29 ms·

Why is lead a requirement for piston-engine aircraft? Land vehicles seem to be doing just fine without leaded fuel

FL410 5y ago
Because airplanes still use engine designs from 70 years ago. Your next question is: why? Same reason it takes 12 years to get a replacement fuel approved.
bombcar 5y ago
Piston-based aircraft are built to survive numerous failures that wouldn't be a major issue for a land-based vehicle (or a boat) - both of those just kind of sit there when the power generator fails. A plane turns into a glider at that time. Off the top of my head, they mainly use magnetos instead other forms of ignition, have dual mag setups so that one can fail and the engine will still run. And lead-based fuel lubrication is simpler than other forms. Some work is being done on diesel: https://en.wikipedia.org/wiki/Aircraft_diesel_engine#Modern_developments https://en.wikipedia.org/wiki/Aircraft_diesel_engine#Modern_... - and jets of course run off of kerosene basically. And a brand-new plane is really hard to build out cost-wise, much of the aviation fleet is very, very old.
sokoloff 5y ago
There's a multi-part answer. Legally (the subject that requires FAA approval) is that the type-certificate (the legal basis for certification and authority to operate) for these engines specifies the use of 100LL and the standard for 100LL is a composition-based standard, not a performance-based standard. If it doesn't have the prescribed amount of TEL, it's definitionally not 100LL, even if it would meet the performance requirement of the engine. Then, from an engineering standpoint (the subject that companies are working on in order to propose fuels to the FAA via PAFI or GAMI seeking multi-model Supplemental Type Certificate), aircraft engines are stone-age in complexity and are overwhelmingly air-cooled, fixed timing [except during starting], and operate for hours on end at 75%+ of rated power. Your auto is water-cooled, has variable timing, and typically runs at 8-15% of rated power. An aircraft engine has to run reliably with cylinder head temperatures of 460ºF or more (your car's cylinders are much more tightly controlled as a result of the water cooling). Why can't airplanes use a different engine? Airplanes are (legally) certified to use a specific engine and have had extensive flight testing to determine takeoff/landing distances, fuel consumption figures, etc. Those engines are (legally) certified to use a specific fuel [or list of fuels]. Automobile engines have a horrible track record when attempted to be converted to aviation usage. Small, normally aspirated engines are able to run on low-octane, ethanol-free pump gas. The aircraft with high-powered turbo-charged engines represent around 25% of the engines. Because of their higher utilization, those engines burn around 75% of the avgas purchased in a year. (These are the CapeAir fleet, skydive operators, etc.)
- stoltzmann 5y ago
  >The aircraft with high-powered turbo-charged engines represent around 25% of the engines. Because of their higher utilization, those engines burn around 75% of the avgas purchased in a year. Wouldn't turbo-charged engines burn jet A-1 rather than avgas?
  - cjrp 5y ago
    Turbine engines would, but that's different from turbo-charged (piston) engines.
    - stoltzmann 5y ago
      That'd make sense - I'm just wondering how many of those would be in use today. From my limited experience with skydiving operators, I've seen only smaller dropzones run piston planes that take avgas (i.e. C182), or turboprops at larger operations (like a C208). Running a larger plane on avgas would be prohibitively expensive, lead to higher jump ticket prices and in return cause less people to jump there.
      - cjrp 5y ago
        Ah sorry, I didn't see the part about using them as jump planes. Not sure about that, the ones I've seen are usually turbines. Plenty of twin-pistons used for transport though.
        sokoloff 5y ago
        I should have said "high-compression or turbo-charged". The C182 after serial number 65176 uses the O470-U high-compression (8.6:1) engine which does not have an auto-fuel certification. C182s prior to that number use the less efficient, lower compression (7.0:1) O470-R engine, which can run physically unmodified on ethanol-free auto fuel. (Note that what aviation considers high-compression is laughably low-compression in modern automobiles.)
rlpb 5y ago
Certifications work on a per-aircraft-type and per-engine-type basis. Converting a single aircraft type to a different engine is possible, but every single combination would need to be tested (I think this also includes flight testing) and certified individually. This is not economic. So what remains is for the existing fleet to eventually be retired and replaced by newer aircraft that use newer engines. But most light single piston aircraft being sold today still use older engine designs that require leaded fuel! And newer aircraft are so expensive, it's more economical to keep aircraft from the seventies still going. Newer aircraft are expensive because the certification and insurance requirements are extremely expensive. For example, aircraft manufacturers get sued for accidents caused by pilot error on a regular basis, and so they need insurance for that. "Average cost of manufacturer's liability insurance for each airplane manufactured in the U.S. had risen from approximately $50 per plane in 1962 to $100,000 per plane in 1988, according to a report cited by the Bureau of Labor Statistics,[14] a 2,000-fold increase in 24 years." --https://en.wikipedia.org/wiki/General_Aviation_Revitalization_Act https://en.wikipedia.org/wiki/General_Aviation_Revitalizatio... So light aircraft are stuck running ancient engine designs that require leaded fuel. There is some progress. One manufacturer uses a modern engine that runs on jet fuel (basically a diesel piston engine like in some cars). But that doesn't stop new aircraft being extremely expensive, so it looks like we're still stuck with mostly an old fleet that uses lead for a long time yet.
DebtDeflation 5y ago
None of the above answers actually answer your question. Lead was historically used in gasoline for two reasons - 1) to increase the octane rating (a measure of knock resistance) ans 2) to lubricate the valve seats in the cylinder head. There are multiple other chemical additives that can accomplish #1 (the fuel in the article uses one of them). For #2, the solution in the automotive world starting in the 1970s was to use hardened valve seats that didn't require lead in the fuel to lubricate. The actual answer to your question is "because the FAA won't allow it".
p_l 5y ago
Lead isn't actually a requirement for piston-engine aircraft. It's just that most piston engines available with: 1) world-wide certification 2) world-wide base of trained mechanics 3) already widely deployed (you can't just "swap" engines on a plane design) Are piston engines whose original design happened ages ago when leaded gasoline was the cheapest fuel with necessary performance (and that includes not just octane rating, but also things like evaporation rate at given atmospheric pressure - you don't want your fuel tank to explode due to gasoline vapors). Several newer designs in Europe have been playing around with Diesel engines again, because thanks to being heavier fraction it doesn't have as much evaporation problem of gasoline, and also diesels are by design very tolerant of fuel types. Most importantly, a plane with an aviation diesel engine can just as well use non-ethanol-added diesel from car gas station, or any of the Jet-A1 compatible fuels available on airfields (pretty sure everything that isn't accidental leftover of SR-71 blackbird operation will work just fine).
- garaetjjte 5y ago
  >also diesels are by design very tolerant of fuel types While combustion process is tolerant, modern high-pressure pumps not so much. I don't think you can run automotive engine on Jet-A without detrimental effects on lifetime of injection system (as it uses fuel as lubricant). Does these aviation diesels use some specially hardened pumps?

