The cost of fuel is one of the primary reasons why air travel is as expensive as it is. A Boeing 777 flying across the Atlantic can burn as much as 50 tonnes of fuel over the course of its 7-hour flight. Even a 10% increase in engine efficiency could save 5 tonnes of fuel every flight, which would add up rapidly with how many flights are airborne everyday. So, squeezing as much efficiency out of a jet engine is extremely important not only for reducing cost, but also to help alleviate the impact aviation has on global carbon emissions.
Turbofans and bypass ratio
As you saw in the last module, turbofans are the most efficient variant of jet engines due to the fact that they generate the vast majority of their thrust via the fan at the front of the engine which pushes back air to push the engine forward. But why exactly does using the same amount of generated energy in a different way produce different amounts of thrust? Shouldn’t using the exhaust gas to spin engine components actually be less efficient because of frictional losses? Well the issue lies in the fact that the basis of jet propulsion, which relies on expelling the combusted fuel at high velocity to generate thrust, is inherently very inefficient. Perhaps the easiest way to understand this would be to compare turbofan engines with rocket engines, which can be thought of as simplified versions of turbojet engines.
As rockets are based off the same principle as jet propulsion, we can equate them quite easily. If the mass of the propellant being burned in one second is m kg and its velocity is v m/s, then the momentum of the ejecta is going to be mv kg·m/s. This is actually going to be the same as the amount of impulse imparted to the spacecraft, and so can loosely be thought of as proportional to thrust. Now, in order to increase the thrust generated by the engine, there are two options – either the amount of propellant being ejected (m) can be increased or the velocity at which it is being ejected (v) can be increased. As increasing m would mean increasing the fuel consumption (since a greater amount of propellant is being ejected every second), in order to increase efficiency v needs to be increased while keeping m constant.
Since the generated thrust is equivalent to the momentum of the ejected propellant, the only way to increase the amount of thrust generated at the same fuel consumption is to increase the velocity of the ejected propellant. As the mass of the ejecta is constant, fuel consumption is constant for a greater amount of thrust, and so efficiency is higher. However, the amount of energy needed to accelerate that much propellant to that speed is equivalent to the kinetic energy it gains: (1/2)mv^2. Notice that this means that while the amount of thrust generated by accelerating some mass to a velocity v is directly proportional to v, the amount of energy required to accelerate it to that speed is proportional to v^2. This means that increasing exhaust velocity to increase thrust is less efficient than increasing thrust by increasing the reaction mass.
In the context of rocket engines, there is no way around this problem because of the fact that the rocket needs to carry all the propellant that is used as reaction mass to generate thrust. With jet engines though, things are different. The air around the engine can also be used as reaction mass in addition to the mass of the fuel itself. So whereas rockets only have the option of increasing the exhaust velocity at a given fuel flow rate, jet engines also have the option of increasing the reaction mass at the same fuel flow rate by increasing the amount of air being pushed by the engine by not accelerating it as much in the first place.
So, in a nutshell, a turbofan engine is more efficient than a turbojet because accelerating a larger amount of air to a lower velocity requires less energy than accelerating a smaller amount of air to a higher velocity. A turbofan takes advantage of this fact by reclaiming some of the energy used to accelerate the exhaust from the core and using it to accelerate a larger amount of air at a slower speed around the core. Thus, more thrust can be extracted out of the same amount of fuel being burned, which means that over the course of the flight less fuel will have to be burned which will result in a greater efficiency overall.
Fine, but what role does bypass ratio have to play in all this? Well it’s quite simple. The bigger the fan at the front of the engine, the more air it will push in any given time frame, and thus the greater component of the thrust it will generate. Essentially, efficiency can be thought of as directly proportional to the bypass ratio of an engine.
So is bigger always better?
Well not exactly. If making jet engines more efficient was as simple as making them bigger and pushing more air through them, a lot of people would be out of their jobs. There are multiple problems with making engines bigger, and current and next generation engines are reaching the point where increasing efficiency by increasing bypass ratio is going to give diminishing returns.
