38．燃油能量轉變成動能的效率被稱作熱效率或內部效率。象所有熱機一樣，它是受循環壓縮比和燃燒溫度控制的。然而，這種溫度是受渦輪能夠忍受的高溫應力和機械應力限制的。研制盡可能減少這
些限制的新材料和新技術一直是人們追求的目標。
39．動能轉變成推進功的效率被稱作推進效率或外部效率，它受到推進機構浪費的動能的數量的影響。噴氣尾流中浪費的無用能是一種損失，可表示為……，式中（……）是無效速度。因此，很顯然，在飛機低速范圍，純噴氣流浪費的能量比螺旋槳系統要多得多，因而在這個范圍內效率也較低。不過。隨著飛機速度增大，這個因素也發生變化，因為雖然噴氣流繼續從發動機高速噴出，但是它相對于周圍大氣的速度減小了，這樣，就減少了廢能損失。
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
圖21-8 氣溫對一種典型的雙轉子發動機的影響
40．簡單來說，推進效率可表示為：對飛機做功/加給發動機氣流的能量 或簡化為
所做的功/（所做的功+排氣中浪費的功）
所做的功是凈推力乘上飛機速度。因此，從第18段中給出的凈推力方程可推導得出下列方程：
推進效率 …… 采用非堵塞噴管的發動機中(第20章),該方程變為:……
41.當完全膨脹到大氣壓力時，用VJ代表噴氣速度，而這個方程也可用于堵塞噴管條件，英尺省略了噴口壓力(P-P0)A這一項。
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
對轉風扇
槳扇
圖21-9 推進效率和飛機速度的關系圖
224
43．為了獲得良好的推進效率而不采用復雜的螺旋槳系統，內外涵原理(第2章)以各種形式得到利用。采用這一原理時，總輸出的一部分是由噴氣流而不是通過發動機循環的燃氣流提供，而且這是由風扇或一臺級數可變的低壓壓氣機來提供。借助于外涵氣流從一單獨的推進噴管或與渦輪氣流混合后從一共用的噴管排出，這種外涵道空氣可用于降低平均噴氣溫度和速度。
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