TURBOCHARGER

TURBOCHARGER

A turbocharger,is a turbine-driven forced induction device that increases an internal combustion engine's efficiency and power output by forcing extra air into the combustion chamber.

The key difference between a turbocharger and a conventional supercharger is that a supercharger is mechanically driven by the engine, often through a belt connected to the crankshaft, whereas a turbocharger is powered by a turbine driven by the engine's exhaust gas. Compared to a mechanically driven supercharger, turbochargers tend to be more efficient, but less responsive. Twincharger refers to an engine with both a supercharger and a turbocharger.

Turbochargers are commonly used on truck, car, train, aircraft, and construction equipment engines. They are most often used with Otto cycle and Diesel cycle internal combustion engines. They have also been found useful in automotive fuel cells

DIRECT PETROL INJECTION

DIRECT PETROL INJECTION

This diagram shows the layout of the Bosch direct injection system. Direct injection systems differ from conventional port injection in several ways.

The fuel supply system uses two fuel pumps – a conventional electrical fuel pressure pump (in the past dubbed a high pressure pump but now referred to in this system as a low pressure pump) and a mechanically-driven high pressure pump. The low pressure pump works at pressures of 0.3 – 0.5 MPa while the high pressure pumps boost this very substantially to 5 – 12 MPa.

The high pressure fuel is stored in the fuel rail that feeds the injectors. The fuel rail is made sufficiently large that pressure fluctuations within it are minimised as each injector opens. The pressure of the fuel in the injector supply rail is controlled by an electronically-controlled bypass valve that can divert fuel from the high pressure pump outlet back to its inlet. The fuel bypass valve is varied in flow by being pulse-width modulated by the Electronic Control Unit (ECU). A fuel pressure sensor is used to monitor fuel rail pressure.

This diagram shows a cross-sectional view of an injector. Compared with a conventional port fuel injection system, the fuel injectors must be capable of working with huge fuel pressures and also injecting large amounts of fuel in very short periods. The reason for the much reduced time in which the injection can be completed is due to the fact that all the injection must sometimes occur within just a portion of the induction stroke. Conventional port fuel injectors have two complete rotations of the crankshaft in which to inject the fuel charge – at an engine speed of 6000 rpm, this corresponds to 20 milliseconds. However, in some modes, direct fuel injectors have only 5milliseconds in which to inject the full-load fuel. The fuel requirements at idle can drop the opening time to just 0.4 milliseconds. Direct injection fuel droplets are on average only one-fifth the droplet size of traditional injectors and one-third the diameter of a human hair.

The very lean air/fuel ratios at which direct injection systems can operate results in the production of large quantities of oxides of nitrogen (NOx). As a result, direct injected cars require both a primary catalytic converter fitted close to the engine, and also a main catalytic converter - incorporating a NOx accumulator - that is fitted further downstream.

MULTIPOINT FUEL INJECTION

MULTIPOINT FUEL INJECTION

Fuel injection is a method or system for admitting fuel into the internal combustion engine.There are basically two ways of admitting fuel one is through carburetor and other is fuel injection system.But presently the most used injection system are MPFI in petrol engine and CRDI in diesel engine.

Multi point fuel injection is a system wherein fuel is injected into individual cylinders based on commands from "on board engine management system computer"-popularly known as Engine control unit.ECU receives feed back from several sensors like engine speed sensor, fly wheel position sensor, vehicle speed sensor, atmospheric temp. sensor, accelerator pedal position sensor intake airflow sensor. Then ECU control the correct amount of fuel to be injected and the proper time at which the fuel will be injected at any speed and load condition. This ensure maximum power output at minimum fuel.


MPFI systems are of three types, first is BATCHED in which fuel is injected to the cylinders in groups, without precisely bringing together to any particular cylinder’s intake stroke, the second one is simultaneous in which fuel is injected at the same time to all the cylinders and the third one is sequential in which injection is timed to coincide with each cylinder’s intake stroke.

ADVANTAGES OF MPFI

• More uniform air-fuel mixture will be supplied to each cylinder, hence the difference in power developed in each cylinder is minimum. 
• The vibrations produced in MPFI engines is very less, due to this life of the engine component is increased. 
• No need to crank the engine twice or thrice in case of cold starting as happen in the carburetor system. 
• Immediate response, in case of sudden acceleration and deceleration. 
• The mileage of the vehicle is improved. 
• More accurate amount of air-fuel mixture will be supplied in these injection system. As a result complete combustion will take place. This leads to effective utilization of fuel supplied and hence low emission level. 

