Aircraft like Airbus A320neo/A330neo/A350 series have a visible exhaust cone, which is not visible in the older versions of Boeing 747, Airbus A330/A321/A319 etc. Why is this design preferred over the other?
The « cone exposition » is a result from nacelle engine design, and the choice to mix flows or not. The two main strategies are called:
- Mixed exhaust engine resulting to long nacelle hiding exhaust nozzle and plug.
- Separate exhaust engine which is shorter and then allows a view of nozzle component.
The reason to choose one type of exhaust is made by considering overall engine performance (mainly efficiency and fuel consumption), noise but also visual pleasing (for business jet for instance).
Exhaust mixer is generally more efficient (secondary flow is slightly accelerate producing more thrust) and less noisy (primary flow is slow down). They might be also an advantage in material resistance in cooling hot primary flow. See this article for details about advantage of mixed exhaust.
Nevertheless, long nacelle has the overcome to increase weight and drag of the engine and this is worst as the engine is bigger. Note that it is related to turbofan by-pass ratio (BPR). More the BPR is high and bigger the engine diameter is. For commercial aircraft, the trends is to increase more and more the BPR parameter because it is directly correlated to propulsive efficiency. Therefore, there is a kind of « BPR limits » where mixed nozzle benefits do not balance its drawback.
That's why, modern turbofan (high BPR and high diameter) have likely to integrate a separate exhaust nacelle.
It depends on the airframe/engine combination rather than than newer and older airplanes.
Very generally, P&W have mostly had visible exhaust cones when compared to GE and RR, atleast on widebodies.