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PREPUBLICATION COPY â Uncorrected Proofs 85 6 Potential Future Lead-Free Fuels and Propulsion Systems The previous chapter considered the potential for replacing the almost universally used 100LL grade of aviation gasoline (avgas) with other avgas grades that are lead-free or have lower lead content and that are currently available for purchase at some airports (i.e., a proprietary UL94), or at least specified in an American Society for Testing and Materials (ASTM) standard (i.e., 100VLL). A conclusion from the analyses in Chapter 5 is that a reduction in lead emissions by nearly 20 percent, but possibly more than 40 percent, is plausible from these existing unleaded and lower-lead alternatives when used in varying combinations by the current piston-engine fleet. Consideration was also given to the potential for substituting widely available grades of unleaded automotive gasoline based on approvals during the 1980s for thousands of piston- engine aircraft to use motor gasoline (MOGAS). However, gasoline formulations that would meet the specifications of those engines and aircraft certified for MOGAS can no longer be obtained in the automotive fuel market and are unlikely to become available in the future. The higher end of the lead reduction range (e.g., a roughly 40 percent lead reduction) from replacing 100LL would require the substitution of this grade by100VLL and potentially large and widespread airport investments in new fuel storage and dispensing capacity to make UL94 available to those aircraft that can use it. As a practical matter, such investments may not be forthcoming in a general aviation (GA) industry characterized by many small airports with limited capital and a total avgas demand that is already very small, projected by the Federal Aviation Administration (FAA) to decline further, and difficult to subdivide economically into specialty grades. Whether pilots would be motivated to use UL94 is another matter, and a potential obstacle to its increased use if aircraft owners need to expend considerable time, money, and effort to obtain the needed FAA certifications. A solution, at least conceptually, to the vexing problem of having to supply and motivate the use of two avgas gradesâone leaded and one unleadedâis to develop and introduce a lead- free, high-octane avgas that can meet the needs of the entire piston-engine fleet and can fully and quickly replace 100LL (or 100VLL). Currently there is no ASTM specification for an unleaded fuel with a motor octane number (MON) above 94, much less a 100 MON or higher, nor has one been approved in an engine aircraft type certificate. Moreover, even an unleaded fuel that has a 100 MON rating may not satisfy the requirements of some high-performance legacy aircraft that are only able to operate on a leaded fuel having a 100 MON rating because the tetraethyl lead (TEL) additive, as discussed below, provides an anti-knock âbonusâ that may be equivalent to several additional octane numbers. Accordingly, the actual MON of an unleaded replacement fuel may need to exceed 100 (i.e., 100+ MON). A full replacement for 100LL/VLL is sometimes referred to as a âdrop-inâ fuel because it would not require any changes to the existing piston-engine fleet, new FAA certification approvals, modifications to future engines and aircraft, or new investments in fuel storage and dispensing capacity.1 This ideal means of lead mitigation, however, presents formidable technical challenges. To convey these challenges, the chapter considers findings from past research on octane-enhancing TEL alternatives and the history and accomplishments of the 1 The transition to a drop-in fuel could nevertheless require steps to prevent inadvertent mixing of the fuel with residual leaded grades in aircraft fuel tanks and airport fueling systems.
PREPUBLICATION COPY â Uncorrected Proofs 86 FAAâs Piston Aviation Fuels Initiative (PAFI) collaborative with fuel suppliers and the GA industry. PAFI was created in 2013 for the specific purpose of hastening the development and availability of an unleaded 100+ MON avgas that can satisfy the entire piston-engine fleet without introducing new adverse safety, environmental, or health impacts. To date, however, the collaborative has not yielded the desired drop-in fuel. Following the discussion of PAFI, and what has been learned more generally about the challenges associated with developing and deploying a satisfactory unleaded, high-octane fuel, consideration is given to retrofitting existing GA aircraft and changing the future fleet so that more aircraft can use existing grades of unleaded avgas or other lead-free sources of energy. While technology refinements are making some traditional lead-free propulsion systems, such as diesel and gas turbine engines, better suited to small aircraft, continued advances in battery- and hybrid-electric motor technologies also hold promise for farther-out applications. An uncertain development and implementation pathway that is likely to take decades to transition the legacy fleet suggests that waiting for fundamentally different small aircraft technologies to solve the aviation lead problem would be a mistake. Nevertheless, successive generations of new GA aircraft that do not require leaded avgas could make transitioning away from leaded avgas more manageable. SEARCHING FOR AN UNLEADED 100+ MON AVGAS Following passage of the Clean Air Act Amendments in 1990, the U.S. Environmental Protection Agency (EPA) issued regulations to eliminate lead from the gasoline used in on-road engines and vehicles. Although avgas was excluded from the new requirements, concerns about the potential for future actions prompted FAA to undertake more concerted investigations of unleaded fuel formulations at its William J. Hughes Technical Center starting in the mid-1990s (UAT ARC, 2012). In more recent years, as EPA has continued to assess the public health risks associated with lead and the possibility of restricting the use of leaded avgas, FAA has intensified efforts to understand and overcome technical challenges that have hindered the development of a universally usable unleaded avgas. Technical Challenges with Unleaded, High-Octane Fuel Formulations Decades of research have revealed many technical challenges to the development and introduction of an unleaded avgas offering 100+ MON, particularly in finding a suitable chemical additive in a gasoline formulation that provides the required octane level and knock resistance. Major categories of octane-enhancing additives to gasoline are aromatics, oxygenates, aromatic amines, and metals, each of which brings its own set of technical issues and formulation requirements. After screening more than 250 combinations of additives and gasoline formulations during the 1990s and early 2000s, researchers using FAAâs fuel and engine testing facility conducted full-scale engine testing on nearly 50 of the most promising blends. However, when the octane-enhancing additives were used in quantities needed for anti-knock performance, the blends could not meet all other essential performance requirements for properties such as vapor pressure, hot and cold starting capability, material compatibility, water separation, storage stability, freeze point, and toxicity (CRC, 2010). Table 6-1 lists the main categories of octane-enhancing additives in fuel alternatives examined, gives some specific examples of additives, and notes advantages and disadvantages
