Basic Mechanical interview questions

150 questions with answers (MEC-001 to MEC-150). Try each one aloud before reading the answer.

Engineering Mechanics

MEC-001 · What is a force and what is its SI unit?

A force is an interaction that changes or tends to change the state of rest or motion of a body. It is a vector with magnitude, direction and point of application, measured in newtons (1 N = 1 kg·m/s²).

MEC-002 · What is the difference between a scalar and a vector quantity?

A scalar has magnitude only (mass, time, energy, temperature). A vector has magnitude and direction and adds by the parallelogram law (force, velocity, acceleration, moment).

MEC-003 · What is the resultant of a force system?

The single force (and, if needed, couple) that produces the same external effect on a body as all the original forces acting together. For concurrent forces it is found by vector addition: R = √((ΣFx)² + (ΣFy)²).

MEC-004 · State Lami's theorem.

If three coplanar concurrent forces keep a body in equilibrium, each force is proportional to the sine of the angle between the other two: P/sin α = Q/sin β = R/sin γ.

MEC-005 · What are the conditions of equilibrium for a coplanar force system?

ΣFx = 0, ΣFy = 0 and ΣM = 0 about any point. The first two prevent translation and the third prevents rotation.

MEC-006 · What is a free body diagram?

A sketch of a body isolated from its surroundings, showing every external force and moment acting on it, including support reactions, weight and friction. It is the first step in any statics or dynamics problem.

MEC-007 · What is the moment of a force, and what does Varignon's theorem state?

Moment = force × perpendicular distance from the point to the line of action (N·m). Varignon's theorem says the moment of a resultant about any point equals the sum of the moments of its components about the same point.

MEC-008 · What is a couple?

Two equal, opposite, parallel, non-collinear forces. It produces pure rotation with no resultant force, and its moment (F × arm) is the same about every point in the plane.

MEC-009 · What is the difference between static and kinetic friction?

Static friction acts before sliding starts and can rise up to μs·N. Kinetic friction acts during sliding and equals μk·N. Usually μk < μs, which is why it takes more force to start sliding than to keep sliding.

MEC-010 · What is the angle of repose?

The maximum inclination of a plane at which a body rests without sliding. It equals the angle of friction, so tan θ = μs.

MEC-011 · What is the difference between centroid and center of gravity?

The centroid is the geometric center of an area or volume. The center of gravity is the point where the resultant weight acts. They coincide for a homogeneous body in a uniform gravity field.

MEC-012 · State Newton's three laws of motion.

1st: a body stays at rest or in uniform motion unless acted on by a net force. 2nd: F = ma (rate of change of momentum). 3rd: every action has an equal and opposite reaction on a different body.

Strength of Materials

MEC-013 · What is the difference between stress and strain?

Stress is internal resisting force per unit area, σ = F/A, measured in Pa (N/m²). Strain is deformation per unit original length, ε = ΔL/L, and is dimensionless.

MEC-014 · State Hooke's law and define Young's modulus.

Within the elastic (proportional) limit, stress is proportional to strain: σ = Eε. E is Young's modulus, about 200 GPa for steel and 70 GPa for aluminium.

MEC-015 · What is Poisson's ratio?

The ratio of lateral strain to longitudinal strain under uniaxial load, ν = −ε_lateral/ε_axial. It is about 0.3 for steel, 0.33 for aluminium, and its theoretical maximum for isotropic materials is 0.5 (incompressible).

MEC-016 · How are E, G and K related?

E = 2G(1 + ν) = 3K(1 − 2ν), where G is the modulus of rigidity (shear) and K is the bulk modulus. Combining them gives E = 9KG/(3K + G).

MEC-017 · Describe the key points on a mild steel stress–strain curve.

Proportional limit, elastic limit, upper and lower yield points, strain hardening region, ultimate tensile strength (peak of engineering curve), necking, and fracture.

MEC-018 · What is proof stress (0.2% offset yield)?

For materials without a clear yield point (aluminium, high-strength steel), yield is defined where a line parallel to the elastic slope, offset by 0.2% strain, meets the curve.

MEC-019 · What are shear force and bending moment in a beam?

Shear force at a section is the algebraic sum of transverse forces on one side. Bending moment is the algebraic sum of moments of those forces about the section. dM/dx = V, so M is maximum where V crosses zero.

MEC-020 · Name the common types of beams.

Simply supported, cantilever, overhanging, fixed (built-in), propped cantilever and continuous beams. The first three are statically determinate; the rest are indeterminate.

