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Posted for Sky2high@aol.com:
Ray,
George is right.
Continental engines, such as the IO 360G, utilize a fuel system where excess fuel at the throttle body is returned to the tank from whence it came. Ergo, it is useful to run the boost pump with the mixture at idle cutoff to circulate enough cool fuel thru the system to minimize or eliminate vaporization in the lines or the heat soaked engine driven pump. In the Skymaster I used to fly, the front boost pump was located in the nose wheel well and not subject to engine compartment heat, the rear pump was mounted on the firewall. Either worked equally well to cool the rest of the fuel system.
Lycoming injected 320/360 engines are usually not equipped with a fuel system that provides a return line, thus one must rely on fuel pressure to minimize vaporization due to heat. There is a period of time after hot engine shutdown during which the fuel in the lines and pump will vaporize and disappear. One way to monitor this is to note the fuel pressure, as measured just downstream of the engine pump, after turning on the master switch. Low pressure may indicate vaporization or a leaky system. The engine driven pump will become heat soaked from the engine, will not cool down in a short period of time because of our closely cowled engine compartments and will not sucessfully pump fuel if its heat keeps vaporizing fuel in the vicinity of the pump. It is useful to turn on the boost pump for a hot start (mixture begins at idle cutoff) in order to raise the pressure of the fuel (NOT to prime the engine), thus minimizing its ability to vaporize in the wrong place. In most Lancairs (320/360 injeted), this is successful because the standard configuration is to provide the firewall mounted pump with gravity-fed cool fuel from the header tank. Other configurations (i.e. over-heated firewall mounted boost-pump direct wing-tank suckers) may not work as well.
It is always interesting to discuss the most modified experimental Lancairs, the 235/320/360 series, in that one man's floor is another man's ceiling. These are not type certificated aircraft where comparisons and techniques are always transferable betwixt the same model airplanes. One must form hypotheses and then test them on one's own aircraft. 'Tis, after all, experimental.
Ray, you also said: <<<When it comes to fuel pumps I believe the engine driven diaphragm pump will always pump fuel. >>>>
Beliefs are good until they fail to be supported by facts. Whilst idly taxiing a long time for takeoff at a hot Oshkosh, my fuel pressure, as provided by the engine pump, dropped below the minimum 12 psi resulting in a wake-up idiot light. This heat induced vaporization problem was solved by activating the boost pump, returning the pressure to 26 psi. This is also why most checklists call for the boost pump to be ON during takeoff and approach to landing - wouldn't want the engine to cough, sputter or wheeze at the wrong time.
Scott Krueger AKA Grayhawk
Sky2high@aol.com
II-P N92EX IO320 Aurora, IL (KARR)
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