History
The Wright Brother’s “Wright Flyer” used a single piston engine to turn two propellers. Variations of piston engines would continue to evolve, becoming much more powerful. Piston engines would reach the limit of their development during the Second World War when a new type of aircraft engine would be introduced to the skies over Europe. By 1944, two aircraft, the British built Gloster Meteor and the German built Messerschmidt Me-262, would take to the skies powered by a new type of engine, the turbojet. The Me-262 in particular proved the advantages of the turbojet engines when they were sent after American bomber formations that were escorted by one of the most advance piston engine fighters of the war, the North American P-51 Mustang. The Mustang was fast, with a top speed of about 450 miles per hour, but the Me-262 was about 100 miles per hour faster. The Me-262s would come in at full speed and tear through the bomber formations, claiming several bombers and a few of the escort fighters before the fighter pilots or the defensive gunners on the bombers could even react. Fortunately for the allies the Me-262 was a case of too little too late. There were too few of the new jet fighters to make a difference in the outcome of the war and too late because by 1944-45, the manufacturing centers of Nazi Germany had been bombed to oblivion. Because of the constant bombing the manufacturing process were not always the greatest and the Me-262’s engines were unreliable and very fragile, a disadvantage that American and British fighter pilots exploited to some success. The Me-262 did prove that piston engines had reached their limit and that the jet was the way of the future. While most aircraft designers after the war embraced the turbojet for military aircraft, few would look at applying the technology to civilian aircraft. Many believed that the turbojet was economically unfeasible for airline use. That idea would soon change however thanks to one enterprising British airplane manufacturer.
In 1947, the British de Havilland Company changed the world with the first flight of world’s first jet powered commercial airliner. The de Havilland DH.106 Comet was powered by four turbojet engines and had a top speed of around 500 miles per hour, cutting flight times in half versus piston engine airliners. The Comet proved that a jet powered airliner was economically feasible and set off a scramble among other airplane manufacturers to build a jet airliner to compete with it. By the mid-1950s three American companies would debut their own jet airliners, Boeing with its 707 airliner, McDonnell-Douglas with its DC-8, and Convair with its 880 and 990 airliners. All of these aircraft were initially powered by turbojet engines and one major drawback with the early turbojets became apparent, they were too underpowered for a large commercial airliner during takeoff and they were gas guzzlers. In order to alleviate the lack of power drawback during takeoff many of the turbojets had a water injection system that would force water into the engine to fool it into creating more power, but that process would mean very smoky and very, very noisy takeoffs. There had to be a better engine for the job, and one was found. Enter the turbofan.
The Wright Brother’s “Wright Flyer” used a single piston engine to turn two propellers. Variations of piston engines would continue to evolve, becoming much more powerful. Piston engines would reach the limit of their development during the Second World War when a new type of aircraft engine would be introduced to the skies over Europe. By 1944, two aircraft, the British built Gloster Meteor and the German built Messerschmidt Me-262, would take to the skies powered by a new type of engine, the turbojet. The Me-262 in particular proved the advantages of the turbojet engines when they were sent after American bomber formations that were escorted by one of the most advance piston engine fighters of the war, the North American P-51 Mustang. The Mustang was fast, with a top speed of about 450 miles per hour, but the Me-262 was about 100 miles per hour faster. The Me-262s would come in at full speed and tear through the bomber formations, claiming several bombers and a few of the escort fighters before the fighter pilots or the defensive gunners on the bombers could even react. Fortunately for the allies the Me-262 was a case of too little too late. There were too few of the new jet fighters to make a difference in the outcome of the war and too late because by 1944-45, the manufacturing centers of Nazi Germany had been bombed to oblivion. Because of the constant bombing the manufacturing process were not always the greatest and the Me-262’s engines were unreliable and very fragile, a disadvantage that American and British fighter pilots exploited to some success. The Me-262 did prove that piston engines had reached their limit and that the jet was the way of the future. While most aircraft designers after the war embraced the turbojet for military aircraft, few would look at applying the technology to civilian aircraft. Many believed that the turbojet was economically unfeasible for airline use. That idea would soon change however thanks to one enterprising British airplane manufacturer.
