How did aircraft engines like the 707 become more fuel efficient and powerful into the 777?
In: Technology
There are a lot of factors, but the biggest was a fundamental shift in design. To understand that, you need to know how jet engines work.
All jet engines have a common design in that they use specialized fans called *compressors* to compress the air coming in before it’s fed into the *combustor*, where fuel is added and ignited. The hot exhaust spins another series of fans called a *turbine*; this energy is used to drive the rest of the engine shaft, including the compressors, making the engine self-sustaining as long as fuel and air are available.
However, there are several designs of jet engine. The simplest is the one we just described, called a *turbojet*. All of the thrust comes from the exhaust being blown out of the back end of the engine. However, most civil aircraft today use a more sophisticated design called a *turbofan*. In this case, the spinning power of the engine shaft is used to turn a fan at the front of the engine. The air being blown by that fan doesn’t go through the jet engine, instead bypassing the compression, combustion, and turbine sections; this bypass air is what gives a turbofan its thrust. Turbofan engines are divided into *low-bypass* and *high-bypass* designs, the difference being how much air blown by the fan goes through the engine versus past it.
The Boeing 707 used Pratt & Whitney JT3D and later JT4A turbojets. These were early jet engine designs called *low-bypass* engines. The fans were relatively small, and most of the air they moved went through the jet’s combustion process, meaning that much of the thrust energy the fan produced couldn’t be harnessed.
However, as material science and designs improved over the years, engine design shifted to *high-bypass turbofans* rather than the early low-bypass designs. These are the engines you see on modern airliners, with their large front openings and very visible, wide-diameter fans. In these engines, the fans produce a lot of pressure, and most of the air they move does not go through the engine’s core, instead bypassing it and being sent straight out the back of the engine as thrust.
The advantage of high-bypass engines is that their design allows them to produce a great deal of thrust for comparatively little fuel. While they don’t have the absolute power of a turbojet – meaning that rapid acceleration and super-high thrust aren’t possible – they do have efficiency; it’s the difference between a sprinter (turbojet) and a marathoner (turbofan).
Today, engine efficiency improvements have largely tapered off because we’ve become very good at designing engines that don’t waste thrust and harvest as much energy as possible from the fuel being burned. In large measure, you can thank extensive computer modeling of the fluid dynamics within the engine for that; by visualizing air flow and the fluid dynamics of the compressor and turbine blades, we’ve been able to tune them to minimize inefficiency.
Of course, engine design is only a part of making an aircraft more fuel efficient, the rest being on how the aircraft itself is designed and built. But insofar as the engines themselves, the move from low-bypass to high-bypass engines was the biggest contributor, and advances in how those high-bypass engines are built has provided the rest.
It’s all about bypass.
707 engines are, like military fighter jet engines, non-bypass. All the air that’s compressed by the front blades feeds combustion. This is great for thrust-to-weight ratio, but not efficient at less than max power. Alas, an airliner spends almost all it’s time at less than max, and fuel economy is really important to an airline.
777 engines are high bypass. Most of the air that’s compressed by the front blades pushes out the back just like the exhaust. Bypass engines don’t grow in power output as fast, the 777 engine might be 20x the fan cross-sectional area of the 707, but it only makes 5x as much thrust at max. What it does make is lots of thrust at the most economical cruise settings, where airliners spend most of their time flying.
All-bypass, what are generally called propeller airplanes, turn the front fan in a way that’s unconnected to combustion. They are even more economical, but not easy to scale up to 707 passenger capacity.
although a lot to it, three things – technology, the size of engines and the ability to increase the pressure in the fans
I think it’s got a lot to do with the efficiency of the compressor fan blades, the size of the engine and the fact that it uses high bypass, meaning that a lot of the thrust is generated by the front fan blowing air out of the rear of the engine. The more efficient compressor system squashes the air more so that it burns the fuel more economically and adds loads more thrust for less fuel per mile. Couple that with better aerodynamics of the airframe, and you have a more efficient aircraft.
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