Plasma arc machining

In this article, we have to discuss plasma arc machining how it works and its application and main advantages and also disadvantages of this process.

PAM : 

When a flowing gas is heated sufficiently high temperature to become partially ionized, it is known as 'plasma'. This virtually a mixture of free electrons, positively charged ions and neutral atoms.
PAM is a material removal process in which the material is removed by ionized gas of high temperature (11,000 to 30,000 C) which applied by a high-velocity jet on the workpiece.

Working principle :

The principle of plasma arc machining is in a plasma torch, known as the gun or plasma-Tron, a volume of gas such as H₂, N₂, O₂, etc. is passed through a small chamber in which a high-frequency spark (arc) is maintained between the tungsten electrode (cathode) and the copper nozzle (anode), both of which are water-cooled. In certain torches, an insert gas-flow surface rounding the main flame is provided to shield the gas from the atmosphere. The high-velocity electrons generated by arc collide with the gas molecules and produce dissociation of diatomic molecules of the gas resulting in ionization of the atoms and causing large amounts of thermal energy to be liberated. The plasma forming gas is forced through a nozzle duct of the torch in such a manner as to stabilize the arc. Much of the heating of the gas takes place in the constricted region of the nozzle duct resulting into relatively high exit gas velocity and very high core temperature up to 30,000 C. the plasma jet steams effectively blows the material and the high-velocity gas stream effectively blows the molten metal away.

Accuracy:

• This is a roughing operation to an accuracy of about 15 mm with a corresponding surface finish.
• Accuracy on the width of slots and diameter of holes is ordinary from +0.8 mm to -0.9 mm on 6 to 30 mm thick plates, and +3 mm to -3 mm on 100 to 150 mm thick plates.

Applications of PAM:

• This is chiefly used to cut stainless steel and aluminium alloys. 
• Profile cutting of metals, particularly of these metals and alloys, has been the most prominent commercial application of PAM. 
• PAM has been used successfully in turning and milling of materials which are hard and difficult to machine.

Advantages :

• The main advantages of this process are that it is almost equally effective on any metal, regardless of its hardness of refractory nature.
• There being no contact between the tool and work piece, only a simply supported work piece structure is enough.

Disadvantages :

• The main disadvantages of this process are the metallurgical change of the surface.
• Safety precautions are necessary for the operator and those in near by areas. 
• This adds additional cost. 

Difference between electrochemical series and galvanic series

The electrochemical series is an orderly listing of standard half-cell electrode potential and the galvanic series is an orderly listing of corrosion potential for different metals and alloys. They both are based on thermodynamic analysis. 

Let us have a deep insight into the discuss the electrochemical and galvanic series and the difference between them.  

Electrochemical series :

• Electrode potentials are measured by dipping pure metals in their salt solution of IM concentration, without any oxide films on them.
• The position of a metal in the electrochemical series is fixed.
• It gives no information regarding the positions of alloys.
• The position of metal is permanently fixed in this series.
• this series comprises metals and non-metals.
• It predicts the relative displacement tendencies.
• It is absolute.
• It is quantitative.
• it is a series only for pure metals.
• It is used for theoretical calculations.

Galvanic series :

• This series was developed by studying corrosion of metals and alloys in unpolluted seawater, without their oxide films, if any removed.
• In galvanic series, the position of a given metal may shift.
• Their corrosion can be studied from this series since alloys are included in galvanic series
• The position of metal, when present in the form of an alloy, is different from that of pure metal.
• This series comprises metals and alloys.
• It predicts the relative corrosion tendencies.
• It is relative.
• It is qualitative.
• It is a series for pure metals and alloys also.
• It is used for practical applications.

For video description >> https://www.youtube.com/watch?v=QzGcI2R93jk&t=15s

Difference between orbit and orbital

The orbit and orbit of the two comparable terms often confuse individuals. You must have seen in pictures that an atom is simple, in which electrons are like our planets revolving around a nucleus which may be considered as our sun. Actually, the truth is much more complex. 