PREPUBLICATION COPY â Uncorrected Proofs 87 associated with each example as cited by FAA.2 A major disadvantage cited, particularly for the non-metals, is assuring material compatibility. Aircraft fuel systems, from the tank to the carburetor or fuel injector, have many rubber components, synthetic elastomers, sealants, carbon fiber, composite components, and surface treatments that are potentially susceptible to damage by fuel additives. Ensuring material compatibility, however, is further complicated by the demographics of the legacy fleet. Because the average age of aircraft in the piston-engine fleet is about 50 years, many of the manufacturers of the aircraft and their engines and components no longer exist, and their material specifications have been lost to history. Other disadvantages, as listed in the table, involve concerns about toxicity, groundwater contamination, and fouling of engines and other aircraft components, particularly for metals that otherwise offer good anti- knock performance. TABLE 6-1 Examples of Octane Enhancers in Fuel Alternatives Under Development to Replace TEL in Avgas Category Examples Advantages Disadvantages Aromatics Toluene, Mesitylene Low toxicity Known combustion issues, exhaust sooting, some material incompatibility concerns. Oxygenates Ethyl tert-butyl ether (ETBE), Methyl tert-butyl ether (MTBE), Ethanol Good octane enhancer Aircraft range impacts, potential groundwater contamination, water solubility, odor, restrictions on transportation across some state lines. Aromatic Amines Aniline, m-Toluidine Good octane enhancer Aggressive toward elastomers, polysulfide sealant, fuel bladder, and some paints. Engine deposits may be an issue, possible cold-flow issues. Metals Methylcyclopentadienyl manganese tricarbonyl (MMT) Good to excellent octane enhancer. Compatible with most materials. Most similar to TEL additive in existing 100LL. Toxicity concerns, issues with engine deposits, plug fouling, and UV sensitivity. Approved for use in small quantity in auto fuel. SOURCE: Adapted from FAA presentation to committee, November 2019. As noted above, candidate additives may also fall short in their ability to fully replace TELâs anti-knock enhancement in excess of 100 MON. While 100 MON is widely used as a shorthand for satisfactory detonation avoidance, there is evidence that the presence of lead in 100LL grants some additional detonation margin over a lead-free 100 MON grade. For instance, a 2010 study by the Coordinating Research Council (a petroleum and automotive equipment 2 FAA presentation to the committee, November 19, 2019.
PREPUBLICATION COPY â Uncorrected Proofs 88 industry collaborative), which was informed by work at FAAâs Technical Center, concluded that high-performance engines (depending on power output and configuration) can require unleaded fuels in excess of 100 MON to achieve knock-free operation and that the added resistance of TEL may be as high as 3 MON (CRC, 2010). If these and other technical challenges can be met to identify a qualifying unleaded 100+ MON fuel, development of an ASTM specification would be a required next step, followed by aircraft and engine types needing to be certified to use the fuel. Testing performed to identify the fuel would aid in the development of the ASTM specification and inform FAA certification. The latter could be hastened appreciably by the recent authority granted by Congress (under Section 565 of the 2018 Reauthorization Act) for FAA to permit broad-based authorizations of a satisfactory unleaded fuel. Once these authorizations are granted, aircraft and engine manufacturers could update their service recommendations to include the new fuel alternative. Even with expedited approvals for the use an unleaded 100+ MON avgas, the deployment of the unleaded fuel could require major changes to fuel refining processes that will take time and investments to implement. If those requirements are accompanied by a need to use more expensive fuel ingredients this could have implications on the fuelâs price, availability, and competitiveness of production, especially if there are proprietary aspects. Additionally, transitional steps would need to be taken while the limited amount of refining and refueling infrastructure is converted from a leaded to an unleaded fuel distribution, storage, and dispensing system. In the case of the automotive sector, for instance, the downstream conversion from leaded to unleaded gasoline was made easier by most filling stations having multiple dispensers and underground storage tanks, allowing both fuels to be offered simultaneously. As discussed in the previous chapter, similar redundancy does not exist throughout much of the aviation sector. PAFI Collaborative After nearly two decades of searching for an unleaded drop-in avgas without success, in 2012 FAA established the Unleaded Avgas Transition Aviation Rulemaking Committee (UAT ARC) comprised of GA trade and membership associations, aircraft and engine manufacturers, fuel producers, and EPA.3 In doing so, FAA acknowledged that the development and deployment of a satisfactory and safe drop-in avgas would require a coordinated public- and private-sector effort. In response, UAT ARC recommended the use of FAAâs Technical Center for centralized testing of candidate fuels offered by developers accompanied by a deliberate process for soliciting and selecting the fuels to be tested. A purpose of centralized testing was to generate standardized qualification and certification data that could be used to support development of ASTM specifications and FAA fleetwide certifications, thereby eliminating potentially redundant testing and shortening the time for fuel development and mass introduction. UAT ARC further recommended that FAA establish a technical review board to evaluate the feasibility of the candidate fuels and a special fuels program office dedicated to implementing the recommendations through the creation of an FAA-industry collaborative, which became PAFI. In response to these recommendations, in June 2013 FAA issued a solicitation for proposers of unleaded fuels to participate in the testing program and formed a government- industry PAFI Steering Group to establish fuel evaluation and testing protocols and to coordinate and oversee the evaluation program. Fuel developers were given 1 year to submit data packages for fuel formulations for prescreening based on considerations such as chemical makeup, 3 See https://www.faa.gov/regulations_policies/rulemaking/committees/documents/media/UATARC-1312011.pdf.
