ME - Mechanical Engineering


Online Magazine for Mechanical Engineers

Material Requirement Planning (MRP) is a computer based inventory management technique used for determining the quantity and timing for the acquisition of dependent demand items needed to satisfy master schedule requirements.

The Terminology used in MRP are:
Dependent Demand: Demand for components that is derived from the demand of other items.
Parent and component items: A parent is an assembly made up of basic parts or components. The parent of one subgroup may be a component of higher-level parent.
Lot Size: The quantity of items required for an order. The order may be either purchased from a vendor or produced in house. Lot sizing is the process of specifying the order size.
Time Phasing: Scheduling to produce or receive an appropriate amount (log) of material so it will be available in the time periods.
Time Bucket: Time bucket is the time period used for planning purposes in Material Requirement Planning (MRP) usually 7 days (week).
Requirements: Projected needs for raw materials, components, sub-assemblies or finished goods. Gross requirements are total needs from all sources, whereas net requirements are ‘net’ after allowing for available inventory.
Requirement explosion: The breaking down (exploding) of parent items into components parts that can be individually planned and scheduled.
Bill of Materials: A list of all components (sub-assemblies and materials) that go into assembled items. It frequently includes the part numbers and quantity required per assembly.
Scheduled Receipt: Materials already ordered from vendor or in-house shop. The MRP shows both the quantity and project time of receipt.
Lead-time Offset: The supply time or number of time buckets between releasing an order and receiving the materials.
Planned Order Release: The plan to initiate the manufacture of materials or purchase of materials so that they will be received on schedule after the lead-time offset.

Different Lot Sizing Techniques in MRP are:
  1. Lot for Lot (LFL): Order the exact amount of the net requirement each period.
  2. Single Lot: Order quantity is equal to the total requirement and only one order is to be placed.
  3. Economic Order Quantity (EOQ): Order an EOQ or ERL amount.
  4. Least Unit Cost (LUC): Order the net requirement for the current period or current plus next or current plus next two and so on, depending upon which gives the lowest unit cost.
  5. Minimum Cost Period (MCP): Order quantity is selected in such a way the cost period must be minimum.
  6. Part-Period Algorithm (PPA): Use the ratio of ordering and carrying costs to derive a part-period number and use the number as a criterion for cumulating requirements.
  7. Period Order Quantity (POQ): Divide the Economic Order Quantity (EOQ) into the annual demand and order that many times per year.
Heat is a form of energy which transfers between bodies which are kept under thermal interactions. When a temperature difference occurs between two bodies or a body with its surroundings, heat transfer occurs.
Heat transfer occurs in three modes:
  1. Conduction
  2. Convection and
  3. Radiation
modes of heat transfer
Conduction:
In Conduction, heat transfer takes place due to temperature difference in a body or between bodies in thermal contact, without mixing of mass. The rate of heat transfer through conduction is governed by the Fourier's law of heat conduction.
Q = -kA(dT/dx)
Where, 'Q' is the heat flow rate by conduction
'K' is the thermal conductivity of body material
'A' is the cross-sectional area normal to direction of heat flow and
'dT/dx' is the temperature gradient of the section.

Convection:
In convection, heat is transferred to a moving fluid at the surface over which it flows by combined molecular diffusion and bulk flow. Convection involves conduction and fluid flow. The rate of convective heat transfer is governed by the Newton's law of cooling
Q = hA(Ts-T∞)
Where 'Ts' is the surface temperature
'T∞' is the outside temperature
'h' is the coefficient of convection
heat transfer occurs in three modes, they are conduction, convection and radiation

Radiation:
In radiation, heat is transferred in the form of radiant energy or wave motion from one body to another body. No medium for radiation to occur. The rate of heat radiation that can be emitted by a surface at a thermodynamic temperature is based on Stefan-Boltzmann law.
Q = σ.T⁴
Where, 'T' is the absolute temperature of surface
'σ' is the Stefan-Boltzmann constant.
The law of heat conduction is also known as Fourier's law. Fourier's law states that the time rate of heat transfer through a material is proportional to the negative gradient in the temperature and to the area.

