Ever cleaner jet engines from CFM International
Ever cleaner jet engines from CFM International
Keywords: Engines, Aerodynamics, Environment
There are already many engines whose speeds cannot be increased significantly because of aerodynamic disturbance. Innovation, therefore, comes less in radical advances than in perfecting known areas of application. Aero engine manufacturers have decided to develop a new engine with a modular, simple and expandable architecture, capable of integrating technological innovations and of being adapted to new requirements.
"Today, the major concern of engine manufacturers is not only to develop engines with high technical and economic performance but to meet increasingly strict environmental standards for pollution emission and noise nuisance",explained Pierre Bry, Marketing Manager at CFM International. CFM International is a joint venture between French engine manufacturer Snecma Moteurs and General Electric, to produce the CFM 56 engine which powers aircraft with 18,500-34,000 pounds thrust. "Over the last 20 years, we have reduced engine noise by 20 EPNdB(Effective Perceived Noise in decibels, a unit of measurement used in acoustic certification). The regulations in force (Chapter 3) have already ensured the withdrawal of the noisiest aircraft and Chapter 4, which comes into force in 2006, will see a strengthening of the standard by 10 EPNdB. We also have to consider the discharge of nitrogen oxides, carbon monoxide, volatile organic compounds and smoke", continued Bry. Having developed an engine which claims to consume 20 per cent less fuel than its competitors, Snecma has introduced a novel combustion system to the CFM 56. According to Snecma, a double annular combustion chamber will make it possible to reduce emissions of nitrogen oxide by more than 40 per cent. The company is currently refining this technique to make it cheaper and simpler.
Thanks to progress achieved in metallurgy and techniques for cooling the turbine blades; the temperatures of the gases leaving the combustion chamber have increased from less than 1,000°C to around 2,000°C, making the engines more efficient. But the increase in temperature is not without effect on the resistance of the materials, which then have to offer the particular characteristics of resistance to corrosion and creep. Research into materials is therefore a priority research direction, particularly as they directly affect the mass of the unit and therefore the fuel consumption (30 per cent of operating costs).
Snecma, which up to now has been using special steel alloys based on nickel,chromium and molybdenum or titanium, is now turning to metal matrix composites(metal alloys reinforced with carbon, boron or silicon carbide fibres). Aluminium was introduced for the external housing. Other materials such as magnesium are also being investigated.
Subjected to strong forces, such as shock, load factors during turning and gyroscopic effects during direction changes, the bearings can cause a large amount of damage if they fail. "We are working on a development of the intershaft bearing to turn the two parts (high pressure and low pressure shafts)of the engine in the opposite direction in order to optimise the aerodynamic interface between the high and low pressure turbines and to improve efficiency",added Bry.
The aerodynamics offers other opportunities. The latest version of the CFM 56(65C/P), which recently entered service on the A340-300, incorporates metallic hollow swept blades.
The improvement in the aerodynamics of the blades and shroud and the incorporation of grooving on the housing of the low pressure compressor(booster) makes it possible to foresee in the future the elimination of bypass valves – complex and expensive components whose current role is to ensure engine stability. Lastly, by greatly reducing the number of turbine blades through aerodynamic optimisation, future engines will offer significantly lower maintenance costs.
The four-engined configuration, which is necessary for propulsion of a 560 tonne jet like the A380, has restarted co-operation within the transatlantic Alliance on the development of the GP 7200 engine, originally intended to power the Boeing 747 X but put on ice in favour of the Sonic Cruiser. Snecma, which is mainly responsible for components in the high pressure compressor, and for the low pressure compressor through its Techspace Aero subsidiary, has developed a nine-stage high pressure compressor with blades formed from the bulk material to reduce weight and cut maintenance costs. The blades were modified for some stages to optimise mass balancing.
In the transonic field, the prospects for aero engine manufacturers therefore remain limited. Revolutions could rather be expected in the supersonic field,where an effective and economical engine still remains to be developed. On the other hand, changes could come quickly in turboprop craft, which, thanks to fast propeller engines known as transonic propfans, will offer performances close to those of jet engines, with consumption reduced by around 20 per cent.
For further details, contact: FTPB. Tel: +44 (0) 207 235 5330; Fax: +44 (0)207 235 2773.