MEC-021 · What is the neutral axis?

The line in a beam cross-section where bending stress and strain are zero. For linear elastic bending it passes through the centroid of the section.

MEC-022 · What is section modulus?

Z = I/y_max, where I is the second moment of area about the neutral axis. Maximum bending stress is σ = M/Z, so a larger Z means a stronger section in bending.

MEC-023 · State the bending equation.

M/I = σ/y = E/R, where M is bending moment, I second moment of area, σ bending stress at distance y from the neutral axis, E Young's modulus and R radius of curvature.

MEC-024 · State the torsion equation for a circular shaft.

T/J = τ/r = Gθ/L, where T is torque, J polar moment of inertia, τ shear stress at radius r, G modulus of rigidity and θ angle of twist over length L.

MEC-025 · What is thermal stress?

Stress caused when thermal expansion or contraction is restrained. For a fully restrained bar, σ = E·α·ΔT, independent of length.

MEC-026 · What is resilience and modulus of resilience?

Resilience is strain energy stored in a body up to the elastic limit. Modulus of resilience is that energy per unit volume, u = σ_y²/(2E).

Thermodynamics

MEC-027 · State the laws of thermodynamics.

Zeroth: two bodies each in thermal equilibrium with a third are in equilibrium with each other, which defines temperature. First: energy is conserved. Second: entropy of an isolated system never decreases and heat flows spontaneously hot to cold. Third: entropy of a perfect crystal tends to zero as T → 0 K.

MEC-028 · What are open, closed and isolated systems?

An open system (control volume) exchanges mass and energy, e.g. a turbine. A closed system exchanges energy but not mass, e.g. gas in a sealed piston. An isolated system exchanges neither.

MEC-029 · What is the difference between intensive and extensive properties?

Intensive properties do not depend on system size (T, p, density). Extensive properties scale with mass (V, U, H, S). Dividing an extensive property by mass gives a specific (intensive) property.

MEC-030 · What is the difference between a point function and a path function?

A point (state) function depends only on the end states, e.g. p, T, U, H, S. A path function depends on the process followed; heat and work are path functions and have inexact differentials.

MEC-031 · What is a quasi-static process?

A process carried out infinitely slowly so the system passes through a continuous series of equilibrium states. It can be drawn as a line on a p–v diagram and is the idealization behind reversible work, W = ∫p dV.

MEC-032 · What is enthalpy?

h = u + pv, the sum of internal energy and flow work. For steady-flow devices and constant-pressure processes, heat or work equals the change in enthalpy.

MEC-033 · What is entropy?

A state property measuring molecular disorder or energy dispersal, defined by dS = δQ_rev/T (J/K). It increases in every real process in an isolated system.

MEC-034 · What are cp and cv, and how are they related for an ideal gas?

cp is specific heat at constant pressure and cv at constant volume. For an ideal gas cp − cv = R and γ = cp/cv; for air γ ≈ 1.4 and R ≈ 0.287 kJ/kg·K.

MEC-035 · What is the difference between isothermal and adiabatic processes?

Isothermal: temperature constant, pv = constant for an ideal gas, heat transfer occurs. Adiabatic: no heat transfer, pv^γ = constant if also reversible, so temperature changes.

MEC-036 · What makes a process irreversible?

Friction, unrestrained expansion, heat transfer across a finite temperature difference, mixing of different substances, and inelastic deformation. Any of these generates entropy.

MEC-037 · What is the difference between a heat engine, a refrigerator and a heat pump?

A heat engine converts heat into work. A refrigerator uses work to remove heat from a cold space. A heat pump uses work to deliver heat to a warm space; the device is the same as a refrigerator but the useful output differs.

MEC-038 · State the Kelvin–Planck and Clausius statements of the second law.

Kelvin–Planck: no cyclic device can produce work while exchanging heat with only one reservoir. Clausius: no cyclic device can transfer heat from a colder to a hotter body without work input. The two are equivalent.

MEC-039 · What are the triple point and critical point of water?

Triple point: 0.01 °C and 611.657 Pa, where solid, liquid and vapour coexist. Critical point: 373.95 °C and 22.064 MPa, above which liquid and vapour cannot be distinguished.

MEC-040 · What is dryness fraction?

x = mass of vapour ÷ total mass of a wet steam mixture. x = 0 is saturated liquid, x = 1 is dry saturated vapour, and mixture properties follow h = h_f + x·h_fg.

Fluid Mechanics

MEC-041 · What is a fluid?

