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
控制杆
燃油进口
孔
活塞
油滤
弹簧调节
96．然而，这种溢流喷嘴系统涉及一种多少改型过的供油和控制系统，这种系统在改型前曾用于先前的几种喷嘴。必须要有一种手段来排去溢流和在各种发动机工作状态下控制溢流的总量。这种系统的缺点是当大量燃油回流到进进口时可能会产生过多的热量。这种热量最终会导致燃油变质。
图10-17 一种可调进口(或Lubbock)
的燃油喷嘴
燃油加温
100．在许多发动机上，燃油冷却的滑油散热器(第8章)安排在低压燃油泵和燃油油滤进口之间(图10-13)，优点是热可由滑油传给燃油，防止油滤元件被冰粒堵塞。当用这种方法传热不足时，燃油应流
过第二个热变换器，此时燃油从恒温控制的空气流吸收热量，空气流则引自压气机。
燃油改变的影响
101．从一种燃油换为另一种燃油对发动机的主要影响是比重和一加仑燃油所发出的热值的变化。对于核准用于燃气涡轮发动机的所有燃油来说，实际上每磅燃油的热值都是相同的，只要对比重加以比较就可以获得每加仑燃油热值的比较值。
102．比重的变化对离心压力式发动机转速调节器(第15段)有明显的影响，因为比重的增加使作用在调节器膜片上的离心压力增大。这样就减小了调节器控制的转速，所以凋节器应当重新调整。
103．当比重减小时，作用在膜片上的离心压力减小，使调节器控制的转速增加，结果，驾驶员必须通过手操纵油门来控制最大转速，防止发动机超转，直到调节器得以重新调整为止。液压机械式调节器(第23段)与离心式调节器相比，对比重不那么敏感，所以为许多燃油系统所选用。
97.空气雾化喷嘴(图10-19)使喷射的燃油携带一部分燃烧室的一股空气(第4章)。空气雾化喷油使其它种类喷嘴产生的局部富油得以避免。因此既减少了积碳的形成，又减少了排气冒烟。空气雾化喷嘴另一个优点是燃油雾化要求的压力低，可以采用重量较轻的齿轮泵。
98．为了保证所有喷嘴流过等量的燃油，通常要求使用流量分配器(图10-20)以补偿在低燃油压力下油路中的重力压头。
99．某些燃烧系统使燃油在进入燃烧区时汽化(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
图10-18
双油路燃油喷嘴和增压活门
主燃油
压气机供气
主燃油
初级孔
油滤
主孔
防止孔积碳的空气流
压力增加时
增压活门打开
来自油门的燃油进口
发动机停车开关
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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