Mechanical Engineering: Thermodynamics, Fluids, Strength and Manufacturing
What the mechanical paper tests
KPTCL mechanical questions test thermodynamics, fluid mechanics, strength of materials, theory of machines and manufacturing processes. Numerical questions usually use steady-flow energy equations, Bernoulli, stress-strain relations, or efficiency definitions.
Core rules
- First law for a cycle: net heat supplied = net work done; for a closed system, Q−W=ΔU.
- Carnot efficiency η=1−T_L/T_H, with absolute temperatures in kelvin.
- Refrigeration COP=refrigeration effect/work input; COP is not limited to 1 like efficiency.
- Continuity: A₁V₁=A₂V₂ for incompressible flow; Bernoulli: p/ρg+V²/2g+z=constant along a streamline.
- Bending stress σ=My/I; torsion τ=Tr/J; power transmitted by shaft P=2πNT/60.
Worked example
A Carnot engine operates between 500 K and 300 K. η=1−300/500=1−0.6=0.4, or 40%. If 1000 J is supplied from the hot reservoir, maximum work=0.4×1000=400 J and heat rejected=600 J. Using Celsius values such as 227°C and 27°C without converting gives a wrong ratio.
Traps
Using gauge pressure where absolute pressure is required; forgetting kelvin in Carnot; applying Bernoulli across a pump without adding pump head; and mixing shaft power formula units (N in rpm, T in Nm gives watts after 2πNT/60).