How Come Newer and More Powerful Jet Engines are Quieter

If we think about engines like the Pratt & Whitney JT8D found on the Boeing 727, 737-200, and DC-9, we can imagine a loud roaring engine. However, as aviation progresses, we see more powerful engines like the CFM56, IAE V2500, and Rolls Royce Trent 700, which have become even quieter. Now, you even have larger engines such as the Rolls Royce XWB and Pratt & Whitney PW1100G engines, which are even quieter yet more powerful than the engines in the past. So how have newer and more powerful jet engines on commercial planes become quieter?

That’s because of a higher bypass ratio and more advanced noise reduction techniques.

For your reference, you might want to read about why commercial jet engines are getting larger and the difference between a turbojet engine and a turbofan to become more familiar with the topic.

High bypass turbofan engines are the go-to choice for commercial airliners these days due to their efficiency and lower noise levels. These engines have an extra component called a fan at the front, which is driven by the turbine. This fan draws in a large volume of air and accelerates it, creating a separate stream of air that bypasses the combustion chamber. This bypass air is responsible for a significant part of the engine’s thrust, while only a smaller part comes from the exhaust gases produced by burning fuel.

The larger volume of bypass air helps to reduce the overall noise level. The fan also acts as a noise shield, reducing the core engine noise and the jet noise produced at the exhaust. So, the high bypass turbofan engines are not just efficient; they’re also quieter.

How Engine Noise is Reduced

The bypass ratio is a crucial factor in determining how quiet an engine will be. It measures the amount of air that bypasses the core of the engine compared to the air passing through the combustion chamber. In high-pass turbofan engines, this ratio is much higher than in pure turbojets. Modern commercial turbofan engines can have a bypass ratio as high as 8:1 or even more. The higher the bypass ratio, the quieter the engine operation, as more air is directed around the combustion chamber rather than through it.

Engineers have also gone the extra mile to make these engines even quieter. They use acoustic liners inside the engine nacelle to absorb and dissipate noise. The nacelle design itself is optimized to minimize noise radiation. Some engines even use chevron-shaped nozzle designs to alter the flow patterns of the exhaust gases, reducing the noise they generate. Boeing planes that uses engines with chevrons are the 787, 737 MAX, and 747-8.

Another factor contributing to quieter engines is the lower exhaust velocity in high bypass turbofan engines. Slower-moving exhaust gases create less turbulence and noise when they mix with the surrounding air. This is another reason why these engines are preferred in commercial aviation.

Meeting Regulations and Minimizing Impact

Noise reduction is not just about passenger comfort; it’s also a regulatory requirement. High bypass turbofan engines are designed to meet noise standards set by organizations like the International Civil Aviation Organization and local aviation authorities. Quieter engines mean less noise pollution and a reduced impact on communities near airports.

Whereas in the past, speed mattered more than efficiency and being environmentally friendly, today’s planes are more eco-friendly and quieter. However, speed isn’t sacrificed. In fact, the Boeing 747-8 has a top speed of 988 kph, which is close to that of the Convair 990 at 990 kph. Nevertheless, with IATA’s goal of net-zero emissions by 2050, I expect newer engines like the Rolls Royce Ultrafan and the upcoming GE9X to be more powerful and yet quieter.