Mechanical Engineering - IESL Part II
What this domain tests
The IESL Part II Mechanical paper examines the core mechanical syllabus at professional-entry level: engineering thermodynamics, fluid mechanics, solid mechanics and machine design, manufacturing and materials. Questions are quantitative. Marks go to SI-consistent working, stated assumptions, and design decisions defended with a factor of safety - not only to the final number.
Core rules and concepts
- Steady-flow energy equation per unit mass: q − w = Δh + Δke + Δpe. For an adiabatic turbine or nozzle, w = h₁ − h₂ and enthalpy drop drives the work output.
- Bernoulli with losses: p₁/ρg + v₁²/2g + z₁ = p₂/ρg + v₂²/2g + z₂ + h_L. Head loss is always positive in the flow direction.
- Reynolds number Re = ρvD/μ fixes the regime (laminar below about 2300 in a pipe) and therefore the friction factor used in Darcy–Weisbach, h_f = f(L/D)(v²/2g).
- Bending and torsion: σ = My/I and τ = Tr/J. For a solid circular shaft J = πd⁴/32, hence τ_max = 16T/(πd³).
- Design check: working stress is compared with yield or ultimate strength divided by a factor of safety; fatigue requires stress-concentration factors and an endurance limit.
Worked example
A solid shaft of diameter 60 mm transmits 150 kW at 300 rpm. Find the maximum torsional shear stress.
ω = 2πN/60 = 2π(300)/60 = 31.42 rad/s. Torque T = P/ω = 150 000/31.42 = 4775 N·m. Then τ = 16T/(πd³) = 16(4775)/(π × 0.06³) = 1.13 × 10⁸ Pa ≈ 113 MPa. The selected material must therefore have a shear yield strength comfortably above 113 MPa once the safety factor is applied.
Traps that cost marks
- Mixing millimetres and metres inside d³ - a factor-of-1000 error in stress.
- Substituting radius into τ = 16T/(πd³), which is written for diameter.
- Treating a gauge pressure as absolute, or forgetting that gauge pressure is what a manometer reads.
- Applying Bernoulli across a pump, valve or long pipe without adding or subtracting the relevant head term.
- Reporting a stress without comparing it with the allowable value when the question asks whether the design is safe.