Airplane Engines - Decreasing Fuel Consumption and Exhaust Gas Emissions in Transportation - page 152

10.6 Summary and Recommendations: Combustion
Process in a Jet Engine

The first fuel-efficient engines with higher bypass ratios were introduced in the
1970s and 1980s. They reduced HC and CO emissions but increased the NO and
NO 2 output. In contrast to unburned substances, NO and NO 2 concentrations are
higher in full load intervals because of the higher air and fuel throughput, and the
higher pressure and temperature. The aim is to reduce NO and NO 2 levels by 50%
of the current level within 5-10 years.
The new approach involves improving the uniformity of fuel injection, mixing
fuel and air, and reducing emissions. NO x levels may be reduced to 50-70% of the
present amount by using multiple combustion zones in radial and axial configu-
rations, which permit optimal local temperatures and burning times in the

combustor.
High bypass turbofan engines are generally quieter than the earlier low bypass
engines. Using multi-stage fans significantly increases thrust and velocity of
exhaust gases. The combination of a higher BPR and a higher turbine inlet tem-
perature further improves thermal efficiency and lowers SFC.
Both economic and safety considerations limit the installation of new com-
bustor designs to old engines. A new construction is often introduced as a package
that includes modifications to the combustion chamber, to the fuel nozzles, and to
the engine control. Positive steps must prevent intermixing of new and old com-
ponents during maintenance. Of course, other requirements such as better fuel
consumption, lower peak cycle temperatures, reduced NO, NO 2 , and particle
emissions, and high durability with low costs must also be balanced if combustion
technology is retrofitted.
Micro sensors in the combustion chamber could become important in the future
jet engine technology because they could add additional signals to the current
sensor technology and control the first deteriorations in the burning process. Self
Diagnosis can be a further developed stage in addition to recent Emission Index
technology.

References

1. Götsch E (2000) Luftfahrzeugtechnik. Einführung, Grundlagen, Luftfahrzeugkunde, 1st edn.
Motorbuch Verlag, Stuttgart. ISBN: 3-613-02006-8
2. Jet force. http://en.wikipedia.org/wiki/Jet_force
3. Almond P Aviation. Könemann Verlag. ISBN: 3-8331-2560-8. http://www.gettyimages.com
4. Turbojet. http://en.wikipedia.org/wiki/Turbojet
5. MTU: Turbofan. http://www.mtu.de/de/globals/glossary/T/turbofan/index.html
6. Turboshaft. http://en.wikipedia.org/wiki/Turboshaft
7. Propfan. http://en.wikipedia.org/wiki/Propfan
8. Mantelstromtriebwerk. http://de.wikipedia.org/wiki/Turbofan
9. Thrust specific fuel consumption. http://en.wikipedia.org/wiki/Thrust_specific_fuel