Orbits and orbital are different paths of atoms. In this article, let us have a deep insight into the difference between those two similar types of term orbit and orbital.

Orbit :

• Orbit is a fixed circular path, described by a moving electron around a nucleus.
• There is an orbit for any two bodies with a certain mass.
• It has a two-dimensional representation.
• The position and velocity of a moving electron, in an orbit, can be calculated with great accuracy.
• The distance of the orbit from the nucleus for a given electron is fixed.
• The movement of an electron in an orbit is known definitely.
• The maximum capacity of any orbit is 2n² electrons, where n is the number of orbits.
• All orbits are circular and disc-like.
• The concept of the well-defined orbit is against Heisenberg's principle. 
• Orbits are non-directional which is why they can't shape of the molecule.
• An orbit shows an exact position of an electron within an atom.
• Orbits are the definite paths that a moving object traces out.

Orbital :

• Orbital is a three-dimensional region in space around the nucleus, where there is a maximum probability of finding an electron.
• An orbital exists only for an atom and an electron.
• It has a three-dimensional representation.
• The position and velocity of moving electrons, in an orbital, can be determined within the limits of Heisenberg's uncertainty principle.
• It is impossible to know the exact trajectory of an electron in an orbital of an atom.
• Only the probability of finding an electron can be traced with the help of Heisenberg's principle.
• The maximum capacity of any orbital is two electrons.
• Orbital may be spherical (s), dumb-bell (p) and so on in shape.
• The concept of the well-defined orbital is in agreement with Heisenberg's principle.
• Orbitals have a directional character that is why they can account for the shapes of molecules.
• It does not specify an exact position of an electron.
• Orbitals are the regions where the object is likely to be found or move about.

Explore more information: 

1. Difference between PSLV and GSLV

Difference between galvanizing and tinning

Galvanizing is an iron or steel sheet coating process with a thin zinc coat to avoid rusting. Tinning is a coating tin over parts of iron or steel. Both the coating method is used to avoid corrosion. Let us have a deep insight into the difference between galvanizing and tinning. 

Galvanizing :

• Zinc is used for coating.
• Since this method is more electropositive than iron, zinc sacrificially protects the iron and does not allow the iron to pass into the outside solution.
• Zinc continues to protect the underlying iron through galvanic cell action in the galvanized article, even if the zinc layer is punctured or broken somewhere.
• Galvanized containers can not be used to store acidic foods as zinc reacts with extremely toxic zinc-forming food acids.

Tinning :

• Tin is used for coating.
• Due to its noble nature and higher corrosion resistance, Tin protects the base metal, iron from corrosion.
• Tin is non-toxic and protects the iron at the base until the coating is perfect. Any coating break creates fast iron corrosion.
• Tin-coated containers can be used to store any food as tin is non-toxic, protecting the metal from corrosion, and preventing food poisoning.

Difference between addition polymerization and condensation polymerization

There are mainly two types of polymerization. Let us check it out both below. 

What is Addition polymerization?

It is a reaction that produced a product, which is an exact multiple of the original monomeric molecule.

It is also called a chain polymerization.

• It requires two like molecule. 
• Kinetic long linear chain reaction.
• Very fast reaction time. 
• The polymer produced thermoplastic.
• Example - Polythene, Orlon

What is Condensation polymerization?

It is a reaction occurring between simple polar group-containing monomers with the formation of polymer and eliminating of small molecules like water and HCL etc.

• It requires two, unlike molecule. 
• Intermolecular reaction. 
• Slow reaction time. 
• Chemo setting plastic produced. 
• Example - Decron, Bakelite, Nylon 6

Let us have a deep insight into the difference between addition and condensation polymerization. 