Q = -kA(dT/dx)

Where: 'Q' is the heat flow rate by conduction (W·m−2)
'k' is the thermal conductivity of body material (W·m−1·K−1)
'A' is the cross-sectional area normal to direction of heat flow (m2) and
'dT/dx' is the temperature gradient (K·m−1).

  • Negative sign indicates that he heat flow is in the direction of negative gradient temperature and that serves to make heat flow positive.
  • Thermal conductivity 'k' is one of the transport properties. Other are the viscosity associated with the transport of momentum, diffusion coefficient associated with the transport of mass.
  • Thermal conductivity 'k' provides an indication of the rate at which heat energy is transferred through a medium by conduction process.

Assumptions of Fourier equation:

  • Steady state heat conduction.
  • One directional heat flow.
  • Bounding surfaces are isothermal in character that is constant and uniform temperatures are maintained at the two faces.
  • Isotropic and homogeneous material and thermal conductivity 'k' is constant.
  • Constant temperature gradient and linear temperature profile.
  • No internal heat generation.

Features of Fourier equation:

  • Fourier equation is valid for all matter solid, liquid or gas.
  • The vector expression indicating that heat flow rate is normal to an isotherm and is in the direction of decreasing temperature.
  • It cannot be derived from first principle.
  • It helps to define the transport property 'k'.
A Psychrometric chart can graphically represents the thermodynamic properties of air-water vapour mixture. Standard psychrometric charts are bounded by the dry-bulb temperature line (X - axis) and the vapour pressure or humidity ratio (Y - axis). The Left Hand Side of the psychrometric chart is bounded by saturation line.
Below image shows the schematic diagram of a psychrometric chart.
Psychrometric charts are readily available for standard barometric pressure of 101.325 kPa at sea level and for normal temperatures (0-50ºC).
Closed cycle gas turbine and open cycle gas turbine can be compared in 11 different criterias.
The 11 criterias are cycle of operation, working fuel used, type of fuel used, manner of heat input, quality of heat input, efficiency, part load efficiency, turbine blade life of a turbine, control on power production and cost of turbine plant installation.

S.no
Criterion
Closed Cycle Gas Turbine
Open Cycle Gas Turbine
1 Cycle of operation It works on closed cycle. The working fluid is recirculated again and again. It is a clean cycle. It works on open cycle. The fresh charge is supplied to each cycle and after combustion and expansion. It is discharged to atmosphere.
2 Working fluid The gases other than the air like Helium or Helium-Carbon dioxide mixture can be used, which has more favourable properties. Air-fuel mixture is used which leads to lower thermal efficiency.
3 Type of fuel used Since heat is transferred externally, so any type of fuel; solid liquid or gaseous or combination of these can be used for generation of heat. Since combustion is an integral part of the system thus it requires high quantity liquid or gaseous fuel for burning in a combustion chamber.
4 Manner of heat input The heat is transferred indirectly through a heat exchanger. Direct heat supply. It is generated in the combustion chamber itself
5 Quality of heat input The heat can be supplied from any source like waste heat from some process, nuclear heat and solar heat using a concentrator. It requires high grade heat energy for generation of power in a gas turbine.
6 Efficiency High thermal efficiency for given lower and upper temperature liquids. Low thermal efficiency for same temperature limits.
7 Part load efficiency Part load efficiency is better. Part load efficiency is less compared to Closed cycle gas turbine.
8 Size of plant Reduced size per MWh of power output. Comparatively large size for same power output.
9 Blade life Since combustion products do not come in direct contact of turbine blade, thus there is no blade fouling and longer blade life. Direct contact with combustion products, the blades are subjected to higher thermal stresses and fouling and hence shorter blade life.
10 Control on power production Better control on power production. Poor control on power production.
11 Cost Plant is complex and costly. Plant is simple and less costly.

