Mechanical Engineering
Mechanical Engineering in SSC JE covers thermodynamics, IC engines, strength of materials, fluid mechanics, manufacturing, and power plants. The paper favours definitions, cycle efficiencies, and direct formula application.
Core rules
- Absolute pressure = gauge pressure + atmospheric pressure; use kelvin for all gas-law and cycle calculations.
- For a heat engine, thermal efficiency η = W_net / Q_in = 1 − Q_out/Q_in. Carnot efficiency η_C = 1 − T_L/T_H with absolute temperatures.
- In a four-stroke engine, one power stroke per two revolutions; in a two-stroke, one per revolution.
- Brake power is output at the shaft; indicated power is inside the cylinder; mechanical efficiency = BP/IP.
- Bending stress σ = M y / I; for a rectangular section I = bd³/12, so maximum stress occurs at y = d/2.
- Bernoulli's equation applies to steady, incompressible, frictionless flow along a streamline.
Worked example
A Carnot engine receives heat at 627 °C and rejects at 27 °C. Convert: T_H = 627 + 273 = 900 K; T_L = 27 + 273 = 300 K. η_C = 1 − 300/900 = 1 − 1/3 = 2/3 = 66.7%. The reasoning is to convert to absolute temperature first; using 627/27 would give a meaningless negative or inflated value.
Traps
- Entering Celsius directly into Carnot or gas-law formulas.
- Confusing brake power with indicated power, or forgetting mechanical efficiency.
- Using diameter instead of radius in area and stress calculations.
- Treating gauge pressure as absolute in compressor or steam problems.
- Mixing up two-stroke and four-stroke firing frequency.