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"There are two primary fuels in use in aviation: avgas and\nJet-A.&nbsp; They are similar in nature to\nautomotive fuel (unleaded gasoline) and diesel, respectively, but have some key\ndifferences in both their properties and ability to be used in aviation\napplications. So, what is the difference between avgas, jet\nfuel, diesel, and auto fuel? \n\n\n\nAvgas, auto fuel, and\ndiesel are designed for piston engines, whereas jet fuel is designed for\nturbine engines. Avgas and jet fuel are formulated for the specific operating\nconditions and engine performance demands associated with aviation. \n\n\n\nThese fuels are all different in the types of engines they\nare designed for and the combustion and ambient temperature ranges in which\nthey are designed to operate. Knowing the difference between these fuels is\nimportant not only for the safety of flight, but it’s also a pretty common\nquestion your non-aviation friends will ask you. In this post we’ll give you\nall of the information you need to know to give a solid answer to anyone who\nwants to know the differences between all of these fuels. \n\n\n\nWhat is the difference between avgas, jet fuel, auto fuel, and diesel? \n\n\n\nIn order to understand the specific differences of these\nfuels, we need to first discuss the two main types of fuel. There are\neffectively two different types of fuel in use in combustion engines, “compression fuels” and “ignition fuels”.&nbsp; \n\n\n\nCompression fuels,\nfor the purposes of this discussion are those that require highly compressed\nfuel air mixtures in order to burn properly, and ignition fuels are those that\nare ignited using some type of spark or other ignition source.&nbsp; \n\n\n\nCompression fuels are Jet-A (and its equivalents), diesel and kerosene.&nbsp; Ignition fuels are aviation (AvGas) and automotive (MoGas) gasolines.\n\n\n\n\n\n\n\nWhat Differentiates Compression Fuels and Ignition Fuels?\n\n\n\nIgnition fuels and compression fuels are differentiated by a host of material properties that are a result of the refining process.&nbsp; Refer to “Why Does Jet A Fuel Cost Less than 100LL (Avgas) Fuel?” for additional details on the refining process.\n\n\n\nIgnition Fuels\n\n\n\nSimply put, ignition fuels are a mixture of lighter, more volatile compounds that require a specific ignition source to burn controllably.\n\n\n\nBecause ignition fuels are created later in the refining\nprocess, the carbon compounds that make up the fuel are more consistent and\nsimultaneously more energetic and less stable.&nbsp;\nIgnition fuels will tend to ignite unevenly under sufficient pressure, a\ncondition known as knock or detonation, which can quickly lead to engine\ndamage. \n\n\n\nGasolines are\nconsidered ignition fuels, and require a spark plug to initiate the combustion\nprocess.\n\n\n\nCompression Fuels\n\n\n\nCompression fuels on\nthe other hand are created earlier in the refining process and contain a wider\nvariety of carbon chains that are more stable under a wider range of\ntemperatures and pressures.&nbsp; \n\n\n\nAdditionally, because the auto ignition temperature for\nthese fuels is lower, the associated pressures required inside combustion\nengines are lower, resulting in simpler engine designs.&nbsp; \n\n\n\nDiesel, Kerosene, and\nJet-A are all examples of such fuels. Compression of the fuel is used to\ninitiate the combustion process eliminating the need for full time ignition\nsystems within the engines that use these fuels. \n\n\n\nWhat types of engines use each type of fuel?\n\n\n\nCombustion engines\ncome in two “flavors:” piston and turbine.&nbsp;\nPiston engines use the familiar cylinder and piston arrangement and use\neither two or four piston strokes per ignition event.&nbsp; Turbine\nengines continuously compress air, ignite it within a static combustion\nchamber and expel the exhaust product through an expansion turbine to drive the\nintake compressor.&nbsp; \n\n\n\nAircraft apply these engines differently depending on a\nrange of design factors and requirements.