Difference : 

• In addition polymerization, only growth reaction adds repeating units one at a time to the chain while in condensation polymerization any two molecular species present can react.
• The number of units decreases steadily throughout the reaction in case of addition polymerization while monomer disappears early in reaction in case of condensation polymerization.
• In case of additional polymerization, a high molecular mass polymer is formed at once, while in the case of condensation polymerization, polymer molecular mass rises steadily throughout the reaction.
• Longer reaction times have a little effect on molecular-weight but give higher yields in case of addition polymerization while in case of condensation polymerization to obtain high molecular-weight and longer reaction time is essential.
• The reaction mixture contains only monomers, high polymers and about 10^-8part of growing chains in case of addition polymerization while all type of molecular species is present at any stage in case of condensation polymerization.

What is calorific value

In this article, we have to discuss the calorific value and different types of units of calorific value and some basic information about that. First of all, we want to know 

What is calorific value?

• The calorific value of a fuel is the total quantity of heat liberated when a unit mass of the fuel is burnt completely. 

Units of calorific value :

Units of calorific value is calories per gram or kilocalories per kg this both used mostly in case of solid or liquid fuel.

In the case of gaseous fuels the unit used is kilocalorie per meter.

It is usually denoted by joules per kilogram.

Calorific value mainly two types :

1. Higher or gross calorific value 
2. Lower or net calorific value 

Advantages and disadvantages of liquid fuels

Liquid fuels are combustible or energy-generating molecules that can be harnessed to generate mechanical energy, generating usually kinetic energy, they also have to take their container shape. Instead of the fluid, the fumes of liquid fuels are inflammable. Let us have a deep insight into the pros and cons of liquid fuels in this article. 

Advantages of liquid fuels :

• They possess higher calorific value per unit mass than solid fuels.
• They burn without forming dust, ash, clinkers etc.
• Their firing is easier and also fire can be extinguished easily by stopping the liquid fuel supply.
• They are easy to transport through pipes.
• They can be stored indefinitely without any loss.
• The flame produced by burning liquid fuels can easily be controlled by adjusting the liquid fuel supply.
• Liquid fuels are generally handled by pipes and one man can easily regulate a large number of furnaces simultaneously.
• They are clean in use and economic in labour.
• Loss of heat to the chimney is very low due to greater cleanliness.
• They require less excess of air for complete combustion.
• They require less furnace space for combustion.
• There is no wear and tear of grate bars and cleaning of fires, unlike solid fuels.
• They can be used as internal combustion fuels.

Disadvantages of liquid fuels :

• The cost of liquid fuel is relatively much higher as compared to solid fuels.
• Costly special storage tanks are required for storing liquid fuels.
• There is a greater risk of fire hazards, particularly in the case of highly inflammable and volatile liquid fuels.
• Liquid fuels give bad odour.
• For efficient burning of liquid fuels, specially constructed burners and spraying apparatus are required.
• Choking for sprayers is a drawback of oil firing.

Advantages and disadvantages of solid fuels

Solid fuel is the various forms of solid material that can be burnt to release energy, providing heat and light through the process of combustion.

The solid fuels find little practical application at present because of the problems in handling the fuel as well as in disposing of, the solid residue or ash after combustion. 

Solid fuels compared to gaseous and liquid fuels, solid fuels are quite difficult to handle and storage and feeding are quite cumbersome. Let us have a deep insight into the pros and cons of solid fuels in this article. 

Advantages of solid fuels :

• They are easy to transport.
• They are convenient to store without any risk of spontaneous explosion.
• Their cost of production is low.
• They possess moderate ignition temperature.

Disadvantages of solid fuels :

• Their ash content is high.
• They burn with clinker formation.
• They combustion operations cannot be controlled easily.
• Their cost of handling is high.
• Their large proportion of heat is wasted during combustion.
• Their thermal efficiency is low.
• Their calorific value is lower as compared to that of liquids fuels.
• They require a large excess of air for complete combustion.
• They cannot be used as internal combustion engine fuels.

Characteristics of a good fuel

A good characteristic of good fuel is depending upon the following factors :

• High calorific value 

A fuel should possess high calorific value since the amount of heat liberated and temperature attained thereby depends upon the calorific value.