PREPUBLICATION COPY â Uncorrected Proofs 89 performance properties, emissions, toxicology properties, and projected production and distribution potential. The plan called for a year-long Phase 1 evaluation in which fuels that passed the prescreening criteria (focused on identifying unacceptable flaws) would be subject to laboratory and rig testing at the Technical Center and two or three of the most promising candidates would be selected for Phase 2 evaluations that would involve testing on specific and representatively configured engine and aircraft models to assess their suitability across as much of the existing piston-engine fleet as possible. The ambitious aim of Phase 2, expected to take about 2 years to complete, was to generate data for development of an ASTM specification and to certify most of the existing fleet to operate on the fuel. FAA formed a technical committee to serve as the primary evaluator of the fuels, with guidance and technical input from an advisory committee of aircraft and engine manufacturers. According to periodic FAA updates posted on its website4 and provided in study committee briefings, PAFI received 17 fuel proposals from six fuel developers by August 2014. By January 2016, prescreening and Phase 1 testing was completed, and two fuel formulations, one from Shell5 and another from Swift Fuels,6 were selected to participate in Phase 2âs extensive tests on about 15 engine and 10 aircraft models. In June 2018, FAA reported that Phase 2 testing of the two formulations, which began in mid-2016, was being suspended with the aircraft test program approximately one-third complete and the engine test program about halfway complete. Because of proprietary agreements in the PAFI process, FAA was not able to report the specific reasons for the suspended testing but noted unacceptable aspects of the two fuels that required the fuel suppliers to conduct further research and development to find solutions. In early September 2018, FAA announced that Swift Fuels had decided to pursue the development and testing of its candidate fuel outside the PAFI structure. FAA further announced that Shellâs efforts to mitigate the issues identified in Phase 2 testing appeared promising and that Phase 2 testing would resume, including testing on material compatibility, durability, detonation, and performance issues, before additional aircraft testing would be conducted. However, in June 2019 FAA reported that engine test results with the optimized Shell fuel were not successful and that additional refinements to the fuel would be required before testing could resume. In reporting the status of the testing of Shellâs fuel, FAA pointed out that the PAFI experience had further revealed the magnitude of the technical challenge in finding an acceptable unleaded drop-in fuel.7 The agency announced that the scope of PAFI would be expanded to support the needed fuels research and development while also attracting developers of other candidate fuels for evaluation, including fuel formulations not proposed during the original 2013 solicitation. In its August 2020 PAFI update, FAA reported that developers of new fuels would be asked to complete the following prescreening tests prior to a proposed fuel being accepted for more extensive testing through PAFI: â¢ Successful completion of a 150-hour engine endurance test on a turbocharged engine using PAFI test protocols or other procedures coordinated with FAA; 4 FAA updates on PAFI progress are available at https://www.faa.gov/about/initiatives/avgas. 5 See https://www.shell.com/business-customers/aviation/aviation-fuel/avgas.html. 6 See https://www.swiftfuels.com/swift-100r. 7 See https://www.faa.gov/about/initiatives/avgas.
PREPUBLICATION COPY â Uncorrected Proofs 90 â¢ Successful completion of an engine detonation screening test using the PAFI test protocols or other procedures coordinated with FAA; and â¢ Successful completion of a subset of the material compatibility tests using the PAFI test protocol or other procedures coordinated with FAA. FAA has offered the Technical Centerâs engine testing services to developers to perform these prescreening evaluations, with testing tentatively scheduled to resume in 2021 depending on developments with the coronavirus pandemic. FAA has also continued to emphasize that it stands ready to support other fuel applicants who have decided to pursue engine and airframe approvals that would allow the use of their fuel formulations through traditional certification processes separate from PAFI.8 During this study, the committee became aware of at least two initiatives in addition to those of Shell and Swift Fuels to develop an unleaded 100 MON fuel. Phillips 66 and Afton Chemical are developing a fuel that contains manganese to replace TEL, a proprietary scavenger formulation, and an antioxidant.9 LyondellBassel is also developing an unleaded high-octane fuel. In addition, General Aviation Modifications, Inc. (GAMI) claims to be developing a fuel formulation intended to replace 100LL.10 However, the committee could not find publicly reported technical information on GAMIâs fuel or its development status. During the summer of 2020, Swift Fuels announced that FAA certification testing and ASTM fuel specifications were in progress for an unleaded 100 MON fuel, named 100R.11 However, details on the fuel and its testing status are not publicly available for review. ENGINE MODIFICATIONS AND CONVERSIONS FOR UNLEADED AVGAS As discussed in Chapter 5, FAA has estimated that 43 percent of the existing piston-engine fleet cannot be operated safely using an avgas grade that has an octane rating lower than 100 MON. For at least a subset of these high-performance aircraft, it seems likely that it would be technically possible to engineer changes to their engines to enable operations with unleaded, lower octane avgas, such as through modifications to lower the compression ratio coupled with ignition calibration changes. Several modern high specific-output turbocharged engines are certified for operation using UL94, such as the Continental TSIO-550K12 and Rotax 914 and 915 series (which can use an 85 MON fuel),13 which suggests it would be technically feasible to retrofit at least some aircraft in the legacy fleet. In such cases, a challenge would be to limit any penalty to payload, range, ceiling, and runway performance that would discourage conversion investments by the aircraft owner. Ensuring safety, of course, is paramount, and any planned retrofit would require the aircraft owner to undertake the necessary testing to obtain an FAA supplemental type certificate (STC). Where conversions to a lower performance engine raise safety issues, such as by reducing performance during critical phases of flight (e.g., takeoff), the technical challenge of obtaining approval for a retrofit could be formidable. 8 See https://www.faa.gov/about/initiatives/avgas. 9 See https://www.phillips66aviation.com/about-us/news/industry-news/focused-on-the-future-of-avgas-ul100-qa. 10 See https://gami.com/g100ul/g100ul.php. 11 See https://www.swiftfuels.com/swift-100r. 12 See http://www.continental.aero/uploadedFiles/Content/Engines/Gasoline_engines/550AvGas-SpecSheet.pdf. 13 See https://www.flyrotax.com/produkte/detail/rotax-915-is-isc-2.html.
