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I wonder how hard it would be to adapt a plane to run on propane? You'd need to find somewhere to keep a cylindrical tank (so not the wings) and ideally you'd

I wonder how hard it would be to adapt a plane to run on propane? You'd need to find somewhere to keep a cylindrical tank (so not the wings) and ideally you'd need a liquid-cooled engine so the coolant could heat the gas vapouriser. You'd get 115 octane fuel, and no CO / HC / NOx / SOx or other nasties in the exhaust, just CO2 and water.

mulmen 4y ago
Not just a cylindrical tank. A pressure vessel. Anything that can store propane is going to weigh a lot more than a rubber bladder in the metal the plane is already made of.
- Gordonjcp 4y ago
  It's not really under much pressure, about 8-10 bar. It's in the same ballpark as a light truck tyre pressure.
  - mulmen 4y ago
    Have you seen the payload numbers for light aircraft? Payload with full fuel on a Cessna 172 is only 369lb. How much does a vessel that can withstand 10 bar weigh? A light truck tire weighs about 50lb. Recall that this vessel must withstand the temperature, pressure, and vibration extremes of aircraft application. Light truck tires aren't typically filled with flammable gasses. 10bar is a terrifying amount of pressure when it comes in your direction.
    - Gordonjcp 4y ago
      Not sure, to be honest. I can lift the 95 litre tank in my Range Rover with one hand, so probably about 20-30kg? When it's full there's about 40kg of gas in, so that's very much a two-man lift. But bear in mind that's something like 6mm steel and intended to be literally indestructible. The dinky fibreglass gas bottles you get for patio heaters and forklifts are about 5kg and take an 11kg fill, which isn't a lot. 10 bar is not much more than the pressure in your car tyre, and roughly same as the average road bike tyre (currently running about 12 in my road bike). If you disconnect a 10 bar compressed air line, it hisses a bit. If you disconnect a 10 bar LPG line you get a lot of very cold gas on your hand, and it'll take ages to get the mercaptan-y smell off you.
      - mulmen 4y ago
        1bar is 14.5psi. Are we doing the same conversion? 10 bar is literally 4x the pressure in my car tire and 50% over that of my road bicycle. My air compressor operates at around 10 bar but the hose is regulated down to 2.5 bar and blasts debris all over the driveway when I disconnect it. If I don't firmly hold the air tool it shoots out like a missile. 30 extra kg is a loss of about 20% of effective payload. And this says nothing of the loss in power density.
- maximilianburke 4y ago
  Not even a rubber bladder in many cases; there are plenty of places in wings, fuselages, and empennage that make for great fuel tanks if they're just sealed up. Wet wings are quite common.
p_l 4y ago
Considerably problematic due to storage specifics and probably annoying mess in terms of maintenance handling.
- Gordonjcp 4y ago
  I figure that finding somewhere to mount the tank would be a pain in the arse. Maintenance is far easier than with petrol systems because it's not constantly trying to set you on fire.
  - p_l 4y ago
    Umm, Propane is constantly trying to set you on fire, in fact harder than gasoline and kerosene, because by default when it escapes it's going to switch to gas phase and, being heavier than air, will pool instead of nicely getting away, making it much worse explosion danger than AVGAS or Jet-A1/Diesel. Any leaks in unventilated space create risk of fuel-air explosion. Moreover, even comparatively "low" pressure systems where propane/lpg is used in gas form have pressure considerably higher than atmospheric and from the point of dangers and maintenance issues are a high pressure system just waiting to create a nice FAE lightshow. High efficiency liquid injection systems would be even funnier (FSVO). With the default state of propane being gas, and 5%-20% mix being apparently sweet spot for explosion, you're probably going to have to redesign everywhere that fuel lines go with an eye for propane vapours. Not fun at all.
    - Gordonjcp 4y ago
      Not really, no. You just run the propane lines roughly where the petrol lines run. They run at very very low pressures - in the tank it's less than about ten bar, usually eight or so. The fuel rail pressure in a modern petrol injection engine is far higher, and when you get into DI engines as you say they become hilariously high pressure. There's not really any drama when you want to maintain it - if you want to disconnect a fuel line, you just close off the tank valve, run the engine until it cuts out from fuel starvation, and the line is more or less empty. You probably blast more HC into the air when you spray your deodorant in the morning ;-) Given how oldschool aero engines are, I would think a suitable gas conversion would be a single-point mixer and vapouriser like on my old Range Rover (similar level of technology - massive low-compression overhead valve boat anchor). The only thing is you would need a liquid-cooled engine to get a coolant loop to feed around the vapouriser, or you'd need to come up with some other clever solution. Aircooled industrial engines running on gas used to just heat the vapouriser directly with the exhaust, but you're not allowed to do that any more!
chrismartin 4y ago
Physically? A lot harder than converting it to unleaded gasoline. For one, existing aircraft are designed around storing the heavy fuel near the center of lift/mass. (One benefit, the aircraft keeps its trim as the fuel is burned.) This video by Real Engineering [0] is a great overview. You could fit a propane cylinder into the tail of a Cessna 172, but now there's a lot more weight aft, the aircraft might be difficult to trim and exhibit unsafe handling characteristics. This same packaging issue exists with hydrogen-powered aircraft. Retrofitting is infeasible, lifting body designs are one possible solution. Administratively, getting the FAA to certify it? Hahahahahaha. [0] https://www.youtube.com/watch?v=C_BK7PRugK4 https://www.youtube.com/watch?v=C_BK7PRugK4
stephen_g 4y ago
The option of using an aero- Diesel engine to be able to run from Jet-A1, which is plentiful at many airports seems to be a good approach, and far preferable to having a heavy, bulky pressure vessel onboard. Do you have a reference for lower NOx emissions from propane? My understanding is that most NOx is a product of combustion in air (although it can come from impurities in the fuel). As for SOx, natural gas can contain H2S which produces these, is it normally significantly less for propane?
- Gordonjcp 4y ago
  There's no H2S in propane, although there's a little methyl mercaptan in it as an odourant so it "smells of gas". As for the NOx emissions I'm not sure why it's lower as in theory it should burn hotter - LPG runs lean which is why there's no CO, but that should make more NOx.
jabl 4y ago
The whole point of this decades-long unleaded AVGAS saga is to develop a fuel that is a drop-in replacement for the current leaded AVGAS. A fuel that needs to be stored either in a pressure tank or chilled does not fulfill that requirement. Now, if you're developing a new plane from scratch, I guess propane in theory could be an option. But why? For aircraft, energy density of the fuel is important, and propane does pretty well here, slightly better in fact than gasoline or kerosene (jet fuel). But in practice more than offset by the requirement to store the fuel pressurized. And then you have the whole fuel infrastructure issue, how are you gonna convince airports to stock propane considering they're already struggling to find an economic case for AVGAS due to the long-term decline of general aviation. I think if you're going to develop a new plane, it would make sense to make it use the fuel that's available, namely Jet A1 (well, maybe electric for some usecases). Sure, it's not usable in an Otto engine, but since you're developing a new plane anyway, power it by a diesel engine or turbine.
- Gordonjcp 4y ago
  Yes, and indeed in the mid-to-late 90s a lot of manufacturers experimented with Peugeot XUD engines in aircraft - various Merc ones too, but they were a bit too "high precision" for aircraft use and rattly old Citroën BX diesels were about perfect :-)
  - jabl 4y ago
    Currently some of the most widely used diesel aviation engines, the Thielert/Technify/Continental 1.7/2.0 liter one, as well as the Austro E4, are both based on a Mercedes car engine.