• Moderate ignition temperature 

Ignition temperature id the lowest to which the fuel must be pre-heated so that it starts burning smoothly.
Low ignition temperature is dangerous for storage and transport of fuel, hence it can cause fire hazards. While high ignition temperature causes difficulty in igniting the fuel, but fuel is safe during storage and transport. Hence, an ideal fuel should have moderate ignition temperature.

• Low moisture content 

The moisture content of the fuel reduces the heating value and involves a loss of money because it is paid for at the same rate as the fuel. Hence, fuel should have low moisture content.

• Low non-combustible matter content

After combustion, the non-combustible matter content generally in form of ash or clinker.
That also reduces the heating value, besides the additional cost of storage, handling and disposal of the waste products produced.

• The moderate velocity of combustion 

If the rate of combustion id low then the required high temperature may bot be possible, because a part of the heat liberated may get radiated, instead of raising the temperature. While too high combustion rates are also not required.

• Products of combustion should not be harmful 

Gaseous products of combustion should not pollute the atmosphere. 

• Low cost 

A good fuel should be readily available in bulk at a cheap rate.

• Easy to transport 

Fuel must be easy to handle, store and transport at a low cost must be necessary. Solid and liquid fuel can easily be transported but gaseous fuel is costly and can even cause fire hazards.

• Combustion should be easily controllable 

Combustion of the fuel should be easy to start or stop. when it is required. Hence, it must be controllable.

• Should no undergo spontaneous combustion 

Spontaneous ignition can cause fire hazards.

• Storage cost in bulk should be low 
• Should burn in air with efficiency, without a highly smoke
• In case of solid fuel, the size should be uniform so that combustion is regular

VTVT engine technology

In this article we will discuss about the VTVT engine techology. So let first you must know the VTVT full form.

VTVT is a technology which has ability to have independent control of the intake and exhaust valve in an internal combustion engine.

VTVT engine technology gives you class mileage and optimum performance for driving experience.

In this technology the timing of intake and exhaust will be depend upon the engine requirement. The timing of both valve will be programmed in different load criteria that the same as Hyundai do in VVT technology.

Function of lubricants

To supply lubricating oil between the moving parts is simply termed as lubrication.

Any substance introduced between two moving or sliding surface with a view to reducing the frictional resistance between them is called lubricants.

Functions of lubricants :

• It reduces surface deformation, wears, and tear because the direct contact between the rubbing surfaces is avoided.
• It reduces the loss of energy in the form of heat.
• It also acts as a coolant.
• It reduces waste of energy so that the efficiency of a machine is enhanced.
• It reduces the expansion of metal by local frictional heat.
• It avoids the seizure of moving surfaces since the use of lubricant minimizes the liberation of frictional heat.
• It avoids or reduces unsmooth relative motion of the moving or sliding parts.
• It reduces the maintenance and running cost of the machine.
• Sometimes it acts as a seal.

Difference between physical adsorption and chemical adsorption

What is Physical adsorption?

It is one in which the adsorbed molecules are held to the surface of the adsorbent by weak physical or van der Waal's forces.

This types of adsorption are completely reversible.

It is also called physisorption or van der Waal's adsorption.

Example: Adsorption of various gases by charcoal belongs to this type.

What is Chemical adsorption?

It is the force which holds the adsorbed molecules is of valence type.

This type of adsorption is irreversible.

It is also called chemisorption or activated adsorption.

Example: Adsorption of oxygen on tungsten is an example of this type.

Let us have a deep insight into the difference between physical and chemical adsorption. 

