Rolls-Royce is preparing technologies for the next generation of business jet turbofans and the design engineers’ motto seems to be “smaller, faster, leaner.” Karsten Mühlenfeld, Rolls-Royce executive v-p of engineering and technology for civil small and medium engines, provided AIN with details on future designs that will feature swifter development cycles, near-perfect reliability and reduced acquisition costs.
As one of the main drivers is reducing fuel burn, the engine maker’s specialists in Dahlewitz, Germany, are endeavoring to make the core engine more compact, since a smaller core greatly facilitates design of a lighter engine. At the heart of the next generation of engines will be a new high-pressure (HP) spool nearly half the size of those in today’s core engines. It will feature a pressure ratio of about 23:1, much greater than the current 16:1. (Increasing the pressure ratio improves efficiency.) The compressor will have 10 stages and the turbine will have two.
However, smaller blades in a smaller engine create a challenge: preventing air leaks. “If blades are inserted into disk slots, there are still under-platform gaps and, as the blades have gaps between each other, during each run they can slightly reposition themselves and therefore tip clearance can slightly vary,” Mühlenfeld explained. The smaller the blade, the more difficult it is to have a good command of clearances. In response, Rolls-Royce is developing blade-integrated disks (blisks) for all compressor stages, rather than for just a few.
As a result of the smaller rotor size, rotors in the HP system of future engines could spin at more than 26,000 rpm, far in excess of rotation speeds found in large civil engines. So designing the rotors for the lowest vibration possible becomes increasingly important. Again, blisks are a response. Moreover, residual imbalances are reduced through the design of optimized squeeze film dampers into bearing housings.
In the low-pressure spool, more compression is also needed. Just as GE is doing on the Passport engine, Rolls-Royce is pursuing a blisk fan concept. “Manufacturing the blade and disk as one unit will allow more airflow and higher pressure ratios,” Mülhenfeld asserted.
In combustion, the company is targeting lean burn for the long term. “Lean burn combustion quite simply involves burning the fuel in an excess of air to significantly reduce the formation of NOx [nitrous oxide]; the operation of the system is based on premixing the fuel and air inside the fuel injector prior to entering the combustor,” Mühlenfeld said. The injectors use fuel staging, with a pilot and a main fuel injector. The pilot maintains combustion stability at low power conditions, while the main injector reduces NOx emissions at high power conditions. However, Mühlenfeld sees major challenges in the complexity of a fuel-staged system and the integration of controls.
To meet its “best reliability from day one” goal, Rolls-Royce will keep using the lessons learned on in-service engines. “This data will be used and crosschecked during the development phase,” Mühlenfeld said. In addition, the engine is run in endurance tests in conditions well beyond those experienced in service. Special rig tests for accessories have been developed with suppliers.
Not content with lighter, more efficient and more dependable engines, Rolls-Royce is trying to bring them to market faster, too. “There is a mutual benefit for us and the airframer to carry out flight testing of a new airframe with engines that are mature and pre-certified,” Mühlenfeld pointed out.
All this foreseeable progress should be made within strict engine cost limits. Corporate jets are flown less intensively than airliners and, for the owner, the balance between acquisition and maintenance costs is different, Mühlenfeld said. The pressure on reducing the purchase price is even more intense in business jet engines. Rolls-Royce is thus looking for lower-cost materials and, for example, is developing a new, lower-cost alloy for HP turbine disks, a high-strength nickel alloy, called “Allvac 718 plus.”