Thermal efficiency vs Propulsive efficiency : Who will win?

If you love aviation, then flightglobal is one publication that you can’t miss. Ever since I have been in this industry, I am a regular visitor and reader of flightglobal. I specially gobble up anything related to gas turbine engine.

And have come to love and eagerly wait for the yearly commercial engine report that flightglobal comes up. This years update is great, specially the war between Leap 1A vs PW1100G

Here’s an brief extract.

The blisks, the new materials and the two- stage high-pressure turbine allow CFM to vastly improve the thermal efficiency of the Leap, yielding a double-digit improvement in fuel efficiency with a conventional architecture for a narrowbody aircraft engine.

If the Leap architecture is intended to optimise the thermal efficiency of the engine, P&W’s PW1100G is mostly aimed at improving propulsive efficiency. There are generally two airflows in a turbofan engine – one that travels through the core of the engine and one which bypasses the core. The former is used mainly to drive the engine, although a small amount generates thrust. The latter, or bypass airflow, generates the majority of thrust.

A simple way to make the engine more efficient in generating thrust is to increase the amount of airflow that bypasses the engine core, or the bypass ratio. The only way to increase the bypass flow is to enlarge the diameter of the inlet fan, which is connected by a shaft to its power sources in the low-pressure turbine.

In a conventional engine architecture such as the Leap, the low-pressure turbine and inlet fan rotate at the same speed. As the inlet fan diameter widens, the tips of the blades spin faster than the speed of sound, reducing efficiency, and causing noise and vibration problems.

Instead, P&W introduces a reduction gear on the shaft that decouples the rotation speed of the high-pressure turbine and the inlet fan, allowing the latter to spin at one-third the speed of the former. As a result, the PW1100G has a bypass ratio of 12:1, twice the 6:1 ratio of the V2500. The reduction gear also reduces the load on the low- pressure turbine. The job of spinning the inlet fan and booster stages on the CFM Leap requires seven stages in the low-pressure turbine. The PW1100G inlet fan is 10cm (4in) wider than the Leap-1A, but uses only three stages in the low-pressure turbine.

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