38．燃油能量转变成动能的效率被称作热效率或内部效率。象所有热机一样，它是受循环压缩比和燃烧温度控制的。然而，这种温度是受涡轮能够忍受的高温应力和机械应力限制的。研制尽可能减少这
些限制的新材料和新技术一直是人们追求的目标。
39．动能转变成推进功的效率被称作推进效率或外部效率，它受到推进机构浪费的动能的数量的影响。喷气尾流中浪费的无用能是一种损失，可表示为……，式中（……）是无效速度。因此，很显然，在飞机低速范围，纯喷气流浪费的能量比螺旋桨系统要多得多，因而在这个范围内效率也较低。不过。随着飞机速度增大，这个因素也发生变化，因为虽然喷气流继续从发动机高速喷出，但是它相对于周围大气的速度减小了，这样，就减少了废能损失。
termined ambient air temperature. Above this energy wasted by the propelling mechanism. Waste
temperature the fuel flow is automatically controlled to prevent turbine entry temperature limitations from being exceeded, thus resulting in reduced thrust and, overall, similar curve characteristics to those shown in fig. 21-8. In the instance of a triple-spool engine the pressure ratio is expressed as P4/P1. i.e. H.P. compressor delivery pressure/engine inlet pressure.
PROPULSIVE EFFICIENCY
37.
Performance of the jet engine is not only concerned with the thrust produced, but also with the efficient conversion of the heat energy of the fuel into kinetic energy, as represented by the jet velocity, and the best use of this velocity to propel the aircraft forward, i.e. the efficiency of the propulsive system.
38.
The efficiency of conversion of fuel energy to kinetic energy is termed thermal or internal efficiency and, like all heat engines, is controlled by the cycle pressure ratio and combustion temperature. Unfortunately, this temperature is limited by the thermal and mechanical stresses that can be tolerated by the turbine. The development of new materials and techniques to minimize these limitations is continually being pursued.
39.
The efficiency of conversion of kinetic energy to propulsive work is termed the propulsive or external efficiency and this is affected by the amount of kinetic
energy dissipated in the jet wake, which represents a W(vJ . V)2
loss, can be expressed as where (vJ-V)
2gis the waste velocity. It is therefore apparent that at the aircraft lower speed range the pure jet stream wastes considerably more energy than a propeller system and consequently is less efficient over this range. However, this factor changes as aircraft speed increases, because although the jet stream continues to issue at a high velocity from the engine its velocity relative to the surrounding atmosphere is reduced and, in consequence, the waste energy loss is reduced.
40. Briefly, propulsive efficiency may be expressed as:
Work done on the aircraft or simply
Energy imparted to engine airflow
Work done
Work done + work wasted in exhaust
Work done is the net thrust multiplied by the aircraft speed. Therefore, progressing from the net thrust equation given in para. 18, the following equation is arrived at: Propulsive efficiency =
. W(v . V).
V.(P -P0)A + J .
g
..
. W(vJ . V). W(vJ . V)2
V.(P -P0)A + .+
. g . 2g
图21-8 气温对一种典型的双转子发动机的影响
40．简单来说，推进效率可表示为：对飞机做功/加给发动机气流的能量 或简化为
所做的功/（所做的功+排气中浪费的功）
所做的功是净推力乘上飞机速度。因此，从第18段中给出的净推力方程可推导得出下列方程：
推进效率 …… 采用非堵塞喷管的发动机中(第20章),该方程变为:……
41.当完全膨胀到大气压力时，用VJ代表喷气速度，而这个方程也可用于堵塞喷管条件，英尺省略了喷口压力(P-P0)A这一项。
In the instance of an engine operating with a non-choked nozzle (Part 20), the equation becomes:
WV(vJ . V)
1
WV(vJ . V) + 2W(vJ . V)2
2V
Simplified to :
V + vJ
41. This latter equation can also be used for the choked nozzle condition by using vj to represent the jet velocity when fully expanded to atmospheric pressure, thereby dispensing with the nozzle pressure term (P-P0)A.
Performance
42. Assuming an aircraft speed (V) of 375 m.p.h. and a jet velocity (vj) of 1,230 rn.p.h., the efficiency of a turbo-jet is:
2 × 375 = approx. 47 per cent375 + 1,230
On the other hand, at an aircraft speed of 600 m.p.h. the efficiency is:
2 × 600 = approx. 66 per cent600 + 1,230
Propeller efficiency at these values of V is approxi-mately 82 and 55'per cent, respectively, and from
对转风扇
桨扇
图21-9 推进效率和飞机速度的关系图
224
43．为了获得良好的推进效率而不采用复杂的螺旋桨系统，内外涵原理(第2章)以各种形式得到利用。采用这一原理时，总输出的一部分是由喷气流而不是通过发动机循环的燃气流提供，而且这是由风扇或一台级数可变的低压压气机来提供。借助于外涵气流从一单独的推进喷管或与涡轮气流混合后从一共用的喷管排出，这种外涵道空气可用于降低平均喷气温度和速度。
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