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Fuel injection - pressure of fuel. 3
- Thread starter Nabla1
- Start date
- Thread starter
- #3
Thanks, so we have something like:
Tank -> Valve -> Pump -> Valve -> High pressure fuel line -> Fuel injector?
I'm interested in fuel systems for jet engines, where rather than a pulsed injection, the injection is continuous, and in particular I want to know how the system works for controlling the flow rate through the injection valve (And hence engine thrust).
The pump continuously ADDS pressure to the fuel line, while the injection valve continuously REMOVES it, so there must be some kind of servomechanism involving a pressure sensor in the fuel line for controlling the final flow rate through the injection valve.
Either the servo is between the pressure sensor and the pump (where the injection valve is a simple on/off action), the pressure sensor and the injection valve (where the valve would be a proportional solenoid valve), or there would be some system varying both the pump AND the valve.
Can anyone explain to me how it's acheived?
Thanks
Tank -> Valve -> Pump -> Valve -> High pressure fuel line -> Fuel injector?
I'm interested in fuel systems for jet engines, where rather than a pulsed injection, the injection is continuous, and in particular I want to know how the system works for controlling the flow rate through the injection valve (And hence engine thrust).
The pump continuously ADDS pressure to the fuel line, while the injection valve continuously REMOVES it, so there must be some kind of servomechanism involving a pressure sensor in the fuel line for controlling the final flow rate through the injection valve.
Either the servo is between the pressure sensor and the pump (where the injection valve is a simple on/off action), the pressure sensor and the injection valve (where the valve would be a proportional solenoid valve), or there would be some system varying both the pump AND the valve.
Can anyone explain to me how it's acheived?
Thanks
btrueblood
Mechanical
or
tank -- pump --- injector
^ |
| relief/return/regulating valve
------------|
tank -- pump --- injector
^ |
| relief/return/regulating valve
------------|
if what you want is constant flow, i sugest some fluid dynamics namely bernoulli equation, velocity at the injector exit is a function of pressure, mass flow (for non compressible flow ie liquids) is proporcional to velocity, in a car injection system the flow is controled by the time the injector is open and fuel line pressure is held constant by the pump and a fuel pressure valve, in your case you can vary mass flow by changing pressure, you can change pressure with a valve or an electronic controller (pump speed). Fuel pumps in jet engines are usually positive displacemente so the valve solution would be bad, u can vary the pump speed or bipass some fuel back to tank.
not so much, no.
if you're looking at a common rail fuel system for an indirect-injection gasoline engine, then you'll likely have a pump which supplies fuel from an unpressurized tank to a pressurized rail. Rail pressure is regulated by a valve which allows fuel to return to the tank. Fuel injection quanitity is determined by pressure and injector duty cycle. Max pressure in in the neighborhood of 60 psi.
on some diesel common rail systems, max pressure may be 29000psi. Fuel is taken from an unpressurized tank and slightly pressurized by a transfer pump, then delivered via low-pressure supply lines to the high pressure pump. Pressurizing excess fuel is wasteful, so high-pressure pump delivery to the rail is limited (either by limiting fuel intake to the pump, or by allowing excess fuel to return from the pump to the tank). If rail pressure exceeds a design limit, typically there is a relief mechanism to prevent damage to the rail. Fuel quantity is determined by the injector delivery profile, which can be controlled electronically (within bounds). It is sometimes the case that the fuel injectors return some amount of low pressure fuel to the tank for cooling purposes.
I don't know specifically how the system on a jet engine works, but jet engines would seem to pre-date electronic pressure sensors and servos by several decades. My guess would be that fuel delivery pressure is determined by pump delivery rate and injector restriction (and perhaps a relief valve or two).
if you're looking at a common rail fuel system for an indirect-injection gasoline engine, then you'll likely have a pump which supplies fuel from an unpressurized tank to a pressurized rail. Rail pressure is regulated by a valve which allows fuel to return to the tank. Fuel injection quanitity is determined by pressure and injector duty cycle. Max pressure in in the neighborhood of 60 psi.
on some diesel common rail systems, max pressure may be 29000psi. Fuel is taken from an unpressurized tank and slightly pressurized by a transfer pump, then delivered via low-pressure supply lines to the high pressure pump. Pressurizing excess fuel is wasteful, so high-pressure pump delivery to the rail is limited (either by limiting fuel intake to the pump, or by allowing excess fuel to return from the pump to the tank). If rail pressure exceeds a design limit, typically there is a relief mechanism to prevent damage to the rail. Fuel quantity is determined by the injector delivery profile, which can be controlled electronically (within bounds). It is sometimes the case that the fuel injectors return some amount of low pressure fuel to the tank for cooling purposes.
