Gas Turbines - Introduction — Page 212, Lesson 282

Right, let's pick this up. We've already followed the core gas path through the core of the engine, so now I want to look at the big picture of the high bypass turbofan, and specifically the airflow through it. First, a key point on efficiency. The propulsive efficiency of both the low and high bypass ratio engines is much greater than that of the pure turbojet at the speeds normally associated with jet transport aircraft. And this also follows for the specific fuel consumption, which is appreciably lower for the high ratio bypass engine. So the high bypass engine is both more efficient and burns less fuel per unit of thrust at cruise speeds. Now, why did we go down this route? The experience gained through manufacturing and operating the low bypass ratio type of engine proved that engines dealing with larger comparative airflows and lower jet velocities could give propulsive efficiencies comparable to those of turboprops, and greater than turbojets, at normal cruising speeds. That was the advent of the fan jet engine. Let's look at the classic example: the triple spool front fan turbojet, the Rolls Royce RB 211. The air enters the intake and passes immediately into the low pressure compressor, more commonly called the fan. Here its pressure is raised before it splits to go either through the bypass duct or into the intermediate pressure compressor, the amount depending upon the bypass ratio. The thrust of this type of engine is almost completely dependent on the bypass airflow, which has a high mass and relatively low velocity. That's why its propulsive efficiency is so good. The air which passes through the intermediate and high pressure compressors has a great deal of energy added in the combustion chambers, but this energy is required to drive the compressors. Finally, the rearmost, or the low pressure turbine, is responsible for extracting virtually all of the energy that remains in the gas stream to drive the front fan. If it is efficient in doing its job, then there should be only residual thrust remaining when the hot gases emerge from the turbine. So the whole design philosophy is: move a large mass of air slowly for thrust, and use the hot core just to keep the fan spinning.