Figure 7-17: Effect of pressure ratio on NOx-specific fuel
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Figure 7-17: Effect of pressure ratio on NOx-specific fuel |
Between 1965 and 1975, low-smoke combustors were developed and incorporated into low bypass ratio engines commonly used in early commercial jet aircraft. These changes virtually eliminated visible smoke trails from aircraft. Introduction of higher bypass ratio engines of the late 1960s and early 1970s-with their significantly improved SFCs-marked a new and important step in reducing CO2 and water vapor emissions from aircraft. These engines also emitted much lower levels of HC and CO at low power ("idle") setting as a result of improved fuel/air mixing and relatively high levels of pressure and temperature in the combustor at this condition. Improved fuel/air mixing in annular combustors of the new engines also reduced take-off smoke. The trend toward even higher bypass engines, with their improved fuel efficiency, continues today, responding not only to the initial and continuing commercial and operational pressures but also to increasing concern about the effect of CO2 on the environment.
Between 1975 and 1985, new combustor design features that had better fuel atomization and circumferential fuel staging at idle led to further reductions in HC and CO emission levels. A typical example of such improvements in HC and CO emission is presented in Table 7-3, which indicates the scale of some of the benefits that have already been made toward abatement of urban air pollution burdens (Bahr, 1992).
At that time, however, there was little change in the NOx levels; although, as Table 7-3 shows, they were well within ICAO standards, which applied to landing and take-off operations (ICAO, 1981). Emissions regulation for HC, CO, and NOx are based on the total mass of each species produced over the ICAO landing/take-off cycle, which is intended to represent typical aircraft operations in the vicinity of the airport. The mass of each species (in grams) is divided by the take-off thrust of the engine (in kilo Newtons) so that different size engines can be reasonably compared; thus, the resulting units are g kN-1. ICAO landing/take-off cycle, measurement procedures, and emissions standards are discussed further in Section 7.7.1.
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Table 7-4: Typical basic performance and operational requirements of a modern aircraft engine combustor. |
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| Item | Requirement | Value | Max/Min |
| 1 | Combustion efficiency - At takeoff thrust (%) - Idle thrust (%) |
99.9 99.0 |
(Min) (Min) |
| 2 | Low-pressure light-off capability (MPa) | 0.03 | (Max) |
| 3 | Lean blowout fuel/air ratio (at low engine power conditions) | 0.005 | (Max) |
| 4 | Ground light-off fuel/air ratio (with cold air, cold fuel) | 0.010 | (Max) |
| 5 | Total pressure drop-compressor exit to turbine inlet (%) | 5.0 | (Max) |
| 6 | Exit gas temperature distribution - Pattern factor - Profile factor |
0.25 0.11 |
(Max) (Max) |
| 7 | Combustion dynamics [dynamic pressure range/inlet air pressure (%)] | 3 | (Max) |
| 8 | Liner metal temperature (K) | 1120 | (Max) |
| 9 | Cyclic life to first repair (cycles) | 5000 | (Min) |
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Table 7-5: Combustion developments linked to emissions performance. |
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| Requirements* | Emissions Implications and Compromise Required |
| 1, 8, and 9 |
Lowering of liner cooling flows improves "idle" efficiency (low HC and CO) and ability to reduce NOx but must be balanced against effect on liner temperatures and life/durability |
| 2, 3, and 4 | Wide range of fuel/air ratios and long dwell times in combustor favor good light-off performance but must be balanced against need to control HC and CO emissions at low power and NOx formation rates at high power |
| 5 and 7 | Low overall and dynamic pressure losses required to minimize SFC losses but must be balanced by need to retain good mixing for low emissions and good liner cooling |
| 6 | Long mixing lengths improve exit profiles and pattern factors but require more cooling air and increase NOx formation times |
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*See Table 7-4 for item requirement numbering scheme. |
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Other reports in this collection |
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