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  #1  
Old 01-07-2016, 09:53 PM
vluvelin vluvelin is offline
 
Join Date: Dec 2013
Location: Montreal
Posts: 205
Default RSA fuel injection desighn pressure

Can any of knowledgeable guys explain the reason
why RSA systems designed to operate at around 15 psi

if the pressure would be 30-40 psi like in multiport EFI
or 80-100psi like VW CIS injection fuel boiling issue wouldn't exist
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  #2  
Old 01-07-2016, 10:54 PM
tim2542 tim2542 is offline
 
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Location: Redding,Ca
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Default 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

Last edited by tim2542 : 01-07-2016 at 11:05 PM.
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  #3  
Old 01-08-2016, 07:40 AM
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DanH DanH is offline
 
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Location: 08A
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Default

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.
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  #4  
Old 01-08-2016, 10:58 AM
Don at Airflow Don at Airflow is offline
 
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Posts: 316
Default 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
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  #5  
Old 01-08-2016, 11:41 AM
moosepileit moosepileit is offline
 
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Location: Floyds Knobs, IN
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Default

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.
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  #6  
Old 01-08-2016, 12:32 PM
Don at Airflow Don at Airflow is offline
 
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Default 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
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  #7  
Old 01-08-2016, 07:17 PM
vluvelin vluvelin is offline
 
Join Date: Dec 2013
Location: Montreal
Posts: 205
Default 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
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Last edited by vluvelin : 01-08-2016 at 07:20 PM.
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  #8  
Old 01-09-2016, 01:43 PM
Don at Airflow Don at Airflow is offline
 
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Default 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
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  #9  
Old 01-11-2016, 11:13 PM
vluvelin vluvelin is offline
 
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Location: Montreal
Posts: 205
Default Found my calculation mistake

Thank you Don!!!
Now I understand why BOSCH CIS injectors made differently
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  #10  
Old 01-12-2016, 06:39 AM
Don at Airflow Don at Airflow is offline
 
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Default

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.

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