Page 15 - Nov 2015 Volume 9, Number 11

Ask the Expert
Fuel System Operation
Marc Wolf, a Southern California long-time pilot, instructor, and King Air magazine reader, has requested that I write about the different King Air PT6 shutdown fuel purge systems that have been used through the years. I appreciate the suggestion of this interesting topic and will address it here.
First, realize that kerosene is actually more difficult to burn than most people realize. Did your high school chemistry teacher show you the trick of throwing a lighted match into a bucket of kerosene? It probably boggled your mind that the liquid extinguished the match harmlessly, and that no big “Kaboom!” resulted. Either the liquid must be very hot or atomized into a fine mist before combustion is easily achieved.
The fuel nozzles in the PT6 are the devices that atomize the jet fuel into a fine mist that is easily combusted. For this to occur, a tiny, properly-
shaped orifice and very high fuel
pressure upstream of the orifice
are both necessary. The Minimum
Pressurizing Valve in the Fuel
Control Unit (FCU) won’t permit
any fuel to leave the FCU and head
for the nozzles without at least 80
psi, and in typical cruise operation
we may see pressures near 400 psi.
But when the fuel flow is terminated for the purpose of engine shutdown, very quickly the pressure in the fuel manifold drops dramatically and we lose the shove that sent the fuel through the nozzles with enough force to achieve the desired atomization. Now the remaining fuel merely dribbles though the nozzles, entering the still-hot combustion chamber as liquid instead of atomized vapor. This causes more than one problem. First, the high combustion chamber temperature tends to boil off the lighter weight “hydro” part of this hydrocarbon fuel, leaving the heavier weight carbon behind. That carefully designed and meticulously manufactured nozzle orifice now can
NOVEMBER 2015
become partially clogged with leftover burned carbon residue...called “coke.” The overall result goes by the name of “coking” of the fuel nozzles and the result is a bad distribution of fuel, non-uniform temperature distribution in the combustion chamber, and eventual premature and/or expensive hot section repairs.
The second problem caused by having the last vestiges of fuel dribble, not spray, in the combustion chamber is the appearance of disconcerting white smoke coming out of the engine’s exhaust stacks following shutdown. Those dribbles of liquid jet fuel hit the hot combustion chamber liner surface and are evaporated or boiled into fuel vapor, or fuel “steam.” That is the white smoke we see – vaporized, unburned, jet fuel. The coking of the nozzles that likely preceded the appearance of the smoke is damaging. The smoke is harmless. But, it surely gives
at Shutdown
by Tom Clements
KING AIR MAGAZINE • 13