Turbine vs Piston Thermodynamics
Patrick, Wed Dec 04 2019, 02:34AMIn my time working on turbine aircraft both hobby and military (5k$-120k$ engines) between 5 to 82 lbs of thrust at 300 mph or less i get a fuzzy feeling in my gut. A feeling that magically shrinking a 111 inch diameter GEnx turbine (58k lbs thrust) down to 4 inch diameter (82 lbs thrust), doesn't allow the expected linear decrease in thrust per volume, per unit of mass or per specific fuel consumption.
So considering natural scaling laws make that kind of comparison useless, I ask if the Brayton cycle describes this difference.
Further more if we compare a tiny turbine using the brayton cycle (not otto) to a mid-sized piston engine, would we find a region on the graph where their power and efficiency start to overlap ?
Sorry if this is a messy question Im not good at thermodynamics.
Re: Turbine vs Piston Thermodynamics
2Spoons, Wed Dec 04 2019, 04:44AM
your answer is on the first page
I think the other thing you need to consider is that viscous effects have a bigger impact as you reduce size. And you are probably aware that thrust is not directly related to energy ( M.dV vs M.(dV)^2)
2Spoons, Wed Dec 04 2019, 04:44AM
your answer is on the first page
I think the other thing you need to consider is that viscous effects have a bigger impact as you reduce size. And you are probably aware that thrust is not directly related to energy ( M.dV vs M.(dV)^2)
Re: Turbine vs Piston Thermodynamics
Patrick, Wed Dec 04 2019, 04:54AM
ok thats pretty complicated, but im reading it...
Patrick, Wed Dec 04 2019, 04:54AM
ok thats pretty complicated, but im reading it...
Re: Turbine vs Piston Thermodynamics
2Spoons, Wed Dec 04 2019, 04:55AM
Short answer : the efficiency of the Otto and Brayton cycles are the same, but Otto is best for a reciprocating engine and Brayton is better suited to continuous flow engines
2Spoons, Wed Dec 04 2019, 04:55AM
Short answer : the efficiency of the Otto and Brayton cycles are the same, but Otto is best for a reciprocating engine and Brayton is better suited to continuous flow engines
Re: Turbine vs Piston Thermodynamics
Patrick, Sun Dec 15 2019, 06:44AM
do the tips of the rotating turbine blades in the combustion section exceed the local speed of sound while surrounded by flame ?
Patrick, Sun Dec 15 2019, 06:44AM
do the tips of the rotating turbine blades in the combustion section exceed the local speed of sound while surrounded by flame ?
Re: Turbine vs Piston Thermodynamics
2Spoons, Sun Dec 15 2019, 07:51PM
I wouldn't think so - you'd end up with shockwaves which I would have thought to be detrimental to an engine.
2Spoons, Sun Dec 15 2019, 07:51PM
I wouldn't think so - you'd end up with shockwaves which I would have thought to be detrimental to an engine.
Re: Turbine vs Piston Thermodynamics
Patrick, Sun Dec 15 2019, 11:09PM
i found this . . . and wsa looking at the pressure and velocity part of the turbine section. 150 psi @ 1400 ft/s
Patrick, Sun Dec 15 2019, 11:09PM
i found this . . . and wsa looking at the pressure and velocity part of the turbine section. 150 psi @ 1400 ft/s
Re: Turbine vs Piston Thermodynamics
2Spoons, Mon Dec 16 2019, 04:19AM
Turns out speed of sound in a gas is only dependent on temperature (which I didn't know).
In the turbine section, with temps of 1090C(2000F) speed of sound is 740m/s (2400ft/s.)
2Spoons, Mon Dec 16 2019, 04:19AM
Turns out speed of sound in a gas is only dependent on temperature (which I didn't know).
In the turbine section, with temps of 1090C(2000F) speed of sound is 740m/s (2400ft/s.)
Re: Turbine vs Piston Thermodynamics
Plasma, Mon Dec 16 2019, 05:38AM
Its also got to do with the chemical make up, H2,CO2,CH4 all have different numbers. The pressure is related to mocules by temperature.
They get close to the speed of sound with winglet's and taper.
If you can design a combustion chamber to handle large pressure differences and still work, worth aiming for.
Plasma, Mon Dec 16 2019, 05:38AM
Its also got to do with the chemical make up, H2,CO2,CH4 all have different numbers. The pressure is related to mocules by temperature.