In 1947, the British de Havilland Company changed the world with the first flight of world’s first jet powered commercial airliner. The de Havilland DH.106 Comet was powered by four turbojet engines and had a top speed of around 500 miles per hour, cutting flight times in half versus piston engine airliners. The Comet proved that a jet powered airliner was economically feasible and set off a scramble among other airplane manufacturers to build a jet airliner to compete with it. By the mid-1950s three American companies would debut their own jet airliners, Boeing with its 707 airliner, McDonnell-Douglas with its DC-8, and Convair with its 880 and 990 airliners. All of these aircraft were initially powered by turbojet engines and one major drawback with the early turbojets became apparent, they were too underpowered for a large commercial airliner during takeoff and they were gas guzzlers. In order to alleviate the lack of power drawback during takeoff many of the turbojets had a water injection system that would force water into the engine to fool it into creating more power, but that process would mean very smoky and very, very noisy takeoffs. There had to be a better engine for the job, and one was found. Enter the turbofan.
The Turbofan
The turbofan was the first leap in jet engine technology over the turbojet. The main difference between the two engines is that turbofans allow air to bypass the engine core where as in a turbojet; all air that goes into the engine goes through the core. Turbofans could generate more power while using less fuel and were quieter than turbojets. Boeing and McDonnell-Douglas where some of the first aircraft manufacturers to start building commercial aircraft with turbofans installed, both choose the Pratt and Whitney JT3D engines. This not only increased the takeoff performance; it decreased the fuel consumption and as a side effect of that, increased range. Engines like the JT3D and the later JT8D, which powered the Boeing 727 and 737 as well as the McDonnell-Douglas DC-9, are known as a low-bypass ratio turbofans because the ratio of bypass airflow to non-bypass airflow is very small, the next evolution in turbofans would led to the creation of the high-bypass ratio turbofan.
High-bypass ratio turbofans have very large ratio of bypass airflow to non-bypass airflow versus low bypass ratio turbofans. The high-bypass ratio turbofans offer increased power and decreased noise and fuel consumption. The first patent for the high-bypass ratio turbofan was filed by Siegfried H. Decher and Dale H. Rauci on February 26, 1965 (Decher and Rauci, 1965). A few years later, the first aircraft powered by the new high bypass ratio turbofans would take to the skies. These aircraft were the new generation of wide body airliners (that means they have more than six abreast seating which would require more than one aisle in the cabin). These aircraft would include the Boeing 747, McDonnell-Douglas DC-10, Lockheed L1011 TriStar, and the Airbus A300, which was the first aircraft built by European manufacturer Airbus. These aircraft had even more range thanks in part to the new type of engine and where much quieter than their predecessors. High bypass ratio turbofans have been used on almost every aircraft manufactured since the late 1960s. As the years have passed by, the bypass ratios have increased and as a result so have engine diameters; a Boeing 777 engines have the same diameter as the fuselage of a Boeing 737. All of this adds up to increased weight, as you can imagine the engines on a Boeing 777 are very heavy and more weight leads to drag and loss of fuel efficiency. There is also the matter of harmful CO2 (carbon dioxide) and NOx (nitrogen oxide compounds) emissions that aircraft engines produce. In order to address these issues, there is a lot of attention and effort being spent on technologies to reduce the negative impacts of the emissions and weight issue, and one company is heavily invested in finding solutions to these problems.
Making Turbofans Better
Because of how jet engines operate, by mixing air and fuel and burning it, one of the byproducts is harmful emissions, mainly CO2, NOx, and SOx (sulfur oxide compounds). Aviation emissions account for only about 2% of the total greenhouse gas emissions, but this is expected to grow to 3% by 2050 (Parker, 2009). There are new policies that include how to better manage air traffic that can reduce emissions, but one of the most looked at ways of reducing emissions is using technology. Since the 1960s there has been a 70% reduction in CO2 and a 50% reduction in NOx emissions, most of this was out the airlines’ cost saving desires but a lot of it was also to stricter ICAO (International Civil Aviation Organization) regulations since the 1980s (Parker, 2009). One of the companies taking a big step in emission reduction is British engine manufacturer Rolls-Royce in response to Advisory Council for Aeronautical Research in Europe (ACARE) Vision 2020 agenda which strives for a 50% reduction in CO2 emissions though airframe, engine, and air traffic management improvements (Parker, 2009). This represents a challenge since one of the hardest things to do is bring technology to market; this requires an extensive research and development budget. Rolls-Royce spends roughly two thirds of an £800 million per year R&D budget on technologies for reducing environmental impact (Parker, 2009). However, simply throwing money at a problem will not make it go away, you need to have a plan set up for it that not only monitors what will be going on today, but also in a few years and a few decades down the line. Rolls-Royce has done that with their Vision 5, Vision 10, and Vision 20 programs. These look at technology that is already available, what will be available within the decade and technologies that are only theoretical or in very early stages of development, respectively (Parker, 2009). Vision 5 has already given the world two new technologies in recent years; the hollow, swept back titanium fan blades that are used on the Rolls-Royce Trent 900 engines for the Airbus A380 and the electric Environmental Control and Anti-Ice systems in the Trent 1000 engines that power Boeing’s 787 Dreamliner, both help reduce emissions in both aircraft (Parker, 2009). All of this research as brought up numerous ways to reduce emissions; while some are better than others they all have their merits.
