Van's Air Force
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RSA fuel injection desighn pressure
- Thread starter vluvelin
- Start date
completly different systems
The RSA is a constant dribble system, 15 PSI would be ample to keep the fuel from boiling but down stream of the servo the fuel system is open into the injector, there is nothing to contain the pressure so it just drains into the intake port. Metering is done by adjusting pressure to an orifice in the injector and by orifice size. So boosting pressure beyond what's needed can't be done without flooding the engine. Pretty crude but reliable and simple.
Any modern car with sequential multiport injection uses computer controlled injectors with 30-40 psi on them to meter the fuel, and timing of the fuel delivery. The pulse time of the injector opening can be precisely controlled via the computer in "closed loop" mode (feed back from the o2 sensor, throttle position, map, temperature etc.) to occur just as the intake valve is opening, and then finely control the mixture by changing the pulse duration based on feed back from various sensors. Pretty cool stuff.
The newest systems, i.e. Direct injection like Fords "ecobboost" using very high fuel pressure can deliver the fuel, finely atomized, directly into the cylinder only when we're ready for combustion. What's cool is pre ignition or detonation risk is gone as there is nothing in the cylinder ignite yet. Turbo boost, compression ratios and ignition timing can now go off the chart allowing much higher power levels than before.
Tim Andres
The RSA is a constant dribble system, 15 PSI would be ample to keep the fuel from boiling but down stream of the servo the fuel system is open into the injector, there is nothing to contain the pressure so it just drains into the intake port. Metering is done by adjusting pressure to an orifice in the injector and by orifice size. So boosting pressure beyond what's needed can't be done without flooding the engine. Pretty crude but reliable and simple.
Any modern car with sequential multiport injection uses computer controlled injectors with 30-40 psi on them to meter the fuel, and timing of the fuel delivery. The pulse time of the injector opening can be precisely controlled via the computer in "closed loop" mode (feed back from the o2 sensor, throttle position, map, temperature etc.) to occur just as the intake valve is opening, and then finely control the mixture by changing the pulse duration based on feed back from various sensors. Pretty cool stuff.
The newest systems, i.e. Direct injection like Fords "ecobboost" using very high fuel pressure can deliver the fuel, finely atomized, directly into the cylinder only when we're ready for combustion. What's cool is pre ignition or detonation risk is gone as there is nothing in the cylinder ignite yet. Turbo boost, compression ratios and ignition timing can now go off the chart allowing much higher power levels than before.
Tim Andres
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Actually, the specified supply pressure for an RSA is more like 20~50 psi. However, as noted, that pressure does not continue to the individual nozzles. Nozzle pressure is very low at idle. The pressure in the divider and nozzle lines does rise with increasing fuel delivery. Here's an RSA chart for the 360/390:
"Dribble" is a fair description at idle delivery, but not with the noise handle forward. There is no boiling at higher fuel flows. The injector line pressure has nothing to do with metering. That's a function of pressure delta across a jet in the fuel control body.
The currently available EAB EFI is not sequential. The injectors cycle without regard for piston position, putting fuel in the intake port all the time, just like the Bendix-style injection.
The key difference between the two systems, in the context of the OP's question, is really just the physical location of the pressure drop due to metering. With a Bendix, it's in the fuel control (the servo) with a slight additional drop at the divider. With the EFI, it's an injector function; pressure is maintained to the point of discharge. The practical difference is better hot idle quality with EFI.
"Dribble" is a fair description at idle delivery, but not with the noise handle forward. There is no boiling at higher fuel flows. The injector line pressure has nothing to do with metering. That's a function of pressure delta across a jet in the fuel control body.
The currently available EAB EFI is not sequential. The injectors cycle without regard for piston position, putting fuel in the intake port all the time, just like the Bendix-style injection.
The key difference between the two systems, in the context of the OP's question, is really just the physical location of the pressure drop due to metering. With a Bendix, it's in the fuel control (the servo) with a slight additional drop at the divider. With the EFI, it's an injector function; pressure is maintained to the point of discharge. The practical difference is better hot idle quality with EFI.