I don't know specifically how the system on a jet engine works, but jet engines would seem to pre-date electronic pressure sensors and servos by several decades. My guess would be that fuel delivery pressure is determined by pump delivery rate and injector restriction (and perhaps a relief valve or two).
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2
- #8
thruthefence
Aerospace
Jet engines,including turboprop & turboshaft, use a device called a FCU, or fuel control unit. The work in conjunction with a high pressure fuel pump. They are both driven from the engine's accessory drive,often coaxialy, with the FCU containing flyweight governor elements, as well as it's own fuel pump element. The FCU sees engine speed, thru it's drive,compressor discharge air pressure, from the engines compressor, and throttle angle, or power setting, commanded by the pilot, and on some FCUs, ambient temperature. This is what is referred to as a "hydromechanical FCU. What fuel that is not needed, is bypassed within the pump/FCU.There are fuel schedules for starting, idle, and "on speed" fuel flows. More fuel, more heat in the turbine section, more heat, more power. the FCU controls this. Technology has moved beyond this with FADEC, "Full authority digital engine control" This is a " fly by wire" system, with no mechanical control from the pilot to the engine. A digital computer takes the various engine parameters, and meters fuel accordingly.
patprimmer
New member
thruthefence
The quality of your answer far exceeds the quality of the OP. I found it quite informative.
Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers for professional engineers
The quality of your answer far exceeds the quality of the OP. I found it quite informative.
Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers for professional engineers
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1
- #12
Excellent post by thruthefence.
If I may add; The fuel injection nozzles into a turbine engine combustor, or combustors, depending on the engine design, do not "open" as per sae or operate at any specific pressure. That is, they are always "open" and their purpose is solely to atomise the fuel, and spray it at the correct orientation within the combustor. Often, injection nozzles will have two circuits, primary and secondary, where the primary circuit is used for light off, and idle conditions. This primary circuit ensures a well atomised spray pattern at lower pressures supplied by the FCU/HP fuel pump, up to 80-120 psig. At higher pressures delivered by the FCU/HP pump as commanded by a higher power settng, the secondary circuit opens up via a simple spring loaded shuttle valve. At 100% power, typical fuel rail pressures may be in the 900 psig range. When the pilot, or control system commands "stop", the FCU/HP pump diverts fuel away from the fuel rail, and another shuttle valve dumps any residual pressure away from the rail, preventing the injection nozzles from dribbling. Jet engines once spinning and combustion is stable, output is strictly a function of fuel BTU input. More BTUs in, more RPM and Mass airflow and working gas expansion ratio.
j79guy
If I may add; The fuel injection nozzles into a turbine engine combustor, or combustors, depending on the engine design, do not "open" as per sae or operate at any specific pressure. That is, they are always "open" and their purpose is solely to atomise the fuel, and spray it at the correct orientation within the combustor. Often, injection nozzles will have two circuits, primary and secondary, where the primary circuit is used for light off, and idle conditions. This primary circuit ensures a well atomised spray pattern at lower pressures supplied by the FCU/HP fuel pump, up to 80-120 psig. At higher pressures delivered by the FCU/HP pump as commanded by a higher power settng, the secondary circuit opens up via a simple spring loaded shuttle valve. At 100% power, typical fuel rail pressures may be in the 900 psig range. When the pilot, or control system commands "stop", the FCU/HP pump diverts fuel away from the fuel rail, and another shuttle valve dumps any residual pressure away from the rail, preventing the injection nozzles from dribbling. Jet engines once spinning and combustion is stable, output is strictly a function of fuel BTU input. More BTUs in, more RPM and Mass airflow and working gas expansion ratio.
j79guy
- Thread starter
- #13
Thanks to all for the wonderful replies, it's made things alot clearer for me.
One possible scenario I thought about was that the injectors were used solely for atomization (Being either on or off, or constantly on), and didn't control the flow rate, so the only control for such a system must have been to control the pressure in the fuel line, and now I know this is indeed the case and have some excellent ideas of how this is acheived.
PedroG, yes I have thought about it, and it's just a bit of a hobby/ambition in my spare time really. I would probably be alright electronics-wise, to program an MCU, and wire up a control circuit, given the right electronic valves and pumps, but I'm eager to learn more about the mechanical side of things, and this thread has answered alot of questions.
I know there are companies like "JetCat" that make small turbojets for use in model planes, so they obviously have an effective fuel injection system, but I'm unsure as to how the complexity of such a system would compare to an actual real-size system, and also where to obtain miniature electronic pumps and valves, If I were to build my own.
Thanks all.
One possible scenario I thought about was that the injectors were used solely for atomization (Being either on or off, or constantly on), and didn't control the flow rate, so the only control for such a system must have been to control the pressure in the fuel line, and now I know this is indeed the case and have some excellent ideas of how this is acheived.