They get close to the speed of sound with winglet's and taper.
If you can design a combustion chamber to handle large pressure differences and still work, worth aiming for.
Re: Turbine vs Piston Thermodynamics
Patrick, Mon Dec 16 2019, 06:33AM
Patrick, Mon Dec 16 2019, 06:33AM
Plasma wrote ...
If you can design a combustion chamber to handle large pressure differences and still work, worth aiming for.
I think the rotary engine holds promise for 500-750 lbs class drones. I was thinking of conventional propellers with the tips cut off running in a tube (i.e. f-16 ish inhaling)If you can design a combustion chamber to handle large pressure differences and still work, worth aiming for.
Re: Turbine vs Piston Thermodynamics
Plasma, Mon Dec 16 2019, 07:00AM
In a tube cut off, its the tsr tip speed ratio, the tips will turn slower cut off, but a comment I made that Ashsmall ruled off a rackets system that allows more air one the tube, say 1:10 ratio, will allow more mass, which turbines, cyntrycill pumps allow but not pressure which piston's are good.
I had a idea of using a blower to supply the air for methane to heat pipes were, turpentine got raised to 600°C then a piston compressor injected into the chamber.
It broke even, but 200kg would need to be upscaled.
Plasma, Mon Dec 16 2019, 07:00AM
In a tube cut off, its the tsr tip speed ratio, the tips will turn slower cut off, but a comment I made that Ashsmall ruled off a rackets system that allows more air one the tube, say 1:10 ratio, will allow more mass, which turbines, cyntrycill pumps allow but not pressure which piston's are good.
I had a idea of using a blower to supply the air for methane to heat pipes were, turpentine got raised to 600°C then a piston compressor injected into the chamber.
It broke even, but 200kg would need to be upscaled.
Re: Turbine vs Piston Thermodynamics
Patrick, Tue Dec 17 2019, 07:02PM
We typically cut the tips off a triple blade prop then put it in a tube as in an A-10 or F-16. That was before the premade electric turbines became cheap enuff for any idiot to buy.
i was thinking of a 18" diameter triple blade variable pitch prop driven by a rotary wankel engine.
something like this :
but this small version :
General characteristics
Type: Aircraft Wankel engine
Dry weight: 27 kg (60 lb)
Performance
Power output: 60 kW (80 hp)
Oil consumption: lossless
Power-to-weight ratio: 2.2 kW/kg (1.3 hp/lb)
60lbs @ 80HP = 1.3 hp/lb is really tempting compared with the 8" diameter turbines which are really under throttled most of the time for a M 0.9 capable drone which normally cruises and operates at M 0.3 (80% mission time).
Turbines can beat otto/diesel cycle engines in overall speed and power density, but they dont seem to scale in physical size or specific fuel consumption which is paramount above all other features in sub 1000 lbs drones.
the dimensions are surprisingly good:
Type: Twin rotor, four stroke Wankel aircraft engine, 105 hp @ 7,500 rpm
Displacement: 588 cc (35.9 cu in)
Length: 720 mm (28.3 in)
Width: 530 mm (20.9 in)
Height: 420 mm (16.5 in)
Dry weight: 54 kg (119.0 lb)
For a piston aircraft engine, though not by comparing turbine to piston directly. Absolute power density always favors the turbine even with the ancillary operating junk we build into the plane the get the turbine to run, which isnt needed for the piston power plants.
Patrick, Tue Dec 17 2019, 07:02PM
We typically cut the tips off a triple blade prop then put it in a tube as in an A-10 or F-16. That was before the premade electric turbines became cheap enuff for any idiot to buy.
i was thinking of a 18" diameter triple blade variable pitch prop driven by a rotary wankel engine.
something like this :
but this small version :
General characteristics
Type: Aircraft Wankel engine
Dry weight: 27 kg (60 lb)
Performance
Power output: 60 kW (80 hp)
Oil consumption: lossless
Power-to-weight ratio: 2.2 kW/kg (1.3 hp/lb)
60lbs @ 80HP = 1.3 hp/lb is really tempting compared with the 8" diameter turbines which are really under throttled most of the time for a M 0.9 capable drone which normally cruises and operates at M 0.3 (80% mission time).