Because airplanes still use engine designs from 70 years ago. Your next question is: why? Same reason it takes 12 years to get a replacement fuel approved.

bombcar 5y ago

Piston-based aircraft are built to survive numerous failures that wouldn't be a major issue for a land-based vehicle (or a boat) - both of those just kind of sit there when the power generator fails. A plane turns into a glider at that time. Off the top of my head, they mainly use magnetos instead other forms of ignition, have dual mag setups so that one can fail and the engine will still run. And lead-based fuel lubrication is simpler than other forms. Some work is being done on diesel: https://en.wikipedia.org/wiki/Aircraft_diesel_engine#Modern_developments https://en.wikipedia.org/wiki/Aircraft_diesel_engine#Modern_... - and jets of course run off of kerosene basically. And a brand-new plane is really hard to build out cost-wise, much of the aviation fleet is very, very old.

sokoloff 5y ago

There's a multi-part answer. Legally (the subject that requires FAA approval) is that the type-certificate (the legal basis for certification and authority to operate) for these engines specifies the use of 100LL and the standard for 100LL is a composition-based standard, not a performance-based standard. If it doesn't have the prescribed amount of TEL, it's definitionally not 100LL, even if it would meet the performance requirement of the engine. Then, from an engineering standpoint (the subject that companies are working on in order to propose fuels to the FAA via PAFI or GAMI seeking multi-model Supplemental Type Certificate), aircraft engines are stone-age in complexity and are overwhelmingly air-cooled, fixed timing [except during starting], and operate for hours on end at 75%+ of rated power. Your auto is water-cooled, has variable timing, and typically runs at 8-15% of rated power. An aircraft engine has to run reliably with cylinder head temperatures of 460ºF or more (your car's cylinders are much more tightly controlled as a result of the water cooling). Why can't airplanes use a different engine? Airplanes are (legally) certified to use a specific engine and have had extensive flight testing to determine takeoff/landing distances, fuel consumption figures, etc. Those engines are (legally) certified to use a specific fuel [or list of fuels]. Automobile engines have a horrible track record when attempted to be converted to aviation usage. Small, normally aspirated engines are able to run on low-octane, ethanol-free pump gas. The aircraft with high-powered turbo-charged engines represent around 25% of the engines. Because of their higher utilization, those engines burn around 75% of the avgas purchased in a year. (These are the CapeAir fleet, skydive operators, etc.)