mulmen 4y ago

Not just a cylindrical tank. A pressure vessel. Anything that can store propane is going to weigh a lot more than a rubber bladder in the metal the plane is already made of.

Gordonjcp 4y ago
It's not really under much pressure, about 8-10 bar. It's in the same ballpark as a light truck tyre pressure.
- mulmen 4y ago
  Have you seen the payload numbers for light aircraft? Payload with full fuel on a Cessna 172 is only 369lb. How much does a vessel that can withstand 10 bar weigh? A light truck tire weighs about 50lb. Recall that this vessel must withstand the temperature, pressure, and vibration extremes of aircraft application. Light truck tires aren't typically filled with flammable gasses. 10bar is a terrifying amount of pressure when it comes in your direction.
  - Gordonjcp 4y ago
    Not sure, to be honest. I can lift the 95 litre tank in my Range Rover with one hand, so probably about 20-30kg? When it's full there's about 40kg of gas in, so that's very much a two-man lift. But bear in mind that's something like 6mm steel and intended to be literally indestructible. The dinky fibreglass gas bottles you get for patio heaters and forklifts are about 5kg and take an 11kg fill, which isn't a lot. 10 bar is not much more than the pressure in your car tyre, and roughly same as the average road bike tyre (currently running about 12 in my road bike). If you disconnect a 10 bar compressed air line, it hisses a bit. If you disconnect a 10 bar LPG line you get a lot of very cold gas on your hand, and it'll take ages to get the mercaptan-y smell off you.
    - mulmen 4y ago
      1bar is 14.5psi. Are we doing the same conversion? 10 bar is literally 4x the pressure in my car tire and 50% over that of my road bicycle. My air compressor operates at around 10 bar but the hose is regulated down to 2.5 bar and blasts debris all over the driveway when I disconnect it. If I don't firmly hold the air tool it shoots out like a missile. 30 extra kg is a loss of about 20% of effective payload. And this says nothing of the loss in power density.
maximilianburke 4y ago
Not even a rubber bladder in many cases; there are plenty of places in wings, fuselages, and empennage that make for great fuel tanks if they're just sealed up. Wet wings are quite common.

p_l 4y ago

Considerably problematic due to storage specifics and probably annoying mess in terms of maintenance handling.