PREPUBLICATION COPY â Uncorrected Proofs 91 Considering that any re-engine program would be both costly to the owner and technically challenging to implement, lower-cost retrofit options may deserve exploration. One such option is an anti-detonation injection (ADI) device that injects a water-methanol mixture into the induction system to cool the combustion event during very high load operating conditions, such as takeoff and initial climb. The ADI concept is decades old, once employed by several piston-engine military aircraft.14 During the 1980s, FAA granted a number of ADI STCs for families of aircraft and engines (e.g., IO-470 and IO-520 families, Cessna 188 and 210, Beech Baron)âand examples of these aircraft with STCs remain in the fleet today. Originally marketed as a way to use less expensive automotive gasoline, ADI systems are still available and thus could enable the safe use of unleaded 94 MON fuel in at least some aircraft that otherwise require 100 MON. ADI conversion kits can be purchased and installed at a fraction of the cost of a full engine retrofit, but would nevertheless require outlays for the testing required to obtain the needed STC. By and large, the major investments required for development, testing, and installation suggest that an engine retrofit program targeted to an aging legacy fleet would not appear to be a promising way to reduce aviation lead. Considering that if fuel developers are successful in introducing an unleaded 100+ MON avgas, then some of these large investments in engine retrofits will have been made for naught. An additional factor that may warrant consideration in assessing this option is the incentive structure created by the General Aviation Revitalization Act of 1994 (P.L. 103-298).This legislation addressed the substantial impact of product liability litigation on GA aircraft and engine manufacturers, and the resulting increases in the price of aircraft. The act set a limit of 18 years on product liability claims post-purchase, but one provision is that this 18-year liability period is reset for manufacturers of the installed modifications.15 Hence if a major retrofit were performed on any aircraft built more than 18 years ago, which is the vast majority of the legacy fleet, owners of these older, modified aircraft could pursue liability claims, and owners of newer aircraft that are modified could have an extended product liability period. Such new and extended liabilities could limit manufacturer interest in retrofit programs. As an aside, the conversion of the motor vehicle fleet in the United States to unleaded gasoline during the 1980s and 1990s might be viewed as a model for converting the piston- engine GA fleet from leaded avgas to unleaded alternatives. For reasons explained in Box 6-1, however, the factors that prompted and enabled this conversion for automotive vehicles do not have strong parallels in the GA sector, particularly because of the need for backward avgas compatibility, the higher rate of automotive fleet turnover, and the large size of the automotive fuel market. Retrofitting existing motor vehicles was not necessary because of rapid turnover and the ability of older vehicles to run on unleaded gas without safety issues. 14 Todd Petersen. Unpublished document. Anti-Detonation Injection & Low Octane Fuel. See https://www.flyinpulse.com/user/file/73220.pdf. 15 The statute has a rolling date for any new component, system, subassembly, or other part which replaces or is added to the aircraft and which causes the accident. In these circumstances, the statute runs from the date the new component is added or replaced.
