Introduction to Mechanical Engineering 


               

Who are mechanical engineers? What are the main subjects that comprise typical?

mechanical engineering curriculum.

Ans: Mechanical engineers analyze their work using the principles of motion, energy, and force—ensuring that designs function safely, efficiently, and reliably, all at a competitive cost.

The main subjects that comprise typical mechanical engineering curriculum.

       I.          Heat transfer, thermodynamics and energy system.

     II.          Combustion and the environment.

    III.          Bio-medical and engineering fluid mechanics.

   IV.          Manufacturing engineering.

     V.          Ground vehicle system.

   VI.          Mechanical design.

 VII.          Transportation system.

 

Define following terminologies (any three):

Stored energy ii) Specific heat ii) Enthalpy iv) Heat Engine.

Stored energy: Potential energy is stored energy. stored energy in any form, including chemical, gravitational and electrical energy.

Specific heat: The quantity of heat required to raise the temperature of one gram of a substance by one Celsius degree.

Heat engine: Heat engine is a device that convert heat to work.

 

Do mass is an intensive property or extensive property? Why?

Intensive: these properties don’t depend on the mass or quantity of the system

Example:

         I.          Boiling point.

       II.          Density.

      III.          Specific volume.

     IV.          Freezing point temperature.

 

 

 

Extensive: The properties of the system that depend on the mass or quantity of the system.

Example of extensive properties are

Volume, pressure, mass, internal energy etc.

 

What is an isothermal process? Write an expression for the work done during an

isothermal process.

 

Isothermal process: The isothermal process which is thermodynamic process in which the temperature of a system remains constant.

W=[P1V1-P2V2/Y-1]

 

Write the differences among Pump, Compressors & Blower.

Blower:

·        A blower is a machine for moving volumes of a gas with moderate increase of pressure.: a more powerful fan, if you will. By changing the angle of the blades, a blower will be able to push air in any direction you want it.

Compressor:

·        A compressor is a machine for raising a gas – a compressible fluid – to a higher level of pressure., actually making the air denser by cramming air into a small space.

Pump:

·        A pump is a machine for raising a liquid – a relatively incompressible fluid – to a higher level of pressure or head.

 

 With the help of P-V and T-S diagram write down differences between Otto Cycle and

Diesel Cycle.

 

l No.

Otto Cycle

Diesel Cycle

1.

Otto cycle has low thermal efficiency.

The diesel cycle has high thermal efficiency.

2.

It has a low compression ratio.

But This one works on a high compression ratio.

3.

Otto cycle is also called a Constant volume cycle.

The diesel cycle is called a constant pressure cycle.

4.

Otto cycle system is light in weight. But

The diesel cycle is heavy in weight.

5.

The explosion process takes place at constant volume process and

In the diesel engine, the explosion process takes                          place at a constant pressure process.

6.

The Spark Plug is used here for igniting the charge.

A fuel injector is used here for igniting the charge.

7.

A mixture of air and fuel is entered in suction stroke.

Only air is entered in suction stroke.

8.

Here the working fuel is used as Petrol.

The working fuel is used as Diesel.

 

What is meant by saturation temperature and saturation pressure?

Saturation temperature: The temperature at which vaporized (boiling) starts to occur for a given pressure is called saturation temperature.

Saturation pressure: The pressure at which vaporized starts to occur for a given temperature is called saturation pressure.

Explain how the Wet steam, dry saturated steam and superheated steam is produced.

Wet steam: Wet steam generated by heating water which then turn to steam at the first stage. If additional heat is added to wet steam at a set pressure, temperature remains the same unit of the liquid is evaporated. Only then does the rise above saturation allowing the formation of wet steam.

Dry steam: Dry steam results when the water is heated to the boiling point and then vaporized with additional heat.

Superheated steam: If dry steam is further heated above the saturation point it become superheated steam.

 

 

 

What is a steam boiler? How they are classified?

A steam boiler is a device used create steam by applying heat energy to water.

Purpose of steam boiler

·      For generating power in steam engine and steam turbine.

·      In process industries for various processes.

 

 

 

 

Mainly two types of steam boiler

Ø  Fire tube boiler: A fire tube boiler is a type of boiler in which the water is flowing outside the tube and hot gases which are produced by the combustion chamber are inside the tube.

Ø  Water tube boiler: A water tube boiler is a type of boiler in which the water is flowing inside the tube and hot gases which are produced by the combustion chamber are outside the tube.

 

What are the differentiating features between a water tube and a fire tube boiler?

Water tube boiler

Fire tube boiler

Water flow inside the tube and flue gases outside the tube.

Hot flue gases flow inside the tube and the water outside the tube.

It is suitable for larger power plant.

It is not suitable for larger power plant.

The boiler pressure limited up to 165 bar.

The boiler pressure limited up to 24 bar.

The water tube boiler has a higher rate of steam production.

The fire tube boiler has a lower rate of steam production.

The bursting chances are more.

Bursting chances are less.

 

What is the boiler mounting and accessories? 

Boiler mounting: Boiler mounting are a set of safety device installed for the safe operation of a boiler.

Boiler mounting devices are

·       Water level indicator: The function of a water level indicator is to indicate the level of water in the boiler constantly.

·       Safety valve: The function of safety valve is to safeguard the boiler from the danger of pressure higher the safe working pressure.

