As we saw in Part 1 of this series, the air entering the core of a jet engine does not go straight to the combustion chamber. Instead, it is first highly compressed. Compression is one of the most powerful ways engineers have to improve jet engine efficiency.

Compressing the air as much as possible does three important things:

1. It improves combustion efficiency: Dense, compressed air allows fuel to burn more completely. There is more oxygen contained in a smaller volume.
2. It increases thermal efficiency: According to thermodynamics, higher compression lets the engine extract more useful work from the same fuel. This is called thermal efficiency.
3. It reduces fuel consumption: More energy is converted into thrust instead of wasted as heat.

In simple terms, better squeezing, leads to better burning, which results in better efficiency.

The amount of “squeeze” provided by the compressor is called the pressure ratio (PR). It is simply the ratio of the air pressure leaving the compressor to the air pressure entering it or

\[ PR = \frac{P_{out}}{P_{in}}=P_{out}:P_{in} \]

A PR of 20:1 means the air pressure is increased 20 times (it is 20 times greater when exiting the compressor than when entering). Modern GE Aerospace engines exceed 25:1 – levels that would have seemed impossible decades ago.

Compression happens in stages, not all at once. Trying to compress all the air at once would stall the flow and shut the engine down. A compressor consists of a series of rotating blades (rotors) alternating with stationary vanes (stators). Each pair of rotors and stators adds a small pressure increase. Together, dozens of these stages multiply the pressure dramatically.

A modern turbofan has 2 distinct compressor sections:

• The low-pressure compressor (LPC): This is immediately after the fan and has fewer stages (normally about 3 - 6) and larger diameter blades. It provides a moderate pressure increase (2:1 to 4:1) at low speeds (between 3,000 and 5,000 RPM). Its main job is to precondition the air to increase the efficiency of the high-pressure compressor.
• The high-pressure compressor (HPC): This has more stages (normally about 8 - 14) and smaller diameter blades. It spins faster (between 10,000 and 15,000 RPM) and can achieve PRs of 15:1 to 25:1.

This design lets the LPC spin slower (at a similar speed to the fan) while the HPC spins faster for peak efficiency, enabling overall PRs of 40:1 to 50:1.

Increasing the PR sounds simple – just add more stages and make the blades spin faster. However, as we have come to expect, this is no free lunch. Each increase introduces new problems.

• Heat builds up quickly: Compressing air raises its temperature. At very high compression, air exiting the compressor is extremely hot and places the materials under enormous thermal stress. Too much heat reduces component life and reliability. Therefore, component cooling becomes essential.
• Aerodynamic stability decreases: When air gets highly compressed, it is more prone to flow separation, compressor stall, and surge (a violent reversal of airflow). Preventing this requires very precisely shaped rotor blades, stator vanes that can be adjusted and advanced control systems to make these adjustments. This all adds cost and complexity to the design.
• Mechanical loads increase: Higher compression needs faster spinning blades which, as we saw in part 2, greatly increases the centripetal forces (remember that $ F=m\omega^2 r $). Therefore, the compressor blades experience huge forces trying to tear them apart every second the engine runs. This requires making blades that are light, strong, and fatigue-resistant - a major materials engineering challenge.
• Manufacturing precision matters more: To more effectively achieve high compression ratios, the gaps between rotor blades and stator vanes must be as small as possible. All the components must also be perfectly and uniformly shaped. Tolerances need to be measured in fractions of a millimeter and advanced manufacturing and quality control are essential but time consuming and expensive.

But engineers don’t maximize compression blindly. At some point, adding more stages adds too much weight, more intricate blade designs with tighter tolerances raise costs and complexity, and higher temperatures reduce component durability. Therefore, the best design is not actually the highest PR design - it’s the optimum PR design.

Instead of making the rotors as a central hub with many attached blades, rotors are made as blisks. “Blisk” is a portmanteau of "blade" and "disk" and is a rotor disk and blade machined out of a single piece of material.

The benefits of blisks include weight savings (there is no need for blade attachments), improved aerodynamics (there are no gaps between the blades and the hub), and better reliability (there is far less chance of a blade detaching). But there is a trade-off. If one blade is damaged, the entire blisk may need to be replaced.

Secondly, engines like the GEnx, have compressor blades of titanium aluminide (TiAl). TiAl has several advantages over traditional titanium alloys. It is lighter (4 g/cm³ vs 4.5 g/cm³), more heat-resistant (up to 800°C vs about 550°C), stiffer and reacts less with oxygen and nitrogen at high temperatures, meaning that the compressor can run faster for improved PRs.

However, titanium aluminides are more brittle than traditional titanium alloys and need more complex and expensive manufacturing techniques.

Read the article The Evolution of Compressor Blades: From Steel to Titanium to Ceramic Composites to learn more.