According to a report by the simpleflying website on December 19, British aero-engine manufacturer Rolls-Royce announced on Monday that it will begin testing its UltraFan technology demonstrator (super fan engine) early next year. The report mentioned that a major innovation of the UltraFan engine is the use of a power gearbox, which has a rated power of 50 megawatts. This also marks that two of the three strongest civilian large-bypass-ratio turbofan engine manufacturers in the world, Pratt & Whitney of the United States and Rolls-Royce of the United Kingdom, have determined that "gear-driven turbofan engines" will be their future development direction.
UltraFan engine
gear transmission turbofan engine What are the advantages and why is it so attractive?
For an airline, fuel costs account for about one-third of its total operating costs . If the fuel consumption rate of passenger planes can be reduced, it will undoubtedly save airlines a lot of money. Therefore, the fuel consumption rate of passenger aircraft engines has become the most important indicator.
research shows that under the same conditions, the larger the bypass ratio, the lower the fuel consumption rate of the turbofan engine. In order to reduce fuel consumption as much as possible, civil aviation passenger aircraft engine manufacturers have been trying various methods to improve its bypass ratio. High-bypass ratio turbofan engines developed in the 1980s generally have a bypass ratio of only about 5.0; engines developed in the mid-to-late 1990s have a bypass ratio of about 8.0 (GE90); engines developed in the early 21st century have a bypass ratio of 10.0 to 11.0.
Parameters of some Pratt & Whitney engines
Research shows that when the bypass ratio of a turbofan engine increases to 8~9, the increase in quality and cost caused by increasing the number of stages may offset the benefits brought by the increase in the bypass ratio. In addition, the development of the existing technology route is getting closer and closer to the limit, and it is increasingly difficult to continue to increase the bypass ratio.
In addition, increasing the bypass ratio also brings a problem to the engine: the low fan speed causes the low-pressure compressor (supercharger stage) and the low-pressure turbine (power turbine) to work at unfavorable speeds.
This is because large bypass ratio turbofan engines are generally connected by multi-stage low-pressure turbines through connecting rods, and the rotation speeds of the two are the same. Because the diameter is relatively much smaller, for the low-pressure turbine, its optimal speed range is much higher than that of the fan. However, due to the drag of the fan, the speed is reduced, resulting in a significant decrease in its working efficiency. As the bypass ratio increases, the problem becomes more and more serious.
So is there any way to solve this contradiction?
In fact, smart R&D personnel have already thought of disconnecting the direct connection between the low-pressure turbine and the fan, and using the reduction gearbox to achieve different speeds between the fan and the low-pressure turbine, so that they are both in the best working speed range as possible.
UltraFan engine realizes differential operation of the fan and low-pressure turbine through the gearbox
After increasing the low-pressure turbine speed, the operating efficiency of the low-pressure turbine can also be greatly improved, and fewer stages can be used to achieve the same power output, which means that the structure can be reduced in weight.
As a result, the U.S. company Pratt & Whitney began exploring this technology route early in the 1980s and has mass-produced multiple models. The British company Rolls-Royce also proposed its own "geared turbofan engine" (GTF engine) development plan, and the UltraFan engine is its result.
Although the geared turbofan engine is good, the technical difficulty is not small.
In principle, the gear-driven turbofan engine is based on the traditional large bypass ratio turbofan engine and just adds a gearbox behind the fan or booster stage. It is very simple.
It is not difficult to build a fan drive gearbox, but it is not that simple to build a fan drive gearbox with high reliability, high stability and high durability.
Due to limited space, the power density of the fan drive gearbox is too high, which requires extremely high transmission efficiency and heat dissipation efficiency. This means extremely high requirements are placed on the materials, tooth profiles, tolerances, wear allowances, design limits, etc. of the transmission gears, as well as the performance limits, efficiency targets, durability and other parameters of the entire reducer.
Generally speaking, if you want to create a gearbox that meets the requirements, you must work hard on these two aspects:
The first is that the transmission efficiency must be efficient enough, generally exceeding 99%.
