Mechanical Engineering
Mechanical Engineering in IESL Part I covers thermodynamics, fluid mechanics, strength of materials, mechanisms, manufacturing and basic machine design. Expect compact calculations where efficiency, units and gauge-versus-absolute pressure change the result.
Core rules and concepts
- First law for steady flow: q − w = Δh + Δke + Δpe. For a cycle, net work equals net heat.
- Carnot efficiency η = 1 − TL/TH; Otto cycle η = 1 − r^(1−γ), where r is compression ratio.
- Pump power: hydraulic power = ρgQH; shaft power = ρgQH/ηpump; motor input divides by motor efficiency.
- Bending: σ = My/I; torsion: τ = Tr/J. Maximum shear and bending moments occur at different sections.
- Fluid flow: Darcy–Weisbach hf = fLV²/(2gD); Reynolds number determines laminar or turbulent flow.
- Manufacturing: tolerances, fits, casting defects and weld quality are common descriptive topics.
Worked example
A pump delivers 0.02 m³/s against 30 m head with 75% pump efficiency. Hydraulic power = 1000 × 9.81 × 0.02 × 30 = 5.89 kW. Shaft power = 5.89 / 0.75 = 7.85 kW. If the motor efficiency is 90%, motor input = 7.85 / 0.90 = 8.72 kW. The reasoning is to calculate hydraulic power first, then divide by each efficiency in series.
Traps that cost marks
- Placing efficiency in the numerator instead of the denominator.
- Using gauge pressure where absolute pressure is required, especially in NPSH or steam problems.
- Mixing mm and m in stress or deflection formulas.
- Treating Carnot efficiency as an achievable real-engine value.