&nbsp;\nLarge modern aircraft generally use\nturbines and piston engines are favored by small personal aircraft.&nbsp; \n\n\n\nTypes of Fuel for Turbine Engines\n\n\n\nTurbine engines use compression fuels, and in aviation these\nengines (which can be Turbojets, Turbofans, Turboshafts, or Turboprops – the\ndifference between these is a function of what types of machinery are driven by\nthe expansion turbine) all run on Jet-A and its military and international\nequivalents. \n\n\n\nSome aircraft permit the use of aviation gasoline as an\nemergency fuel, but such operations are very strictly limited and induce a\nsignificant reduction in the operating life of the engine.&nbsp; \n\n\n\nTurbine engines are\ndesigned around the use of Jet-A because of the fuel’s design to be highly\nstable and predictable in a wide range of environments, particularly at extreme\ntemperatures and its high energy density.\n\n\n\nJet-A is also safer to handle in large quantities as a\nresult of its much higher flashpoint, and when dealing with large quantities of\nfuel, it is beneficial to utilize a fuel that will not ignite easily outside of\nthe engine combustion chamber.&nbsp;\nCommercial and military operations benefit massively from the reduced\nstorage precautions required to store vast quantities of liquid fuel. &nbsp;As a result the broader turbine engine world\nhas standardized around this type of fuel. \n\n\n\nTypes of Fuel for Piston Engines\n\n\n\nPiston engines are the second category of aviation engine in\ncommon use, and depending on their design may use ignition or compression\nfuels.&nbsp; Piston engines may burn\nautomotive or aviation gasoline (ignition fuels, though few can use automotive\nfuel… more on that later), or for aviation diesel engines, Jet-A (compression\nfuel).&nbsp; \n\n\n\nThe standard fuel\nused in aviation piston engines is aviation gasoline, or AvGas.&nbsp; AvGas as a technical term covers a wide range\nof different types of aviation fuel, but the most commonly used avgas is 100LL\n(“One Hundred Low Lead”).&nbsp; \n\n\n\nThe Experimental Aviation Association (“EAA”) has recently\npromoted inexpensive STCs (supplemental type certificates) allowing the use of\nalcohol free automotive fuel in certain aviation piston engines.&nbsp; The approved engines typically are approved\nfor the use of lower octane fuels that are no longer produced specifically for\naviation engines.&nbsp; \n\n\n\nThese STCs are popular with the owners of older light\naircraft and with homebuilders and it offers a potentially lower cost source of\nfuel.&nbsp; \n\n\n\nThe rarest piston fuel is by far Jet-A.&nbsp; Aviation diesel engines are an abnormality in\nthe piston aircraft fleet, as the technical challenges associated with building\nlightweight aircraft diesel engines are substantial.\n\n\n\nWhat are the differences between compression fuels? \n\n\n\nKerosene is a generic name for a wide range of fuel products, of which Jet-A is a tightly standardized version.&nbsp; As a fuel product, Kerosene uses a wider range of quality and chemical property standards, which is acceptable for most terrestrial and marine applications.&nbsp; Because aviation requires consistent fuel performance across multiple engines and airframes and in rapidly changing environments, Jet-A is more precisely refined to remove impurities and ensure consistent performance.\n\n\n\n\n\n\n\nIt is apparent from the table above that Jet-A is refined to\nmuch tighter tolerances.&nbsp; This is\nnecessary to ensure the fuel meets the necessary performance across the\noperating envelope of turbine aircraft.&nbsp; \n\n\n\nCommercial transport aircraft operate in cruise at temperature well below -47°C (fuel / oil heat exchangers raise the fuel temperature – see “How Does Fuel Aircraft Fuel Not Freeze During Flight?” or watch the 2-minute version below), making consistent thermal properties mandatory for Jet-A fuel.&nbsp; The slightly lower autoignition temperature allows for more reliable performance within the combustion chamber of the turbine engine itself. \n\n\n\n\nhttps://www.youtube.com/watch?v=YvSuIc_HgIQ\n\n\n\n\nFinally, the thermodynamic output of the fuel must also be\nconsistent in order to ensure that flight planning manuals provide accurate\ndata, as lower heating values produce lower thrust output, and reduced cruise\nperformance.