7.4.1.2. Propulsive and Overall Efficiency

The power produced by engine thrust is the product of thrust and flight velocity (V). The ratio of this useful power to the increment in kinetic energy given to the flow in passing through the engine is the propulsive efficiency (hp); a good approximation of this variable may written in the form hp = 2V/(V + Vj), where Vj is the jet velocity.

High levels of thermal efficiency require the T4/T2 ratio to be as high as possible with appropriate pressure ratio and component efficiencies. In the case of a simple turbojet gas turbine engine, this requirement would mean that the jet velocities are also relatively high. For example, for a ratio of T4/T2 = 5.6 together with an engine pressure ratio of 40, the jet velocity Vj would be about 817 ms^-1, (Cumpsty, 1997). At Mach 0.85 at 10.7 km, when the flight velocity V is 252 ms^-1, the relationship shown above gives a propulsive efficiency of only about 47%. Thermal efficiency at this condition is about 48%, so ho= hp x htherm = 0.48 x 0.47 = 23% for the assumptions of this simplified example. For typical aircraft, overall efficiency ranges between 20 and 40%.

The most practical method of raising overall efficiency is to lower the jet velocity and thereby increase propulsive efficiency; this approach has been adopted in the bypass engine used so widely today. In this engine design, hot gases leaving the core turbine expand through further turbine stages, which drive the fan mounted in front of the core compressor. In most modern engines, the pressure ratio across the fan at cruise condition is about 1.6, giving a bypass ratio of about 6 and a jet velocity of about 400 ms^-1. At this jet velocity, propulsive efficiency is about 77% at a cruise speed of Mach 0.85 at 10.7 km. Unfortunately, losses associated with the inefficiency of the fan and the turbine driving it inevitably reduce these benefits somewhat, so a typical value for the overall efficiency of such an engine is currently about 30 to 37% at cruise. Increasing the bypass ratio clearly offers the prospect of further increases in propulsive efficiency, but this approach has to be weighed against the penalties of increased size and weight of the installed engine and associated changes in drag (see Section 7.2 for further discussion of airframe efficiency and installation effects). Other prospects for increasing propulsive efficiency (e.g., propellers and unducted fans) are discussed in Section 7.4.3.

This brief review of theoretical considerations summarizes thermodynamic and aerodynamic constraints for engine designers in the continuing drive toward more efficient engines for current and future requirements. Although today's most advanced engines have bypass ratios in the range 5 to 9, there will be efforts to continue to try to increase them. Despite their attractions, however, bypass ratios much in excess of 9 are likely to require a gear box between the power turbine and the fan, or a novel configuration (a topic taken up further in Section 7.4.3), as well as imposing installation and weight problems.

7.4.2. Historical Trends

The foregoing discussion briefly reviewed the ways in which basic engine cycle considerations can influence design trends of gas turbines. Practical confirmation of such trends may be obtained by looking at historical trends of principal parameters affecting the performance of gas turbines since they were first introduced. Economic and aircraft range considerations have been uppermost in engine designers' minds. Figure 7-9 clearly shows the impressive progress made in reducing thrust-specific fuel consumption (mass flow rate of fuel burned per unit of thrust) with time. The engines of 1960 to 1970 vintage were either turbojets or first-generation low bypass ratio turbofans (Figure 7-7) with relatively high levels of fuel consumption. The period from 1970 to the mid-1980s saw the introduction of second-generation turbofan engines, which are generally referred to as high bypass ratio engines, which had significantly better fuel consumption than the earlier engines. Improvements in fuel consumption for third generation engines were smaller.

Important improvements in the understanding of complex aerodynamic flows within turbomachinery have been achieved over the past 25 years through mathematical modeling and parallel advances in experimental techniques. Figure 7-10 provides clear evidence of the benefits of that work in the rising trend of overall pressure ratio with time. In line with the fundamental relationships presented in Section 7.4.1, this trend has contributed significantly to the reduced fuel consumption shown in Figure 7-9.

Parallel work in the fields of combustion technology and materials have contributed to increasing levels of peak cycle temperatures-as shown in Figure 7-11, in which the trend of combustor exit temperature is plotted against the same time scale. (Note that this is the maximum temperature, which occurs at take-off. Temperatures are generally lower at cruise.) This increasing temperature trend has contributed not only to the improved fuel consumption trend shown in Figure 7-9-by virtue of improved thermal efficiency-but also to the rise in thrust-to-weight ratio, leading to higher payload for the same overall aircraft weight. The impact of increasing temperatures on various pollutant emissions is described in Section 7.5.

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