A substance that deforms continuously under any shear stress, however small. Liquids and gases are fluids; a solid resists shear by a finite static deformation.

MEC-042 · Define density, specific weight and specific gravity.

Density ρ = mass/volume (kg/m³; water ≈ 1000). Specific weight γ = ρg (N/m³; water ≈ 9.81 kN/m³). Specific gravity is the ratio of a fluid's density to water's density and is dimensionless.

MEC-043 · What is the difference between dynamic and kinematic viscosity?

Dynamic viscosity μ relates shear stress to velocity gradient, τ = μ·du/dy, in Pa·s. Kinematic viscosity ν = μ/ρ, in m²/s. Water at 20 °C has μ ≈ 1.0 × 10⁻³ Pa·s.

MEC-044 · What is the difference between Newtonian and non-Newtonian fluids?

Newtonian fluids have constant viscosity regardless of shear rate (water, air, oil). Non-Newtonian fluids do not: shear-thinning (paint, blood), shear-thickening (cornstarch slurry), or Bingham plastics with a yield stress (toothpaste).

MEC-045 · State Pascal's law.

Pressure at a point in a fluid at rest is the same in all directions, and pressure applied to an enclosed fluid is transmitted undiminished throughout it. It is the basis of hydraulic presses and jacks.

MEC-046 · What are absolute, gauge and vacuum pressure?

Absolute pressure is measured from perfect vacuum. Gauge pressure = absolute − atmospheric. Vacuum pressure = atmospheric − absolute, used when absolute pressure is below atmospheric (101.325 kPa at sea level).

MEC-047 · State Archimedes' principle.

A body immersed in a fluid experiences an upward buoyant force equal to the weight of fluid displaced. A body floats when buoyancy equals its weight.

MEC-048 · What is the Reynolds number and what does it indicate?

Re = ρVD/μ, the ratio of inertial to viscous forces. In pipe flow, Re < ~2000 is laminar, above ~4000 is turbulent, and in between is transitional.

MEC-049 · State the continuity equation.

Conservation of mass for flow: ρ₁A₁V₁ = ρ₂A₂V₂. For incompressible flow it becomes A₁V₁ = A₂V₂, so velocity rises where area falls.

MEC-050 · State Bernoulli's equation and its assumptions.

p/ρg + V²/2g + z = constant along a streamline. It assumes steady, incompressible, inviscid flow along a streamline with no shaft work or heat transfer.

MEC-051 · What are surface tension and capillarity?

Surface tension is the force per unit length at a liquid surface caused by unbalanced molecular attraction (water ≈ 0.073 N/m at 20 °C). Capillary rise is h = 4σ cos θ/(ρgd); water rises in glass tubes while mercury is depressed.

MEC-052 · What is a manometer?

A device that measures pressure by balancing it against a column of liquid. Types include piezometer, U-tube, differential and inclined manometers; Δp = ρgh.

MEC-053 · Distinguish steady/unsteady and uniform/non-uniform flow.

Steady flow: properties at a point do not change with time. Uniform flow: velocity does not change with position along the flow at an instant. Flow through a long constant-diameter pipe at constant rate is steady and uniform.

Heat Transfer

MEC-054 · How does heat transfer differ from thermodynamics?

Thermodynamics tells how much energy is transferred between equilibrium states. Heat transfer tells how fast it is transferred, using rate laws and temperature gradients.

MEC-055 · What is thermal conductivity? Give typical values.

k is the rate of heat conduction per unit area per unit temperature gradient, in W/m·K. Typical values: copper ≈ 400, aluminium ≈ 205, carbon steel ≈ 50, water ≈ 0.6, air ≈ 0.026.

MEC-056 · State Fourier's law of heat conduction.

q = −kA·dT/dx. The heat flow rate is proportional to area and temperature gradient; the minus sign shows heat flows down the gradient, from hot to cold.

MEC-057 · What is the convective heat transfer coefficient?

h in Newton's law of cooling, Q = hA(T_s − T_∞), in W/m²·K. It depends on fluid properties, velocity and geometry, not on the material alone; natural convection in air gives about 2–25, forced water convection hundreds to thousands.

MEC-058 · What is the difference between natural and forced convection?

Natural convection is driven by buoyancy from density differences caused by temperature. Forced convection uses an external driver such as a fan or pump, so it gives higher h.

MEC-059 · What is a black body and what is emissivity?

