推力和马力之间的 比较
飞行中的发动机推力
前飞速度的影响
加力燃烧对发动机 推力的影响
高度的影响
温度的影响
推进效率
油耗和功率重量的关系
Fuel consumption and power-to-weight relationship 225
INTRODUCTION
1. The performance requirements of an engine are obviously dictated to a large extent by the type of operation for which the engine is designed. The power of the turbo-jet engine is measured in thrust, produced at the propelling nozzle or nozzles, and that of the turbo-propeller engine is measured in shaft horse-power (s.h.p.) produced at the propeller shaft. However, both types are in the main assessed on the amount of thrust or s.h.p. they develop for a given weight, fuel consumption and frontal area.
2.
Since the thrust or s.h.p. developed is dependent on the mass of air entering the engine and the accel-eration imparted to it during the engine cycle, it is obviously influenced, as subsequently described, by such variables as the forward speed of the aircraft, altitude and climatic conditions, These variables influence the efficiency of the air intake, the compressor, the turbine and the jet pipe; conse-quently, the gas energy available for the production of thrust or s.h.p. also varies.
3.
In the interest of fuel economy and aircraft range, the ratio of fuel consumption to thrust or s.h.p. should be as low as possible. This ratio, known as the specific fuel consumption (s.f.c.), is expressed in pounds of fuel per hour per pound of net thrust or
s.h.p. and is determined by the thermal and propulsive efficiency of the engine. In recent years considerable progress has been made in reducing
s.f.c. and weight. These factors are further explained in para. 46.
5.热效率和推进效率在很大程度上还影响着压气机和涡轮的尺寸，因而在输出给定的情况下，这些效率能决定发动机的重量和直径。
4.热效率常被称作发动机的内部效率，而推进效率称作外部效率。在第37段中描述的这个推进效率解释了为什么飞机在低速时纯喷气发动机的效率不如涡轮螺桨发动机，因而导致涡轮风扇发动机以及最近的桨扇发展。
7.为了能对相似的一些发动机的性能进行比较，有必要以常用的形式使随高度和天气条件而发生的空气温度和压力的变化标准化。在在使用中，对标准大气有几种不同的定义，最为通用的是国际标准大气（I.S.A.).这是根据温度的递减率约为每1000英尺1.98K，导致从海平面的288.15K（15℃）降至36，089英尺（对流层顶）的216.65K（-56.5℃）。在这个高度以上，直至65，617英尺国际标准大气温度不变。在海平面国际标准大气的标准压力为14.69磅/平方英寸，在对流层顶，它降至3.28磅/平方英寸（参阅图21-10国际标准大气表）
6.这些和其他因素在曲线和图表中做了介绍，它们是从基本的气体定律（第2章）中计算出来的，并在试车台和试飞或在高空试验台上模拟飞行条件下得到实验证明。为了进行这些计算，采用专门的符号来表示发动机各部位的压力和温度；例如，利用图21-1中表示的符号，压气机的总增压比为P3/P1。这些符号因发动机类型不同而稍有不同；例如，对高涵道比发动机，还要当采用加力（第16章）时，就使用更多的符号。
4.
Whereas the thermal efficiency is often referred to as the internal efficiency of the engine, the propulsive efficiency is referred to as the external efficiency. This latter efficiency, described in para. 37, explains why the pure jet engine is less efficient than the turbo-propeller engine at lower aircraft speeds leading to development of the by-pass principle and, more recently, the propfan designs.
5.
The thermal and the propulsive efficiency also influence, to a large extent, the size of the compressor and turbine, thus determining the weight and diameter of the engine for a given output.
6.
These and other factors are presented in curves and graphs, calculated from the basic gas laws (Part 2), and are proved in practice by bench and flight testing, or by simulating flight conditions in a high altitude test cell. To make these calculations, specific symbols are used to denote the pressures and tem-peratures at various locations through the engine; for
Performance
instance, using the symbols shown in fig. 21-1 the P3
overall compressor pressure ratio is . These
P1 symbols vary slightly for different types of engine; for instance, with high by-pass ratio engines, and also when afterburning (Part 16) is incorporated, additional symbols are used.
7. To enable the performance of similar engines to be compared, it is necessary to standardize in some conventional form the variations of air temperature and pressure that occur with altitude and climatic conditions. There are in use several different definitions of standard atmospheres, the one in most common use being the International Standard Atmosphere (I.S.A.). This is based on a temperature lapse rate of approximately 1.98 K. degrees per 1,000ft,, resulting in a fall from 288.15 deg.K. (15 deg.C) at sea level to 216.65 deg.K (-56.5 deg.C.) at 36,089 ft. (the tropopause). Above this altitude the
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