PREPUBLICATION COPY â Uncorrected Proofs 92 BOX 6-1 Conversion to Unleaded Gasoline by Automobiles and Challenges for Aviation The complete conversion of the automotive fleet in the United States from leaded to unleaded gasoline, which was completed less than 30 years ago, might be viewed as a model for such a conversion by the piston-engine fleet. The circumstances in the two sectors, however, were very different. As explained below, a key difference is that the automotive transition to vehicles that can use only unleaded fuel was not hampered by the need for leaded gasoline by legacy vehicles. As fleet turnover caused the number of legacy vehicles that could use leaded fuel to decline dramatically within two decades, so too did the demand for leaded fuel and any interest in supplying it. By comparison, in order to completely eliminate lead from avgas, aviation is constrained by the absence of an unleaded fuel that can satisfy a significant portion of its legacy fleet, which can be expected to remain significant for decades because of very slow turnover. Automotive Experience At the start of the 1970s, the national light-duty automotive fleet consisted of about 130 million vehicles, all satisfied with a range of leaded fuels with different octane levels. However, in anticipation of more stringent requirements in 1975 for allowable tailpipe emissions of hydrocarbons (HCs) and carbon monoxide (CO), automotive original equipment manufacturers (OEMs) determined that oxidation of HCs and CO through the use of catalytic converters would be essential for regulatory compliance and the catalytic converters, in turn, would require the use of unleaded fuel so as not to poison the catalyst. (Attempts to develop lead-tolerant catalysts proved fruitless.) Unleaded fuel with comparable octane levels became available quickly to accommodate catalyst-equipped vehicles entering the fleet from model year 1975. Because about 15 million new vehicles were added to the fleet each year, there was a rapidly developing market for unleaded gasoline. The unleaded grades could be used by older vehicles, but to prevent potential inadvertent misfueling of the new vehicles with catalysts, they were equipped with a narrower filler neck and unleaded gasoline was dispensed through a smaller diameter gas pump nozzle. The leaded gasoline pump nozzle would not fit through this filler neck. Subject to a phaseout schedule from the U.S. Environmental Protection Agency (EPA), some leaded gasoline was supplied until January 1996 to satisfy older vehicles, a small number of new cars that could comply with the 1975 standards while using leaded fuel, and gasoline- powered trucks without catalytic converters. Meanwhile, because of the millions of new vehicles added to fleet every year and the average service life of automobiles at the time was less than 12 years, the number of vehicles that could use leaded fuel had dropped precipitously by the late 1980s, such that the demand for leaded gas had all but dried up. Thus, the automotive conversion to unleaded gasoline can be characterized as having a forward focus. That is, an unleaded grade was introduced to facilitate the use of a new generation of emission control systems and leaded fuel was phased out during two decades to allow for fleet turnover to largely alleviate backward compatibility issues. Conversion Challenge for General Aviation The piston-engine GA fleet of about 170,000 active aircraft is satisfied with one standard leaded fuel, as was the case for automobiles in 1970. However, the GA fleet turns over at much slower
PREPUBLICATION COPY â Uncorrected Proofs 93 rate, with low attrition (as average aircraft lives span many decades) and the addition of only about 1,000 new piston aircraft per year. Going forward, those 1,000 new aircraft that are not already able to use an unleaded fuel could potentially be equipped to run on UL94, and if a UL100 were developed, they could use that fuel too. Therefore, the more vexing problem is how to accommodate the aircraft in the legacy fleet that now require a 100 MON gasoline, which is available only in leaded form. Retrofitting these aircraft to reduce octane requirement, such as through engine rebuilds to lower compression ratio and the adoption of electronic ignition, is not a promising route because the low average value of a legacy aircraft.a a FAA estimated that aircraft in the piston-engine fleet in 2014 had an average value of about $60,000, while the average value of an aircraft in the pre-1984 portion of that fleet was $44,000. See https://www.faa.gov/regulations_policies/policy_guidance/benefit_cost/media/econ-value-section-5-resto.pdf. NON-GASOLINE PROPULSION SYSTEM DEVELOPMENTS The committee cannot estimate when and if an unleaded 100+ MON avgas will be developed and introduced to safely accommodate the current and future piston-engine fleet, and as the preceding discussion makes clear the prospects of retrofitting substantial portions of the legacy fleet appear to be limited for technical and economic reasons. Aviation technology, however, is not static and even with low annual fleet turnover there may be opportunities to reduce lead emissions at least gradually from the GA sector through the development and introduction of new propulsion systems that do not depend on gasoline. Many of these technologies would raise the cost of aircraft but offer certain other advantages that may be compensating, particularly when applied to the commercial and working sector of the GA fleet, which accounts for a disproportionate share of avgas consumption and resulting lead emissions. In the sections that follow, a number of non-gasoline propulsion options are discussed, starting with the most technically ready systems such as diesel and turbine engines and then considering electric propulsion, which may be the most promising of all due to rapid advances in energy storage and onboard power generation and the potential for such systems to meet the low weight, size, and cost requirements of small aircraft. Although the discussion and examples given are not comprehensive, the systems described are indicative of a GA sector whose future direction is almost certain to be shaped in some way by propulsion technologies other than the traditional gasoline-powered, spark-ignition engine. It is important to note that the discussion of lead-free propulsion technologies does not include consideration of other potential emissions or environmental effects (e.g., changes in non-lead emissions such as greenhouse gases and fine particulate matter) that might be associated with their broad implementation in GA aircraft. Diesel Propulsion Diesel cycle, compression-ignition aircraft engines date back to the earliest days of powered aviation because of their attractive qualities of reduced flammability, increased thermodynamic efficiency, and higher energy density than gasoline. For instance, the Packard Motor Car Company was awarded the National Aeronautic Associationâs Collier Trophy in 1931 for the introduction of a nine-cylinder, air-cooled, diesel aviation engine.16 Nevertheless, even though it 16 See https://naa.aero/awards/awards-and-trophies/collier-trophy/collier-1930-1939-winners.