Gordonjcp 4y ago
I figure that finding somewhere to mount the tank would be a pain in the arse. Maintenance is far easier than with petrol systems because it's not constantly trying to set you on fire.
- p_l 4y ago
  Umm, Propane is constantly trying to set you on fire, in fact harder than gasoline and kerosene, because by default when it escapes it's going to switch to gas phase and, being heavier than air, will pool instead of nicely getting away, making it much worse explosion danger than AVGAS or Jet-A1/Diesel. Any leaks in unventilated space create risk of fuel-air explosion. Moreover, even comparatively "low" pressure systems where propane/lpg is used in gas form have pressure considerably higher than atmospheric and from the point of dangers and maintenance issues are a high pressure system just waiting to create a nice FAE lightshow. High efficiency liquid injection systems would be even funnier (FSVO). With the default state of propane being gas, and 5%-20% mix being apparently sweet spot for explosion, you're probably going to have to redesign everywhere that fuel lines go with an eye for propane vapours. Not fun at all.
  - Gordonjcp 4y ago
    Not really, no. You just run the propane lines roughly where the petrol lines run. They run at very very low pressures - in the tank it's less than about ten bar, usually eight or so. The fuel rail pressure in a modern petrol injection engine is far higher, and when you get into DI engines as you say they become hilariously high pressure. There's not really any drama when you want to maintain it - if you want to disconnect a fuel line, you just close off the tank valve, run the engine until it cuts out from fuel starvation, and the line is more or less empty. You probably blast more HC into the air when you spray your deodorant in the morning ;-) Given how oldschool aero engines are, I would think a suitable gas conversion would be a single-point mixer and vapouriser like on my old Range Rover (similar level of technology - massive low-compression overhead valve boat anchor). The only thing is you would need a liquid-cooled engine to get a coolant loop to feed around the vapouriser, or you'd need to come up with some other clever solution. Aircooled industrial engines running on gas used to just heat the vapouriser directly with the exhaust, but you're not allowed to do that any more!

chrismartin 4y ago

Physically? A lot harder than converting it to unleaded gasoline. For one, existing aircraft are designed around storing the heavy fuel near the center of lift/mass. (One benefit, the aircraft keeps its trim as the fuel is burned.) This video by Real Engineering [0] is a great overview. You could fit a propane cylinder into the tail of a Cessna 172, but now there's a lot more weight aft, the aircraft might be difficult to trim and exhibit unsafe handling characteristics. This same packaging issue exists with hydrogen-powered aircraft. Retrofitting is infeasible, lifting body designs are one possible solution. Administratively, getting the FAA to certify it? Hahahahahaha. [0] https://www.youtube.com/watch?v=C_BK7PRugK4 https://www.youtube.com/watch?v=C_BK7PRugK4

stephen_g 4y ago

The option of using an aero- Diesel engine to be able to run from Jet-A1, which is plentiful at many airports seems to be a good approach, and far preferable to having a heavy, bulky pressure vessel onboard. Do you have a reference for lower NOx emissions from propane? My understanding is that most NOx is a product of combustion in air (although it can come from impurities in the fuel). As for SOx, natural gas can contain H2S which produces these, is it normally significantly less for propane?

Gordonjcp 4y ago
There's no H2S in propane, although there's a little methyl mercaptan in it as an odourant so it "smells of gas". As for the NOx emissions I'm not sure why it's lower as in theory it should burn hotter - LPG runs lean which is why there's no CO, but that should make more NOx.

jabl 4y ago

The whole point of this decades-long unleaded AVGAS saga is to develop a fuel that is a drop-in replacement for the current leaded AVGAS. A fuel that needs to be stored either in a pressure tank or chilled does not fulfill that requirement. Now, if you're developing a new plane from scratch, I guess propane in theory could be an option. But why? For aircraft, energy density of the fuel is important, and propane does pretty well here, slightly better in fact than gasoline or kerosene (jet fuel). But in practice more than offset by the requirement to store the fuel pressurized. And then you have the whole fuel infrastructure issue, how are you gonna convince airports to stock propane considering they're already struggling to find an economic case for AVGAS due to the long-term decline of general aviation. I think if you're going to develop a new plane, it would make sense to make it use the fuel that's available, namely Jet A1 (well, maybe electric for some usecases). Sure, it's not usable in an Otto engine, but since you're developing a new plane anyway, power it by a diesel engine or turbine.

Gordonjcp 4y ago
Yes, and indeed in the mid-to-late 90s a lot of manufacturers experimented with Peugeot XUD engines in aircraft - various Merc ones too, but they were a bit too "high precision" for aircraft use and rattly old Citroën BX diesels were about perfect :-)
- jabl 4y ago
  Currently some of the most widely used diesel aviation engines, the Thielert/Technify/Continental 1.7/2.0 liter one, as well as the Austro E4, are both based on a Mercedes car engine.