Beams are generally horizontal structural members which transfer loads horizontally along their length to the supports where the loads are usually resolved into vertical forces. Beams are used for resisting vertical loads, shear forces and bending moments.

Different types of beams can be classified based on the type of support.

The four different types of beams are:
  1. Simply Supported Beam
  2. Fixed Beam
  3. Cantilever Beam
  4. Continuously Supported Beam

Simply Supported Beam
If the ends of a beam are made to rest freely on supports beam, it is called a simply (freely) supported beam.
Simply Supported Beam

Fixed Beam
If a beam is fixed at both ends it is free called fixed beam. Its another name is built-in beam or encastre beam.
Fixed Beam

Cantilever Beam
If a beam is fixed at one end while the other end is free, it is called cantilever beam.
Cantilever Beam

Continuously Supported Beam
If more than two supports are provided to beam, it is called continuously supported beam.
Continuously Supported Beam
In casting process, first few castings will be inspected dimensionally and THE pattern is qualified afterwards, only few random inspection will be done. Every casting must be inspected for finding out the defects in casting process.
Different methods of inspection for finding out defects in metal casting process are
  1. Visual Inspection
  2. Hydrostatic Pressure Test
  3. Magnetic Particle Inspection
  4. Dye - Penetrant Inspection
  5. Coin Testing
  6. Radiographic Examination
  7. Ultrasonic inspection

Visual Inspection
Common defects such as surface roughness, obvious shifts, omission of cores and surface cracks can be detected by a visual inspection of the casting. Cracks may also be detected by hitting the casting with a mallet and listening to the quality of the tone produced.

Hydrostatic Pressure Test
  • The Hydrostatic pressure test is conducted on a casting to be used as a pressure vessel.
  • In this test, first all the flanges and ports are blocked.
  • Then the casting is filled with water, oil or compressed air, Thereafter, the casting is submerged in a soap solution when any leak will be evident by the bubbles that come out.

Magnetic Particle Inspection
The Magnetic particle test is conducted to check for very small voids and cracks at or just below the surface of a ferromagnetic material. The test involves inducing a magnetic field through the section inspection. this done, the powdered ferromagnetic material is spread out onto the surface. The presence of voids or cracks in the section results in an abrupt change in the permeability of the surface; this, in turn, causes a leakage in the magnetic field. The powdered particles accumulate on the disrupted magnetic field, outlining the boundary of a discontinuity.

Dye - Penetrant Inspection
The dye - penetrant method is used to detect invisible surface defects in a nonmagnetic casting. The casting is brushed with, sprayed with, or dipped into a dye containing a fluorescent material. The surface to be inspected is the wiped, dried and viewed in darkness. The discontinuous in the surface will then be readily discernible.

Coin Testing
By hitting with coin on to the component and by hearing the sound coming from the casing, the presence of defect can be estimated.

Radiographic Examination
The radiographic method is expensive and is used only for subsurface exploration. In this, both X-rays and γ-rays are used. With γ-rays, more than one film can be exposed simultaneously; however, x-ray pictures are more distinct. Various defects, like voids, nonmetallic inclusions, porosity, cracks and tears, can be detected by this method. The defects being less dense, film appears darker in contrast to the surrounding.

Ultrasonic Inspection
In the Ultrasonic method, an oscillator is used to send an ultrasonic signal through the casting. such as signal is readily transmitted through a homogeneous medium. However, on encountering a discontinuity, the signal is reflected back. This reflected signal is then detected by an ultrasonic detector. The time interval between sending the signal and receiving its reflection determines the location of the discontinuity. the method is not very suitable for a material with a high damping capacity (e.g. cast iron) because in such a case the signal gets considerably weakened over some distance.
Syllabus for Graduate Aptitude Test in Engineering (GATE) Mechanical Engineering (ME) Examination.

