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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 t

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!
  - p_l 4y ago
    TIL. There's still a problem that running the engine till fuel starvation is not exactly a practical option, but then arguably you could make it so that standard shut down depended on at-tank cut off valve (going to make certification a PITA). And of course you can't reuse original fuel runs because for majority of aircraft, the tank won't fit where existing tanks are. I'm still unconvinced that you could avoid considerable reconstruction of the fuel lines - there are reasons why propane/LPG have different hazard classification, and what is easily allowed on a ground vehicle can be a big nope in an airplane. On the more practical side, it would be dead in the water from legal issues - you'd have to recertify the plane, possibly deal with the fact that there's AFAIK no preexisting baseline for certification, and unlike 100UL you'd also add pilot & mechanic retraining - at this point you might have better ROI by shooting for the stars and going the cryogenic hydrogen route. While there are some CNG & LPG conversions around, pretty much everything seems to fall under experimental classification - something wildly different from the goal of getting G100UL declared fully equivalent to AVGAS 100LL, which would allow everyone to just switch supply.
    - Gordonjcp 4y ago
      Well in an automotive install you'd have a shutoff valve at the tank multivalve that you can physically screw shut, and a solenoid valve to allow fuel out to the line. Then up front there will be a filter and safety valve with another solenoid valve, and finally a solenoid on the vapouriser too. If I want to take a vapouriser off I'd just unplug the solenoid for the safety valve, start the engine, let it cut out, and then unscrew the pipe. Similarly if I wanted to work on the safety valve, I'd shut the tank off and let it empty the line running to the back of the car. The gas lines we use now are thin nylon with a tough PVC jacket over them, about as thick as a pencil (6mm inside bore) and flexible enough to tie your boots with ;-) It's a long way from stiff fragile copper line.

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!
- p_l 4y ago
  TIL. There's still a problem that running the engine till fuel starvation is not exactly a practical option, but then arguably you could make it so that standard shut down depended on at-tank cut off valve (going to make certification a PITA). And of course you can't reuse original fuel runs because for majority of aircraft, the tank won't fit where existing tanks are. I'm still unconvinced that you could avoid considerable reconstruction of the fuel lines - there are reasons why propane/LPG have different hazard classification, and what is easily allowed on a ground vehicle can be a big nope in an airplane. On the more practical side, it would be dead in the water from legal issues - you'd have to recertify the plane, possibly deal with the fact that there's AFAIK no preexisting baseline for certification, and unlike 100UL you'd also add pilot & mechanic retraining - at this point you might have better ROI by shooting for the stars and going the cryogenic hydrogen route. While there are some CNG & LPG conversions around, pretty much everything seems to fall under experimental classification - something wildly different from the goal of getting G100UL declared fully equivalent to AVGAS 100LL, which would allow everyone to just switch supply.
  - Gordonjcp 4y ago
    Well in an automotive install you'd have a shutoff valve at the tank multivalve that you can physically screw shut, and a solenoid valve to allow fuel out to the line. Then up front there will be a filter and safety valve with another solenoid valve, and finally a solenoid on the vapouriser too. If I want to take a vapouriser off I'd just unplug the solenoid for the safety valve, start the engine, let it cut out, and then unscrew the pipe. Similarly if I wanted to work on the safety valve, I'd shut the tank off and let it empty the line running to the back of the car. The gas lines we use now are thin nylon with a tough PVC jacket over them, about as thick as a pencil (6mm inside bore) and flexible enough to tie your boots with ;-) It's a long way from stiff fragile copper line.