Aviation News, Thoughts and Pictures
Engines are the heart of any aircraft. As much as the aerodynamics of an aircraft are important, without the engines to power them, they are useless.
At the end of February, GE announced that it’s test engine core achieved the highest temperature in a Jet engine compressor and turbine. By boosting the temperature of the engine, the combustion can be carried out more efficiently. They estimate that the core could contribute to as much as a 25% increase in efficiency and boosting flight ranges by 30 percent.
The problem with hotter jet engines is how to physically build them. Material selection is currently the limiting factor for how hot the engine core can become, as the actual temperatures sustained in modern engines are above their melting points. By bleeding in cooler air from other sections of the engine, the actual temperature of the materials can be controlled, but lowering the core temperature reduces efficiency.
History of Materials in Early Jet Engines
We can trace a lot of post World War II aviation advances to research and development done by the Nazis during the war. Possibly the most important technology developed by the Germans was their Junker Jumo 004 and BMW 003 turbojet engines.
One of the problems all early WW II era jet engines had, of either Allied or Nazi make, was getting the proper materials. With the technology available to them at the time, Nickel alloys offered the highest operating temperatures, but getting the nickel in wartime was difficult. Everything from Tanks to Ships needed the Nickel to achieve the strength needed to function. Small amounts could be made available for research and development, but no for mass production.
BMW was able to put holes into the turbine blades of the hot section of the engine. By pumping cooler air (which was still hotter than an oven) into the blades, lower quality materials could be used. Unfortunately, this still did not completely solve their temperature problems.
Pilots, new to jets during the war, and in the years afterwards, found that rapid, or sudden advances of the throttle made the engines overheat. While the materials allowed for stable operation, adding more fuel to the fire (pun in intended) increased the temperature too rapidly for the cooling methods to keep up.
Modern Jet Engines
We come back to today, and find jet engines operating at temperatures that would make any post-war engineer envious. But the nature of jet engine has changed. Rather than turbojets, the most common engines today are turbofans. Now, jet engines are used to provide shaft torque to spin fan blades.
Today’s cutting edge advancement in Turbofan engines is adding a gearbox between the fan and jet engine core. The big advantage of this system is that it allows the jet engine core to spin at it’s optimal speed for power and fuel economy, and lets the fan spin at it’s most efficient speed. This removes the compromise and balance between combustion efficiency and aerodynamic efficiency.
Now, these engines have existed for some time, but they are generally lower thrust, and relatively low gear ratios around 3-to-1. The Pratt & Whitney PW1000G is one of, if not the largest geared turbofan engine, with up to 33,000 pounds of thrust. It has been selected to be used on the A320neo for many airlines, and pursuing another 1000 engines for the remaining planes.
Advantages of Geared Turbofan Engines
Aside from the efficiency benefits, by running the core and fan at more efficient speeds, geared turbofans offer the capability for lower engine noises. This is partially due to the lower presses created in the engine, but as air traffic grows, and the footprint of airports and their supporting facilities expands, noise becomes an issue the closer planes get to houses.
Noise isn’t something that people will generally think of when talking about jet engines. It has become almost expected that jets will be loud. In the United States there exists a regulation that requires Jet engines to not produce noise above a certain level. The FAA has defined four Stages that classify the amount of noise a jet can make on takeoff, based on its takeoff weight. There is a handy chart in Malcom J. Crocker’s Handbook of Acoustics on page 889 that shows the limits for these Stages. You can find this through Google Books, by searching for the book and author.
The takeoff noise level is measured at 2000m to the side of the runway, meaning that it is not as loud as right next to the jet. While a jet engine might produce 150 or more dB on takeoff, from 2000 meters away, the noise may be under 100 dB. The Stage 2 Noise limit maxes out at around 108dB, being the loudest for large aircraft approaching a takeoff weight of 1 million pounds.
How does a jet engine become quieter?
The first way jet engines reduce noise is by increasing the fan bypass ratio. The cooler air sucked through the fan encases the hot gasses exiting the core, and helps muffle it. The larger the bypass ratio, the better the noise reduction.
For engines with low bypass ratios, or even turbojet engines, hush kits can be installed to reduce engine noise. The Boeing 727, and early versions of the Boeing 737 using turbojet engines all had hush kits installed, which helped reduce the take off noise, but at the cost of range and fuel efficiency. As an example, the Gulfstream II hush kits add more than 230 pounds, and reduced range by 1.6%.
Disadvantages of Geared Turbofan Engines
The largest disadvantages with geared turbofans are the weight and complexity that the gearbox introduces to the engine. The weight, while significant is more than wiped out by the efficiency increase of the engine. The one remaining concern is complexity. This manifests itself in several places, primarily in maintenance and increased potential failure modes. These costs and disadvantages must be weighed against the benefits of the system, but it seems that Geared Turbofan Engines offer enough to make them a worthwhile investment.
Geared turbofans seem to offer the path to the future as airlines become increasingly aware of fuel consumption and costs. I personally think that applying the gearing technology to large diameter engines like those powering the 777, A380, 747, and others, will be the next challenge for engine makers. By possibly combining GE’s hotter jet engine core, and gearing, engine efficiency might increased by upwards of 30 percent, if the estimates put forward by engine makers are considered.