Subaru running on LPG

[Victor Bravo]

Flight school training in the local area would be a valid test case, just like the electric Alpha guys are doing in central CA.

Modify a C-172 with a composite filament tank, held down into the existing rear seat mounting points and cargo tie-downs, so the extra weight of the tank doesn't over-gross or rear-CG the airframe.

The relative merits, costs, good vs. bad of Propane will show itself in the data very very quickly.

Also, the same experiment could be run at glider towing operations, and most of the towplanes already have room for a liquid hopper right on the CG.

 

[Vigilant1]

Flight school training in the local area would be a valid test case, just like the electric Alpha guys are doing in central CA.

Modify a C-172 with a composite filament tank, held down into the existing rear seat mounting points and cargo tie-downs, so the extra weight of the tank doesn't over-gross or rear-CG the airframe.

A mogas STC would probably save them more money, if they can get one. In the US, propane is almost twice as expensive as mogas on a BTU equivalent basis. Propane burns cleaner, which might reduce some maintenance and wear in our "high bypass" engines. But we can buy a lot of oil changes for the difference in fuel cost.
ETA: Chart of historical prices of fuels, by Gasoline Gallon Equivalent (GGE) On a $ per GGE basis, propane is the most expensive fuel in their survey, though it is still cheaper than 100LL.

 

[Twodeaddogs]

With regard to having to heft a heavy weight in and out of an aircraft, try a battery in a commercial aircraft like an A320 or an A330. They are heavy (up to 80 lbs), always located in an awkward place and in the case of an A330, you need to haul them out with a rope. they are not routinely charged in situ and have to be taken out to be charged and serviced. Same goes for oxygen tanks; located right in the nose, so they always arrive early at the crash scene. In an airliner, they are also changed out for servicing and inspection. So the procedures that would be used for a propane tank effectively already exist and thousands of mechs are schooled in their care.

 

[rv7charlie]

How many million passenger-miles between changes, in an A3XX? As you said; done by trained mechanics. With a light a/c, you're talking about doing it after every flight. What will your mechanic charge to R&R your fuel tank after every flight?

 

[BBerson]

most expensive fuel in their survey, though it is still cheaper than 100LL.

Propane is cheaper in the summer. Used for home heating in winter so demand is greater.
Propane is normally half of mogas here ($1.50-$2.00) (no road tax)
When 100LL is banned it might be worth a look if you have a high compression engine or just want to operate without lead pollution.

 

[Vigilant1]

Propane is normally half of mogas here ($1.50-$2.00) (no road tax)

But a gallon of propane has much lower weight and much lower energy than a gallon of gasoline. The GGE prices account for that. In Oct of 2020, (fall, not summer or winter) propane was about $3.75 per GGE, while gasoline was about $2.15. Per the DoE chart, national avg prices for propane, on an equivalent energy basis, have consistently been much higher than gasoline.
An aside: Can folks who burn gasoline in non-highway use get a refund of highway/road taxes?
In the US, unless local prices are much different from national averages, I can't see an economic case for preferring propane to gasoline in road vehicles or airplanes.

 

[Brünner]

Flight school training in the local area would be a valid test case, just like the electric Alpha guys are doing in central CA.

Modify a C-172 with a composite filament tank, held down into the existing rear seat mounting points and cargo tie-downs, so the extra weight of the tank doesn't over-gross or rear-CG the airframe.

While the idea has merit, such modification would make the plane an Experimental, and no student would be interested in it if the hours can't be logged - at least here.
Also, here in beer country XP planes are a major PITA to operate.

The relative merits, costs, good vs. bad of Propane will show itself in the data very very quickly.

Also, the same experiment could be run at glider towing operations, and most of the towplanes already have room for a liquid hopper right on the CG.

This might be a more feasible and realistic proposition.

 

[Brünner]

But a gallon of propane has much lower weight and much lower energy than a gallon of gasoline. The GGE prices account for that. In Oct of 2020, (fall, not summer or winter) propane was about $3.75 per GGE, while gasoline was about $2.15. Per the DoE chart, national avg prices for propane, on an equivalent energy basis, have consistently been much higher than gasoline.
An aside: Can folks who burn gasoline in non-highway use get a refund of highway/road taxes?
In the US, unless local prices are much different from national averages, I can't see an economic case for preferring propane to gasoline in road vehicles or airplanes.

