I’ve been lurking here for some time now, and I’ve been moved to contribute because of constant, seemingly wilful misunderstandings on the subject of jet engines. For all the good it will do me, I’m throwing my weight in with many other members of this forum, including two with Master’s Degrees in Engineering from Oxbridge. To this I add my own experience, which is currently three weeks short of an MEng of my own, in Aeronautical Engineering, with a dissertation on Gas Turbine performance.
To avoid misunderstandings:
By ‘Jet Engine’ I refer to the incarnation of the Open Cycle Gas Turbine in which thrust is generated by the expulsion of gas from the engine, due to momentum and pressure considerations.
I risk repeating what has been said before, but I want to be sure that there is no room at all for misunderstanding (you see the theme here).
There are two distinct forms that the open cycle aero gas turbine takes - the turbofan and the turbojet. The turbojet consists of an intake, a compressor (possibly multi-stage), a combustor, a turbine (again, possibly multistage) and an exhaust nozzle. The turbofan has all these stages, except that there is a fan between the intake and the first compressor stage. In the context of the gas turbine, this is what a ‘fan’ is. Thus, a turbofan has a fan, whilst a turbojet does not.
The nature of the fan is also different to that of a compressor. A compressor compresses air which will pass through the combustor, raising its temperature and pressure (at the risk of adding confusion, up to 25% by mass of this air will be drawn from the high pressure combustor to be used for cooling of the HP turbine stage). A fan will compress all the air that enters the engine, not all of which will progress to the compressor / combustor / turbine, but will instead bypass them, providing thrust only by virtue of the work done on it by the fan. This is why turbofans are also called ‘bypass engines’. On a large Civil engine, up to about 90% by mass of the flow will bypass the core.
It is true to say that a part of the compressor can also be called a ‘compressor fan’, but this would never be referred to as a ‘fan’ without the qualifying ‘compressor’ term, and is unusual terminology when referring to overall cycle aspects, as the discussions on this forum are. I feel I can say ‘unusual’, as I have heard it used no more than once in a reputable academic or industrial context, despite having read in excess of 200 journal articles and textbooks on the Gas Turbine in the past nine months. Thus, it is perverse to use ‘fan’ to mean ‘compressor fan’ in the context of the previous discussions in this forum, as it is not a fan as is understood in this context, any more than ‘gas’ in the American automotive sense can be thought of as referring to butane.
In summary:
No fan: turbojet.
Fan: turbofan.
Please can we have no more of this silliness so we can spend more time arguing about why playing Call of Duty gives more insight into modern warfare than actually fighting in real wars.