A black body absorbs all incident radiation and emits the maximum possible at a given temperature, E_b = σT⁴. Emissivity ε (0–1) is the ratio of a real surface's emission to a black body's at the same temperature.

MEC-060 · What is thermal diffusivity?

α = k/(ρc_p), in m²/s. It measures how quickly a material's temperature responds to a change; high α (metals) means heat spreads fast during transient conduction.

MEC-061 · What is thermal resistance?

The temperature difference divided by heat flow, R = ΔT/Q (K/W). For a plane wall R = L/(kA), for convection R = 1/(hA); series resistances add like electrical resistors.

MEC-062 · What is the purpose of a fin?

A fin is an extended surface that increases heat transfer area, used where the convection coefficient is low. Examples: motorcycle cylinder fins, heat sinks and finned-tube radiators.

MEC-063 · Name the types of heat exchangers by flow arrangement.

Parallel flow (both fluids in the same direction), counter flow (opposite directions) and cross flow (perpendicular). By construction: double-pipe, shell-and-tube, plate and compact finned types.

MEC-064 · What is the overall heat transfer coefficient U?

U combines all resistances between two fluids so that Q = UAΔT. For a thin plane wall, 1/U = 1/h₁ + L/k + 1/h₂, plus fouling resistances if present.

MEC-065 · What do the Nusselt and Prandtl numbers represent?

Nusselt number Nu = hL/k is the ratio of convective to conductive heat transfer across the fluid layer. Prandtl number Pr = ν/α = μc_p/k compares momentum diffusivity to thermal diffusivity (air ≈ 0.7, water ≈ 7 at room temperature).

Theory of Machines & Vibrations

MEC-066 · What are a kinematic link and a kinematic pair?

A link is a rigid body that transmits motion to other parts. A kinematic pair is two links in contact that permit a definite relative motion, such as a revolute (pin) or prismatic (slider) joint.

MEC-067 · What is the difference between a lower pair and a higher pair?

A lower pair has surface contact (pin joint, slider, screw). A higher pair has point or line contact (cam–follower, gear teeth, ball bearing).

MEC-068 · What is the difference between a mechanism and a machine?

A mechanism is a kinematic chain with one link fixed that transforms motion, e.g. a four-bar linkage. A machine is a mechanism or set of mechanisms that also transmits useful force or energy to do work, e.g. a lathe.

MEC-069 · How do you find the degrees of freedom of a planar mechanism?

Gruebler's (Kutzbach) equation: F = 3(n − 1) − 2j − h, where n is links including the frame, j lower pairs and h higher pairs. F = 1 gives a constrained mechanism, F = 0 a structure.

MEC-070 · State Grashof's law for a four-bar linkage.

If s + l ≤ p + q (shortest plus longest ≤ sum of the other two), at least one link can make full rotations. Fixing the link adjacent to the shortest gives a crank–rocker; fixing the shortest gives a double crank.

MEC-071 · Name inversions of the single slider-crank chain.

Reciprocating engine mechanism, Whitworth quick-return, crank and slotted-lever quick-return, rotary (Gnome) engine, oscillating-cylinder engine and the hand pump.

MEC-072 · What are the common types of cams and followers?

Cams: disc (radial), cylindrical, wedge and face cams. Followers: knife-edge, roller, flat-faced and spherical, either radial or offset, and either translating or oscillating.

MEC-073 · What is the difference between a flywheel and a governor?

A flywheel smooths speed fluctuations within each cycle by storing kinetic energy. A governor controls mean speed over many cycles by adjusting the fuel or energy supply to match load.

MEC-074 · Define module, pitch circle and pressure angle for gears.

Module m = d/z (mm), pitch diameter over number of teeth. The pitch circle is the imaginary circle that rolls without slip with the mating gear's. Pressure angle is the angle between the line of action and the common tangent, typically 20°.

MEC-075 · Name the main types of gears.

Parallel shafts: spur, helical and herringbone. Intersecting shafts: straight and spiral bevel. Non-intersecting, non-parallel shafts: worm and worm wheel, hypoid and crossed helical. Rack and pinion converts rotation into linear motion.

MEC-076 · What are the types of gear trains?

Simple (one gear per shaft), compound (two or more gears on an intermediate shaft), reverted (input and output coaxial) and epicyclic or planetary (some gear axes move around a central axis).

MEC-077 · What are free, forced and damped vibrations?

Free vibration occurs after an initial disturbance with no external force, at the natural frequency. Forced vibration is sustained by a periodic external force. Damped vibration loses energy to friction or viscous resistance, so amplitude decays.