Section 1: General Aptitude
Verbal Ability: English grammar, sentence completion, verbal analogies, word groups, instructions, critical reasoning and verbal deduction.
Numerical Ability: Numerical computation, numerical estimation, numerical reasoning and data interpretation

Section 2: Engineering Mathematics
Linear Algebra: Matrix algebra, systems of linear equations, eigenvalues and eigen vectors.
Calculus: Functions of single variable, limit, continuity and differentiability, mean value theorems, indeterminate forms; evaluation of definite and improper integrals; double and triple integrals; partial derivatives, total derivative, Taylor series (in one and two variables), maxima and minima, Fourier series; gradient, divergence and curl, vector identities, directional derivatives, line, surface and volume integrals, applications of Gauss, Stokes and Green’s theorems.
Differential equations: First order equations (linear and nonlinear); higher order linear differential equations with constant coefficients; Euler-Cauchy equation; initial and boundary value problems; Laplace transforms; solutions of heat, wave and Laplace's equations.
Complex variables: Analytic functions; Cauchy-Riemann equations; Cauchy’s integral theorem and integral formula; Taylor and Laurent series.
Probability and Statistics: Definitions of probability, sampling theorems, conditional probability; mean, median, mode and standard deviation; random variables, binomial, Poisson and normal distributions.
Numerical Methods: Numerical solutions of linear and non-linear algebraic equations; integration by trapezoidal and Simpson’s rules; single and multi-step methods for differential equations.

Section 3: Applied Mechanics and Design
Engineering Mechanics: Free-body diagrams and equilibrium; trusses and frames; virtual work; kinematics and dynamics of particles and of rigid bodies in plane motion; impulse and momentum (linear and angular) and energy formulations, collisions.
Mechanics of Materials: Stress and strain, elastic constants, Poisson's ratio; Mohr’s circle for plane stress and plane strain; thin cylinders; shear force and bending moment diagrams; bending and shear stresses; deflection of beams; torsion of circular shafts; Euler’s theory of columns; energy methods; thermal stresses; strain gauges and rosettes; testing of materials with universal testing machine; testing of hardness and impact strength.
Theory of Machines: Displacement, velocity and acceleration analysis of plane mechanisms; dynamic analysis of linkages; cams; gears and gear trains; flywheels and governors; balancing of reciprocating and rotating masses; gyroscope.
Vibrations: Free and forced vibration of single degree of freedom systems, effect of damping; vibration isolation; resonance; critical speeds of shafts.
Machine Design: Design for static and dynamic loading; failure theories; fatigue strength and the S-N diagram; principles of the design of machine elements such as bolted, riveted and welded joints; shafts, gears, rolling and sliding contact bearings, brakes and clutches, springs.

Section 4: Fluid Mechanics and Thermal Sciences
Fluid Mechanics: Fluid properties; fluid statics, manometry, buoyancy, forces on submerged bodies, stability of floating bodies; control-volume analysis of mass, momentum and energy; fluid acceleration; differential equations of continuity and momentum; Bernoulli’s equation; dimensional analysis; viscous flow of incompressible fluids, boundary layer, elementary turbulent flow, flow through pipes, head losses in pipes, bends and fittings.
Heat-Transfer: Modes of heat transfer; one dimensional heat conduction, resistance concept and electrical analogy, heat transfer through fins; unsteady heat conduction, lumped parameter system, Heisler's charts; thermal boundary layer, dimensionless parameters in free and forced convective heat transfer, heat transfer correlations for flow over flat plates and through pipes, effect of turbulence; heat exchanger performance, LMTD and NTU methods; radiative heat transfer, Stefan-Boltzmann law, Wien's displacement law, black and grey surfaces, view factors, radiation network analysis.
Thermodynamics: Thermodynamic systems and processes; properties of pure substances, behaviour of ideal and real gases; zeroth and first laws of thermodynamics, calculation of work and heat in various processes; second law of thermodynamics; thermodynamic property charts and tables, availability and irreversibility; thermodynamic relations.
Applications: Power Engineering: Air and gas compressors; vapour and gas power cycles, concepts of regeneration and reheat. I.C. Engines: Air-standard Otto, Diesel and dual cycles. Refrigeration and air-conditioning: Vapour and gas refrigeration and heat pump cycles; properties of moist air, psychrometric chart, basic psychrometric processes. Turbomachinery: Impulse and reaction principles, velocity diagrams, Pelton-wheel, Francis and Kaplan turbines.