PedroG, yes I have thought about it, and it's just a bit of a hobby/ambition in my spare time really. I would probably be alright electronics-wise, to program an MCU, and wire up a control circuit, given the right electronic valves and pumps, but I'm eager to learn more about the mechanical side of things, and this thread has answered alot of questions.
I know there are companies like "JetCat" that make small turbojets for use in model planes, so they obviously have an effective fuel injection system, but I'm unsure as to how the complexity of such a system would compare to an actual real-size system, and also where to obtain miniature electronic pumps and valves, If I were to build my own.
Thanks all.
Nabla1; Most, if not all RC turbines that I have experience with use propane as the fuel, greatly simplifying the fuel system. We here too, use vapourous propane as the main fuel when testing our aeroderivitive gas turbine engines, up to 30,000 IGHP. It's clean, simple, and requires minimal controls to produce excellent results. On-wing fuel systems, by virtue of having to handle liquid fuel, as much more complex, and have been steadily developed since the mid 1930's
There are many good reference manuals available explaining the basics of aircraft turbine engine fuel systems, in finer detail than can be described here. If you want to play with a small hobby turbine, take all the necessary safety precautions and consider vapourous propane as the fuel. Homebuilt turbines made from turbocharger components have an especially poor track record, with the majority blowing up in spectacular fashion.
j79guy
There are many good reference manuals available explaining the basics of aircraft turbine engine fuel systems, in finer detail than can be described here. If you want to play with a small hobby turbine, take all the necessary safety precautions and consider vapourous propane as the fuel. Homebuilt turbines made from turbocharger components have an especially poor track record, with the majority blowing up in spectacular fashion.
j79guy
there's lots of people building jets from turbos and documenting the process on the internet, if you search for DIY gas turbine on google you will find lots of info, theres a yahoo group to. i think you will find this informative and inspiring i also recomend some reading about turbines and compresible flow (for the nozzle), running is not same thing as running well XD.
good luck with your project.
good luck with your project.
- Thread starter
- #16
PedroCG: Thanks for the link and the info, theres definately some useful stuiff in there. However, I'm looking to build a smaller axial flow type engine, similar to the jetcat turbines.
j79guy: Thanks also for the info, I'll definitely consider propane as a fuel, as opposed to kerosene, and I'll have to weigh up the pro's and con's of each. Also, any chance you could provide me with some links to those references you mention? I've been looking for some good info about axial compressor design, and have only managed to find bits and pieces of info. If you ould recommend a good book that would help alot too.
Thanks to all for the help
j79guy: Thanks also for the info, I'll definitely consider propane as a fuel, as opposed to kerosene, and I'll have to weigh up the pro's and con's of each. Also, any chance you could provide me with some links to those references you mention? I've been looking for some good info about axial compressor design, and have only managed to find bits and pieces of info. If you ould recommend a good book that would help alot too.
Thanks to all for the help
hi, i've read bits of this book explains the thermodynamics simply, i dont know how deep it goes in the design process but he talks of all types of compressors and turbines, about the axial compressor, a single stage is probably not going to make the required pressure ratio, a centrifugal would do much better. you migth also want to read something about fluid pumps and turbines, imcompressible flow should be simpler to understand...
turbomotor
Mechanical
Hello Nabla1,
The practical performance characteristics (primarily sensitivity to clearances) of a centrifugal flow compressor are much more suitable to achieving a running turbine engine in a small size (less than about 6 inch wheel OD) than those of an multistage axial flow compressor.
However, I applaud your interest in doing an purely axial flow model aircraft turbine engine.
I've read the book that PedroCG mentions (above), and I like it. But for a clear straightforward presentation of turbomachinery including tradeoffs of this config versus that config, I still like Shepherd best. It is an old book, but very useful. ("Principles of Turbomachinery", Shepherd, MacMillan, 1956)
The practical performance characteristics (primarily sensitivity to clearances) of a centrifugal flow compressor are much more suitable to achieving a running turbine engine in a small size (less than about 6 inch wheel OD) than those of an multistage axial flow compressor.
However, I applaud your interest in doing an purely axial flow model aircraft turbine engine.