MEC-078 · What is natural frequency and what is resonance?

For a spring–mass system, ωn = √(k/m) rad/s, the frequency at which it vibrates freely. Resonance occurs when the excitation frequency equals the natural frequency, causing very large amplitudes limited only by damping.

Machine Design

MEC-079 · What is the factor of safety?

FOS = failure stress (yield for ductile, ultimate for brittle) ÷ allowable working stress. It covers uncertainty in loads, material properties, manufacturing and analysis; typical values range from about 1.5 to 4.

MEC-080 · What are the main steps in the machine design process?

Recognize the need, define the problem and specifications, synthesize concepts, analyze loads and stresses, select materials, size the parts, evaluate and optimize, then prepare drawings and prototype for testing.

MEC-081 · What types of loads act on machine members?

Static (steady), impact or shock (suddenly applied), and fluctuating or cyclic loads (reversed, repeated, alternating). A suddenly applied load produces twice the static stress in an elastic member.

MEC-082 · What is a key and what types are used?

A key is a machine element that locks a hub to a shaft to transmit torque. Types include sunk (rectangular or square), feather, Woodruff, gib-head, saddle and tangent keys, plus splines for higher torque.

MEC-083 · What is the difference between rigid and flexible couplings?

Rigid couplings (sleeve, flange, clamp) need accurate alignment and transmit torque without accommodating misalignment. Flexible couplings (bush-pin, jaw, gear, Oldham, universal joint) absorb misalignment and shock.

MEC-084 · What is the difference between a bolt, a screw and a stud?

A bolt passes through clearance holes and is tightened with a nut. A screw threads into a tapped hole in one of the parts. A stud is threaded at both ends, one end screwed permanently into a part and a nut used on the other.

MEC-085 · What are lap and butt riveted joints?

In a lap joint the plates overlap and are riveted together. In a butt joint the plate edges meet and are joined by one or two cover (strap) plates. Butt joints avoid the eccentric load path of lap joints.

MEC-086 · What are the common types of welded joints?

Butt, lap, T, corner and edge joints, made with groove (butt) welds or fillet welds. Fillet welds are sized by leg length and designed on throat area, throat = 0.707 × leg.

MEC-087 · What is spring rate and what types of springs exist?

Spring rate k = F/δ (N/mm). Types: helical compression and extension, torsion, leaf (laminated), disc (Belleville), spiral and garter springs.

MEC-088 · What is the difference between sliding and rolling contact bearings?

Sliding (journal) bearings support the shaft on a lubricant film, suit high loads, shock and very high speed, and run quietly. Rolling bearings (ball, roller) have low starting friction, are compact and standardized, but have finite fatigue life.

MEC-089 · Compare belt, chain and gear drives.

Belts suit large center distances, absorb shock and slip under overload, but have no exact ratio. Chains give a positive ratio at moderate distances. Gears give a positive, compact, high-efficiency drive for short center distances.

MEC-090 · What is the difference between a clutch and a brake?

A clutch connects or disconnects two rotating shafts to transmit torque. A brake absorbs kinetic energy of a moving member, usually by friction, to slow or stop it. Both are designed on friction torque and heat dissipation.

Manufacturing Processes

MEC-091 · How are manufacturing processes classified?

Casting (shaping liquid), forming (plastic deformation), machining (material removal), joining (welding, brazing, fastening), powder metallurgy, additive manufacturing, and finishing and heat treatment.

MEC-092 · What is casting and what is a pattern?

Casting pours molten metal into a mould cavity where it solidifies into shape. A pattern is a replica of the part, slightly oversized, used to form the cavity in the mould.

MEC-093 · What are pattern allowances?

Shrinkage (for solid contraction), machining (extra stock), draft (taper for withdrawal), distortion (to offset warping) and shake (rapping) allowance, which is negative.

MEC-094 · Name the parts of a sand mould gating system.

Pouring basin, sprue (vertical channel), sprue well, runner, ingates, and risers. The riser feeds liquid metal to the casting as it shrinks during solidification.

MEC-095 · What is the difference between hot working and cold working?

Hot working is done above the recrystallization temperature, so no strain hardening occurs and large deformations are possible. Cold working is below it, giving better finish, accuracy and strength through work hardening, but needs more force.

MEC-096 · How do rolling, extrusion and wire drawing differ?

Rolling reduces thickness by passing stock between rotating rolls. Extrusion pushes a billet through a die to make a constant cross-section. Drawing pulls stock through a die to reduce diameter, as in wire and tube making.

