Jet engines have been powering all the larger commercial aircraft since the introduction of the Comet back in 1949. Since then the engine industry has been busy improving the efficiency of the engines in order to make them more fuel efficient, less polluting and more silent than predecessors. With the development of the Solar Impulse, a solar powered aircraft, the question arises more often: is there a future for jet engines?
The first thing that one should wonder is: when exactly is the future? Is it in five years, ten years or even fifty years? Looking at the development of electrical powered engines, there is a possibility that jet engines will disappear in about fifty years. This is just a rough estimation, because the development of electrical powered engines is going very slowly. When looking at the future in five years from now, much more information is available. How are the engine manufacturers dealing with the strict demands from the industry and the regulators?
The demand of the industry is very clear: engines should be less fuel consuming and more quiet. Less fuel consumption means less fuel costs, which still is one of the biggest expenses of an airline. Landing with aircraft equipped with more quiet engines means less landing charges, which are categorised in weight and noise categories.
In order to protect the environment and energy supply, the Advisory Council for Aviation Research and Innovation in Europe (ACARE) have set some goals for 2020. This is a 90% drop in NOx emission, a 10dB noise reduction and a 50% drop in fuel consumption in comparison to the latest aircraft in 2000. The newest aircraft back in 2000 were equipped with engines like the CFM56, GE90, Trent 600 or the GP7200. The Committee on Aviation Environmental Protection (CAEP), a technical committee of the ICAO Council, has set milestones over the years to decrease emission of harmful gasses, as well as ACARE.
When looking at today’s newest engines, there is a lot of development on a less fuel consuming and less polluting combustor. One of the newest developments is the TAPS III combustor of the GE9X engine, for the Boeing 777X. This combustor is an improvement of the TAPS II combustor, which is found in the GEnx engine for the Boeing 787. Looking at the fan there are a lot of different methods to make the fan more efficient. Pratt & Whitney is using a gearbox to lower the RPM of the fan compared to the low pressure turbine. P&W advertises that the gearbox does not contain life limited parts. Rolls Royce uses a triple shaft design to lower the RPM of the fan. For the compressor, increasing the compression ratio with less stages is a challenge. Less stages means less maintenance and thus less maintenance costs. General Electric has managed to create a 60 to 1 compression ratio in the GE9X engine with a three stage booster and an eleven stage high pressure compressor. Rolls Royce manages to achieve a 50 to 1 compression ratio with an eight stage intermediate pressure compressor and a six stage high pressure compressor. The bypass ratio increases on all of the newest engines, which is a logical development, in order to make the turbofan more efficient. The use of additive manufactured components is increased as well. For instance, the GE9X and the CFM LEAP engine both have printed fuel nozzle tips.
ACARE has set a new set of goals for 2050 which even sharpen the goals of 2020: a 75% reduction of CO2 emissions, a 90% reduction in NOx emissions and a noise reduction of 65%, compared to new aircraft in 2000. So engine manufacturers are already back on developing even more efficient and less polluting engines. The development will go on until a new type of engine is developed which is even more efficient than the turbofan engine.
Such an engine type could come from the AHEAD-project (Advanced Hydro Engines for Aircraft Development). They have developed a hybrid engine with a lot of new innovations, never seen before in today’s gas turbines. For instance, they have created a smaller fan with the same propulsive power and bypass ratio as a conventional engine, making it even more efficient. This efficiency is reached with the use of counter rotating fans. The most unique design point is that the engine contains two separate combustion chambers. In the first one, liquid hydrogen is combusted. With the better flammability of hydrogen, a leaner combustion could be achieved. In the second combustion chamber, a flameless combustion of biofuel is taking place. This combustion is possible with the product of the first combustion chamber: water vapour. All this makes the engine less CO and NOx emitting, making it a much cleaner and efficient engine and meeting the ACARE 2050 demands. This engine design is made for a blended-wing-body aircraft design. Could this be the future for jet engines? The implementation of a blended-wing-body design would demand a great change in the airport infrastructure, even greater than the change for the implementation of the Airbus A380.