Don at Airflow
Well Known Member
Bendix fuel injection operation
TIM2542 ?So boosting pressure beyond what's needed can't be done without flooding the engine? That would be a Continental fuel injection system, not Bendix or Airflow. So perhaps you should consider attending our FI-101 Class. Next one in March, 4 through 6. This will give you a complete understanding on how the Bendix or Airflow Performance fuel injection works, the reason why parts are designed the way they are, and how the various components operate and interact with each other.
Dan has. He gets it.
Don
TIM2542 ?So boosting pressure beyond what's needed can't be done without flooding the engine? That would be a Continental fuel injection system, not Bendix or Airflow. So perhaps you should consider attending our FI-101 Class. Next one in March, 4 through 6. This will give you a complete understanding on how the Bendix or Airflow Performance fuel injection works, the reason why parts are designed the way they are, and how the various components operate and interact with each other.
Dan has. He gets it.
Don
moosepileit
Well Known Member
I thought the only 15psi fuel pump pressure system in light GA was the Bendix Pressure Carb. Used an oddball mechanical fuel pump with a typical backup of a Christen Wobble that contained a off/on/aux valve and a small paper cartridge fuel filter with a quick drain.
Alternative was a Blue Holley 15 PSI electric pump, but I used that for my smoke oiler.
Alternative was a Blue Holley 15 PSI electric pump, but I used that for my smoke oiler.
Don at Airflow
Well Known Member
PS-5 operating pressure
Actually it?s not an odd ball pump pressure. Back in the day of GO-435, 480?s, E-185 and E 225?s PS-5?s were the main stay. There were a lot of engine driven diaphragm or vane pumps and Airborne electric pumps cranking out 12-15 PSI to make these systems work. The PS-5 is a pretty good metering system with lots of adjustments. Problem is now that Precision has jacked up the price of the diaphragms and parts so much it?s around $3500.00 or more to overhaul one. You can install an Airflow multi-port fuel injection system for almost the cost to rebuild one of these PS-5?s and not have to worry about fuel distribution or a vapor return.
Holly Blue pumps got a lot of people in trouble too, again because they didn?t understand how they worked and how the fuel system in the aircraft has to work with an engine driven fuel pump in the system.
Don
Actually it?s not an odd ball pump pressure. Back in the day of GO-435, 480?s, E-185 and E 225?s PS-5?s were the main stay. There were a lot of engine driven diaphragm or vane pumps and Airborne electric pumps cranking out 12-15 PSI to make these systems work. The PS-5 is a pretty good metering system with lots of adjustments. Problem is now that Precision has jacked up the price of the diaphragms and parts so much it?s around $3500.00 or more to overhaul one. You can install an Airflow multi-port fuel injection system for almost the cost to rebuild one of these PS-5?s and not have to worry about fuel distribution or a vapor return.
Holly Blue pumps got a lot of people in trouble too, again because they didn?t understand how they worked and how the fuel system in the aircraft has to work with an engine driven fuel pump in the system.
Don
No guys it is not what i wanted to to know
I am talking about idle on hot day
The problem with fuel boiling in the injector lines it is not new problem
I am trying to understand why not to reduce nozzle restrictor size
and increase max servo pressure to compensate for reduced fuel-flow.
basically to shift graph that Dan posted in to higher pressure.
from design of RSA servo input pressure can be safely increased to around 45 psi
lets disregard at the moment mechanical pump pressure
lets talk about theory
I am talking about idle on hot day
The problem with fuel boiling in the injector lines it is not new problem
I am trying to understand why not to reduce nozzle restrictor size
and increase max servo pressure to compensate for reduced fuel-flow.
basically to shift graph that Dan posted in to higher pressure.
