Technical Abilities - Mechanical Engineering - CBT 2
What the domain tests
This is the largest scoring block of CBT 2. Coverage: engineering mechanics and strength of materials, thermodynamics and IC engines, fluid mechanics and machinery, theory of machines, manufacturing processes, engineering materials, and power plant basics.
Core relationships
- Stress = force/area in N/m² (pascal); strain is dimensionless; Young's modulus E = stress/strain in the elastic region.
- Carnot efficiency η = 1 − T₂/T₁, with temperatures in kelvin.
- Otto cycle efficiency η = 1 − 1/r^(γ−1), where r is the compression ratio; higher r or higher γ raises efficiency.
- Continuity: A₁V₁ = A₂V₂ for incompressible flow; Bernoulli's equation conserves pressure, velocity and elevation energy.
- 1 bar = 100 kPa; 1 atm = 101.325 kPa; gauge pressure = absolute − atmospheric.
- Gear ratio = N_driven/N_driver = speed ratio inverse of torque ratio.
Worked example
A Carnot engine operates between 500 K and 300 K. Efficiency = 1 − 300/500 = 1 − 0.6 = 0.4, that is 40%. If the sink temperature were lowered to 250 K, efficiency becomes 1 − 250/500 = 50%, showing that a colder sink raises efficiency more cheaply than heating the source.
Traps that cost marks
Using Celsius in Carnot or gas-law formulas - always convert to kelvin first. Confusing gauge and absolute pressure, which flips answers in boiler and compressor problems. Assuming a real engine can reach Carnot efficiency, which is impossible because of friction and finite heat-transfer rates. And mixing UTS (ultimate tensile strength, breaking point) with yield strength (end of elastic behaviour).