MEC-097 · What is the difference between welding, brazing and soldering?

Welding melts the base metals to fuse them. Brazing uses a filler that melts above 450 °C but below the base metal's melting point. Soldering uses a filler melting below 450 °C. Only welding melts the parent metal.

MEC-098 · What is the difference between MIG and TIG welding?

MIG (GMAW) uses a continuously fed consumable wire electrode with inert or active shielding gas and is fast. TIG (GTAW) uses a non-consumable tungsten electrode with separate filler, giving precise, high-quality welds on thin and non-ferrous metals.

MEC-099 · Name common lathe operations.

Turning, facing, taper turning, threading, knurling, parting off, chamfering, drilling, boring and reaming.

MEC-100 · How do drilling, boring and reaming differ?

Drilling creates a hole in solid material. Boring enlarges an existing hole with a single-point tool and corrects its position and straightness. Reaming takes a small cut to give an accurate size and smooth finish.

MEC-101 · What is the difference between up milling and down milling?

In up (conventional) milling the cutter rotates against the feed, chip thickness starts at zero, and the work tends to lift. In down (climb) milling it rotates with the feed, gives better finish and tool life, but needs a backlash-free machine.

MEC-102 · What materials are used for cutting tools?

In order of increasing hot hardness: carbon tool steel, high-speed steel (HSS), cast alloys, cemented carbide, ceramics, cubic boron nitride (CBN) and diamond. Carbides are the most widely used in production.

MEC-103 · What is grinding?

An abrasive machining process in which a rotating wheel of bonded abrasive grains removes small chips. It produces fine finish and tight tolerances and can machine hardened materials.

MEC-104 · What is the difference between blanking and piercing?

Both shear sheet metal with a punch and die. In blanking the punched-out piece is the product; in piercing (punching) the hole is the product and the slug is scrap.

IC Engines

MEC-105 · What is the difference between an internal and an external combustion engine?

In an IC engine, fuel burns inside the cylinder and the combustion gases are the working fluid (petrol, diesel engines). In an EC engine, combustion is outside and heat passes to a separate working fluid (steam engine, Stirling engine).

MEC-106 · What is the difference between the Otto and Diesel cycles?

The Otto cycle adds heat at constant volume and is the ideal cycle for spark-ignition engines. The Diesel cycle adds heat at constant pressure and is the ideal cycle for compression-ignition engines.

MEC-107 · How does a two-stroke engine differ from a four-stroke engine?

A four-stroke engine completes a cycle in two crank revolutions with separate intake, compression, power and exhaust strokes. A two-stroke completes it in one revolution using ports and scavenging, giving more power per weight but poorer efficiency and higher emissions.

MEC-108 · What is compression ratio?

r = (V_s + V_c)/V_c, total cylinder volume at BDC divided by clearance volume at TDC. Typical values are about 8–12 for SI engines and 15–22 for CI engines.

MEC-109 · Define bore, stroke, swept volume and clearance volume.

Bore is cylinder diameter D. Stroke L is piston travel between TDC and BDC. Swept volume V_s = (π/4)D²L. Clearance volume is the space left above the piston at TDC.

MEC-110 · How do SI and CI engines differ?

SI engines draw in a fuel–air mixture, ignite it with a spark, and control load by throttling. CI engines draw in air only, inject fuel into hot compressed air for self-ignition, and control load by fuel quantity.

MEC-111 · What is the difference between carburetion and fuel injection?

A carburetor meters fuel by the pressure drop of air through a venturi. Fuel injection meters fuel with pressurized injectors under electronic control, giving more accurate mixture, better efficiency and lower emissions.

MEC-112 · What are TDC and BDC?

Top dead center is the piston position nearest the cylinder head, where volume is minimum. Bottom dead center is the farthest position, where volume is maximum. Valve and ignition timing are referenced in crank degrees from these points.

MEC-113 · What are indicated power, brake power and mechanical efficiency?

Indicated power is developed by the gas inside the cylinder. Brake power is delivered at the crankshaft. Mechanical efficiency = BP/IP, and the difference IP − BP is friction power.

MEC-114 · What is knocking in an SI engine?

Auto-ignition of the end gas ahead of the flame front, producing sharp pressure waves, a metallic ping, and possible piston damage. It limits the compression ratio of SI engines.

MEC-115 · What do octane and cetane numbers indicate?

Octane number measures a petrol's resistance to knock (higher is better for SI engines). Cetane number measures a diesel's ignition quality, or readiness to self-ignite (higher is better for CI engines).

