Options for Reducing Lead Emissions from Piston-Engine Aircraft (2021) / Chapter Skim
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6 Potential Future Lead-Free Fuels and Propulsion Systems
6 Potential Future Lead-Free Fuels and Propulsion Systems
Pages 105-124
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From page 105... ...
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 ASTM International standard (i.e., 100 octane very low lead aviation gasoline [100VLL]
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From page 106... ...
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.
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From page 107... ...
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.
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From page 108... ...
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.
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From page 109... ...
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.
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From page 110... ...
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.
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From page 111... ...
In reporting the status of the testing of Shell's 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: • Successful completion of a 150-hour engine endurance test on a turbocharged engine using PAFI test protocols or other procedures coordinated with FAA; • Successful completion of an engine detonation screening test using the PAFI test protocols or other procedures coordinated with FAA; and • Successful completion of a subset of the material compatibility tests using the PAFI test protocol or other procedures coordinated with FAA.
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From page 112... ...
During summer 2020, Swift Fuels announced that FAA certification testing and ASTM International 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.
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From page 113... ...
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.
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From page 114... ...
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.
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From page 115... ...
Thus, the automotive conversion to unleaded gasoline can be characterized as having a forward focus. That is, an unleaded grade was introduced to facili tate the use of a new generation of emission control systems and leaded fuel was phased out during the course of two decades to allow for fleet turnover to largely alleviate backward compatibility issues.
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From page 116... ...
Many of these technologies would raise the cost of aircraft but offer certain other advantages that may compensate, 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, onboard power generation, and the potential for such systems to meet the low weight, size, and cost requirements of small aircraft.
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From page 117... ...
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 engine's high compression that contributes to about 30 percent less fuel consumption per horsepower output.
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From page 118... ...
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 (Conrad, 2019)
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From page 119... ...
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.
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From page 120... ...
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)
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From page 121... ...
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.
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From page 122... ...
. 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.
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From page 123... ...
. 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)
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From page 124... ...
https://www.flyingmag.com/ inside-aviation-diesel-revolution. UAT ARC (Unleaded Avgas Transition Aviation Rulemaking Committee)
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