Engine Construction and Major Modules
What this domain tests
Construction questions ask you to identify what each module does, how rotors and stators are arranged, and why clearances, cooling and bearing support matter.
Core rules
- Compressors are axial (many stages, high mass flow), centrifugal (one or two stages, high pressure rise per stage), or a combination. Rotor blades add tangential velocity; stator vanes diffuse it into pressure. Variable stator vanes and bleed valves protect surge margin at low speed.
- Combustors are can, can-annular or annular. Annular gives even temperature distribution and shorter length. Turbine nozzle guide vanes turn and accelerate gas onto turbine blades; turbine blades extract work and are cooled by compressor bleed air.
- A twin-spool engine has N1 (LP compressor or fan) and N2 (HP compressor); a three-spool adds N3. Each spool has its own shaft and bearings. Roller bearings take radial loads; ball bearings take radial and axial loads.
- Casings contain the gas path and carry engine mounts; tip clearance is a direct efficiency penalty.
Worked example
An engine has 8 compressor stages and an overall pressure ratio of 16. If each stage had the same pressure ratio, per-stage PR = 16^(1/8) = 1.414. Reasoning: pressure ratios multiply, so the nth root gives the equal-stage value. Real stages are matched differently, with later stages often at lower pressure ratio to avoid surge.
Traps
- Thinking turbine blades and compressor blades are interchangeable: turbine blades expand hot gas, compressor blades compress cool air.
- Forgetting that cooling air for the turbine comes from the compressor, which costs cycle efficiency.
- Describing bearings as 'oil-free' or ignoring thrust loads.
Key points
- Axial compressor: rotor adds velocity, stator diffuses; variable stator vanes and bleed valves protect surge margin.
- Annular combustors give even temperature and short length.
- Turbine blades are cooled with compressor bleed air.
- N1 is the LP spool, N2 the HP spool; ball bearings take axial loads, roller bearings radial loads.
- Tip clearance and casing clearance directly affect efficiency.