I've read the book that PedroCG mentions (above), and I like it. But for a clear straightforward presentation of turbomachinery including tradeoffs of this config versus that config, I still like Shepherd best. It is an old book, but very useful. ("Principles of Turbomachinery", Shepherd, MacMillan, 1956)
Nabla1: Aircraft Gas Turbine Engines by Irwin Treager comes to mind. Excellent reference book for those starting out on learning basic aircraft turbine engine fuel systems.
turbomotor is spot on, when you are discussing propulsion turbine engines with a mass inlet airflow of 7-10 Lb/Sec or less, axial compressors have a tough time reaching the efficieny of a properly designed centrifugal compressor. Clearly, there are adventages to axial type, such as smaller frontal area/diameter of the engine, allowing for a lower drag airframe, however you indicate that you are interested in a hobby type application, not cutting edge military aircraft, thus I recommend building your first engine around a centrifugal compressor. Perhaps later when you have accumulated experience with building a few engines, you could logically move to an axial flow type. Without labouring the subject too much, the disadvantages to axial flow are:
- Low compression ratio per stage. Using modern wide-chord type compressor blades of NACA C4 profile, or better, will still only give 1.3:1 compression raise per stage, under ideal conditions. Most engines after running a bit and accumulating some fouling of the compressor blades, this compression ration perstage falls off significantly. Chances are that in building your first axial flow compressor, unless you have access to manufacturing equipment that we are not aware of, the compression ratio per stage will be somewhat more modest, perhaps 1.15:1. In any case, you will need more that just a few stages to get a reasonable total compression ratio with an axial flow. Centrifugal compressors on the otherhand, in a hobby type application, can draw on literally dozens of proven turbocharger wheels, which have over 3:1 compression ratio potential right out of the box.
- Tip losses. Every single axial flow compressor blade and stator vane requires a certain tip clearance, depending on materials used in the construction of your engine, rpm, and compression ratio. You can envision this tip clearance and the losses resulting therefrom. Centrifugals only require clearance to the wheel face, and in enclosed type wheels, this is eliminated as well.
- Toughness. Ingest the tiniest of objects into an axial flow compressor and you will deteriorate the performance, or worse, damage the blading/stators. Centrifugals you can throw the proverbial cat through, and come out unscathed.
The Rolls Royce/Allison 250 engine line comes to mind. The earlier maodels hade several stages of axial, and a single centrifugal as the last stage. Later models reduced the number of axial stages, and The latest and largest model of 250 has a single centrifugal flow only.
Having said all this, if you still have your heart set on an axial flow compressor, I suggest checking out the Latham supercharger for automotive applications. Originally designed in the 1950's, the design is still kicking around and being produced by a small company in California(?)
j79guy
turbomotor is spot on, when you are discussing propulsion turbine engines with a mass inlet airflow of 7-10 Lb/Sec or less, axial compressors have a tough time reaching the efficieny of a properly designed centrifugal compressor. Clearly, there are adventages to axial type, such as smaller frontal area/diameter of the engine, allowing for a lower drag airframe, however you indicate that you are interested in a hobby type application, not cutting edge military aircraft, thus I recommend building your first engine around a centrifugal compressor. Perhaps later when you have accumulated experience with building a few engines, you could logically move to an axial flow type. Without labouring the subject too much, the disadvantages to axial flow are:
- Low compression ratio per stage. Using modern wide-chord type compressor blades of NACA C4 profile, or better, will still only give 1.3:1 compression raise per stage, under ideal conditions. Most engines after running a bit and accumulating some fouling of the compressor blades, this compression ration perstage falls off significantly. Chances are that in building your first axial flow compressor, unless you have access to manufacturing equipment that we are not aware of, the compression ratio per stage will be somewhat more modest, perhaps 1.15:1. In any case, you will need more that just a few stages to get a reasonable total compression ratio with an axial flow. Centrifugal compressors on the otherhand, in a hobby type application, can draw on literally dozens of proven turbocharger wheels, which have over 3:1 compression ratio potential right out of the box.
- Tip losses. Every single axial flow compressor blade and stator vane requires a certain tip clearance, depending on materials used in the construction of your engine, rpm, and compression ratio. You can envision this tip clearance and the losses resulting therefrom. Centrifugals only require clearance to the wheel face, and in enclosed type wheels, this is eliminated as well.
- Toughness. Ingest the tiniest of objects into an axial flow compressor and you will deteriorate the performance, or worse, damage the blading/stators. Centrifugals you can throw the proverbial cat through, and come out unscathed.
The Rolls Royce/Allison 250 engine line comes to mind. The earlier maodels hade several stages of axial, and a single centrifugal as the last stage. Later models reduced the number of axial stages, and The latest and largest model of 250 has a single centrifugal flow only.
Having said all this, if you still have your heart set on an axial flow compressor, I suggest checking out the Latham supercharger for automotive applications. Originally designed in the 1950's, the design is still kicking around and being produced by a small company in California(?)
j79guy
thruthefence
Aerospace
J79guy, Regarding the latest incarnation of my beloved Allison/Rolls Royce 250, To what do you attribute the increased efficiency of centrifugal compressors these days? Better computer modeling? More accurate machining? Better combustion driving the compressor?
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