stoltzmann 5y ago
>The aircraft with high-powered turbo-charged engines represent around 25% of the engines. Because of their higher utilization, those engines burn around 75% of the avgas purchased in a year. Wouldn't turbo-charged engines burn jet A-1 rather than avgas?
- cjrp 5y ago
  Turbine engines would, but that's different from turbo-charged (piston) engines.
  - stoltzmann 5y ago
    That'd make sense - I'm just wondering how many of those would be in use today. From my limited experience with skydiving operators, I've seen only smaller dropzones run piston planes that take avgas (i.e. C182), or turboprops at larger operations (like a C208). Running a larger plane on avgas would be prohibitively expensive, lead to higher jump ticket prices and in return cause less people to jump there.
    - cjrp 5y ago
      Ah sorry, I didn't see the part about using them as jump planes. Not sure about that, the ones I've seen are usually turbines. Plenty of twin-pistons used for transport though.
      - sokoloff 5y ago
        I should have said "high-compression or turbo-charged". The C182 after serial number 65176 uses the O470-U high-compression (8.6:1) engine which does not have an auto-fuel certification. C182s prior to that number use the less efficient, lower compression (7.0:1) O470-R engine, which can run physically unmodified on ethanol-free auto fuel. (Note that what aviation considers high-compression is laughably low-compression in modern automobiles.)

rlpb 5y ago

Certifications work on a per-aircraft-type and per-engine-type basis. Converting a single aircraft type to a different engine is possible, but every single combination would need to be tested (I think this also includes flight testing) and certified individually. This is not economic. So what remains is for the existing fleet to eventually be retired and replaced by newer aircraft that use newer engines. But most light single piston aircraft being sold today still use older engine designs that require leaded fuel! And newer aircraft are so expensive, it's more economical to keep aircraft from the seventies still going. Newer aircraft are expensive because the certification and insurance requirements are extremely expensive. For example, aircraft manufacturers get sued for accidents caused by pilot error on a regular basis, and so they need insurance for that. "Average cost of manufacturer's liability insurance for each airplane manufactured in the U.S. had risen from approximately $50 per plane in 1962 to $100,000 per plane in 1988, according to a report cited by the Bureau of Labor Statistics,[14] a 2,000-fold increase in 24 years." --https://en.wikipedia.org/wiki/General_Aviation_Revitalization_Act https://en.wikipedia.org/wiki/General_Aviation_Revitalizatio... So light aircraft are stuck running ancient engine designs that require leaded fuel. There is some progress. One manufacturer uses a modern engine that runs on jet fuel (basically a diesel piston engine like in some cars). But that doesn't stop new aircraft being extremely expensive, so it looks like we're still stuck with mostly an old fleet that uses lead for a long time yet.

DebtDeflation 5y ago

None of the above answers actually answer your question. Lead was historically used in gasoline for two reasons - 1) to increase the octane rating (a measure of knock resistance) ans 2) to lubricate the valve seats in the cylinder head. There are multiple other chemical additives that can accomplish #1 (the fuel in the article uses one of them). For #2, the solution in the automotive world starting in the 1970s was to use hardened valve seats that didn't require lead in the fuel to lubricate. The actual answer to your question is "because the FAA won't allow it".

p_l 5y ago

Lead isn't actually a requirement for piston-engine aircraft. It's just that most piston engines available with: 1) world-wide certification 2) world-wide base of trained mechanics 3) already widely deployed (you can't just "swap" engines on a plane design) Are piston engines whose original design happened ages ago when leaded gasoline was the cheapest fuel with necessary performance (and that includes not just octane rating, but also things like evaporation rate at given atmospheric pressure - you don't want your fuel tank to explode due to gasoline vapors). Several newer designs in Europe have been playing around with Diesel engines again, because thanks to being heavier fraction it doesn't have as much evaporation problem of gasoline, and also diesels are by design very tolerant of fuel types. Most importantly, a plane with an aviation diesel engine can just as well use non-ethanol-added diesel from car gas station, or any of the Jet-A1 compatible fuels available on airfields (pretty sure everything that isn't accidental leftover of SR-71 blackbird operation will work just fine).

garaetjjte 5y ago
>also diesels are by design very tolerant of fuel types While combustion process is tolerant, modern high-pressure pumps not so much. I don't think you can run automotive engine on Jet-A without detrimental effects on lifetime of injection system (as it uses fuel as lubricant). Does these aviation diesels use some specially hardened pumps?