&nbsp; The criticality of\nconsistent performance drives these tighter tolerances.&nbsp; While\na typical aircraft turbine would burn generic kerosene, engine performance\nwould be reduced and unpredictable. \n\n\n\nThe difference between diesel fuel and Jet-A is similar to that between Jet-A and kerosene, but of an even greater magnitude.&nbsp; Diesel fuel is designed and refined for use in diesel piston engines; it contains higher concentrations of impurities, and does not tolerate the wide temperature extremes needed in aviation.&nbsp; \n\n\n\n\n\n\n\nReviewing the data in Error! Reference source not found. shows that the autoignition\nand freezing points of automotive diesel are substantially different than those\nof Jet-A.&nbsp; These differences are tied to\nthe chemical makeup of the two fuels, with Jet-A being refined at higher\ntemperatures, thus removing the sulfur compounds and providing a more uniform\nset of carbon molecules.&nbsp; \n\n\n\nJet-A is essentially\nan extremely high-quality diesel fuel, and indeed running a typical automotive\ndiesel engine on Jet-A is likely possible.&nbsp;\n\n\n\n\nHowever, the chemical compounds within diesel fuel have\nlubricating properties that ensure proper operating of fuel injectors, pumps\nand other mechanical components; the higher chemical purity of Jet-A strips\naway these lubricating compounds, resulting in more consistent performance, and\na wider useful temperature envelope, but poorer mechanical properties.&nbsp; \n\n\n\nTurbine engines would\nfunction using diesel fuel, but at the cost of reduced performance and the buildup\nof harmful deposits within the engine’s fuel system. &nbsp;\n\n\n\nAdditionally the higher vapor pressure of diesel fuel may\ninterfere with proper atomization of the fuel within the combustion chamber\nleading to a host of other issues. &nbsp;There\nare turbine engines designed to use diesel fuel, but these engine have distinct\ndesign requirements from aviation turbines. \n\n\n\nWhat are the differences between ignition fuels? \n\n\n\nThe common “ignition fuels” in use in aviation are AvGas and\nAuto Fuel (MoGas).&nbsp; The base chemical\ncomponents of these fuels are virtually identical, as both are fundamentally\ngasoline and are extracted from the refining process at the same point.&nbsp; The\ndifference between MoGas and AvGas is essentially a matter of a particular\nadditive: Lead. \n\n\n\nAvGas contains\ntetraethyl lead (TEL), to improve engine component lubrication, and boost the octane\nrating of the fuel.&nbsp; This type of\noctane boosting was common in automotive engines until 1975, when environmental\nconcerns surrounding lead contamination resulted in the phase-out of lead in\nautomotive gas.&nbsp; \n\n\n\nOctane rating of the\nfuel is a measure of the fuel’s resistance to detonation (known as knock in\nautomotive engines).&nbsp; Detonation is an\noperating condition in which the fuel ignites spontaneously under compression\nprior to normal firing of the spark plug.&nbsp;\nThis results in severe pressure and temperature spikes within the\nengine, and quickly leads to severe engine damage.&nbsp; \n\n\n\nTetraethyl lead’s\nfunction as an octane booster is intended to allow higher engine performance at\nhigh power setting, particularly with the use of forced induction\n(superchargers and turbochargers).&nbsp;\nTetraethyl Lead reduces wear in valve assemblies with the engine.&nbsp; The build-up of lead on valve guides and\nseats helps to lengthen the service life of these components.\n\n\n\nA wide range of\naviation engines are compatible with unleaded fuels such as MoGas, presuming\nthe engine manufacturer has approved the use of unleaded fuel and the\nassociated octane rating is met.