from design of RSA servo input pressure can be safely increased to around 45 psi
lets disregard at the moment mechanical pump pressure
lets talk about theory
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Don at Airflow
Well Known Member
fuel injection system design
Injector nozzles are fixed orifice air bled nozzles. Most 4 cylinder Lycoming?s use a diaphragm fuel pump for the engine driven pump which gives you a operating pressure of 22-26 PSIG inlet fuel pressure. You cannot really disregard the mechanical pump in this discussion. But if you do this gives you an idea of what you are up against. Looking at a vapor pressure chart for avgas you will see that avgas boils at around 130 degrees F at sea level. With a stock set of injector nozzles (.028) in a four cylinder Lycoming at an idle flow of 1.2 GPH you will have a nozzle back pressure of 0.04 PSI. So you can see right away that if the fuel temperature in the nozzle lines gets to 130 degrees the fuel will be boiling in the nozzle lines. Just for kicks say you want to raise the boiling temperature to 160 F. You need around 17 PSIA to do this or 2.3 PSIG. Putting 0.010 restrictors in the nozzles will give you 2.5 PSI nozzle back pressure at 1.2 GPH the problem here is you need 451 PSI to push 16 GPH through the system at take off flow. Remember the 25 PSI we have available? But 160 degrees probably won?t really fix the problem since on landing the engine is around 300 on the CHT and 180 on the oil, plus there?s no air blowing over the top of the engine at idle when your taxing and the engine is radiating heat like crazy. So you say lets bump the temperature to 200 F. So now you need around 30 PSIA or 15.3 PSIG to keep the fuel from boiling. Stick a set of .006 restrictors in the nozzles and that will get you 19.6 PSI nozzle back pressure (that will keep it from boiling) but you need 3485 PSI to push the fuel through the system at take off power.
Get it.
There?s ways to get around this, and we?ve done it, but no use telling the competition how it?s done.
Don
Injector nozzles are fixed orifice air bled nozzles. Most 4 cylinder Lycoming?s use a diaphragm fuel pump for the engine driven pump which gives you a operating pressure of 22-26 PSIG inlet fuel pressure. You cannot really disregard the mechanical pump in this discussion. But if you do this gives you an idea of what you are up against. Looking at a vapor pressure chart for avgas you will see that avgas boils at around 130 degrees F at sea level. With a stock set of injector nozzles (.028) in a four cylinder Lycoming at an idle flow of 1.2 GPH you will have a nozzle back pressure of 0.04 PSI. So you can see right away that if the fuel temperature in the nozzle lines gets to 130 degrees the fuel will be boiling in the nozzle lines. Just for kicks say you want to raise the boiling temperature to 160 F. You need around 17 PSIA to do this or 2.3 PSIG. Putting 0.010 restrictors in the nozzles will give you 2.5 PSI nozzle back pressure at 1.2 GPH the problem here is you need 451 PSI to push 16 GPH through the system at take off flow. Remember the 25 PSI we have available? But 160 degrees probably won?t really fix the problem since on landing the engine is around 300 on the CHT and 180 on the oil, plus there?s no air blowing over the top of the engine at idle when your taxing and the engine is radiating heat like crazy. So you say lets bump the temperature to 200 F. So now you need around 30 PSIA or 15.3 PSIG to keep the fuel from boiling. Stick a set of .006 restrictors in the nozzles and that will get you 19.6 PSI nozzle back pressure (that will keep it from boiling) but you need 3485 PSI to push the fuel through the system at take off power.
Get it.
There?s ways to get around this, and we?ve done it, but no use telling the competition how it?s done.
Don
Don at Airflow
Well Known Member
Yes the Bosch CIS injection is a completely different type of metering system. It has it's draw backs too. The basic theory is the CIS system keeps the pressure drop constant across a variable orifice (metering valve actuated by big air flapper valve). The Bendix system has a fixed area and changes the pressure drop across it (pressure drop is the result of a balance of the forces of the air diaphragm and fuel diaphragm). CIS uses a pintle valve type nozzle to atomize the fuel, the Bendix uses an air bled nozzle.
Don
Don
BOSCH CIS injectors and RSA servo
Thank you Don.