One method of reducing emissions is by increasing bypass ratios in jet engines, however you can only increase the ratio so much before the weight and drag created by the engine offset the fuel and emissions savings. One solution is by using open rotor design on mainly short haul aircraft such as the Boeing 737 and Airbus A320. There are issues with certification, maintenance, and integration as the exposed fan blades need a bit more room than a traditional engine. Increasing temperature and engine combustion pressure is another option, however this is limited by the materials that make up the engine and there is also the undesirable consequence of increasing NOx emissions. There is research going into making engine components out of a single nickel alloy crystals, which are stronger than traditional alloys because they lack grain boundaries which are weak points in the material which would allow higher pressures and temperatures in the engine (Parker, 2009). There is however another school of thought that wants to move away from fossil fuel burning engines and towards electric based propulsion.
Electric Propulsion
The idea of using electric engines has merit. The electric engines would overcome the physical limitations of traditional jet engines in regards to gains in efficiency by using varying speed control by the fact that all of the components are connected to the same drive shaft and that the low pressure, or intake fan, cannot exceed the speed of sound during normal, non-takeoff, operations. An electric engine would use an multiple generators linked to a turbo shaft, with the fan for propeller connected electronically to the generators though an “electrical gearbox” which would allow the faster rotational speeds and less stages necessary, all of this would reduce weight and increase efficiency (Luongo, et al., 2009). It would also enable more control flexibility, kind of like a transmission in a car. The electric engines are a popular contender for the concept of “distributed propulsion” which would use multiple small engines integrated in an aircraft’s structure. While engine technology is one of the main ways to improve aviation sustainability it is not the only way. One of the ways is by looking at what enables jet engines to operate, their fuel.
Top Image Retrieved From http://images.wisegeek.com/close-up-of-jet-engine-on-tarmac.jpg
The turbofan was the first leap in jet engine technology over the turbojet. The main difference between the two engines is that turbofans allow air to bypass the engine core where as in a turbojet; all air that goes into the engine goes through the core. Turbofans could generate more power while using less fuel and were quieter than turbojets. Boeing and McDonnell-Douglas where some of the first aircraft manufacturers to start building commercial aircraft with turbofans installed, both choose the Pratt and Whitney JT3D engines. This not only increased the takeoff performance; it decreased the fuel consumption and as a side effect of that, increased range. Engines like the JT3D and the later JT8D, which powered the Boeing 727 and 737 as well as the McDonnell-Douglas DC-9, are known as a low-bypass ratio turbofans because the ratio of bypass airflow to non-bypass airflow is very small, the next evolution in turbofans would led to the creation of the high-bypass ratio turbofan.
High-bypass ratio turbofans have very large ratio of bypass airflow to non-bypass airflow versus low bypass ratio turbofans. The high-bypass ratio turbofans offer increased power and decreased noise and fuel consumption. The first patent for the high-bypass ratio turbofan was filed by Siegfried H. Decher and Dale H. Rauci on February 26, 1965 (Decher and Rauci, 1965). A few years later, the first aircraft powered by the new high bypass ratio turbofans would take to the skies. These aircraft were the new generation of wide body airliners (that means they have more than six abreast seating which would require more than one aisle in the cabin). These aircraft would include the Boeing 747, McDonnell-Douglas DC-10, Lockheed L1011 TriStar, and the Airbus A300, which was the first aircraft built by European manufacturer Airbus. These aircraft had even more range thanks in part to the new type of engine and where much quieter than their predecessors. High bypass ratio turbofans have been used on almost every aircraft manufactured since the late 1960s. As the years have passed by, the bypass ratios have increased and as a result so have engine diameters; a Boeing 777 engines have the same diameter as the fuselage of a Boeing 737. All of this adds up to increased weight, as you can imagine the engines on a Boeing 777 are very heavy and more weight leads to drag and loss of fuel efficiency. There is also the matter of harmful CO2 (carbon dioxide) and NOx (nitrogen oxide compounds) emissions that aircraft engines produce. In order to address these issues, there is a lot of attention and effort being spent on technologies to reduce the negative impacts of the emissions and weight issue, and one company is heavily invested in finding solutions to these problems.