PREPUBLICATION COPY â Uncorrected Proofs 94 was used in a number of aircraft and airships, the diesel engine never gained traction in the GA sector, perhaps eclipsed by the rapid pace of development in the automotive sector of gasoline- powered engines. As automotive gasoline engine technology developed, the growing GA market benefited from engine advances and the widespread availability of inexpensive automotive gasoline. Weight was also a factor, as gasoline engines generally have better power-to-weight ratios than the heavier diesel engine and aircraft flight performance is highly dependent on weight. While diesel engines can generally run on jet fuel (Jet-A) with adequate cetane number, the unavailability of this fuel at most small airports is likely to have been an additional reason the diesel engine never caught on, and this factor may be a deterrent to its future popularity for GA uses.17 Modern diesel aviation engines that offer up to 300 horsepower are nevertheless available and have been in production for use in retrofits and new GA-type aircraft for several decades, buttressed by continual advances in weight-competitiveness, reliability, and performance capabilities. Diesel propulsion is more common in Europe than in the United States, perhaps because the availability of 100LL can be limited in European airports, while jet fuel is more widely available. Performance and Cost Considerations A direct comparison between gasoline and diesel propulsion can be difficult to make because of differences in airframe/engine installations, engine weight, and fuel weight, which must also account for the higher energy content of jet fuel relative to avgas (~11 percent more BTUs per gallon) and the diesel engineâs high compression that contributes to about 30 percent less fuel consumption per horsepower output. From a performance standpoint, diesel engines are competitive with gasoline engines having similar horsepower, but also offer some distinct advantages. As noted, the diesel cycle is more energy efficient. Moreover, the engines are normally liquid cooled, so temperature control is easier, and without magnetos, ignition and engine control can be managed with a full authority digital engine control (FADEC) system. The modern diesel engine is sufficiently advanced that it has the potential to be used in new aircraft or retrofitted in some existing aircraft in the GA fleet. However, the conversion of a small, 4-cylinder, gasoline-powered aircraft to a comparable diesel-powered system (including the addition of water cooling, necessary propeller and engine display modifications, and flight testing) would be costly, estimated by FAA to be about $100,000, which is more than the market value of an average small aircraft in the legacy fleet, which FAA estimated in 2017 was about $60,000.18 Accordingly, such conversions are not likely to be a practical option except among the most heavily utilized GA aircraft that operate in airports where diesel or jet fuel are available. The cost to retrofit an existing airframe would include the engine acquisition cost, the airframe and installation modifications, and the flight test program before final acceptance for an STC, which in many cases would exceed the value of the existing aircraft. While conversions would 17 An August 2018 article in Flying magazine outlines many of the benefits of diesel technology with an emphasis on the absence lead emissions, but suggests that one reason GA pilots have shown limited interest in newer diesel options is the ready availability of relatively inexpensive 100LL. Mark, R. 2018. Inside the Diesel Revolution. Flying. August 1. See https://www.flyingmag.com/inside-aviation-diesel-revolution. 18 The $100,000 retrofit cost is based on an estimate provided by FAA in a November 19, 2019, briefing to the committee. The average value of piston-engine aircraft was estimated by FAA, as noted in Chapter 2. See https://www.faa.gov/regulations_policies/policy_guidance/benefit_cost/media/econ-value-section-5-resto.pdf.
PREPUBLICATION COPY â Uncorrected Proofs 95 seem to have many economic and practical drawbacks, the same challenges do not exist for new aircraft that would be optimized for diesel propulsion. Current Production and Future Prospects While the economic viability of a diesel propulsion system would seem greater for new aircraft, the airframe and engine makers considering such a system must weigh the sales potential in light of the upfront investments required to develop and produce the aircraft, including absorbing the cost of an extensive certification process. There are nevertheless several diesel aircraft engines in production worldwide that are designed for GA aircraft categories. For instance, the engine manufacturer Continental offers a range of diesel engines providing up to 300 horsepower, which the company states have logged more than 7 million flight hours by the more than 6,000 units delivered.19 Continental offers an STC for a diesel modification of the Cessna 172 and several other aircraft that are heavily used for pilot training. The other major GA engine maker in the United States, Lycoming, also offers a diesel engine derived from a European model.20 The 200 horsepower engine is presently in operation in an unmanned aircraft, but the company states that it intends to certify it for piloted aircraft. Other aviation engine makers developing diesel engines include EPS, which offers engines in the 320 to 420 horsepower range,21 and DeltaHawk, which makes a diesel engine that provides 180 horsepower.22 Currently, many of these diesel engine variants are intended for conversions, because there are few diesel aircraft in production in the United States. Although Cessna developed and certified the diesel-powered Turbo Skyhawk JT-A, it ended production in 2018. Outside the United States, the Austrian aircraft maker Diamond Aircraft Industries sells a twin-engine aircraft, the DA-42 NG, which is powered by the Austro AE-300 turbo diesel engine, and this aircraft is certified in the United States. Diesel propulsion is thus a proven technology that is available now for existing and new GA aircraft. Its promise as an alternative to gasoline engines will nevertheless continue to depend on advances that reduce weight and cost, as well as its appeal to GA aviators interested in purchasing high-utilization aircraft. The potential for large-scale conversions of legacy aircraft to diesel systems appears low given the unlikely prospects that owners of most aircraft could recoup the high conversion cost. Turbine Propulsion The three general classes of turbine-powered aircraft, which burn jet fuel, are turboprop, turbofan, and turbojet designs. A turboprop aircraft uses a gas turbine to drive a shaft and propeller that provide thrust forces to propel the airplane, with a small amount of thrust from the turbine exhaust. In the turbofan aircraft, the turbine powers a forward-mounted fan system. Depending on the bypass ratio, most thrust is created by the fan, although some thrust is still derived from the jet exhaust. Conversely, a turbojet engine develops all of its thrust from the 19 See https://www.continentalmotors.aero/diesel/diesel-engines.aspx. 20 See https://www.lycoming.com/engines/del-120. 21 AOPA ePublishing staff. 2019. EPA Gives Certification Update on Diesel Engine. January 23. See https://www.aopa.org/news-and-media/all-news/2019/january/23/eps-gives-certification-update-on-diesel-engine. 22 Conrad, J.W. 2019. DeltaHawk Diesel Makes First Flight to AirVenture. July 26. See https://www.eaa.org/airventure/eaa-airventure-news-and-multimedia/eaa-airventure-news/eaa-airventure- oshkosh/07-26-2019-deltahawk-diesel-makes-first-flight-to-airventure.
