining system such as the three-coat epoxy/amine adduct system should be tested.
(2) Filtration Compatibility - The performance of coalescer filters and monitors should be compared to performance with existing fuels such as Specification D910 100LL (see Annex A4).
6.3.2.24 Emissions - CO2 , CO, NOX, PM, benzene, polyaromatic hydrocarbons, total hydrocarbons (THC), vapor emissions and combustion products, by comparison with existing fuels such as D910 100LL under similar test conditions.
6.3.3 Final Materials Compatibility - Engine and aircraft fuel system polymer and metallic materials that are exposed to fuel should be evaluated for compatibility with the new fuel. The results of the compatibility testing should be compared to corresponding results or service experience of existing fuels, and any deviations from current material behavior should be reconciled.
6.3.3.1 Incompatibility may be indicated by unacceptable swelling or shrinkage, delamination, unacceptable hardening or softening, corrosion, or embrittlement.
6.3.3.2 Procedures for materials compatibility testing of aviation gasolines and aviation gasoline additives are provided in Annex A2.
6.3.3.3 A representative listing of piston engine aircraft materials is provided in Annex A3. Those materials tested for the preliminary materials compatibility requirement need not be retested for final materials compatibility.
6.3.3.4 An example procedure for compatibility testing of aircraft composite fuel tank materials may be found in Annex A5.
6.3.3.5 An example procedure for permeability testing of aircraft fuel tank bladder materials may be found in Annex A6.
6.3.4 Component Testing:
6.3.4.1 Flame Speed - Flame speed effects should be evaluated with a representative engine cylinder assembly. Particular attention shall be given to exhaust valve temperature, turbocharger inlet temperature, and exhaust valve seat condition in a worst-case engine or cylinder assembly. A determination of exhaust valve creep life response shall be made. In addition, changes in engine cooling demands, fuel consumption, effective ignition timing, performance, power train stresses, and vibration shall be determined in worst-case engine(s).
6.3.4.2 Fuel-Gauging Equipment:
(1) Modern aircraft use capacitance fuel gauges for reporting the fuel quantity. The gauge operates by using a low-voltage capacitor probe where the fuel acts as the dielectric. A low voltage current is applied to the sensing capacitor and the resulting charge is compared to a reference probe. As the fuel level increases, the charge on the senor probe increases. The difference between the sensing probe and the reference probe is measured using a voltmeter. The voltages are calibrated to fuel load, which is reported on the gauge.
(2) If the dielectric properties of the fuel are different, then the measured voltages will be different resulting in an incorrect fuel load reading. It is known, for example, that the presence of ethanol in the fuel load results in an incorrect fuel reading because the dielectric behavior of the fuel has been changed. Test fuel dielectric constant using Test Method D924 and compare to 100LL baseline.
6.3.5 Fuel Performance Evaluation on Aircraft and Engines:
6.3.5.1 A recommended portfolio of tests to be conducted on specific engine and aircraft models has been developed to evaluate the performance of the candidate fuel under actual operating conditions.
6.3.5.2 Each test is designed to evaluate specific performance characteristics of the candidate fuel but not all tests will necessarily be required for all candidate fuels. The tests to be conducted will be determined based on the properties of the candidate fuel under evaluation. In Table 3, a summary of these tests and the associated fuel performance characteristics the test is intended to address are provided. In the table, the E-series reference engine tests, and the A-series reference aircraft tests. A description of each of these tests and test articles to be used for the tests is provided in the following sections.
6.3.6 Engine Testing - Acceptable engine performance, durability, and operability, when operating with the new aviation gasoline, should be demonstrated on the aircraft piston engines identified in this section.
6.3.6.1 General:
(1) The testing should be performed on a dynamometer-equipped engine test stand that meets industry standards for facility configuration and instrumentation calibration unless otherwise noted.
(2) New production or newly overhauled engines that have not been operated on any fuel other than the test fuel should be used for Test E-1. Test E-2 should be performed with a new engine or an engine remanufactured to new engine specifications. For both tests, the engine should be broken in and exclusively operated on only the test fuel. Limited operation with other fuels may be permitted under controlled conditions if accompanied by purging run periods of acceptable duration.
(3) Each test engine should undergo a performance calibration before conducting the tests specified in the following.
6.3.6.2 Engine Tests and Test Articles - Engine models and the engine tests to be conducted on those models are shown in Table 4.
(1) Test E-1: Performance and Detonation
(a) A complete performance characterization of each engine should be performed on a dynamometer-equipped engine test stand to measure brake horsepower, exhaust gas temperature, RPM, and mixture strength relative to the performance charts published in the engine operating instructions.
(b) Detonation testing should be conducted in accordance with the procedures described in Specifications D6424 and D6812. A detonation measurement system that uses piezoelectric sensors that are flush mounted in the combustion chamber, or a system found to be equivalent, should be used for this testing. Detonation threshold levels and measurement accuracy and sensitivity should be correlated to known systems.
(c) Sea level detonation tests should be performed at rated engine power and engine settings defined by the manufacturer such as recommended climb power, maximum best cruise power, and maximum best economy power. Sea level detonation should be performed on naturally aspirated and turbocharged/supercharged engines.
(d) Detonation tests should be performed at critical altitude for 100 %, 75 % power for turbocharged or supercharged engines.