Section 5: Materials, Manufacturing and Industrial Engineering
Engineering Materials: Structure and properties of engineering materials, phase diagrams, heat treatment, stress-strain diagrams for engineering materials.
Casting, Forming and Joining Processes: Different types of castings, design of patterns, moulds and cores; solidification and cooling; riser and gating design. Plastic deformation and yield criteria; fundamentals of hot and cold working processes; load estimation for bulk (forging, rolling, extrusion, drawing) and sheet (shearing, deep drawing, bending) metal forming processes; principles of powder metallurgy. Principles of welding, brazing, soldering and adhesive bonding.
Machining and Machine Tool Operations: Mechanics of machining; basic machine tools; single and multi-point cutting tools, tool geometry and materials, tool life and wear; economics of machining; principles of non-traditional machining processes; principles of work holding, design of jigs and fixtures.
Metrology and Inspection: Limits, fits and tolerances; linear and angular measurements; comparators; gauge design; interferometry; form and finish measurement; alignment and testing methods; tolerance analysis in manufacturing and assembly.
Computer Integrated Manufacturing: Basic concepts of CAD/CAM and their integration tools.
Production Planning and Control: Forecasting models, aggregate production planning, scheduling, materials requirement planning.
Inventory Control: Deterministic models; safety stock inventory control systems.
Operations Research: Linear programming, simplex method, transportation, assignment, network flow models, simple queuing models, PERT and CPM.

References:
Mechanical Engineering (ME) Syllabus PDF(gate.iisc.ernet.in)
General Aptitude (GA) Syllabus PDF(gate.iitk.ac.in)
Petrol can be compared with Liquefied Petroleum Gas (LPG) in aspects like fuel consumption amount, octane rating, fuel odour, spark plug life by using fuel, octane number, etc.


S.No
Petrol
Liquefied Petroleum Gas (LPG)
1 Fuel consumption in petrol engine is less compared to Liquefied Petroleum Gas (LPG) Compared to petrol, running the engine on LPG results in around a 10% increase in fuel consumption.
2 Petrol has odour LPG has no odour.
3 Octane rating of petrol is 81 Octane rating of LPG is 110
4 Petrol engine is not as smooth as LPG engine. Due to higher octane rating, the combustion of LPG is smoother and knocking is eliminated and the engine runs smoothly.
5 In order to increase octane number petrol required lead additives. LPG is lead-free with high octane number.
6 The mixture of petrol and air always leaks past the piston rings and washes away the lubricating oil from the upper cylinder wall surfaces in the process. This result in lack of lubricant which causes more wear. It also carries with it unburnt fuel components and falls into the engine oil. Thus the life of petrol engine is short. When LPG leaks past the rings into the crankcase, it does not generate black carbon. Hence, the lubricating layer is not washed away. thereby, the engine life is increased by 50%.
7 Due to formation of carbon deposits on the spark plugs, the life of the spark plugs is shortened. Due to absence of carbon deposits on the electrodes of the spark plugs, the life of the spark plugs is increased.
8 Carburettor supplies the mixture of petrol and air in the proper ratio to the engine cylinder for combustion. The vaporizer functions as the carburettor when the engine runs on LPG. It is a control device that reduces LPG pressure, vaporizes it and supplies to the engine with a regular flow of gas as per the engine requirement.
Fraction of products formed during petroleum refining process are fuel gas, propane, butane, light naptha, heavy naptha, kerosene, middle, distillate, light gas oil middle distillate and heavy gas oil. There apparatus boiling temperatures of fractions are listed in below in tabular form.