MEC-116 · What is brake specific fuel consumption?

BSFC = fuel mass flow rate ÷ brake power, commonly in g/kWh. Lower BSFC means better efficiency; brake thermal efficiency = 3.6 × 10⁶/(BSFC × CV) with BSFC in g/kWh and CV in J/g.

Refrigeration & Air Conditioning

MEC-117 · What is a ton of refrigeration?

The cooling rate that freezes one short ton (2000 lb) of water at 0 °C in 24 hours. 1 TR ≈ 3.517 kW ≈ 211 kJ/min ≈ 12,000 Btu/h.

MEC-118 · What is the coefficient of performance (COP)?

COP = desired output ÷ work input. For a refrigerator COP_R = Q_L/W; for a heat pump COP_HP = Q_H/W = COP_R + 1. Unlike efficiency, COP is usually greater than 1.

MEC-119 · Name the four main components of a vapour compression refrigeration system.

Compressor (raises refrigerant pressure and temperature), condenser (rejects heat), expansion device (drops pressure), and evaporator (absorbs heat from the refrigerated space).

MEC-120 · What properties should a good refrigerant have?

High latent heat, suitable saturation pressures (slightly above atmospheric in the evaporator), low specific volume, chemical stability, non-toxic and non-flammable, zero ozone depletion potential and low global warming potential.

MEC-121 · How does a vapour absorption refrigeration system work?

It replaces the compressor with an absorber, pump and generator. The refrigerant (ammonia or water) is absorbed into a liquid (water or lithium bromide), pumped to high pressure, and driven out by heat, so it runs mainly on heat instead of shaft work.

MEC-122 · Name some common refrigerants and their designations.

R-134a (HFC), R-410A (HFC blend), R-32, R-717 (ammonia), R-744 (CO₂), R-290 (propane), R-600a (isobutane), and R-1234yf (HFO). Older CFCs like R-12 are phased out for ozone depletion.

MEC-123 · Define dry bulb, wet bulb and dew point temperatures.

Dry bulb is the air temperature from an ordinary thermometer. Wet bulb is read with a wetted wick and reflects evaporative cooling. Dew point is the temperature at which water vapour starts to condense when air is cooled at constant pressure.

MEC-124 · What is the difference between relative humidity and specific humidity?

Relative humidity φ = partial pressure of vapour ÷ saturation pressure at the same temperature, in %. Specific humidity (humidity ratio) ω is mass of water vapour per kg of dry air, ω = 0.622 p_v/(p − p_v).

MEC-125 · What is a psychrometric chart?

A chart of moist air properties at a fixed pressure, plotting humidity ratio against dry bulb temperature with lines of RH, wet bulb, enthalpy and specific volume. It is used to analyze heating, cooling, humidification and mixing processes.

MEC-126 · What are typical human comfort conditions?

Roughly 22–26 °C dry bulb and 40–60% relative humidity with low air velocity (about 0.1–0.25 m/s), depending on clothing and activity.

MEC-127 · What is the function of the expansion device?

It reduces refrigerant pressure from condenser to evaporator level and controls the flow to the evaporator. Types include capillary tubes, thermostatic expansion valves (TXV) and electronic expansion valves.

Engineering Materials

MEC-128 · What is the difference between ductile and brittle materials?

Ductile materials deform plastically before fracture and give warning (mild steel, copper, aluminium). Brittle materials fracture with little plastic deformation (cast iron, glass, ceramics), typically with elongation below about 5%.

MEC-129 · What is the difference between ferrous and non-ferrous metals?

Ferrous metals are iron-based (steels, cast irons), strong and inexpensive but prone to rust. Non-ferrous metals contain little or no iron (aluminium, copper, titanium, nickel alloys) and are often lighter or more corrosion-resistant.

MEC-130 · How do steel and cast iron differ by composition?

Steel is an iron–carbon alloy with up to about 2.1% carbon (most engineering steels below 1%). Cast iron has roughly 2.1–4% carbon, often with silicon, making it castable but less ductile.

MEC-131 · What are BCC, FCC and HCP crystal structures?

BCC: atoms at cube corners and center, e.g. α-iron, chromium. FCC: corners and face centers, e.g. aluminium, copper, γ-iron; it is the most ductile. HCP: hexagonal close-packed, e.g. zinc, magnesium, titanium. FCC and HCP have packing factor 0.74, BCC 0.68.

MEC-132 · What is hardness and how is it measured?