NOTE 1 - Testing is performed based on where the engine is most likely to detonate. For normally aspirated engines, this is at sea level at which the highest manifold air pressure is achieved. For turbo/supercharged engines, this may be a critical altitude. The critical altitude is the highest altitude at which the stated power is attained. This is where the turbo/supercharger works the hardest and therefore produces the highest manifold air temperature. Sea level testing is required for both normally aspirated and turbo/supercharged engines.
(2) Test E-2: Durability and Operability Test - This is a long-duration engine test to evaluate the candidate fuel relative to durability, operability, deposit formation, starting, cooling, mixture distribution, and valve-train operation. The test should evaluate the fuel over the complete range of temperature and altitude conditions specified in the manufacturer's operating instructions. The following requirements apply to this test:
(a) This test may be performed in a propeller test stand only if evidence of acceptable performance calibration is provided.
(b) The test duration should be at least 300 h and include throttle transients, cruise conditions, and cold and hot starts. The specific duty cycle should be defined by the fuel sponsor and reviewed with the task force members for acceptability.
(c) Do not switch between unleaded and leaded fuels during the test.
(d) The test should address the ability of the fuel to form a combustible mixture under all operating conditions.
(e) The impact of the candidate fuel on engine cooling should be evaluated during the test.
(f) The test should evaluate the impact of fuel deposits from the new fuel on the octane demand of the engine and pre-ignition tendency of the engine. Periodic detonation measurements with a leaded reference or baseline fuel at 50 h intervals should be included.
(g) Combustion (including peak cylinder pressure) may impact structural loading of critical engine components. If combustion pressure is different or ignition timing shall be adjusted or both, the resulting effect on both crankshaft torsional vibration and bottom end loading (crankcase, main bearings, and crankshaft) shall be addressed to document resulting loads and stresses are within allowable engine limits.
(h) Upon completion of the test, a comparison of pre- and post-test engine hardware condition and measurements should be conducted. Recommended measurements are shown in Table 5. For locations and direction of measurements, refer to the original equipment manufacturer (OEM) engine overhaul manual. The engine should be examined for evidence of discoloration of fuel- and oil-wetted parts, valve sticking, combustion chamber deposits, fuel system deposits, oil system deposits, exhaust system deposits, turbocharger deposits, carburetor deposits, and induction system.
6.3.7 Aircraft Flight Testing - Aircraft performance, operability, and range when operating with the new aviation gasoline should be demonstrated on the aircraft identified in this section.
6.3.7.1 General - The aircraft fuel system should be flushed with a solvent-acting solution or fuel before initiation of flight testing. When operating with two fuels, the new fuel should be kept segregated from the reference fuel.
6.3.7.2 Aircraft Tests and TestArticles - Aircraft models and the flight tests to be conducted on those models are shown in Table 6.
6.3.7.3 A-1 Performance, Operability, and Durability Test - Aircraft flight testing should be conducted to evaluate the impact of the new fuel on aircraft performance, operability, and durability. The aircraft should be operated at conditions representing the extreme corners of the operational envelope specified by the OEM in the aircraft flight manual or pilot's operating handbook. The aircraft should be tested for a duration long enough to address the following items in a wide range of temperature conditions, including sufficient operation to identify trends in engine performance with accumulated operating time:
(1) Evaluate impact on fuel system materials;
NOTE 2 - Non-operating time may be more important to evaluate compatibility than operating time.
(2) Demonstrate starting with hot fuel and demonstrate cold fuel altitude relights;
(3) Demonstrate smooth throttle transients at most severe operating envelope conditions;
(4) Comparison of induction system icing susceptibility between the new fuel and a reference fuel;
(5) Evaluate for normal engine operation during all approved aircraft maneuvers, for example, takeoff and landing, balked landing, and so forth;
(6) Measure and document the power produced during flight operation;
(7) Document any changes to equipment calibration that is required for the new fuel;
(8) Measure and document take off and cruise fuel consumption;
(9) Monitor and record any effects on engine cooling (CHT);
(10) Monitor and record any effects on exhaust gas temperature (EGT); and
(11) Monitor and record any effects on turbocharger performance.
6.3.7.4 A-2 Hot Weather Operation Test - Aircraft flight testing should be conducted to evaluate aircraft performance under high-temperature conditions. The following should be addressed during this testing:
(1) Hot Fuel Test - Evaluate the performance of the aircraft with hot fuel during taxiing, takeoff, and climb.
(2) Vapor Lock Test - Demonstrate fuel system pumping capability in hot weather conditions.
6.3.7.5 A-2b Cold Weather Operation Test - Aircraft flight testing should be conducted to evaluate aircraft performance under low-temperature conditions. The following should be addressed during this testing:
(1) Starting and operability with cold fuel under cold weather conditions.
(2) Restarting at altitude with cold fuel under cold weather conditions.
6.3.7.6 A-3 Detonation and Range Test - Aircraft flight testing should be conducted to demonstrate adequate anti-knock capability of the new fuel and evaluate any impact on aircraft range when operating with the new fuel. The following should be addressed during this testing:
(1) Detonation testing should be conducted at high altitude under worst-case ambient conditions. The type, make, and accuracy of the detonation test instrumentations should be reviewed with the task force for acceptability.
6.3.8 ASTM Research Report and ASTM Specification - The research report should provide data and summarize results of above fuel and engine testing. The specification should be sufficient to control the performance of the fuel as a finished product. Both the research report and specification should be balloted to the ASTM subcommittee and committee for approval.
7. Keywords
7.1 additive evaluations; additive qualifications; alternative fuels; approval protocols; ASTM International; aviation gasolines; fuel additives; fuel evaluations; fuel qualifications; material compatibility