S.no
Fraction
Apparatus boiling range temperature °C
Remarks
1 Fuel gas -160 to -44 CH4, C2H6 and propane used as refinery fuel.
2 Propane -40 LPG.
3 Butane -12 to -30 Blended with motor gasoline to increase its volatility.
4 Light Naptha 0 to 150 Motor gasoline for catalytic to increase its volatility.
5 Heavy Naptha 150 to 200 Catalytic reforming fuel blended light gas-oil t form jet fuels.
6 Kerosene middle distillate 200 to 300 Domestic Aviation fuel.
7 Light gas oil middle distillate 200 to 315 Furnace fuel oil to diesel
8 Heavy gas oil 315 to 425 Feed for catalytic cracking
9 Vacuum gas oil 425 to 600 Feed for catalytic cracking
10 Pitch >600 Heavy fuel oil asphalt
  • Fuels are any material that store potential energy in forms, which upon burning in oxygen liberates heat energy.
  • Calorific value of fuel is the total quantity of heat liberated when a unit mass or volume of fuel is completely burnt.
  • Higher or gross calorific value (HCV) in the total amount of heat produced when a unit mass/volume of fuel has been burnt completely and the products of combustion have been cooled to room temperature (15°C or 60°F).
  • Lower or net calorific value (LCV) is the heat produced when unit mass (volume) of the fuel is burnt completely and the products  are permitted to escape.
LCV = HCV - Latent heat of water formed

Characteristics of fuels
  • Natural or primary fuels are found in nature such as wood, peat, coal, natural gas, petroleum.
  • Artificial or secondary fuels are prepared from primary fuels charcoal, coal gas, coke, kerosene oil, diesel oil, petrol, etc.
  • Fuels are further classified as
    1. Solid Fuels
    2. Liquid Fuels
    3. Gaseous Fuels
Characteristics of solid fuels
  1. Ash is high.
  2. Low thermal efficiency
  3. Form clinker
  4. Low calorific value and require large excess air.
  5. Cost of handling high
  6. Cannot be used in IC engines.

Characteristics of liquid fuels
  1. High calorific value
  2. No dust ash and clinker
  3. Clean fuels
  4. Less furnace air
  5. Less furnace space
  6. Used in IC engines

Characteristics of Gaseous fuels
  1. Have high heat content
  2. No ash or smoke
  3. Very large storage tanks are required

An ideal fuel should have the following properties:
  1. High calorific value
  2. Moderate ignition temperature
  3. Low moisture content
  4. Low NOn combustible matter
  5. Moderate velocity of combustion
  6. Products of combustion not harmful
  7. Low cost
  8. Easy to transport
  9. Combustion should be controllable
  10. No spontaneous combustion
  11. Low storage cost
  12. Should burn in air with efficiency.
Spark Ignition (SI) engine can be compared with Compression Ignition (CI) engine system in 7 aspects. Those 7 aspects are engine speed, cycle efficiency, fuel used, time of knocking, cycle operation, pressure generated and constant parameter during cycle.

S.no
Aspect
Spark Ignition Engine
Compression Ignition Engine
1 Engine speed SI engines are high speed engines. CI engines are low speed engines.
2 Cycle efficiency SI engines have low thermal efficiency CI engines have high thermal efficiency.
3 Fuel used Petrol is used as fuel, which has high self ignition temperature. Diesel is used as fuel, it has low self ignition temperature.
4 Time of knocking Knocking takes place at the end of combustion. Knocking takes place at the beginning of combustion.
5 Cycle operation SI engine works on otto cycle. CI engine works on diesel cycle.
6 Pressure generated Homogeneous mixture of fuel, hence high pressure is generated. Heterogeneous mixture of fuel, hence low pressure is generated.
7 Constant parameter during cycle Constant volume cycle. Constant pressure cycle.