What is the difference between a high bypass turbofan and a low bypass turbofan?

Question

I see some questions on the site that talk about the relative positives and negatives of a high bypass turbofan (HB) vs a low bypass turbofan (LB). But I don't see anything explaining what actually makes a turbofan engine HB or LB? I assume it has to do with the ratio of air that is bypassing the compressor stages and just going through the fans. But what ratio makes an engine HB v. LB? And why was that ratio chosen?

---

Please note: I am not looking for usage cases, I am looking for differences in design specifications.

Answer 1

The first turbofan, the RR Conway, had just a bypass ratio of 0.25. It was designed for installation into the wing root of airplanes like the HP Victor bomber or the Vickers V-1000 transport project, so RR strived to keep the diameter small. Of course, back then nobody distinguished low- and high-bypass engines. The P&W JT-3D, which powered most of the second generation of jet airliners (Boeing 707 and DC-8), had a bypass ratio of less than 1.5.

But when the new generation of engines (GE CF-6, JT-9D) with bypass ratios of around 5 were developed for the first generation of wide body jets, marketing needed a term to make clear that this was a new generation of engines, and the high-bypass ratio engine category was born. Generally, the threshold is around 5, but note that some CF-6 versions have bypass ratios as low as 4.24 and still fall into the high bypass ratio category. Therefore, Wikipedia sets the limit at a bypass ratio of 4.

There is no technical difference between a low- and a high-bypass engine, and the limit is arbitrary. The jump in bypass ratio from 1.5 to 5 brought significant improvements in specific fuel consumption and noise reduction, but part of the technical progress was also from better materials and aerodynamics, which allowed higher compression ratios and turbine entry temperatures.

Now, almost 50 years later, we see the first engines with bypass ratios above 10 entering service. Designs are being readied with bypass ratios of 12, so marketing will need a new word to describe those creations. With "ultra high bypass" already taken by the unducted fan crowd, the search is on.

Answer 2

There is no particular ratio at which the engine becomes a 'high' bypass, though the it is generally taken to be around ~5:1. The reasons are more historic than technical.

The first large scale development of the high-bypass turbofans was spurred by the USAF's CX-HLS competition, which lead to the C-5 Galaxy. The requirements of the competition (range and fuel efficiency) lead to the development of a new generation of engine by GE and PW, which developed the TF39 and JTF14E respectively. GE won the contract with the TF39 engine, which had a previously unheard of 8:1 bypass ratio.

PW figured out that it better develop a high bypass engine itself or lose the market and developed the JT9D, which had a 5:1 bypass ratio. This engine was the first widely used engine with high bypass ratio (the previous engine had bypass ratios in the range of ~1 or so) and necessitated a special mention, which came to be called high-bypass engines, once it became widely used, especially in the Boeing 747 (Ironically, it was the two losers of the CX-HLS competition PW and Boeing, which won the immediate race in civil aircraft).

Joe Sutter acknowledged the effect of the CX competition in the development of 747:

I should add that fostering large high-bypass engines was all that the USAF C-5 competition contributed to the Boeing 747, as my new airplane would be called.

Not to be left behind, GE developed the CF6 series with bypass ratios of 6:1 and RR developed the RB211-22 which had a bypass ratio of ~5:1. Due to these, the ratio of around 5:1 (or >4:1) is usually taken as the dividing line between the 'high' and 'low' bypass ratio turbofans.

Answer 3

"Bypass" in this context is how much air transits the actual fan stage and is used to directly generate thrust, vs. how much air transits the fan stage and moves into the compressor stage to be burned in the engine.

Low-bypass turbofans don't use a lot of air for thrust; most of the air that transits the fan stage is routed into the engine, compressed, combusted, and exhausted like a standard turbojet. High-bypass turbofans route more air outside the combustion core, so the spinning fan generates most of the thrust (rather than the actual engine). In this case, the turbojet core of the engine is used (mostly) to generate engine rotation, and the fans generate most of the thrust.