&nbsp;\nTypically, these engines have fairly low horsepower outputs and low\ncompression ratios, as such the aircraft they are installed in are typically\nsmall, lightweight aircraft. \n\n\n\nThe greatest hazard for aircraft engines using unleaded fuel\nis typically engine knock during climb at high altitudes, where the fuel air\nmixture is generally leaned to maximize engine performance, and where engine\ntemperatures are highest.\n\n\n\nAdditionally, MoGas\nwhen compared with AvGas, reflects many of the same quality standard\nvariability issues as Kerosene and Jet-A.&nbsp;\nThe tighter quality control processes required for aviation fuels\nensures that greater uniformity is present within AvGas.&nbsp; \n\n\n\nOne example of this is the use of ethanol as a fuel\nadditive.&nbsp; Ethanol is increasingly mixed\nwith automotive fuel to raise the octane rating and improve fuel economy at\nreduced cost to fuel refiners, but the precise mixture ratio is rarely\nplacarded sufficiently to ensure safe aviation use.&nbsp; \n\n\n\nUnfortunately, ethanol is incompatible with nearly all aviation piston engines and their fuel systems.&nbsp; The rubberized components used in aircraft fuel systems, from tanks, to hoses to seals are not compatible with ethanol and quickly deteriorate when ethanol laced fuel is used.&nbsp; \n\n\n\nWhere MoGas is used in aviation engines, it is ethanol free and the octane rating is typically equivalent to or higher than the “premium” fuel found at the local gas station in order to meet the minimum octane requirements of aviation engines. \n\n\n\nAutomotive engines have been designed to utilize a wide\nrange of fuel qualities and tolerate a wide variety of additives to improve\nengine life and durability.&nbsp; The effect\nof these additives is numerous, but in particular, these additional chemicals\nraise the vapor pressure of MoGas, meaning that it evaporates from liquid to\ngaseous at a higher rate relative to AvGas.&nbsp;\n\n\n\n\nThis higher evaporation rate makes MoGas susceptible to\nvapor lock at high altitudes and at high temperatures, making engine starts\nmore difficult and engine failures at low power settings more likely.&nbsp; \n\n\n\nIt is common for MoGas STCs to specify the mixing of certain quantities of AvGas to improve the vapor pressure, or to artificially cap the aircraft’s ceiling when using MoGas.&nbsp; \n\n\n\nQuick Break: Have you subscribed to the Airplane Academy YouTube channel yet? Each week we're putting out videos sharing aviation adventures, ride-along's, answers to commonly asked training questions, and much more! Check it out here.\n\n\n\nRelated Questions\n\n\n\nWhy aren’t more diesel engines used in aviation?\n\n\n\nPiston engine powered\naircraft continue to rely on AvGas as a fuel source because the design\nrequirements of diesel engines incur an inherent weight penalty, and new engine\ndevelopment is expensive and prolonged.&nbsp;\n\n\n\n\nDiesel engines are typically heavier than their gasoline\nequivalents to meet the strength requirements associated with compression\nengine cycles.&nbsp; Automotive and aviation\ndiesels alike nearly exclusively require turbocharging to be effective power\nplants as throttle response is a primary design consideration in both\nindustries.&nbsp; \n\n\n\nAdditionally, aviation diesels are, primarily, derived from\nautomotive diesels and thus must be geared down to drive a propeller.&nbsp; These gearboxes are failure prone, and often\nresult in expensive replacement costs over dramatically shorter time intervals\nthan is typical for gasoline engines.&nbsp; These\nadditional mechanical systems drive up the weight and complexity of the engine,\na serious detriment to the implementation of diesel engines as a primary\npowerplant for piston powered aircraft.&nbsp; \n\n\n\nThis resultant\nincrease in weight generally results in an engine that develops less power for\nthe same basic engine size and weight, resulting in lower aircraft performance.