Take the gearbox of the UltraFan engine as an example. Its diameter is only about 1.2 meters. In order to be able to transmit 50 megawatts of power, its power transmission limit is as high as more than 64 megawatts. Taking 64 MW as an example, if the transmission efficiency is 99%, then the waste heat generated when it works is as high as 640 kilowatts. In fact, its working efficiency is 99.6%, which means that its waste heat is also as high as 256 kilowatts. If the transmission efficiency is too low, on the one hand, the efficiency loss will be too great, and it may even be inferior to a traditional large bypass ratio turbofan engine. On the other hand, too much waste heat will be generated, and the gearbox will be unable to bear it.
UltraFan engine gearbox
Therefore, the gearbox must be precise enough, the engagement between the gears must be precise enough, the stiffness must be high enough, and the gear itself must be wear-resistant enough, otherwise it will be difficult to achieve ultra-efficient transmission for a long time.
The second is the extremely high heat dissipation requirements, which require hundreds of kilowatts or even megawatts of heat dissipation power in a small space.
It is understood that the diameter of the gearbox of the UltraFan engine is only about 1.2 meters, and the whole is a disc with five planetary gears and a sun gear built inside. These gears inevitably generate waste heat during high-speed rotation, which can reach 256 kilowatts according to the above estimate. In order to reserve sufficient safety redundancy, its ultimate heat dissipation capacity must be much greater than 256 kilowatts. Moreover, every time the transmission efficiency of the gearbox decreases by 0.1 percentage point, an additional 64 kilowatts of waste heat will be generated.
Two designs of planetary gearbox
If so much waste heat cannot be discharged in time, the gearbox may be burned in a few seconds. How to design a reasonable and light enough heat dissipation circuit is particularly important.
It can be seen that if you want to create transmission gears that meet the requirements of civil aviation aircraft engines, you need to overcome the three major difficulties of design, materials, and technology. This requires a sufficiently advanced industrial foundation to achieve this.
Unexpectedly, my country's CJ2000 actually planned to get it done in one step.
On December 8, China Shipbuilding Industry Corporation 703 released a promotional report titled "The test mission of Guanghan Transmission's fan drive gearbox for large aircraft was successfully completed." Suddenly many people were surprised: CJ2000 actually planned to use a gearbox to drive the fan!
Photos of the 20 MW fan drive gearbox test site provided by China Shipbuilding Industry Corporation 703
The drive gearbox used in turbofan engines is divided into accessory gearboxes and planetary gearboxes. The former is more commonly used in turboprop and turboshaft engines, and our country has long been able to develop and mass-produce it on its own; the latter is generally used in turbofan engines, and our country has been in a technological blank in this field before. The
report mentioned that "the test task of the first domestic fan drive gearbox for the Chinese commercial engine CJ2000 project undertaken by Harbin Guanghan Transmission Co., Ltd. was successfully completed." "The first" means that this is the first time that this kind of drive gearbox is involved in our country. It is obviously not an accessory gearbox, so there is only one possibility: planetary gearbox!
The key is that Harbin Guanghan Transmission Co., Ltd. has completed the test task of a 20-megawatt fan drive gearbox. Theoretically it can be used on geared turbofan engines with a thrust of up to 18 tons.
Considering that the design thrust of CJ2000 reaches 35 tons, a 40-megawatt fan drive gearbox is required to meet the requirements. Therefore, this set of 20-megawatt fan drive gearbox is likely to be only the model used by the CJ2000 scaled verification machine, and is still in the pre-research stage. To make it practical, at least another 40-megawatt fan drive gearbox needs to be developed.
CJ2000 engine model
If the CJ2000 can obtain an airworthiness certificate in the form of a gear-driven turbofan engine, the fan-driven gearbox technology it eventually develops can directly cover all large-bypass-ratio turbofan engines by reducing its size. This will help COMAC quickly catch up with the world's first-class standards.
To sum up, passing the test of this 20-megawatt fan drive gearbox can only be regarded as the first step in a long journey. Although it is of great significance, it still needs to make persistent efforts to be practical. I wish the R&D personnel of the CJ2000 project to complete the 40-megawatt fan drive gearbox as soon as possible and build the CJ2000 geared turbofan engine as soon as possible.
