PaulMillner
Line Up and Wait
>> I have a IO360 Lycoming with the Surefly variable timing. 8.7 compression ratio. I am guessing that I could run on the 94UL, without a problem.
[First, note that your Surefly is only advancing the timing from stock, not retarding it. There's no detonation detection. The certification standards require you demonstrate 10% fuel flow detonation margin at red line CHT and oil temperature. Flying behind an angle valve IO360 myself, I'd be very surprised if you could make that work. So, you'll need to lower the CHT and oil red line tempeatures, then do climb cooling tests to prove to the FAA that you still have adequate margin for the 100 F day... none of that is cheap, even if it *might* work. Paul]
>> All they know is 'lead bad, must get rid of'. The unintended consequences are rarely a thought.
[I guess IQ impaired children are a consequence that is being considered, and given that, the forbearance for us to get our act together is astonishing. Paul]
>> Their ignorance is matched only by their zeal for quick solutions.
[This is a quick solution? Lead was regulated out of mogas in the early '90's, the industry unleaded avgas task force began work in 1991, and declared failure in 2011... this has been anything but a quick solution. Fortunately, it turns out the task force was trying to solve the wrong problem; the real problem was much more amenable to solution. Paul]
>> It's been decades since I first heard the trope that 20% of the sales base burn 80% of the 100LL. I just wonder if there's been a more recent snapshot. That study is 20-30 years old.
[As you know the FAA does a every-three-year survey of aircraft utilization. My recollection is that their more recent analysis is that 30% of the aircraft are burning 70% of the fuel... that's a distinction that doesn't make much of a difference. See the PAFI papers on their website. Paul]
>> seems like competent FADEC could take care of the knock issue
[It could, but not without reducing performance, which introduces the large cost certification issue, and the very large cost engine modification issue... plus time to comply. Paul]
>> having computer control over our ignition and fuel would let us run a lower octane fuel without losing horsepower. Shouldn’t be hard to develop a system for our engines
[Yikes, I don't think the science supports your assertion, it's been attempted in the engine test cell. Depends on your definition of hard... time, cost, installation details, recertification... Paul]
>> I could understand the lack of an STC for older planes. But the new stuff coming out has done nothing.
[Cessna attempted with the IO580 in the 206, and the IO360 in the 172. But the IO580 blew the heads on the cylinders, and the marketplace didn't want the lower compression 172, folks converted them to higher compression to improve power and fuel efficiency. Cessna responded to the marketplace. Paul]
>> The Diesels do have slightly less horsepower, however all of the ones I know of are turbo, at cruise altitude the diesels end up being far more efficient. The Diamond 62 will carry 7 adults on 12-14 gallons per hour.
[Careful how you measure efficiency. From an engineering and aircraft performance perspective, it's horsepower per pound of fuel. Diesels *appear* to be more efficient because the fuel is heavier, so in HP/gallon it looks better. But, airplanes aren't inherently limited by volume, but by weight... and in horsepower per pound, diesels don't have much or any of an advantage. Paul]
>> My understanding is that that 7:1 is what you have when the turbo isn't ... turboing. The whole point of turbo is to boost more air and fuel into the cylinders - so you have actually higher compression when the turbo is working.
[Not accurate. You might have greater fuel energy packed into the cylinder, but you recover less of it... the thermodynamic power output is definitely dependent on compression ratio, and turbo boost can't compensate for that. Paul]
>> The latest FAA update (8/20/2020) just says, "The FAA, fuel suppliers, and aerospace manufacturers continue to develop high octane, unleaded fuel formulations. The goal of these efforts is to identify fuel formulations that provide operationally safe alternatives to 100LL. The PAFI program continues to support the efforts of fuel producers as they bring forth alternative, unleaded fuels for testing and evaluation." Kind of a kick-the-can-down-the-road update from the FAA: https://www.faa.gov/about/initiatives/avgas/
[Yeah, the FAA isn't leading any longer, which might be a good thing.]
>> Kyle B writes: Infiniti has a variable compression turbo engine that can get up to 14:1 on 91 octane. However, there is quite a bit of tech/complexity involved in achieving that. Direct injection being a big part of that, as well as some very efficient combustion chamber mechanisms. Most of that isn't too useful for marine or aviation applications, since wide power adjustments aren't really common for most aircraft.
>> Notice the cylinder volume and RPM generated by those high compression engines (often motorcycle engines). The cylinder volumes are small, and when the engine is making "real" power, the RPM are way up there. In comparison, our engines make full power at relatively low RPM and have huge cylinder volumes.
>> Why does this matter? Detonation. Cylinders with large volumes are far more prone to detonation than engines at the other end of the spectrum.
>> In small cylinders, the flame front burns through the small cylinder volume fast enough that the fuel burns before the pressure and temperature at the "corners" of the combustion chamber self-ignite the unburnt mixture. In larger cylinder volumes (where the flame front has to travel several times as far), the fuel in the corners (the farthest points from the spark plugs) gets heated and compressed a lot (that's a technical term) before the flame front arrives and you need high octane to make sure the fuel in the corners burns before it detonates.
>> Here's a 1962 paper on the effects of cylinder size on detonation and octane requirements if you don't mind registering: https://www.jstor.org/stable/44469482?seq=1#metadata_info_tab_contents
[Thanks Kyle!]
