96.
The spill spray nozzle system, however, involves a somewhat modified type of fuel supply and control system from that used with the previous types. A means has to be provided for removing the
切向孔
控制桿
燃油進口
孔
活塞
油濾
彈簧調節
96．然而，這種溢流噴嘴系統涉及一種多少改型過的供油和控制系統，這種系統在改型前曾用于先前的幾種噴嘴。必須要有一種手段來排去溢流和在各種發動機工作狀態下控制溢流的總量。這種系統的缺點是當大量燃油回流到進進口時可能會產生過多的熱量。這種熱量最終會導致燃油變質。
圖10-17 一種可調進口(或Lubbock)
的燃油噴嘴
燃油加溫
100．在許多發動機上，燃油冷卻的滑油散熱器(第8章)安排在低壓燃油泵和燃油油濾進口之間(圖10-13)，優點是熱可由滑油傳給燃油，防止油濾元件被冰粒堵塞。當用這種方法傳熱不足時，燃油應流
過第二個熱變換器，此時燃油從恒溫控制的空氣流吸收熱量，空氣流則引自壓氣機。
燃油改變的影響
101．從一種燃油換為另一種燃油對發動機的主要影響是比重和一加侖燃油所發出的熱值的變化。對于核準用于燃氣渦輪發動機的所有燃油來說，實際上每磅燃油的熱值都是相同的，只要對比重加以比較就可以獲得每加侖燃油熱值的比較值。
102．比重的變化對離心壓力式發動機轉速調節器(第15段)有明顯的影響，因為比重的增加使作用在調節器膜片上的離心壓力增大。這樣就減小了調節器控制的轉速，所以凋節器應當重新調整。
103．當比重減小時，作用在膜片上的離心壓力減小，使調節器控制的轉速增加，結果，駕駛員必須通過手操縱油門來控制最大轉速，防止發動機超轉，直到調節器得以重新調整為止。液壓機械式調節器(第23段)與離心式調節器相比，對比重不那么敏感，所以為許多燃油系統所選用。
97.空氣霧化噴嘴(圖10-19)使噴射的燃油攜帶一部分燃燒室的一股空氣(第4章)。空氣霧化噴油使其它種類噴嘴產生的局部富油得以避免。因此既減少了積碳的形成，又減少了排氣冒煙。空氣霧化噴嘴另一個優點是燃油霧化要求的壓力低，可以采用重量較輕的齒輪泵。
98．為了保證所有噴嘴流過等量的燃油，通常要求使用流量分配器(圖10-20)以補償在低燃油壓力下油路中的重力壓頭。
99．某些燃燒系統使燃油在進入燃燒區時汽化(第4章)。
spill and also for controlling the amount of spill flow at various engine operating conditions. A disadvatage of this system is that excess heat may be generated when a large volume of fuel is being recirculated to inlet. Such heat may eventually lead to a deterioration of the fuel.
97. The airspray nozzle (fig. 10-19), carries a proportion of the primary combustion air (Part 4) with the injected fuel. By aerating the spray, the local fuel-rich concentrations produced by other types of spray nozzle are avoided, thus giving a reduction in both carbon formation and exhaust smoke. An additional advantage of the airspray nozzle is that the low
圖10-18
雙油路燃油噴嘴和增壓活門
主燃油
壓氣機供氣
主燃油
初級孔
油濾
主孔
防止孔積碳的空氣流
壓力增加時
增壓活門打開
來自油門的燃油進口
發動機停車開關
Fuel system
pressures required for atomization of the fuel permits the use of the comparatively lighter gear-type pump.
98.
A flow distributor (fig. 10-20) is often required to compensate for the gravity head across the manifold at low fuel pressures to ensure that all spray nozzles pass equal quantities of fuel.
99.
Some combustion systems vaporize the fuel (Part 4) as it enters the combustion zone.
FUEL HEATING
100. On many engines, a fuel-cooled oil cooler (Part 8) is located between the L.P. fuel pump and the inlet to the fuel filter (fig. 10-13), and advantage is taken of this to transfer the heat from the oil to the fuel and thus prevent blockage of the filter element by ice particles. When heat transference by this means is insufficient, the fuel is passed through a second heat exchanger where it absorbs heat from a thermostatically controlled airflow taken from the compressor.
EFFECT OF A CHANGE OF FUEL
101.
The main effect on the engine of a change from one grade of fuel to another arises from the variation of specific gravity and the number of heat units obtainable from a gallon of fuel. As the number of heat units per pound is practically the same for all fuels approved for gas turbine engines, a comparison of heat values per gallon can be obtained by comparing specific gravities.
102.
Changes in specific gravity have a definite effect on the centrifugal pressure type of engine speed governor (para. 15), for with an increase in specific gravity the centrifugal pressure acting on the governor diaphragm is greater. Thus the speed at which the governor controls is reduced, and in consequence the governor must be reset.
103.
With a decrease in specific gravity, the centrifugal pressure on the diaphragm is less and the speed at which the governor controls is increased; in consequence, the pilot must control the maximum
r.p.m. by manual operation of the throttle to prevent overspeeding the engine until the governor can be reset. The hydro-mechanical governor (para. 23) is less sensitive to changes of specific gravity than the centrifugal governor and is therefore preferred on many fuel systems.
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