Difference :

• Chemical adsorption heat evolved is considerably higher than evolved in physical adsorption.
• The heat of adsorption for physical adsorption is 20-40 kcal/mol while 40-400 kcal/mol for chemical adsorption.
• In physical adsorption, molecules are not tightly retained by the adsorbent while molecules are tightly retained by the adsorbent in case of chemical adsorption.
• Adsorption is appreciable only at a temperature below the boiling point of the adsorbate in case of physical while in chemical it occurs at high temperature.
• Multiplayer adsorption occurs in case of physical adsorption while adsorption leads to at most a monolayer in case of chemical adsorption.
• Forces responsible for physical adsorption are very weak while in the case of chemical adsorption it is quite strong.
• In physical adsorption, the rate of adsorption increases with the increase of pressure or of the adsorbate while in case of chemical adsorption rate of adsorption decrease with the increase of pressure or concentration of adsorbate.
• Physical adsorption involves very small or little activation energy while chemical adsorption generally involves appreciable activation energy.
• The equilibrium is established rapidly in a physical case while the establishment of equilibrium requires time in case of chemical adsorption.
• No surface compound takes place in case of physical adsorption while actual surface compound formation between the adsorbent and adsorbate takes place in chemical adsorption.
• Physical adsorption is very specific in nature while chemical adsorption is highly specific in nature.
• Amount of adsorption on a surfaced is more a function of the adsorbate than the adsorbent in case of physical adsorption while the amount of adsorption is characteristics of both adsorbate and adsorbent in chemical adsorption.

You can also check it out : 

• Difference between adsorption and absorption

Difference between adsorption and absorption

Absorption is the process in which a fluid is dissolved by a liquid or a solid whereas adsorption is the process in which atoms, ions or molecules from a substance adhere to a surface of the absorbent. Let us have a deep insight into the difference between adsorption and absorption. 

Adsorption :

• It is a phenomenon of concentration or assimilation of gas or a liquid at the surface of a solid or liquid.
• It is a surface phenomenon.
• It is a fast process.
• Equilibrium is attained easily.
• It depends upon the surface area of the adsorbent. 
• It is an exothermic process.
• It is favoured by low temperature.
• It is steadily increasing and reaches equilibrium.
• It is used in Air conditioning, water purification, synthetic resin. 

Examples : 

• Water vapours adsorbed by silica gel.
• NH₃ is adsorbed by charcoal.

Absorption :

• It is a phenomenon in which the substance assimilated is uniformly distributed throughout the body of the solid or liquid.
• It is a bulk phenomenon.
• It is a slow process.
• Attainment of equilibrium takes some time.
• No such effect is there.
• It is an endothermic process.
• It is not affected by temperature.
• It occurs at a uniform rate.
• It is used in cold storage, ice production, turbine inlet cooling, refrigerant. 

Examples :

• Water vapours absorbed by anhydrous CaCl_2.
• NH₃ is absorbed in water forming NH₄OH.

You can also read about the difference between physical adsorption and chemical adsorption.

Disadvantages of hard water

Disadvantages of hard water in Domestic use :

• Washing :

When we use hard water as a washing purpose the using water does not lather freely with soap and it produces sticky precipitates of calcium and magnesium soaps. This cause wastage of a lot of shops is used. 

• Bathing : Bathing

Hard water does not lather freely with shop solution and produces sticky scum on the body.
Thus, cleansing quality of soap is depressed and lots of water is wasted too.

• Cooking :

Due to the presence of dissolved hardness, the boiling point of water is elevated and it producing slats. Therefore, more fuel and time are required for cooking.

• Drinking :

Hard water causes a bad effect on our digestive system and also the possibility of forming calcium oxalate crystals in urinary tracks is increased.

Disadvantages of hard water in Industrial use : 

• Textile industry :

Hard water cannot produce good quality of lathe. It precipitates of calcium and magnesium soaps adhere to the fabrics. These fabrics when dyed do not produce exact shades of colour.

• Sugar industry :

If hard water used in sugar refining it causes difficulties in the crystallization of sugar and the produced sugar may be deliquescent.

• Dyeing industry :

Because of the use of hard water which yields impure shades and gives spots on the fabrics being dyed.

• Laundry :

Hard water is used in laundry causes much of the soap used in washing and may even cause colouration of clothes.

