Here is a Global Express with the BMW/RR BR710 engines at idle. The oil vapor (not smoke) is clearly visible this day. They do seem to consume just under a pint of oil per hour.
MolaKule Q&A on Aircraft Structures VII Engines
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I’m curious about something.
I remember reading or seeing on a documentary about the manufacture of early jet engines that BMW was in on that game pretty early.
But that association with Rolls Royce that produced engines from some of the Gulfstream business jets (and now, the B-52, right?) is the only mention of BMW WRT aircraft engines that I can recall reading about, since those early turbines from the late 1930s and 40s.
Does anyone know, has BMW kept their hand in building gas turbines all that time? Or was the association with RR a one off thing?
IIRC, BMW's entry into modern turbine engines was with the BR710 series of engines as used on the Gulfstream GV. However it is probably more complex than that. The old Spey and Tay RR engines are very familiar to older Gulfstream folks. The BMW/RR BR710 engines are similar in so many ways, it is easy to think they come from the very same family.
The Pratt engines used on the G600 are not similar in any way. In the end, the RR engines are (in my opinion) operationally superior.
Examples.
1) Our larger and heavier G650ER with the BMW/RR BR725 engines would easily achieve FL510 if one wanted to go up that high, and at FL490 could reach M 0.92. All while consuming about 900 gallons total on a typical trip. (lower fuel burn than our other similar planes)
2) The much more modern Pratt PW815GA on our slightly smaller and lighter (but more modern) G600 was unable to easily get to FL510, and when you did, AOA was a bit ehhhh due to low airspeed, and the engines could not achieve more than about M 0.88 at FL470-490. But at least they used 200 gallons more! However, this does not mean they were underpowered. They made righteous power at normal altitudes. Just that they did not perform as well up high. Some of this is software/FADEC tuning related. As the engine was clearly not working hard up high.
They do this for range reasons.
G600, FL510:
MolaKule
Staff member
Question: Currently, which turbofan engine has the largest fan and nacelle?
Guess, the GE9 on the 777.
MolaKule
Staff member
Yes, the GE9x with 110,000 lbs of thrust will power the Boeing 777X..
"The General Electric GE9X is a high-bypass turbofan engine that has been developed by GE Aerospace for the Boeing 777X. The engine first ran on the ground in 2016 before being tested in the air for the first time a couple of years later. Flight testing took place on the wing of a Boeing 747-400, which was the only aircraft large enough to carry the GE9X (much like its predecessor, the GE90), and certification was then received in late 2019...
One of the standout features of the General Electric GE9X is its sheer size. The engine's diameter is a staggering 161 inches. To put that into perspective, it is larger than the fuselage of the Boeing 737. The fan alone, made up of 16 composite blades, is 134 inches in diameter..."
Quoted from Simple Flying
https://simpleflying.com/boeing-777x-ge9x-largest-engine-ever-built/
Also of note, we can equate static thrust to HP (very roughly) as equal. This engine's core is said to make upwards of 130,000 HP if it were to be used to drive a generator. Aeroderivative peaker plant generators are a real thing, just modify an airliner engine to drive a generator head. Run it on diesel, natural gas, or both.
MolaKule
Staff member
The core of an axial flow jet engine contains the low-pressure compressor, the high-pressure compressor, the combustor, and high- and low-pressure turbines.
The internal shape, from the low-pressure section to the combustor, is one of a converging duct.
The internal shape, from the combustor to the low-pressure turbine, is one of a diverging duct.
Question: In terms of airflow and pressures, what is the significance of those converging and diverging ducts?
The internal shape, from the low-pressure section to the combustor, is one of a converging duct.
The internal shape, from the combustor to the low-pressure turbine, is one of a diverging duct.
Question: In terms of airflow and pressures, what is the significance of those converging and diverging ducts?
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This is a classic venturi.
As the cross section is constricted velocity increases and pressure decreases.
So, pressure decreases as the hot section is reached and then increases after that point at a progressing rate to allow the LPT to capture more of the energy imparted by the burners, kinda like an old compound steam engine.