Turbines can beat otto/diesel cycle engines in overall speed and power density, but they dont seem to scale in physical size or specific fuel consumption which is paramount above all other features in sub 1000 lbs drones.
the dimensions are surprisingly good:
Type: Twin rotor, four stroke Wankel aircraft engine, 105 hp @ 7,500 rpm
Displacement: 588 cc (35.9 cu in)
Length: 720 mm (28.3 in)
Width: 530 mm (20.9 in)
Height: 420 mm (16.5 in)
Dry weight: 54 kg (119.0 lb)
For a piston aircraft engine, though not by comparing turbine to piston directly. Absolute power density always favors the turbine even with the ancillary operating junk we build into the plane the get the turbine to run, which isnt needed for the piston power plants.
Re: Turbine vs Piston Thermodynamics
Plasma, Tue Dec 17 2019, 08:45PM
For a plane its normally four times the motor HP, that the aircraft can weight. So 80*4 =320lb 160 kg. Last I speced that was a 200hp Cessna engine, with three for a VTOL,did get far, the blades were $3000 .
Rotors engine are used for compact power output, but maintance is high, guessing but I'll try design a a car engine say 1100-1300Cc , 80hp is quite a big aircraft, any question not asked?
Plasma, Tue Dec 17 2019, 08:45PM
For a plane its normally four times the motor HP, that the aircraft can weight. So 80*4 =320lb 160 kg. Last I speced that was a 200hp Cessna engine, with three for a VTOL,did get far, the blades were $3000 .
Rotors engine are used for compact power output, but maintance is high, guessing but I'll try design a a car engine say 1100-1300Cc , 80hp is quite a big aircraft, any question not asked?
Re: Turbine vs Piston Thermodynamics
Patrick, Wed Dec 18 2019, 09:02PM
the Mazda car and motor cycle engines are bigger than there wankle air-optimized cousins. These dimensions are perfect for a 600 lbs fixed wing Mach 0.5 drone.
Length: 720 mm (28.3 in)
Width: 530 mm (20.9 in)
Height: 420 mm (16.5 in)
Patrick, Wed Dec 18 2019, 09:02PM
the Mazda car and motor cycle engines are bigger than there wankle air-optimized cousins. These dimensions are perfect for a 600 lbs fixed wing Mach 0.5 drone.
Length: 720 mm (28.3 in)
Width: 530 mm (20.9 in)
Height: 420 mm (16.5 in)
Re: Turbine vs Piston Thermodynamics
Plasma, Wed Dec 18 2019, 10:59PM
I was about to post a single engine in the centre wants to turn to the left, but its twin rotor going opposite directions?
A rotary engine would have better performance, but design wise I would test with a cheap $500 overweight engine.
Aircraft engines have the turbo to match the air desinty,.
Plasma, Wed Dec 18 2019, 10:59PM
I was about to post a single engine in the centre wants to turn to the left, but its twin rotor going opposite directions?
A rotary engine would have better performance, but design wise I would test with a cheap $500 overweight engine.
Aircraft engines have the turbo to match the air desinty,.
Re: Turbine vs Piston Thermodynamics
Patrick, Thu Dec 19 2019, 03:30AM
Im not worried about cost (well other than finding the money). $200,000 is what we're thinking this is going to take for the first three aircraft. Even If its feasible we may not follow through with the fixed wing first. We may focus on our ultra-high altitude thrust vectored rocket. That initial effort and success would help with fund raising for this and other projects. Its also easier to get FAA approval for rocket flights several times a year.
Yes they would counter rotate, but we're thinking of a single engine driving two turbines. So maybe both "internal" turbines turn clockwise, but the rotatory pistons turn clockwise, as you suggest to minimize unwanted torque. Im not sure how the force vectors add up, my brain is still doing some mental drooling from the difficulty.
Patrick, Thu Dec 19 2019, 03:30AM
Im not worried about cost (well other than finding the money). $200,000 is what we're thinking this is going to take for the first three aircraft. Even If its feasible we may not follow through with the fixed wing first. We may focus on our ultra-high altitude thrust vectored rocket. That initial effort and success would help with fund raising for this and other projects. Its also easier to get FAA approval for rocket flights several times a year.
Yes they would counter rotate, but we're thinking of a single engine driving two turbines. So maybe both "internal" turbines turn clockwise, but the rotatory pistons turn clockwise, as you suggest to minimize unwanted torque. Im not sure how the force vectors add up, my brain is still doing some mental drooling from the difficulty.
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