Difference between Two Spool Engines and Three Spool Engines

Basic Knowledge about Two spool Engines and Three Spool Engines

For large commercial aircraft engines, fuel efficiency will always be a primary concern. History has shown that the long-term trend is for fuel costs to increase. More fuel-efficient engines reduce the economic impact of fuel price fluctuations on the operator. Another thing to consider is the much tighter emissions regulations being implemented for CO2, NOx, etc.

Improving commercial aircraft turbine engine efficiency can be achieved with higher bypass ratios, higher cycle pressure ratios, higher cycle temperatures, lower mechanical & thermal losses, and improved controls. The higher cost of using more complex engine systems to get better efficiency is almost always a good bargain for large commercial aircraft engines.

A Two-Spool Engine

A two-spool engine has two concentric shafts that rotate at different speeds: one connects the high-pressure turbine stages to the high-pressure compressor, and the other connects the low-pressure turbine stages to the low-pressure compressor and fan. The reason for doing this is that the low-pressure stages are larger in diameter, so they have higher tip speed for a given rotational speed. At the same time, the temperature is lower, and so too is the speed of sound. So, to keep the flow subsonic, they are designed to rotate at a slower speed.

The Three-Spool Engine

A three-spool engine is the same thing, only with three concentric shafts, the third of which connects an intermediate pressure turbine and compressor. A geared turbofan uses a transmission to reduce the speed of the fan compared to the low-pressure compressor and turbine, instead of a third spool.

This is sometimes used in very high bypass ratio engines, in which the fan is much larger in diameter than the low-pressure compressor.

Advantages of a 3-spool engine

More flexible due to aerodynamic matching at part load
Lower inertia of rotating components
Easier to start as only one spool needs to be turned by the starter
Allows for higher ratios of fan airflow to engine flow. This allows for increased thrust without a corresponding increase in jet velocity and reduction in propulsive efficiency leading to high SFC.

Turbines run closer to optimum speeds

– More efficient

– Easier to start

Note: The older turbojet engine accelerates a small mass of air to a very high velocity to achieve a given amount of thrust. To obtain the same amount of thrust from a turbofan, it takes a much larger mass of air and does not need to accelerate it to as high of a velocity, thus giving a lower SFC.