&nbsp; \n\n\n\nLower performance is not a detriment for certain segments of\nthe aviation industry, particularly in the training sector and Asian and\nAfrican markets, but sufficient sales have not yet materialized to justify the\ndevelopment costs and timelines necessary to certify new piston aircraft under\ncurrent regulatory burdens. \n\n\n\nThe automotive roots of these engines do provide some benefits however, much of the research and development costs of the engine itself have been amortized by the automotive industry, and the engine control systems are typically electronic (i.e. FADEC controlled just like many turbine engines).&nbsp;\n\n\n\n Electronic controls simplify engine operation and allow for extremely efficient operations.&nbsp; However, there is not yet sufficient net operating cost benefit for these engines to make significant sales gains within the piston aircraft fleet, either with OEMs or as aftermarket STCs. \n\n\n\nWhy do aviation piston engines need such a high octane rating?\n\n\n\nConsider briefly your car’s engine. When you drive you spend\nlittle time with the throttle wide open developing maximum power.&nbsp; Additionally, your car’s ECU (engine control\nunit) is effectively a FADEC (full authority digital engine control) carefully\ncontrolling timing, fuel delivery and spark to eliminate nearly all knock, or\ndetonation.&nbsp; \n\n\n\nNow consider the typical aviation engine, which is designed\nto operate at or above 75% maximum continuous power for the entirety of its\nlifecycle, without the aid of electronic controls.&nbsp; It is apparent that if typical automotive\nengines were operated similarly to aviation piston engines, higher octane\nvalues would be required to maintain reliability.&nbsp; \n\n\n\nThe high octane rating requirements of aviation piston engines stem from the pinnacle of piston engine development in the 1940s into the 1950s. &nbsp;Supercharging and turbocharging drove the outputs of piston engines to extreme heights in order to maximize engine performance for military and transport aircraft.&nbsp; \n\n\n\nThe extreme operating pressures and temperatures required fuel that would resist detonation under high engine loads for hours at a time.\n\n\n\nThe requirement for\naviation piston engines to maximize power output under continuous high load is\nthe driving design requirement for the use of high octane rating fuels.&nbsp; While\nsome aircraft engines are capable of operating on lower octanes, “practical”\naircraft that utilize high performance or turbocharged engines are still\nlimited to using high octane fuels to avoid fuel detonation.&nbsp; \n\n\n\nWhy can’t turbine engines use AvGas?\n\n\n\nTurbine powered aircraft operate at environmental extremes\nthat necessitate extremely stable, pure fuels that produce consistent,\npredictable thermodynamic performance.&nbsp;\nIt is entirely reasonable to expect an aircraft operating in Dubai on a\n50°C flight line to climb to cruising altitudes where the air temperature is as\nlow as -60°C or perhaps colder.&nbsp; \n\n\n\nAs discussed previously, the chemical properties and ease of\nstorage of Jet-A and its equivalents make it ideally suited to turbine engine\noperations.&nbsp; AvGas in contrast to Jet-A is contaminated with lead and has a\ndramatically higher vapor pressure; both of these factors make it difficult to\nuse in high temperature environments.&nbsp;\n\n\n\n\nDuring combustion, lead is deposited on combustion liners\nand expansion turbine blades, reducing efficiency aerodynamic efficiency of the\nturbine and increasing the engine temperatures, resulting in decreased engine\nlife.&nbsp; The vapor pressure issue results\nin cavitation and vapor locking of fuel pumps.&nbsp;\nAnyone who has tried to start a hot turbocharged, fuel injected piston\naircraft engine can attest to the difficulty of dealing with vapor lock in a\nfuel system.&nbsp; \n\n\n\nSome turbine engines do permit operation with AvGas as an\nemergency fuel source.&nbsp; Note, that AvGas\nis an emergency fuel only.&nbsp; Steep\nlimitations and impacts to engine service life accompany the use of AvGas as a\nturbine fuel if it is permitted at all.&nbsp; \n\n\n\nAs an example, the King Air 350 PT6A-67A engines permit 150\nhours of AvGas usage over the course of multiple thousands of hours of engine\nservice life.&nbsp; Additional limitations are\nplaced on fuel pump operation and maximum altitudes.