Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
temperature at compressor exit to the point where materials are a limitation; furthermore, the higher temperature increases the problem of cooling the turbine and
increases levels of NO x production in the combustor (as discussed in Section 7.5.2). Increasing the internal pressure ratio at current values of T 4 /T 2 would offer very
little improvement in thermal efficiency.
Figure 7-9: Fuel consumption progress with time.
The increase in turbine inlet temperature over the past 20 years has been the result of research and development, primarily to increase the power-to-weight ratio of the
engine. As Figure 7-8 shows, however, a modest increase in T4/T2 has had an effect on thermal efficiency. At the theoretical limit, by operating at stoichiometric fuelair
ratios in the combustion system, the temperature ratio would be about double that of current engines. The practical difficulties associated with achieving
stoichiometric combustion (largely associated with maintaining structural integrity with materials that lose strength at high temperatures), however, remain formidable.
Furthermore, even if these difficulties could be overcome, the thermal efficiency of the engine would still remain below 60% given current levels of turbomachinery
component efficiencies. Only when hcomp = hturb =100% would thermal efficiency approach 70% for an overall pressure ratio of around 64. Thus, although there are
clear gains to be made by further increases in T4/T2 and thermal efficiency, the full benefits can be achieved only through parallel efforts aimed at raising propulsive
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