[First, note that your Surefly is only advancing the timing from stock, not retarding it. There's no detonation detection. The certification standards require you demonstrate 10% fuel flow detonation margin at red line CHT and oil temperature. Flying behind an angle valve IO360 myself, I'd be very surprised if you could make that work. So, you'll need to lower the CHT and oil red line tempeatures, then do climb cooling tests to prove to the FAA that you still have adequate margin for the 100 F day... none of that is cheap, even if it *might* work. Paul]
>> All they know is 'lead bad, must get rid of'. The unintended consequences are rarely a thought.
[I guess IQ impaired children are a consequence that is being considered, and given that, the forbearance for us to get our act together is astonishing. Paul]
>> Their ignorance is matched only by their zeal for quick solutions.
[This is a quick solution? Lead was regulated out of mogas in the early '90's, the industry unleaded avgas task force began work in 1991, and declared failure in 2011... this has been anything but a quick solution. Fortunately, it turns out the task force was trying to solve the wrong problem; the real problem was much more amenable to solution. Paul]
>> It's been decades since I first heard the trope that 20% of the sales base burn 80% of the 100LL. I just wonder if there's been a more recent snapshot. That study is 20-30 years old.
[As you know the FAA does a every-three-year survey of aircraft utilization. My recollection is that their more recent analysis is that 30% of the aircraft are burning 70% of the fuel... that's a distinction that doesn't make much of a difference. See the PAFI papers on their website. Paul]
>> seems like competent FADEC could take care of the knock issue
[It could, but not without reducing performance, which introduces the large cost certification issue, and the very large cost engine modification issue... plus time to comply. Paul]
>> having computer control over our ignition and fuel would let us run a lower octane fuel without losing horsepower. Shouldn’t be hard to develop a system for our engines
[Yikes, I don't think the science supports your assertion, it's been attempted in the engine test cell. Depends on your definition of hard... time, cost, installation details, recertification... Paul]
>> I could understand the lack of an STC for older planes. But the new stuff coming out has done nothing.
[Cessna attempted with the IO580 in the 206, and the IO360 in the 172. But the IO580 blew the heads on the cylinders, and the marketplace didn't want the lower compression 172, folks converted them to higher compression to improve power and fuel efficiency. Cessna responded to the marketplace. Paul]
>> The Diesels do have slightly less horsepower, however all of the ones I know of are turbo, at cruise altitude the diesels end up being far more efficient. The Diamond 62 will carry 7 adults on 12-14 gallons per hour.
[Careful how you measure efficiency. From an engineering and aircraft performance perspective, it's horsepower per pound of fuel. Diesels *appear* to be more efficient because the fuel is heavier, so in HP/gallon it looks better. But, airplanes aren't inherently limited by volume, but by weight... and in horsepower per pound, diesels don't have much or any of an advantage. Paul]
>> My understanding is that that 7:1 is what you have when the turbo isn't ... turboing. The whole point of turbo is to boost more air and fuel into the cylinders - so you have actually higher compression when the turbo is working.
[Not accurate. You might have greater fuel energy packed into the cylinder, but you recover less of it... the thermodynamic power output is definitely dependent on compression ratio, and turbo boost can't compensate for that. Paul]
>> The latest FAA update (8/20/2020) just says, "The FAA, fuel suppliers, and aerospace manufacturers continue to develop high octane, unleaded fuel formulations. The goal of these efforts is to identify fuel formulations that provide operationally safe alternatives to 100LL. The PAFI program continues to support the efforts of fuel producers as they bring forth alternative, unleaded fuels for testing and evaluation." Kind of a kick-the-can-down-the-road update from the FAA: https://www.faa.gov/about/initiatives/avgas/
[Yeah, the FAA isn't leading any longer, which might be a good thing.]
>> Kyle B writes: Infiniti has a variable compression turbo engine that can get up to 14:1 on 91 octane. However, there is quite a bit of tech/complexity involved in achieving that. Direct injection being a big part of that, as well as some very efficient combustion chamber mechanisms. Most of that isn't too useful for marine or aviation applications, since wide power adjustments aren't really common for most aircraft.
>> Notice the cylinder volume and RPM generated by those high compression engines (often motorcycle engines). The cylinder volumes are small, and when the engine is making "real" power, the RPM are way up there. In comparison, our engines make full power at relatively low RPM and have huge cylinder volumes.
>> Why does this matter? Detonation. Cylinders with large volumes are far more prone to detonation than engines at the other end of the spectrum.
>> In small cylinders, the flame front burns through the small cylinder volume fast enough that the fuel burns before the pressure and temperature at the "corners" of the combustion chamber self-ignite the unburnt mixture. In larger cylinder volumes (where the flame front has to travel several times as far), the fuel in the corners (the farthest points from the spark plugs) gets heated and compressed a lot (that's a technical term) before the flame front arrives and you need high octane to make sure the fuel in the corners burns before it detonates.
>> Here's a 1962 paper on the effects of cylinder size on detonation and octane requirements if you don't mind registering: https://www.jstor.org/stable/44469482?seq=1#metadata_info_tab_contents
[Thanks Kyle!]