• Concrete making :

It affects the hydration of cement and the final strength of the hardened concrete.

• Pharmaceutical industry : 

If hard water used for preparing pharmaceutical products like drugs or injection may produce certain undesirable products in them.

Disadvantages of hard water in steam generation in boilers :

If hard water directly fed into the boilers it may cause many troubles following below :

• Scale and sludge formation 
• Corrosion 
• Priming and foaming 
• Caustic embrittlement 

What is flywheel

What is Flywheel?

Answer :

• A heavy revolving wheel in a machine which is used to increase the machine's momentum and thereby provide greater stability or a reserve of available power.

A flywheel is a heavy rotating body that acts as a reservoir of energy. 

Flywheel used in an internal combustion engine to control the speed.

The energy is stored in the flywheel in the form of kinetic energy. 

The function of flywheel :

• To store and release energy when needed during the work cycle.
• To reduce the power capacity of the electric motor or engine.
• To reduce the amplitude of speed fluctuations.

Flywheel materials :

Flywheel made by grey cast iron. 

Now recently, flywheels are made of high strength steels and composites in-vehicle applications. 

Graphite-fiber reinforced polymer ( GFRP ) is one of the excellent choices for flywheels fitted on a modern car engine.

Difference between herringbone and double helical gears

Herringbone and double helical gears are constructed by combining two identical helical gears of the same pitch circle diameter, module, and a number of teeth, but both gears have an opposite hand of a helix. This offers an advantage of balancing the axial thrust in both the direction by developing opposite thrust reaction providing higher power transmission capacities. 

What is herringbone gear?

No gap is provided between two halves so teeth with left-hand helix touch the teeth with right-hand helix. Its manufacturing is difficult and requires a dedicated machine. 

What is double helical gear?

Small relief gap is provided between two halves so teeth with left-hand helix do not touch the tooth with right-hand helix. Its manufacturing is comparatively easy as it can be cut by hobbing, shaping and even by milling. 

Let us have a deep insight into the difference between herringbone and double helical gears. 

Difference :

• In the case of double helical gears, there is a groove between two helical gears, while herringbone gears are called a gear without a groove.
• A double helical gear is cut on a single gear blank, by a hob with a tool run-out groove between the hands of helices whereas herringbone gear is cut by two cutters, which reciprocate 180⁰ out of phase to avoid clashing. 
• Both the gears allow higher helix angle because there is no thrust force and angle of helix is about to 20⁰ to 45⁰.
• Herringbone gear has less axial strength thus can be used where space is limited whereas double-helical gear requires more axial space due to the presence of relief gap. 

Double helical gear advantages and disadvantages

Advantages of double helical gear :

• They develop opposite thrust reactions thus cancel out the thrust force within the gear itself.
• The net axial force that acts on the bearings is zero.
• Power transmitting capacity is very high.
• By using this high pitch line velocities can be attained.

Disadvantages of double helical gear :

• A double helical gear is expensive as compared to helical gear.
• Balancing of thrust forces depends upon the equal distribution of load between the right and left parts of the gear. Therefore, a high degree of precision is required to locate double-helical gears axially on the shaft.
• No external forces are present.

Where does LPG comes from?

Where does LPG come from?

• Natural gas processing 
• Petroleum refining

LPG comes from drilling oil and gas wells because above both processes are comes during drilling oil and gas wells.

LPG is not made or manufactured separately it is found in combination with other hydrocarbons naturally during the two processes mentioned above.

About 60% of LPG is produced from natural gas and the other 40% during the petroleum refining.

LPG is separated from unprocessed natural gas using refrigeration and LPG is extracted from heated crude oil using a distillation tower.

LPG is stored as a liquid under pressure in gas bottles or in a tank.

Hyundai VTVT full form

What is the full form of VTVT?

Answer: 

• Variable Timing Valve Train

What does VTVT mean?