Resistance to localized plastic deformation (indentation). Common tests: Brinell (steel or carbide ball), Rockwell (depth of indentation, scales B and C), Vickers (diamond pyramid) and Knoop (microhardness).

MEC-133 · What is the difference between strength, toughness and hardness?

Strength is the stress a material withstands before yielding or breaking. Toughness is the energy absorbed before fracture (area under the stress–strain curve). Hardness is resistance to indentation or scratching.

MEC-134 · Name the basic heat treatment processes for steel.

Annealing (slow furnace cooling, softens), normalizing (air cooling, refines grain), hardening (quenching to form martensite), and tempering (reheating below the lower critical temperature to reduce brittleness).

MEC-135 · What makes stainless steel stainless?

At least about 10.5% chromium, which forms a thin, adherent, self-healing chromium oxide film that protects against corrosion. Nickel and molybdenum are added for austenitic structure and pitting resistance.

MEC-136 · What is the difference between malleability and ductility?

Malleability is the ability to deform under compression into thin sheets (gold, lead). Ductility is the ability to be drawn into wire under tension (copper, gold).

MEC-137 · What is a composite material?

A material made of two or more distinct phases, a reinforcement (fibres, particles) in a matrix, giving properties neither has alone. Examples: CFRP, GFRP, concrete and metal matrix composites.

MEC-138 · What is the difference between thermoplastics and thermosets?

Thermoplastics soften on heating and can be remelted and recycled (PE, PP, PVC, nylon). Thermosets cure into a cross-linked network and char rather than melt on reheating (epoxy, phenolics, polyester resin).

MEC-139 · What is corrosion and how is it prevented?

Electrochemical deterioration of a metal by its environment, such as rusting. Prevention includes coatings and painting, galvanizing, cathodic protection, corrosion-resistant alloys, inhibitors and design that avoids water traps and dissimilar metal contact.

Metrology, CAD/CAM & GD&T

MEC-140 · What is the difference between accuracy and precision?

Accuracy is how close a measurement is to the true value. Precision is how close repeated measurements are to each other. A gauge can be precise but inaccurate if it has a systematic bias.

MEC-141 · What is the least count of a vernier caliper?

Least count = 1 main scale division − 1 vernier scale division. For a standard metric caliper it is 0.02 mm (some read 0.05 mm or 0.01 mm digitally).

MEC-142 · What is the least count of a micrometer?

Pitch ÷ number of thimble divisions. With 0.5 mm pitch and 50 divisions, least count = 0.01 mm.

MEC-143 · Define limits, tolerance and allowance.

Limits are the maximum and minimum permissible sizes. Tolerance is the difference between them. Allowance is the intentional difference between the maximum material limits of mating parts (minimum clearance or maximum interference).

MEC-144 · What are clearance, transition and interference fits?

Clearance fit: the shaft is always smaller than the hole. Interference fit: the shaft is always larger, needing press or shrink fitting. Transition fit: depending on actual sizes it may give either small clearance or small interference.

MEC-145 · What is the difference between the hole basis and shaft basis systems?

In the hole basis system the hole has a fixed deviation (H, lower deviation zero) and the shaft tolerance varies to get the fit; it is preferred because holes are made with standard tools. In the shaft basis system the shaft is fixed (h) and the hole varies.

MEC-146 · What are slip gauges?

Precision rectangular steel or ceramic blocks with flat, parallel faces of very accurate thickness. They are wrung together to build up reference lengths for calibration and setting other instruments.

MEC-147 · What is GD&T?

Geometric Dimensioning and Tolerancing is a symbolic language that specifies allowable variation in form, orientation, location and runout of features relative to datums. It is defined by ASME Y14.5 and the ISO GPS standards.

MEC-148 · What is a datum in GD&T?

A theoretically exact point, axis or plane derived from a real datum feature on the part. It establishes the reference frame from which geometric tolerances are measured.

MEC-149 · What do CAD, CAM and CAE stand for?

Computer-Aided Design creates geometry and drawings. Computer-Aided Manufacturing generates toolpaths and machine code. Computer-Aided Engineering analyzes and simulates designs, e.g. FEA and CFD.

MEC-150 · What is the difference between G-codes and M-codes in CNC?

G-codes are preparatory commands that define motion and geometry, e.g. G00 rapid, G01 linear feed, G02/G03 circular arcs. M-codes are miscellaneous machine functions, e.g. M03 spindle on clockwise, M05 spindle stop, M08 coolant on.

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