|Quoting MarkC (Thread starter):|
And I don't buy the argument that larger engines have a larger bypass ratio. 60K engines could be built with the same bypass and pressure ratio as the latest huge fan engines.
Well... they do.
First, lets go to the basic reason why larger engines have lower fuel burn.
1. Tip spacing. Every engine maker has a technological limit on how close they can make the blade tips to the casings. The larger the diameter of the engine (more thrust), the porportionately less work lost with flow around these leaks. Leaks go as core diameter. Core power goes as core diameter squared. In other words, Twice the diameter has twice the leaks but 4X the power. (Ok, I simplified...) Or in other words, half the inefficiency of the tip leakage goes away!
a. compressors. Leaks flow backwards and thus air must be recompressed, adding work and reducing engine thrust.
B. Turbines. Leaks result in hot gasses not producing work.
2. Parts. Basically engine costs go as part count. A high thrust engine pays for added part counts (with their efficiency). For example, most high thrust engines either have two turbine stages powering the high spool. This added overall pressure ratio (OPR) improves the thermodynamic efficiency of the cycle. Or, in the case of RR
, allows for a triple spool. Below a certain thrust, the triple spool doesn't save weight or improve efficiency. Obviously, added turbine stages are matched with more compressor stages. One exception is the CFM for the A340. There an extra low turbine stage was added to power more low turbine compressor stages (compared to the other CFM56's optimized for shorter hops).
3. The ratio of surface area to flow area within the engine drops as the engine gets larger. Surface area within an engine is DRAG
! In other words, a larger engine will have a higher TSFC than a smaller engine if everything else was equal. But it isn't equal (due to tip losses, added turbine/compressor stages, etc.) The advantages just keep adding up for the large engine.
4. Part cost. It usually pays to go with the latest technology in a large engine. Its often not the case in a small engine. For example, fan technologies usually appear first on large engines.
5. Weight. Larger engines weigh less per pound of thrust than a small engine. Weight is the enemy of aircraft. A three/four engine aircraft needs another set of plumbing for bleed/start air. Shaft and bearing weight does not scale up with thrust but rather goes up slowly. In general, the weight of an engine goes up slightly more than the diameter of the engine (parts get thicker), but thrust goes up as the square of the engine diameter. So Large engines have an inherent weight advantage.
6. Mach number. Blades have an optimum mach number that they want to operate in. Yes, technology shifts/broadens that mach number, but that shift is for all engines. A larger engine has a far greater fraction of every airfoil's area (remember, diameter again) near its optimal mach number. So component efficiencies go up with engine size.
7. Nacelle drag. Nacelle drag is purely a function of engine surface area. 3 engines will have more surface area than 2 engines of the same thrust. Just as 4 engines will have more surface area than 3 of the same thrust. Ok, I assume all engines are of the same generation utilizing the same technology.
Because of these efficiencies, the low turbine can do more work in the larger engine. Thus, for the same technology, a larger engine will always have a slightly higher bypass ratio than a smaller engine. Also, 180k of thrust via two engines will *almost* always be cheaper than 3X60k. The exception? If the 60k engine could be produced in much more significant quantities and thus acheive a better "economy of scale" in engineering and production.
Hope this helps,