PREPUBLICATION COPY â Uncorrected Proofs 96 exhaust gas. A powerful turbine engine coupled to a propeller provides for the efficient generation of static thrust for a given horsepower, particularly at lower airspeeds. As a result, turboprops can be used on shorter runways than turbofan aircraft and can be used for applications that require the use of unpaved fields and amphibious service. A number of GA aircraft are turboprop designs, but they are used almost exclusively for short-haul passenger and cargo service and total fewer than 10,000 units (about 5 percent of the GA fleet).23 The Beechcraft Super King Air, Piper PA-46 family, and Cessna 208 Caravan are examples of turboprop GA aircraft. Performance and Cost Considerations Turboprop engines have a number of advantages over gasoline engines. They are considerably lighter in weight for the same power output and can operate at higher altitude. Whereas the performance of normally aspirated piston-engine aircraft drops rapidly above 15,000 feet mean sea level (MSL), the turboprop can be flat rated at sea level power at 25,000 feet MSL or higher. This enables the aircraft to fly at higher speed at altitude and cover distances more rapidly, offering cruising speeds of 200 to 350 mph and ranges in excess of 1,200 miles. There are significant deterrents to the use of turboprops for typical GA applications, including higher non-recurring and recurring engine maintenance costs, the need for additional pilot ratings, and performance characteristics that are not aligned with the needs of local and recreational flying. Even the smallest turboprops can cost $2 million to $4 million, which are multiples of the price of comparably sized piston-engine models. Nevertheless, turboprops have proven utility for a small segment of the high-utilization GA sector and that market could potentially expand if future technology improvements reduce the cost of ownership. While jet fuel is available at many larger airports that serve turbine aircraft, its lack of availability at most smaller airports could be problematic for more widespread use. Current Production and Future Prospects One can find examples of turbine engine makers working on turboprop systems that could have greater attraction for GA uses. Rolls Royce has discussed the development of a RR-500 family of turboprop engines capable of 300+ horsepower, and it has participated with the Mooney Aircraft Company in a market investigation intended to explore this and other alternative power options for private aircraft. Running at full power, the engine would burn approximately 21 to 24 gallons of jet fuel per hour, but at the higher expected airspeeds it has the potential to exhibit similar fuel efficiency (in nautical miles per gallon ) as a gasoline-powered aircraft possessing similar horsepower.24 Focusing on even smaller turboprop aircraft, PBS Aerospace, a company based the Czech Republic, advertises that its TP100 engine is capable of 188 horsepower at cruise speed. The company maintains that the engine is especially well suited to small aircraft and unmanned aerial vehicles, for uses such as search and rescue services, reconnaissance, and agriculture applications. 23 See https://www.bts.gov/content/active-us-air-carrier-and-general-aviation-fleet-type-aircraft. 24 See https://www.avweb.com/air-shows-events/mooney-rolls-royce-look-at-turbine-single and https://www.youtube.com/watch?v=j2nD7Nqh7B4.
PREPUBLICATION COPY â Uncorrected Proofs 97 Electric and Electric Hybrid Propulsion Electric propulsion has accelerated during the past 10 years, with new application concepts for GA, commercial air transport, and even urban air mobility. Several new technologies have enabled electrical propulsion to expand into the aviation domain, including improved battery storage technology and high efficiency/high power density electric motors. Advances are being made along various technology fronts including improved battery storage for pure electric propulsion and for hybrid systems that use battery storage in combination with electric power produced onboard the aircraft through means such as fuel cells and turbo-generators. Performance and Cost Considerations Because space, weight, and power are critical aircraft design parameters, the potential for a battery-powered aircraft propulsion system is largely driven by the energy capacity per weight and volume of the battery system. The lithium ion battery dates back several decades, but its practical applications have grown as energy storage capacity and operating times between recharges have increased. The specific energy of lithium cells is now on the order of 250 watt- hours per kilogram, which still limits aeronautical applications. However, ongoing research into solid state lithium-metal battery technology suggests this figure could double within the next few years and enable the development of longer endurance and longer range aircraft if issues related to safety, operational lifetime, and manufacturability can be overcome. While higher specific energy in batteries is being pursued to enable pure electric aircraft, hybrid propulsion systems that use electricity that is both stored and generated onboard the aircraft are also an option. Several candidate types of power generators are being investigated, including diesel and spark ignition piston engines, hydrogen proton exchange membrane (PEM) fuel cell systems, and regenerative turbo-generators using jet fuel. In a PEM fuel cell, lightweight hydrogen is converted through an electrochemical reaction to produce electricity to drive motors and/or to store in batteries. A regenerative turbo-generator consists of a turboshaft engine in which air entering the compressor is pre-heated through a heat exchanger by the high temperature exhaust exiting the turbine, resulting in high efficiency. This turboshaft engine is then coupled to an electric generator to produce electricity for use in powering electric motors or storing in batteries. Current Production and Future Prospects The GA sector is already benefiting from advances in battery technology and lightweight motors. For example, Bye Aerospace is seeking FAA certification for a two-seat battery electric light aircraft, called the eFlyer, for flight training missions.25 The eFlyer carries lithium-ion batteries and is powered by a Siemens 70 kilowatt continuous power motor. Siemens has also developed a 260 kilowatt, 350 horsepower motor weighing about 104 pounds. A potential indicator of the future prospects for electric propulsion is the recent purchase of the Siemens aircraft electric motor business by Rolls Royce, a major aircraft engine maker intent on furthering the electric and hybrid-electric aircraft market. 25 Lincoln, A. 2019. eFlyer Developmental Prototyoe Flight Tests Confirm Benefits of Electric PropulsionBye Aerospace. October 21. See https://byeaerospace.com/eflyer-developmental-prototype-flight-tests-confirm-benefits- of-electric-propulsion.