Yes CIS metering unit is different but injectors have variable orifice design
it is changing with input pressure. more pressure compress spring and opens orifice.
it looks like same principle would work on RSA with higher pressure in injector line at idle. the only problem i see is operating pressures of RSA Servo and BOSCH injectors
injectors are starting to open at around 50psi depending on part number
and servo max input pressure is maxes at 50psi
as i understand atomization of fuel by injector is depending on high pressure
and it cant operate at lower pressure.
i am coming back to same question
Why RSA servo can not operate at higher pressure????? like 100 psi input
50-80 output?
I never take a part any RSA servo but from what i could find online it operates on deferential pressures and input pressure should not really make difference
Is there possibility to ajust size of main metering jet and diaphragm valve????
Thank you Don.
Yes CIS metering unit is different but injectors have variable orifice design
it is changing with input pressure. more pressure compress spring and opens orifice.
it looks like same principle would work on RSA with higher pressure in injector line at idle. the only problem i see is operating pressures of RSA Servo and BOSCH injectors
injectors are starting to open at around 50psi depending on part number
and servo max input pressure is maxes at 50psi
as i understand atomization of fuel by injector is depending on high pressure
and it cant operate at lower pressure.
i am coming back to same question
Why RSA servo can not operate at higher pressure????? like 100 psi input
50-80 output?
I never take a part any RSA servo but from what i could find online it operates on deferential pressures and input pressure should not really make difference
Is there possibility to ajust size of main metering jet and diaphragm valve????
Don at Airflow
Well Known Member
Bosch CIS and RSA injection systems
You do not understand the principle of operation of the RSA injection system, thus my comment on the first reply. In reality we have operated RSA and our fuel injection systems up to 150 PSI inlet fuel pressure. This does not change the nozzle back pressure (which is dependent on orifice size and fuel flow). And again the CIS injector nozzles are nothing more than a pintle valve design with a cracking pressure of 50 PSI. The pressure drop probably stays fairly constant with flow depending on the spring rate and area of the pintle valve with fuel flow change. And yes the high pressure against the pintle valve is what atomizes the fuel. The fuel metering is done at the Fuel Distributer where an accurate pressure drop (4.5 bar) is held across the precisely sized metering slits that the valve plunger opens and closes. Since you are running a higher pressure drop across the metering slits (65 PSI) than the opening pressure of the pintle valve (50 PSI), it doesn?t matter if one pintle valve nozzle cracking pressure is slightly higher than another (within reason), all nozzles will get equal fuel at any fuel flow. You cannot do this with the current set up of a Bendix system. But there are other solutions to the problem that I will not divulge here.
You do not understand the principle of operation of the RSA injection system, thus my comment on the first reply. In reality we have operated RSA and our fuel injection systems up to 150 PSI inlet fuel pressure. This does not change the nozzle back pressure (which is dependent on orifice size and fuel flow). And again the CIS injector nozzles are nothing more than a pintle valve design with a cracking pressure of 50 PSI. The pressure drop probably stays fairly constant with flow depending on the spring rate and area of the pintle valve with fuel flow change. And yes the high pressure against the pintle valve is what atomizes the fuel. The fuel metering is done at the Fuel Distributer where an accurate pressure drop (4.5 bar) is held across the precisely sized metering slits that the valve plunger opens and closes. Since you are running a higher pressure drop across the metering slits (65 PSI) than the opening pressure of the pintle valve (50 PSI), it doesn?t matter if one pintle valve nozzle cracking pressure is slightly higher than another (within reason), all nozzles will get equal fuel at any fuel flow. You cannot do this with the current set up of a Bendix system. But there are other solutions to the problem that I will not divulge here.
Lest the earlier post slip by unnoticed, this is a shout-out for Don's Fuel Injection 101 class. For anyone running FI, or pondering running FI, the knowledge gained in Don's class is invaluable. You'll learn things that will benefit you and your airplane for years to come. ... Not to mention Don and his lovely bride will feed you well, and you'll likely get to meet and work with other RVers! 
