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rubber created the topic: Fluctuating Fuel Pressure
Dear Bob and Richard,
In page 3.14 you say that in most fuel injected systems, the pressure in the fuel line is measured before the fuel manifold valve. So a fluctuating fuel pressure gauge is a likely indication that vaporisation is occuring.
1. In an engine with a carburettor but no fuel manifold valve, where is the fuel pressure measured?
2. If such an engine were to indicate a fluctuating fuel pressure is it vaporisation which causes this too?
3. Is it possible that fuel pressure fluctuations may also be caused by a tank which is low on fuel?
4. What about failure of the engine-driven fuel pump in a low wing aircraft, or the boost pump? Could this cause a fluctuating fuel pressure gauge?
basketball replied the topic: Re: Fluctuating Fuel Pressure
These are only my thoughts, and they could be totally wrong. so don't take them as gold!
1. I for some reason have in my mind the pressure is measured just before the fuel enters the float chamber.
2. Interesting question again this one, but not really sure.
3. You would think so, especially during turns or sudden acceleration/deceleration where the fuel sploshes around, and if low,to reduce the constant supply of fuel as the point where the fuel is drawn from the tank gets temporarily exposed to air.
4. I would have thought if a pump fails, then the pressure would not fluctuate, unless the pump was splurting, i.e. going on and off again.
Richard replied the topic: Re: Fluctuating Fuel Pressure
1.) In a carburetted aircraft, fuel pressure is measured just before it enters the carburettor.
2.) Vaporisation can cause fuel pressure fluctuations although since the fuel is feeding into the float chamber you may not experience rough running unless the float chamber became depleted because of extended vapor locking in the lines.
3.) Yes, for the reasons basketball described. If the fuel intake port in the tank is exposed to air, you may get air drawn into the lines which will disrupt fuel flow.
4.) The engine driven fuel pump "sucks" fuel towards the engine whereas a boost pump normally "pushes" fuel towards the engine. If the boost pump fails under a high power situation you may get vaporisation in the fuel lines resulting in fluctuationsin fuel pressure. A failure in the engine-driven fuel pump in a low wing aircraft will casue the fuel pressure to drop dramatically with subsequent fuel starvation.
