Compressed LNG for jet engines?

y_p_w

y_p_w

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I know it theoretically works, as natural gas is the preferred fuel for turbine generators. Apparently Aviat has a CNG powered dual-fuel Husky available, but that's a conversion for a reciprocating engine. I'm a bit skeptical about the costs cited here, but I'm sure CNG is cheaper relative to energy than kerosene-based jet fuel. It's also different in that the compressor and fan are driven by electric motors rather than by the turbine. Possibly independently of the turbines. Isn't that what an adaptive cycle engine was supposed to achieve?

Astro Mechanica, an aerospace company founded in 2021, has developed a new type of aircraft engine using electric technology from the automotive industry. The company has successfully completed the first test of its Gen3 engine, reports New Atlas.​
​
The engine design separates the fan and compressor components and drives the compressor separately with high-performance electric motors. A turbogenerator produces electricity that powers the electric motors which in turn drive the compressor and turbofan. This enables variation of air compression for optimal efficiency at different speeds.​

I know range would be limited, but for aircraft often weight is the consideration more than volume. CNG is supposed to be cheaper and cleaner burning than kerosene. And turbine engines are inherently tolerant of all sorts of fuel sources.
 
Danno said:
No way you're getting anywhere near the energy density of jet fuel with a compressed gas (or liquid as mentioned in the article).
And the tank requirements of CNG/LNG add another exponential level of weight and bulk.

Might be useful as a cute demo, to get clicks, that's about it.
Click to expand...

Absolutely I believe they can get it to work, but the question will be about practicality. A jet engine can be powered by almost any kind of liquid or gas fuel as long as the fuel can be fed. I'm thinking that aircraft engines have been bench tested with all manner of fuels, including those that are wildly impractical for transportation, like piped natural gas.
 
A neat experiment or prototype, but like said, jet fuel has so much more BTU content per unit volume than NG.
 
Look at the X axis, the bottom, mj/kg as a measure of energy content by weight. Diesel, gas, LPG and Propane all contain similar energy by mass. With LNG being a little bit better than gas/diesel/jet fuel. We absolutely can operate turbine engines from LNG tanks. Elon Musk does it on every Starship launch. As the turbopumps are methane powered.
1920px-Energy_density.svg.png
 
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This seems theoretical at best. A turbo-fan engine uses the turbine for the one main purpose of driving the fan and compressor. There is no motor that can do that anywhere near as efficient as the multi-stage turbine does.

What is being spoken of in this article is simply making a turboshaft engine, the same thing that is used on propeller driven aircraft and helicopters, and driving a generator. What they do with the electricity after it's been generated by the turbine is varied.

Imagine this, a vertical-lift F35 fighter jet drives a fan (via a turbine-driven shaft) that requires 29,000 shaft horsepower to spin. That's just for one fan on a tiny fighter jet. A huge turbofan engine on a commercial jet liner will require over 80,000 horsepower to turn that big sucker during max output. A gigantic 500 horsepower V-8 Chevy can't even make enough power to start a CF-6.

I think these people are delusional in their theoretical application and viability. But hey, I thought the Segway was the wave of the future too.
 
Just a further clarification since it's too late to edit the title, but it was liquefied natural gas. Apparently it's about 0.74 the energy density per volume compared to gasoline and about 2/3 relative to jet fuel. And it's lighter per unit of energy. But obviously there's the weight of the storage tanks.
 
y_p_w said:
Just a further clarification since it's too late to edit the title, but it was liquefied natural gas. Apparently it's about 0.74 the energy density per volume compared to gasoline and about 2/3 relative to jet fuel. And it's lighter per unit of energy. But obviously there's the weight of the storage tanks.
Click to expand...
How well will lpg work at -40 F ?
 
y_p_w said:
In what way? I don't see how it wouldn't work at low temps. That's well above vaporization temp (-259.6 °F) at sea level. Should be fine at any reasonable temperature that an aircraft will encounter.
Click to expand...
LNG has been pumped directly into the fuel injection nozzles of gas turbine engines. No need to vaporize it. With proper design and handling, it can remain liquid. One of the keys seem to be externally mounted injectors.
 
Cujet said:
LNG has been pumped directly into the fuel injection nozzles of gas turbine engines. No need to vaporize it. With proper design and handling, it can remain liquid. One of the keys seem to be externally mounted injectors.
Click to expand...

I'm thinking someone must have tried this before with an airborn jet aircraft engine. Maybe on something like the GE or Rolls-Royce 747 testbeds. We've got decades of experience using CNG/LNG/natural gas in all manner of different turbines. The ubiquitous GE LM2500 turboshaft is based off the CF6 aircraft engine so it should be possible. Not sure if shaft output scales the same as thrust though.
 
y_p_w said:
I'm thinking someone must have tried this before with an airborn jet aircraft engine. Maybe on something like the GE or Rolls-Royce 747 testbeds. We've got decades of experience using CNG/LNG/natural gas in all manner of different turbines. The ubiquitous GE LM2500 turboshaft is based off the CF6 aircraft engine so it should be possible. Not sure if shaft output scales the same as thrust though.
Click to expand...

The HP produced to drive the fan of a modern turbofan is roughly equivalent to the static thrust. A 90,000 pound thrust engine can be said to be a 90,000HP engine.

However, as I am sure you know, that's not the entire story with regard to aircraft propulsion and HP. The hot exhaust creates thrust, and is added into the HP calculation as it does work once the aircraft is moving. And the faster the plane moves, the more HP the thrust calculates to.

A 1200HP turboprop on a PC12 is said to be 1351HP in real world terms (with tailpipe thrust added in)
A 32,000 pound thrust Gulfstream G600 jet, also said to be 32,000HP produces 37,500 HP worth of work in a typical climb to FL410.

If, for example we wanted to power a Gulfstream G600 (holds 6150 gal JetA in the wings) with LNG, we would need a tank that held 10,878 gallons of LNG.

7FW8EEf.jpg
Here is an 11,000 gallon LNG rail car. Maybe we could carry it under the plane?
3006520-2Scenic.jpg
 
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y_p_w said:
Just a further clarification since it's too late to edit the title, but it was liquefied natural gas. Apparently it's about 0.74 the energy density per volume compared to gasoline and about 2/3 relative to jet fuel. And it's lighter per unit of energy. But obviously there's the weight of the storage tanks.
Click to expand...
Now you have cryogenic fuel which will boil off at some rate regardless of whether the aircraft is operating or not. Having such an aircraft inside a hangar is problematic.

Yes it will operate in the engine, but why? From this standpoint LPG would be better but at a higher cost.
 
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