PREPUBLICATION COPY â Uncorrected Proofs 98 At least two companies, Scaled Power and Turbotech, are developing small regenerative turbo-generators suitable for small GA aircraft. Scaled Power advertises a turbo-generator that it claims offers better performance than piston or fuel cell systems.26 Turbotech, which is based in France, advertises a regenerative gas turbine in either a generator or turboprop configuration.27 Boeing conducted a successful proof of concept flight of a light aircraft using a battery/fuel cell system as early as 200828 and PEM fuel cells are now being used in production automobiles including the Toyota Mirai. The ability of airports to install the needed charging infrastructure, and possibly even hydrogen storage and dispensing systems, could be an important factor driving interest in the future use of battery electric and hybrid electric aviation propulsion. FINDINGS AND RECOMMENDATIONS While a lead-free, high-octane (100+ MON) avgas to fully replace leaded avgas for the entire fleet without requiring changes to aircraft or their engines would be ideal, it faces many challenges that more than 25 years of research into hundreds of fuel formulations has not been able to yet address. While several fuel suppliers are actively trying to develop such a fuel, their prospects for success could not be assessed directly in this study because the fuel formulations and testing results are proprietary. It is uncertain when such a fuel can be developed, tested, and accepted, and the costs associated with its adoption and use are not known, nor are the challenges of deploying it at airports across the country (Finding 6.1). The FAA-industry PAFI collaborative represents a systematic and holistic approach for screening, evaluating, and selecting an acceptable unleaded replacement for leaded avgas for fleetwide use, as well as for overcoming certification and other obstacles to the commercialization and widespread introduction of a lead-free alternative fuel. Although it has not yet yielded a viable replacement, PAFI has led to the development of a fuel testing and evaluation process, prompted supplier interest in developing replacement fuels, and sought solutions to the many regulatory, economic, and other practical challenges associated with developing, introducing, and broadly supplying an unleaded replacement fuel (Finding 6.2). FAA should continue to collaborate with the GA industry, aircraft users, airports, and fuel suppliers in the search for and deployment of an acceptable and universally usable unleaded replacement fuel. The collaboration should be carried out through PAFI or an alternate holistic process for evaluating all the properties and conditions necessary for production, distribution, and safe use of the fuel, including the use of common test protocols and procedures and by making available the needed testing facilities for the development of the data required to support FAA approvals for the fuel to be used by existing piston-engine aircraft (Recommendation 6.1). Retrofitting current aircraft to enable fleetwide use of currently available unleaded fuels and other lead-free means of propulsion would require incentives to develop new technologies for those aircraft where retrofits do not currently exist. Incentives also would need to motivate 26 See http://www.scaled-power.com. 27 See http://www.turbotech-aero.com. 28 Koehler, T. 2008. A green machine. Boeing Frontiers. May. See https://www.boeing.com/news/frontiers/archive/2008/may/ts_sf04.pdf.
PREPUBLICATION COPY â Uncorrected Proofs 99 large and potentially prohibitive investments by aircraft owners in systems such as anti- detonation injection, replacing engines along with other critical components, and undergoing costly recertification processes (Finding 6.3). Tangible success is being demonstrated by aircraft engine makers in creating high- performance gasoline engines that can run on existing unleaded avgas, and innovations in alternative, lead-free propulsion systems (such as diesel, electric, and gas turbine) are showing increasing potential for GA aircraft. Implementation of these new technologies can result in the phasing in of aircraft that do not use leaded fuel and would not be subject to the uncertainty of waiting for an unleaded 100 MON fuel to be developed and deployed widely. Such a technology transition, however, would be limited by the slow turnover rate of GA fleet, barring new incentives to hasten it (Finding 6.4). A clear goal should be established that all newly certified gasoline-powered aircraft after a certain point in time (e.g., within 10 years) are approved to operate with at least one ASTM-approved unleaded fuel. Also, an additional amount of time should be identified by which all newly produced gasoline-powered aircraft, including those currently produced with older type certificates, would attain that same goal. Congressional action to establish the goal and timeframes would ensure achievement of those important results. For example, that congressional action would promote the development of new engine variants compatible with existing unleaded fuels, which could possibly yield prescriptions to support the eventual retrofit of some legacy aircraft and engines as they reach required overhaul milestones (Recommendation 6.2). FAA initiativesâincluding collaborations with industry and other government agencies such as the National Aeronautics and Space Administrationâshould be used to promote the development, testing, and certification of safe and environmentally desirable lead-free emerging propulsion systems (such as diesel, electric, and jet fuel turbine engine) for use in GA aircraft, including the requisite airport refueling and recharging infrastructure. Congressional encouragement and provision of resources as required would ensure the success of those initiatives (Recommendation 6.3). REFERENCES CRC (Coordinating Research Council). 2010. Research Results Unleaded High Octane Aviation Gasoline. CRC Project No. AV-7-07. June. Available at: http://crcsite.wpengine.com/wp- content/uploads/2019/05/AV-7-07-Final-Report-6-18-10.pdf. UAT ARC (Unleaded Avgas Transition Aviation Rulemaking Committee). 2012. FAA UAT ARC Final Report. Part I Body. Unleaded AVGAS Findings and Recommendations. February 17. Available at: https://www.faa.gov/regulations_policies/rulemaking/committees/documents/media/Avga s.ARC.RR.2.17.12.pdf.
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