MolaKule
Staff member
Consider that the converging duct geometry assists the low- and high-pressure compressors as they create a pressure rise toward the combustors; increasing pressures increase temperature for the combustor.
The diverging duct assists the turbines in capturing the energy needed to turn the fan and compressor sections. The turbines transform the gas's chemical energy to mechanical torque, as they exit toward the nozzle, another converging duct. This last converging duct increases air flow and adds to the thrust.
The result is that air flow through the engine increases because of a thermodynamic cycle, causing the air's momentum to increase, thereby satisfying the thrust equation.
The diverging duct assists the turbines in capturing the energy needed to turn the fan and compressor sections. The turbines transform the gas's chemical energy to mechanical torque, as they exit toward the nozzle, another converging duct. This last converging duct increases air flow and adds to the thrust.
The result is that air flow through the engine increases because of a thermodynamic cycle, causing the air's momentum to increase, thereby satisfying the thrust equation.
MolaKule
Staff member
The combustors are positioned immediately after the last high-pressure stage. Fuel is injected inside the combustor to create hot, high-velocity gases.
Question: Name the three types of combustor chamber designs.
Question: Name the three types of combustor chamber designs.
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Annular, Canular, Can annular.
edyvw
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I thought BMW was involved on B717 engines too?
MolaKule
Staff member
Very good.
Here is a slide show from Georgia Tech, see page 4 of 8;
https://seitzman.gatech.edu/classes/ae4451/combustors.pdf
See Operational Principle here for more info on Pressure, Temperature, and Flow Rate:
https://eaglepubs.erau.edu/introductiontoaerospaceflightvehicles/chapter/turbojet-engines/
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I think I was 16 or so when I did a study on jet engines. Also had a job for awhile building some after burner parts no not at 16 
. Yeah some tough metals.
. Yeah some tough metals.
A gas turbine engine runs or self sustains, when we combust fuel, because? It is a very, very simple question, with an incredibly simple answer. But I've seen many engineers misunderstand the basics.
Here is a hint using a picture of a turbocharger compressor and turbine that is properly sized to 'self sustain' as a turbine engine.
Here is a hint using a picture of a turbocharger compressor and turbine that is properly sized to 'self sustain' as a turbine engine.
MolaKule
Staff member
The jet engine utilizes the Brayton thermodynamic cycle. It works with either a centrifugal compressor and turbine or an axial compressor and turbine.
Turboprop jet engines, such as the P&W PT6 and later series, use both types: an axial compressor stage and then a centrifugal compressor with an axial flow turbine.
This engine is interesting in that the airflow through the engine is from rear to front, and it has a reversing-flow-direction combustor.
https://insights.globalspec.com/art...-brayton-thermodynamic-cycle-and-efficiencies
https://www.grc.nasa.gov/www/k-12/airplane/brayton.html
https://www.sciencefacts.net/brayton-cycle.html
Turboprop jet engines, such as the P&W PT6 and later series, use both types: an axial compressor stage and then a centrifugal compressor with an axial flow turbine.
This engine is interesting in that the airflow through the engine is from rear to front, and it has a reversing-flow-direction combustor.
https://insights.globalspec.com/art...-brayton-thermodynamic-cycle-and-efficiencies
https://www.grc.nasa.gov/www/k-12/airplane/brayton.html
https://www.sciencefacts.net/brayton-cycle.html
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Yes! Now the question I posed is an interesting one, because we need not combust fuel to self sustain the turbomachinery and have a 'running engine'. For example we could inject pressurized air into the combustion chamber to mimic the expansion of combustion. With no added heat what so ever.
Anybody else want to try and answer my question? I promise the answer is in the picture I posted. Hint, look at the height of the compressor tips vs. the turbine tips. And then think of pressure
MolaKule
Staff member
I must admit I don't understand your question. How can you have a turboshaft engine with useful thrust without some thermodynamic cycle providing power?
How are you defining self-sustaining?
The unit must rotate, pull air in and discharge air out as expected.
What is it about the compressor and turbine that allows the engine to 'run' or self sustain?
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