By Bjorn Fehrm

July 12, 2024, ©. Leeham News: We do an article series about engine development. The aim is to understand why engine development now has longer timelines than airframe development and carries larger risks of product maturity problems.

To understand why engine development has become a challenging task, we need to understand engine fundamentals and the technologies used for these fundamentals.

We covered the problem areas of a compressor last week. Now, we will discuss how modern compressors can have over 90% conversion efficiency from turbine power to air compression.

Figure 1. The gas turbine cycle and its parts. Source: Rolls-Royce: The Jet Engine.

The importance of Compressor and Turbine efficiency

We wrote last week that modern jet engines have compressor pressure ratios at cruise of around 40:1 for single-aisle engines and 50:1 for widebody engines. At takeoff and climb, the engine spins faster to generate more thrust, and we have pressure ratios of 45 to 60.

Higher pressure ratios result in more efficient engines. But we can’t raise a gas turbine’s pressure ratio unless we have high work transfer efficiency for the engine’s compressors and the turbines driving the compressors.

The reason is that if you need to use a lot of the combustion gas energy to drive an inefficient turbine that drives an inefficient compressor, you lose more energy, raising the pressure ratio than you gain by the high-pressure ratio.

So today’s high-pressure ratio engines wouldn’t exist if the efficiency of today’s compressors and turbines weren’t at or above 90%.

Efficient compressors

A key development in achieving these high-pressure ratios has been the aerodynamics analysis that 3D airflow software programs enable for the complex aerodynamics of compressors.

The aerodynamics in a compressor is more complicated than the aerodynamics around an aircraft. The aircraft flies in free air except during takeoff and landing when the airport runway and surroundings affect the aerodynamics. The predominant free-air aerodynamics make the analysis straightforward.

Compressor aerodynamics take place in a channel where the stator walls form the outer limit, and the foot of the blades and vanes form the inner wall. The shrouding of the compressor stage with walls limits the aerodynamic losses of air flowing over the blade tip from the high-pressure side to the low-pressure ahead of the stage.

But the walls also create problems. First, they cause scrubbing friction against the air molecules, which means the airspeed is lower at the top and bottom of the compressor channel. So the blade/vane shape must be continuously adapted to the different airspeeds over the blade/vane.

Also, the walls reflect supersonic pressure waves in a supersonic compressor. The criss-crossing of pressure waves in a compressor stage makes the aerodynamic analysis very complicated.

Compressor rotation speeds

A compressor’s rotation speed is kept as high as possible, as a blade working on the air at high speed can transfer more energy and, thus, pressure into the air than a low-speed blade. The compressor blades have the same compound speed problem as a propeller or fan. At the tips, the axial speed of the air, at about M0.5, is mixed with the blade’s tip speed and creates supersonic flow around the blade.

Compressor designers make the first stages of a compressor have a supersonic tip speed so that the rest of the blade working at subsonic speed doesn’t lose too much efficiency. The relative speed of the blade versus the air doesn’t change much for later stages, but the speed at which the air goes from the smooth subsonic flow to the “collision” type supersonic flow changes with air temperature.

As the compressor air gets hotter at each stage when the pressure ratio increases, the speed at which the flow turns supersonic increases. This means that the later part of a compressor has subsonic flow all the way to the blade tips.

Since the inception of the jet engine, these phenomena have complicated the study and calculation of the aerodynamic flow in the engine compressor. Early compressors had a transfer efficiency well below 70%. In 1970, a transfer efficiency of 80% or better was achieved using new aerodynamic calculation methods.

Modern engines have passed 90% through using sophisticated analysis software to calculate and analyze all aspects of the complicated three-dimensional flow in the compressors.

Compressor maps

Gas Turbine designers use a 3D graph (Figure 2) to illustrate how a compressor’s efficiency varies with airflow and pressure ratio.

Figure 2. Efficiency map of a turbofan high-pressure compressor. Source: GasTurb.

The diagram is not that complicated to read. Imagine it as a ridge with height curves (the efficiency levels). You have the compressor pressure ratio on the Y axis, and the air mass flow through the compressor on the X axis.

At takeoff and climb, we have the highest engine RPM and, therefore, the highest mass flow and pressure ratio (about 57 lb/s and 23 in the graph). Efficiency is then below the 90% curve.

At cruise, the mass flow and pressure ratio are lower, where a lower RPM gives ~50lb/s and PR 20. The designer ensures the compressor’s design is such that its efficiency is the highest here, above 92% for all the different altitudes and speeds (see simulation point legend in the left box).

Then, as the engine throttles down to flight idle (the lowest allowed RPM of the engine during flight), the efficiency drops below the 88% curve. We also see the working line (how the working point of the compressor changes with RPM) starting to head toward the red top boundary of the map. This is the stall line of the compressor.

The designer uses variable stator vanes, and compressor bleed to ensure that the working line stays well clear of the stall line.

An important takeaway from the diagram is that a compressor (and a turbine) only has a limited range of air flow and pressure ratios with maximum efficiency. Outside this sweet spot, there is an increasing aerodynamic mismatch. Still, as this occurs during a limited time (takeoff and climb) or when the fuel consumption is very low (descent RPMs), it’s acceptable.

The compressor designer works to make the top of the ridge as high and long as possible and to curve with the engine’s working line.

When you design an engine, you have a map for the fan, the two compressors, and the two turbines (which look a bit different). The art is to match all these so they have their sweet spot during cruise and do not fall off too much for the climb.

Next week, we look at all the users of compressor air in an engine.