Making Turbofans Better
Because of how jet engines operate, by mixing air and fuel and burning it, one of the byproducts is harmful emissions, mainly CO2, NOx, and SOx (sulfur oxide compounds). Aviation emissions account for only about 2% of the total greenhouse gas emissions, but this is expected to grow to 3% by 2050 (Parker, 2009). There are new policies that include how to better manage air traffic that can reduce emissions, but one of the most looked at ways of reducing emissions is using technology. Since the 1960s there has been a 70% reduction in CO2 and a 50% reduction in NOx emissions, most of this was out the airlines’ cost saving desires but a lot of it was also to stricter ICAO (International Civil Aviation Organization) regulations since the 1980s (Parker, 2009). One of the companies taking a big step in emission reduction is British engine manufacturer Rolls-Royce in response to Advisory Council for Aeronautical Research in Europe (ACARE) Vision 2020 agenda which strives for a 50% reduction in CO2 emissions though airframe, engine, and air traffic management improvements (Parker, 2009). This represents a challenge since one of the hardest things to do is bring technology to market; this requires an extensive research and development budget. Rolls-Royce spends roughly two thirds of an £800 million per year R&D budget on technologies for reducing environmental impact (Parker, 2009). However, simply throwing money at a problem will not make it go away, you need to have a plan set up for it that not only monitors what will be going on today, but also in a few years and a few decades down the line. Rolls-Royce has done that with their Vision 5, Vision 10, and Vision 20 programs. These look at technology that is already available, what will be available within the decade and technologies that are only theoretical or in very early stages of development, respectively (Parker, 2009). Vision 5 has already given the world two new technologies in recent years; the hollow, swept back titanium fan blades that are used on the Rolls-Royce Trent 900 engines for the Airbus A380 and the electric Environmental Control and Anti-Ice systems in the Trent 1000 engines that power Boeing’s 787 Dreamliner, both help reduce emissions in both aircraft (Parker, 2009). All of this research as brought up numerous ways to reduce emissions; while some are better than others they all have their merits.
One method of reducing emissions is by increasing bypass ratios in jet engines, however you can only increase the ratio so much before the weight and drag created by the engine offset the fuel and emissions savings. One solution is by using open rotor design on mainly short haul aircraft such as the Boeing 737 and Airbus A320. There are issues with certification, maintenance, and integration as the exposed fan blades need a bit more room than a traditional engine. Increasing temperature and engine combustion pressure is another option, however this is limited by the materials that make up the engine and there is also the undesirable consequence of increasing NOx emissions. There is research going into making engine components out of a single nickel alloy crystals, which are stronger than traditional alloys because they lack grain boundaries which are weak points in the material which would allow higher pressures and temperatures in the engine (Parker, 2009). There is however another school of thought that wants to move away from fossil fuel burning engines and towards electric based propulsion.
Electric Propulsion
The idea of using electric engines has merit. The electric engines would overcome the physical limitations of traditional jet engines in regards to gains in efficiency by using varying speed control by the fact that all of the components are connected to the same drive shaft and that the low pressure, or intake fan, cannot exceed the speed of sound during normal, non-takeoff, operations. An electric engine would use an multiple generators linked to a turbo shaft, with the fan for propeller connected electronically to the generators though an “electrical gearbox” which would allow the faster rotational speeds and less stages necessary, all of this would reduce weight and increase efficiency (Luongo, et al., 2009). It would also enable more control flexibility, kind of like a transmission in a car. The electric engines are a popular contender for the concept of “distributed propulsion” which would use multiple small engines integrated in an aircraft’s structure. While engine technology is one of the main ways to improve aviation sustainability it is not the only way. One of the ways is by looking at what enables jet engines to operate, their fuel.
Top Image Retrieved From http://images.wisegeek.com/close-up-of-jet-engine-on-tarmac.jpg