Mechanical engineering interviews test core subjects (thermodynamics, strength of materials, heat transfer and manufacturing) and how you apply them to real machines and production problems. These questions suit campus placements, GATE-based PSU interviews and design or production roles.
Fundamentals, definitions and simple scenarios. Good for freshers and warm-ups.
The zeroth law: if two systems are each in thermal equilibrium with a third, they are in equilibrium with each other, which defines temperature. The first law: energy is conserved, ΔU = Q − W. The second law: the entropy of an isolated system never decreases, heat does not flow on its own from cold to hot, and no heat engine can be 100% efficient. The third law: the entropy of a perfect crystal approaches zero as temperature approaches absolute zero.
Stress is the internal force per unit area, σ = F/A, measured in pascals. Strain is the deformation divided by the original length and has no unit. Hooke's law says that within the elastic limit stress is proportional to strain, σ = Eε, where E is Young's modulus, a measure of stiffness. Beyond the elastic limit the material deforms permanently.
A four-stroke engine completes intake, compression, power and exhaust in four piston strokes, which is two crankshaft revolutions, so it gives one power stroke every two revolutions; it is more fuel-efficient and cleaner. A two-stroke engine completes the cycle in two strokes, one revolution, so it gives a power stroke every revolution and has a better power-to-weight ratio, but it burns more fuel and oil and produces more emissions.
Casting pours molten metal into a mould and lets it solidify; it can make large and complex shapes, but may contain porosity and has a coarser grain. Forging shapes metal with compressive force, hot or cold; the grain flow follows the part's shape, giving better strength, toughness and fatigue resistance, but shapes are simpler. Crankshafts and connecting rods are often forged; engine blocks are usually cast.
Applied problems, trade-offs and questions about your own projects.
The Carnot cycle is an ideal, reversible cycle made of two isothermal and two adiabatic processes between a hot and a cold reservoir. Its efficiency is η = 1 − Tc/Th, with temperatures in kelvin. It sets the maximum possible efficiency for any heat engine working between those two temperatures, which is why real engines aim for higher combustion temperatures and lower exhaust temperatures.
Ductile failure happens after significant plastic deformation: the material necks, absorbs a lot of energy and gives warning before breaking, with a cup-and-cone fracture surface, as in mild steel. Brittle failure happens suddenly with little deformation and a flat, granular fracture surface, as in cast iron or glass. Some steels change from ductile to brittle at low temperatures, which is important in design.
The factor of safety is the ratio of the material's failure strength (yield strength for ductile materials, ultimate strength for brittle ones) to the allowable working stress. It covers uncertainty in loads, material properties, manufacturing defects and analysis. Typical values range from about 1.5 to 4 depending on how well the loads are known and the consequences of failure; it is higher for lifting equipment and pressure vessels.
A tolerance is the permitted variation in a dimension. Fits describe how two mating parts go together: clearance (always a gap), transition (may be a gap or overlap) and interference (always overlap, like a press fit), often specified in a hole-basis system such as H7/g6. Geometric Dimensioning and Tolerancing (GD&T) controls form, orientation, location and runout relative to datums, using standards such as ASME Y14.5 or ISO GPS.
System design, deep internals, leadership and tough follow-ups.
First I find the source with vibration analysis, looking at the frequency spectrum (FFT): typical causes are imbalance, misalignment, looseness, bearing faults and resonance. Then I fix the cause by balancing rotors, aligning shafts, tightening and stiffening the structure, and making sure the operating speed is away from natural frequencies. Where vibration cannot be removed, I add damping, isolation mounts or a tuned mass damper.
Conduction is heat transfer through a material by molecular contact, governed by Fourier's law, q = −kA(dT/dx), as through a furnace wall. Convection is heat transfer between a surface and a moving fluid, governed by Newton's law of cooling, q = hAΔT, as in a car radiator. Radiation is heat transfer by electromagnetic waves without a medium, governed by the Stefan-Boltzmann law, q = εσA(T₁⁴ − T₂⁴), as from the sun. Fins improve convection by increasing surface area.
Fatigue is failure under repeated, cyclic loading at stresses below the material's ultimate strength. A crack starts at a stress concentration, grows with each cycle, and the part finally breaks suddenly. Design uses S-N curves and the endurance limit for steels, the Goodman diagram for mean stress, generous fillets instead of sharp corners, good surface finish, and treatments such as shot peening that introduce compressive surface stress.
I define the problem precisely with data, and contain it so defective parts do not reach the customer. Then I find the root cause with tools like the 5 Whys and a fishbone diagram (man, machine, method, material, measurement, environment), and confirm it with data or a trial. I implement corrective and preventive actions, verify that they work over time, and document the whole process, often in an 8D report.