Hyundai has the technology applied to there Hyundai models are mentioned below :

• Hyundai Verna 
• Hyundai i10
• Hyundai i20 

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• CNC full form

• VMC full form

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• DFI full form

• GVWR full form

• HVAC full form

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Spur gear backlash

In this article, we have discussed the backlash of the spur gear. so let us know first 
What is the backlash ?

• Backlash is defined as the amount by which the width of tooth space exceeds the thickness of the engaging tooth measured along the pitch circle.

Backlash is the play a vital role in spur gear designing it plays between the missing teeth and it occurs only when teeth are in mesh.

Why provide backlash?

• Backlash compensates for machining errors 
• Backlash compensates for thermal expansion of teeth.
• Backlash prevent the mating teeth from jamming together.

Methods to provide backlash :

• The centre distance between mating gears is slightly increased.
• The teeth of the gear are cut slightly thinner.

The magnitude of backlash is depended upon the diametral pitch or module and the centre distance.

The magnitude of backlash is very small for gear trains used in precision equipment and instruments.

The backlash and variation in centre distance have no effect on tooth action or velocity ratio.

Application of rolling contact bearing

Rolling contact bearings are used in the following applications.

• Machine tool spindles 
• Automobile front and rear axles 
• Rope sheaves 
• Crane hooks
• Hoisting drums
• Gearboxes 

Application of sliding contact bearing

Sliding contact bearings are used in the following applications :

• Centrifugal pumps 
• Large size electric motors 
• Steam and gas turbine
• Crankshaft bearings in petrol and diesel engine 
• Concrete mixtures
• Rope conveyors 
• Marine industries in installation 

Difference between LPG and CNG

LPG full form Liquefied petroleum gas is a mixture of propane and butane liquefied at 15⁰C and a pressure of 1.7 to 7.5 bar While CNG full form Compressed Natural Gas which is mainly methane compressed at a pressure of 200 to 245 bar.

Now you need to select the best option that would serve as your best friend in your daily activities. So now we have discussed the difference between LPG and CNG that help you decided in which the best for you.

Basic Difference between LPG and CNG:

Abbreviation:

LPG - Liquefied petroleum gas

CNG - Compressed natural gas

Gas:

LPG - Artificial in nature.

CNG - It is natural gas.

Constituents:

LPG - Composed of propane and butane. 

CNG - Major component is methane.

Source:

LPG - Automatically generated while natural gas extraction and also while crude oil refining process.

CNG - Obtained from natural gas, condensate wells, oil well, coal bed methane wells.

Safety:

LPG - It is difficult to get disperse so that the risk of fire is more.

CNG - Easily disperses so that the risk of ignition is minimized.

Environmental Effect:

LPG - Release gas like CO₂ which is greenhouse gas serves as a great cleaner comparing to gasoline.

CNG - Release the minimum amount of greenhouse gases.

Uses:

LPG uses - Heating and cooking in homes, refrigeration, industrial, agricultural, catering, and automobile fuel.

CNG uses - Alternative of gasoline in Automobile.

Properties:

LPG Properties -

• It is highly inflammable.
• It is heavier than air.
• Leakage will accumulate in low lying areas and settle down to the floor.

CNG properties - 

• It is highly inflammable.
• It is lighter than air.
• Disperses quickly during leakage.

Some key difference between LPG and CNG:

• LPG has a higher energy content than CNG.
• LPG density is greater than CNG.
• There is a number of hydrocarbon gases that fall into the category of LPG while in CNG just two most common properties propane and butane are used.
• LPG needs more oxygen as compared to CNG.

LPG requires oxygen to gas ratio - 25 to 1

CNG requires oxygen to gas ratio - 10 to 1 

Explore more information: 

1. Advantages and disadvantages of LPG
2. Which gas present in LPG?
3. What does LPG stand for? 
4. What are the uses of LPG?
5. What is the full form of LPG?
6. LPG acronyms and Abbreviations
7. Difference between petrol and diesel oil