Mechanical Engineering Division: Thermodynamics, Mechanics, and Fluids
Core concepts to master
Candidates must handle thermodynamics (first and second laws, ideal gas relations, Rankine and Brayton cycles, entropy), strength of materials (stress, strain, bending, torsion, Mohr's circle, fatigue), fluid mechanics (Bernoulli, continuity, Reynolds number, friction losses, pumps), and machine design (bearings, gears, shafts, fits). A solid grasp of units and sign conventions matters as much as the formulas.
Traps that appear in questions
- Using gauge pressure where absolute is required in the ideal gas law or in steam tables.
- Confusing stress with strain, or forgetting that Young's modulus links them only in the elastic region.
- Applying Bernoulli across a pump or turbine without adding the work term.
- Reversing the efficiency definition: thermal efficiency = net work out ÷ heat in, not the reverse.
- Ignoring the difference between laminar and turbulent flow - the friction factor formula changes at Re ≈ 2000–4000.
Worked example
A Carnot engine operating between 600 K and 300 K: efficiency = 1 − Tc/Th = 1 − 300/600 = 0.5, or 50%. Rule of thumb: for a beam in bending, maximum bending stress σ = M·y/I, and for a rectangular section I = bh³/12, so σ = 6M/(bh²). For pipe flow, Reynolds number Re = ρvD/μ; below about 2000 flow is laminar, above 4000 fully turbulent.