·       Pressure gauge.

·       Stop valve.

·       Fusible plug: The main purpose of a fusible plug is to extinguish the fire in the furnace of the boiler when the water level in the boiler falls below an unsafe level.

 

Boiler accessories: Boiler accessories are not main part of the boiler. It is used to increase the efficiency of the boiler.

Boiler accessories are

·       Feed pump.

·       Feed water tank.

·       Super heater: The superheater is needed for converting the wet steam into dry steam. Superheater is an important part of the boiler in the power plant.

·       Economizer: In a boiler system, an economizer captures the heat of the flue gas and transfers the heat it to the feed water.

·       Anti-scale dosing pump.

·       Electronic pressure switch.

·       Oil pump.

 

What is the difference between internal combustion and external combustion engines?

I.C Engine

E.C Engine

The Internal combustion engine of fuel takes place inside the working cylinder.

The external combustion engine of fuel takes place outside the working cylinder.

The working fluid used in the IC engine is maybe petrol, diesel, gases.

The working fluid used in the EC engine is maybe steam.

The capital cost of the internal combustion (IC) engine is relatively low.

The capital cost of the external combustion (EC) engine is relatively high.

Fuel cost of the IC engine is relatively high.

Fuel cost of the EC engine is relatively low

IC engine requires less space.

EC engine requires large space than the IC engine

Thermal efficiency is high.

Thermal efficiency is low.

Starting of the IC engine is easy and quick.

Starting of the EC engine requires time.

 

Differentiate between petrol and diesel engine?

Diesel engine

Petrol engine

These engine work on the diesel cycle.

These engine work on the otto cycle.

The fuel is mixed with air inside the cylinder.

The fuel and air are mixed in a carburetor.

Ignition is achieved with the help of the hot compressed air.

Ignition is achieved with the help of the electric spark.

High compression ratio.

Relatively low compression ratio.

Low fuel cost.

High fuel cost.

High power produce.

Relatively low amount of power is produced in a petrol engine.

Generally used in heavy vehicles such as trucks and buses.

Petrol engine used in light vehicles such as motorcycle, car

High maintenance cost.

Comparatively low maintenance cost.

 

 

What is the gas turbine? Classification of the gas turbine?

Gas turbine: Gas turbine is a combustion engine at the heat of a power plant, that can convert natural gas or other fuels to mechanical energy.

Classifications of the gas turbine following are

·        Closed cycle gas turbine.

·        Open cycle gas turbine.

·        Sim- closed gas turbine.

Difference between Gas turbine and Steam turbine?

Difference between Gas turbine and steam turbine following are

Gas turbines

Steam turbines

Its important components are compressor and combustion chamber.

Its important components are accessories and steam boiler.

Mass of gas turbine per kw is less developed.

Mass of steam turbine per kw is more developed.

For installation is less space.

For installation is more space.

Its cost of installation is less.

Its cost of installation is more.

It is easy to start gas turbine.

It is difficult to start steam turbine.

It is easy to control it.

It is difficult to control it.

It does not depend on water supply.

It depends on water supply.

 

List the methods of improving the efficiency and specific output of a simple gas turbine?

Methods to improve the efficiency of the gas turbines are

Regeneration: This is done by preheating compressed air before entering to the combustion chamber with the turbine exhaust in a heat exchanger.

Reheating: The whole expansion in the turbine is achieved in two or more stages and reheating is done after each stage that increase work done.

Intercooler: The compression is performed by two or more stage but between two stages there is intercooler where the cooling take place at constant pressure. To increase network of gas turbine by saving some compression work.

 

What is the unit of refrigeration?

The unit of refrigeration is expressed in terms of ton of refrigeration TR. One ton of refrigeration defined as the amount of refrigeration effect produced during uniform melting of one ton of ice 0°C to the water 0°C in 24 hours.

 

 

 

What is the Coefficient of performance of a refrigeration (COP)?

The coefficient of performance of a refrigerator is defined as the heat remove from cold reservoir (inside the refrigerator) divided by the work done to remove the heat (work done by the compressor).

 

COP= Q/W

Q= Amount the heat remove in the refrigerator.

W= Amount of work done.

 

Draw the layouts of a vapor compression refrigerating system. State the function of each of the component and show the thermodynamic processes on a pressure-enthalpy diagram.

 

 


Vapor compression system: The vapor compression refrigerant cycle as a refrigerant circulated through the system it is alternately compressed and expanded, changing its state from a liquid to a vapor. As the refrigerant changes state, heat is absorbed and expelled by the system, lowering the temperature of the conditioned space.

Stage-1 compressor: in the first stage of the refrigerant system, refrigerant enters the compressor as low pressure vapor. The compressor compressed the refrigerant high pressure and high temperature vapor.

Stage-2 condenser: After leaving the compressor the hot vapor refrigerant enters the condenser and flows through the series tubes. The while refrigerant passing through the condenser gives up its latent heat to the surroundings condensing medium which is normally water and air. Which then change its state to high pressure liquid.

Stage-3 Expansion valve: Its functions are to allow the liquid refrigerant to pass through high pressure and temperature, where it reduces its pressure and temperature.

Stage-4 Evaporator: Its also includes tube coils in which liquid- vapor refrigerants are evaporated at low pressure and temperature and to converted to vapor refrigerants at low pressure and temperature.