The first challenge is the size of the fan itself. With fan diameters exceeding 3 m on modern engines, multiple problems start to arise. The blade tips of the fan reach speeds in excess of the speed of sound, which causes the generation of shockwaves that increase drag and the stress on the blades. Furthermore, as the centrifugal force on the blades grows with the radius, the stress on them also increases, which means that the maximum size of the blades is limited by the tensile strength of the material they’re made of.
Another problem lies in the way the fan is driven. In standard two-spool designs like the one we discussed in the last module, the fan is part of the N1 assembly and is thus linked to the low pressure compressor and the low pressure turbine. In order to drive a greater mass of air through the fan, the turbine must provide a greater amount of power. To provide more power to the turbine, a greater core temperature is needed, which is not possible when the engine is already running at the limit of what current materials can withstand.
These problems can be resolved by removing the link between the rotation speed of the turbine and that of the fan. There’s two ways engine manufacturers are going about this – by increasing the number of spools in the engine from two to three like in the case of the Trent series of engines from Rolls-Royce, or by installing a gearbox that reduces the rotational speed of the fan as compared to the turbine, as is the case with the Pratt & Whitney PW1000G which features a 3:1 gearbox to reduce the fan speed to one-third that of the turbine.
Inevitably however, bigger engines are going to run into the problem of drag. Increasing the diameter of the engine increases the drag acting on it, causing diminishing increases in efficiency around the 12:1 to 15:1 bypass ratio mark, which is already being reached by many engines today. Soon, the increase in drag due to increasing bypass ratio will outweigh the increase in efficiency afforded by it, which means that other ways to increase engine efficiency will need to be developed sooner or later. Yet another issue with extremely large bypass ratio engines is ground clearance. It becomes difficult to mount such large engines with sufficient clearance from the ground, especially with narrow-body airliners like the A320 and 737. The CFM56-3 engine used on some 737 variants famously features a squished nacelle to maximize the clearance between the ground and the engine. Increasing the height of the fuselage above the ground requires heavier and more voluminous landing gear and also increases the complexity of loading and unloading passengers as well as cargo, increasing operating costs for the airport.
The endgame for engine efficiency
With increases in bypass ratio out of the picture, what other ways do we have to build more and more efficient engines? Barring any revolutionary technologies that come out in the near future, there’s essentially only one other way forward to continue reliably increasing engine efficiency.
This path is thermal efficiency. This basically refers to how much useful energy can be extracted out of the heat generated from the combustion of fuel inside the combustion chamber. Without going into details, the laws of thermodynamics tell us that a higher compression ratio (i.e. the ratio of the pressure inside the combustion chamber to that of the ambient pressure) will increase thermal efficiency. Achieving this is not easy, however. Compression causes temperature to rise, which causes important engine components to melt, which causes all sorts of problems. In order to make engines that have higher compression ratios, newer materials will need to be developed that can withstand these pressures and temperatures reliably.
Besides this, improvements in component level design like that of the fan and compressor blades, airflow channels, combustion chamber, turbines and other individual systems can also help increase efficiency, although this is also another area in which refinement on modern engines is at a point where improvements in this area are going to come few and far between. It is much more likely at this point that a new revolutionary technology is going to be needed to create significant leaps in engine efficiency. Technologies like electric propulsion are starting to look really promising for engine technology in the near future, with the main hurdle right now being battery density, especially for longer range flights.
Assignments on building efficient engines
- What are the problems associated with using a gearbox to reduce fan speed?
- Why are alternate fuels like hydrogen and biofuels not used in jet engines?
- The primary factor holding back use of electric propulsion on aircraft is battery density. If improvements in that field are not forthcoming and battery density remains stagnant, what alternate methods can you suggest to partially or fully utilize electricity to power aircraft?