Peter wrote:
Interesting, AA. What engine and what fuel servo does the C172S use?
The engine is an IO-360-L2A. The servo is made by Precision Airmotive, RSA-5AD1, p/n 2576536-2.
I have an RSA5AD1 too and definitely there is no return pipe anywhere. How strange… Where is the return returning from?
I also looked up the fuel system schematics of the Piper Turbo Arrow, and that also has fuel return lines going into the same tank the engine is being fed from.
Here is the fuel system schematics of the LC41 (Columbia 400). Note the position of the fuel flow transducer.
It says “fuel vapor” (vapour). Surely this is liquid fuel being returned, not vapour?
Peter wrote:
I have an RSA5AD1 too and definitely there is no return pipe anywhere. How strange… Where is the return returning from?
Don’t know… I did note at the Precision Airmotive web site that there are lots of different p/n for fuel servos with the same product name. It might be that there are different versions of the RSA5AD1 with and without a return line.
It says “fuel vapor” (vapour). Surely this is liquid fuel being returned, not vapour?
I believe it must be because the way the Vapour Suppression works is by circulating fuel from the tank, through the pump and back to the tank to cool it down and eliminate vapour.
I would be surprised that principle would work like that over the entire temp range of the aircraft. Fuel vapour pressure varies considerably with temperature.
I always thought the reason for a fuel return line is that the pump (either pump) is a constant volume pump (e.g. a gear pump without a spring loaded bypass valve) and since the fuel servo or the carb can only accept so much fuel, there has to be a means of returning the excess to “somewhere”.
It’s fairly obvious that the very simple Lyco engine driven pump cannot be a constant-volume pump – because there is no return line anywhere in the system I have
and similarly with the electric pump
One previous thread is here.
Peter wrote:
I always thought the reason for a fuel return line is that the pump (either pump) is a constant volume pump (e.g. a gear pump without a spring loaded bypass valve) and since the fuel servo or the carb can only accept so much fuel, there has to be a means of returning the excess to “somewhere”.
The reson for the fuel return line in the C172S is different. I quote from the POH:
A fuel return system was incorporated to improve engine operation during extended idle operation in hot weather environments The major components of the system includes an orifice fitting located in the top of the fuel-air control unit (fuel servo), fuel return line, with check valve, and a fuel reservoir tank. The fuel return system is designed to return a metered amount of fuel/vapor back to the fuel reservoir tank. The increased fuel flow, due to the fuel return system, results in lower fuel operating temperatures at the engine inlet, which minimizes the amount of fuel vapor generated in the fuel lines during hot weather operations.
OK; thanks. That is clear. They wanted to prevent a load of fuel sitting under the engine and heating up and vapourising.
So… what prevents that happening in my installation?
Airborne_Again wrote:
A fuel return system was incorporated to improve engine operation during extended idle operation in hot weather environments The major components of the system includes an orifice fitting located in the top of the fuel-air control unit (fuel servo), fuel return line, with check valve, and a fuel reservoir tank. The fuel return system is designed to return a metered amount of fuel/vapor back to the fuel reservoir tank. The increased fuel flow, due to the fuel return system, results in lower fuel operating temperatures at the engine inlet, which minimizes the amount of fuel vapor generated in the fuel lines during hot weather operations.
That is exactly what is achieved by the Vapour Suppression on the Columbia. The Vapour Suppression is actually just low boost setting on the electrical fuel pump to increase the amount of fuel circulating to the mechanical fuel pump and back to the tanks.
The POH says it should be turned on when needed, or above FL180.