&nbsp; \n\n\n\nWhile turbine engines\nwill burn just about anything as a fuel, the impact to engine operations is\nsignificant if AvGas is used.\n\n\n\nAre there Jet-A alternatives?\n\n\n\nThe primary operating\nchallenges associated with Jet-A fuel revolve around low temperatures.&nbsp; There are two primary alternative fuels that\nhelp to lower the freezing point of jet fuel: Jet-B and TS-1.&nbsp; \n\n\n\nJet-B is a 30%\nKerosene, 70% Gasoline mixture that is limited to use in very cold\nenvironments.&nbsp; The high vapor pressure of\nthe gasoline component makes this a highly flammable fuel in warm environments.&nbsp; The gasoline component also lowers the freezing\npoint to -60°C. &nbsp;\n\n\n\nTS-1 is a Russian\nspecification of fuel that similarly has a lower freezing point of -50°C again\nat the sacrifice of warm temperature volatility, with a flashpoint of 28°C. \n\n\n\nOther jet fuel alternatives include an increasingly diverse set of synthetic and biologically derived fuels.&nbsp; \n\n\n\nWidespread adoption of these fuels has yet to occur, though militaries and commercial operators have been experimenting with these types of fuels for well over a decade.&nbsp; Pure synthetic or biofuel fuel loads are not yet approved, with the highest mixing ratio of Jet-A to bio or synthetic fuel being 50%.&nbsp; Additional development work continues on these types of fuel. \n\n\n\nAre there AvGas alternatives? \n\n\n\nThe environmental impact of lead produced by piston aviation\nand the shrinking supply base of 100LL producers and the sole source of\ntetraethyl lead has put 100LL at risk of extinction by logistics or by\nregulation.&nbsp; \n\n\n\nThere are multiple\nefforts to develop lead free high octane replacement gasoline fuels for piston\ndriven aircraft.&nbsp; The FAA is evaluating a\nhandful of candidate fuels in an array of aircraft engines and aircraft types.&nbsp; \n\n\n\nThe 93UL, UL94, UL102, and G100UL are all potential\ncandidates to meet replacement requirements for 100LL.&nbsp; The primary challenges with these fuels are\nvarious: the increased cost, reduced energy density, toxicity, volatility, and\nequipment compatibility.&nbsp; Development\ntesting and certification testing is ongoing with the ultimate goal of\nstandardization on one or two of these fuels for use in piston driven aircraft.\n\n\n\n\nConclusion\n\n\n\nThe differences between AvGas, MoGas, Diesel, Jet-A and\nKerosene, are both subtle and dramatic.&nbsp;\nAvGas and MoGas both require spark ignition engines to be used\neffectively with internal combustion engines.&nbsp;\nThis is a result of their higher ignition temperatures and higher vapor\npressures require careful engine management and chemical additives to manage\nengine knock or detonation.&nbsp; \n\n\n\nIn contrast Diesel, Jet-A and Kerosene, are burned using\ncompression ignition engines. &nbsp;These\nfuels are all much more stable at higher temperatures and are far less\nsusceptible to generating engine knock.&nbsp;\nThe refining process differentiates these different fuel types both for\npurity and for chemical makeup.&nbsp; \n\n\n\nWhile there is some overlap in the use of these fuels across\nengine types, by in large they serve to power specific types of engines, with\ngasolines powering piston engines and the Kerosene based fuels powering turbine\nengines.&nbsp; \n\n\n\nUltimately, AvGas and Jet-A have become standard fuels for the aviation industry as they meet stringent quality and performance metrics that ensure consistent high performance for any engine designed to use these fuels.  As with most of aviation, the mission requirements and planned operating environments play significant roles in defining the engine and thus fuel for to be used in a specific aircraft.\n\n\n\nWhile you’re on the subject… here are other articles on Airplane Academy related to fuel: \n\n\n\nThis is Why Fuel Tanks are Located in Aircraft WingsHow Does Aircraft Fuel Not Freeze During Flight?Why Does Jet A Fuel Cost Less than 100LL (AvGas) Fuel?Why Do Aircraft Have an Unusable Amount of Fuel?"
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