RACK AND PINION GEAR

RACK AND PINION GEAR

                   Rack and pinion gears are used to convert rotation into linear motion.The circular pinion engages teeth on a flat bar - the rack. Rotational motion applied to the pinion will cause the rack to move to the side, up to the limit of its travel.The rack and pinion arrangement is commonly found in the steering mechanism of cars or other wheeled, steered vehicles.A good example of a ‘rack and pinion’ gear system can be seen on trains that are designed to travel up steep inclines. The wheels on a train are steel and they have no way of griping the steel track. Usually the weight of the train is enough to allow the train to travel safely and at speed along the track. However, if a train has to go up a steep bank or hill it is likely to slip backwards. A ‘rack and pinion’ system is added to some trains to overcome this problem. A large gear wheel is added to the centre of the train and an extra track is, with teeth, called a ‘rack’ is added to the track. As the train approaches a steep hill or slope the gear is lowered to the track and it meshes with the ‘rack’. The train does not slip backwards but it is pulled up the steep slope.



Application:-

                  Rack and pinion gears provide a less mechanical advantage than other mechanisms, but greater feedback and steering sensation. A rack and pinion gear gives a positive motion especially compared to the friction drive of a wheel in tarmac. In a rack and pinion railway, a central rack between the two rails engages with a pinion on the engine allowing a train to be pulled up very steep slopes.

                  Rack and pinions gears are commonly used in the steering system of cars to convert the rotary motion of the steering wheel to the side to side motion in the wheels. The steering wheel rotates a gear which engages the rack. As the gear turns, it slides the rack either to the right or left, depending on which way the wheel is turned. Rack and pinion gears are also used in some scales to turn the dial that displays a weight.

MECHANICAL ENGINEERING INTERVIEW QUESTIONS

MECHANICAL ENGINEERING INTERVIEW QUESTIONS

1.)What is enthalpy?
Ans:-Enthalpy is the heat content of a chemical system.

2.) What is a positive displacement pump?
Ans:-A positive displacement pump causes a liquid or gas to move by trapping a fixed amount of fluid or gas and then forcing (displacing) that trapped volume into the discharge pipe. Positive
displacement pumps can be further classified as either rotary­type (for example the rotary vane)or lobe pumps similar to oil pumps used in car engines. These pumps give a non­pulsating output or displacement unlike the reciprocating pumps. Hence, they are called positive displacement pumps.

3.) Why would you use hydraulics rather than pneumatics?
Ans:-Hydraulics is suitable for higher forces & precise motion than pneumatics. This is because hydraulic systems generally run at significantly higher pressures than pneumatics systems.Movements are more precise (repeatable) because hydraulics uses an incompressible liquid to transfer power whilst pneumatics uses gases.Pneumatic systems have some advantages too. They are usually significantly cheaper than hydraulic systems, can move faster (gas much less viscous than oil) and do not leak oil if they
develop a leak.

MECHANICAL ENGINEERING QUESTIONS

MECHANICAL ENGINEERING QUESTIONS

1.)What is the difference between shear center flexural center of twist and elastic center?
Ans:-The shear center is the centroid of a cross­section. The flexural center is the center of twist, which is the point on a beam that you can add a load without torsion. The elastic center is located at the center of gravity. If the object is homogeneous and symmetrical in both directions of the cross ­section then they are all equivalent.

2.) Explain Otto cycle.
Ans:-Otto cycle can be explained by a pressure volume relationship diagram. It shows the functioning cycle of a four stroke engine. The cycle starts with an intake stroke, closing the intake and moving to the compression stroke, starting of combustion, power stroke, heat exchange stroke where heat is rejected and the exhaust stroke. It was designed by Nicolas Otto, a German engineer.

3.)What is ductile­ brittle transition temperature?
Ans:-It is the temperature below which the tendency of a material to fracture increases rather than forming. Below this temperature the material loses its ductility. It is also called Nil Ductility Temperature.

4.)What is isometric drawing?
Ans:-It is a 3­D drawing used by draftsmen, architects etc

MECHANICAL ENGINEERING INTERVIEW QUESTIONS

MECHANICAL ENGINEERING INTERVIEW QUESTIONS

1.)Which conducts heat faster steel copper or brass?
Ans:-Copper conducts heat faster than steel or brass. Any material that is good for conducting heat is also good for electricity in most cases. Wood terrible for transferring heat thus is also insulator for electric.

2.) What is a Process Flow Diagram?
Ans:-A Process Flow Diagram (or System Flow Diagram) shows the relationships between the major components in the system. It also has basic information concerning the material balance for the process.

3.) Where pneumatic system is used?
Ans:-Any system needs redundancy in work needs pneumatics, because the compressor of the pneumatic system has periodical operations (intermittent work, not as hydraulic pump). The compressed air could be accumulated in tanks with high pressures and used even if the compressor failed.