In several European countries the price of LPG is way, way lower than the price of gasoline.
According to my (faulty) memory gasoline was never cheaper than LPG, and I can't see this changing any time soon.

 

[Vigilant1]

In several European countries the price of LPG is way, way lower than the price of gasoline.
According to my (faulty) memory gasoline was never cheaper than LPG, and I can't see this changing any time soon.

Is there a provision for getting a refund on the taxes that are incorporated in the price of gasoline? In many European countries, taxes (retail and at other levels) constitute a significant share of the price motorists pay for gasoline/petrol/Benzin.

 

[Brünner]

Is there a provision for getting a refund on the taxes that are incorporated in the price of gasoline? In many European countries, taxes (retail and at other levels) constitute a significant share of the price motorists pay for gasoline/petrol/Benzin.

No, not that I'm aware of.
If you're a commercial operator you don't pay VAT (I think), but that's about it.

 

[BBerson]

I can't see an economic case for preferring propane to gasoline in road vehicles or airplanes.

Fleet operators and buses buy bulk propane here and use it because the engines last three times the mileage. And propane is "green".
Almost as low carbon as hydrogen, I suppose.

 

[Pops]

Most of the years I was flying the company work airplane, we had our own fuel tank at the county airport. Local autofuel dealer kept it topped off anytime they drove by in other deliveries. We paid wholesale prices and it saved us lots of money. In a poor state like this they make it a major hassle in trying to get the road use tax money back. Then a state inspector will be looking over your shoulder most of the time.
We have a lot of loggers in this area and they have the same problem trying to use "Off road diesel" in their logging equipment. The inspector better not find any in the logging trucks.

 

[Marc W]

There are several propane tanks at my home airport. They are permitted for heating the hangers. I wouldn't bother powering an airplane with it. It just doesn't make sense to my simple mind.

 

[Twodeaddogs]

How many million passenger-miles between changes, in an A3XX? As you said; done by trained mechanics. With a light a/c, you're talking about doing it after every flight. What will your mechanic charge to R&R your fuel tank after every flight?

if you take the example of a permanently fitted oxygen tank that is left in situ and serviced (refilled and given a visual inspection) on every flight, then a propane tank should be no different. An amateur pilot can be trained to pay proper attention to a propane tank, on the not-unreasonable basis that he will kill himself if he doesn't. Permanent oxygen tanks have existed in aircraft since WW 2 so the knowledge base is there.

 

[Yellowhammer]

TBO would be around 10,000 hours as Propane burns so cleanly.

My grandfather converted one of our tractors to propane when fuel prices rose during the Vietnam War. It was still running strong when we sold it in1998. It was a 1958 Ford Tractor.

My grandfather wanted to experiment. He was also an aircraft mechanic during WWII and Korea. In fact, I have all of his certifications. He went from piston to jets. Old School as they get.

Yellowhammer

 

[Brünner]

The octane rating is also rather high, at about 105/110 without any lead added.

 

[berridos]

Personally i only see advantages too. Wonder if lpg could help wankels become more reliable due to lower temps.... Could the usual handicaps of wankels benefit from lpg?
Any opinion on that?

 

[rv7charlie]

Normally aspirated rotaries can run on *very* poor octane gas, so there's no real upside from an octane standpoint; possibly even a detriment due to the slower burn with higher octane fuels.

 

[Vigilant1]

Personally i only see advantages too. Wonder if lpg could help wankels become more reliable due to lower temps.... Could the usual handicaps of wankels benefit from lpg?
Any opinion on that?

Are there particular Wankel reliability issues you are concerned about? At least for the Mazda, the apex seal problems were well solved decades ago. The exhaust systems require use of the right materials, but as far as I know that isn't a reliability issue. They generally run very reliably now, and of course lack the valve issues (and valves ) of piston engines.

 

[Vigilant1]

The octane rating is also rather high, at about 105/110 without any lead added.

CNG is generally cheaper than propane, but since it isn't liquified the required tank volume is much larger than liquid fuels with the same energy capacity.