BME notes (Module 3 & 4)

7/17/2019 BME notes (Module 3 & 4)

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BASICS OF MECHANICAL ENGINEERING KAILAS S C

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MODULE III

REFRIGERATION AND AIR CONDITIONING

VAPOUR COMPRESSION SYSTEM

A simple Vapour Compression system of refrigeration consists of the following basic

components: Compressor, Condenser, Expansion valve and Evaporator.

The function of the compressor is to compress the refrigerant to high pressure. At the time of

compression, the condition of refrigerant is dry vapour. This vapour enters the condenser

where cooling water is circulated. Heat from the refrigerant is removed and it becomes liquid.

This high pressure liquid refrigerant is expanded suddenly when it flows through the

expansion value. The pressure is decreased at this time and some portion of the refrigerant

becomes vapour. The condition of the refrigerant coming out of the expansion valve is wet

vapour. This enters the evaporator and heat is absorbed by the refrigerant and it becomes

dry vapour. This dry vapour enters the compressor and cycle continues.

COP = Heat extracted from the evaporator

Work done by the compressor

VAPOUR ABSORPTION SYSTEM

The working of Vapour Absorption system is similar to vapour compression system. The

difference lies in the compression of refrigerant. In vapour compression, a compressor is used

to compress the dry refrigerant to high pressure whereas in vapour absorption, the

compressor is replaced by a combination of three devices; An absorber, a pump and a

generator.




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The dry vapour refrigerant from the Evaporator enters the absorber and is dissolved in cold

water. This solution of water and refrigerant is called strong solution. This strong solution is

pumped into generator using a liquid pump. In the generator, this strong solution is heated

using heating coils. As a result of heating, the refrigerant vapor is driven out of the solution

as a high pressure vapour. The remaining water in the generator flows back to the absorber.

From the generator, the refrigerant vapour flows to the condenser where cooling water is

circulated. Heat from the refrigerant is removed and it becomes liquid. This high pressure

liquid refrigerant is expanded suddenly when it flows through the expansion value. Thepressure is decreased at this time and some portion of the refrigerant becomes vapour. The

condition of the refrigerant coming out of the expansion valve is wet vapour. This enters the

evaporator and heat is absorbed by the refrigerant and it becomes dry vapour. This dry vapour

enters the absorber and cycle continues.

COP = Heat extracted in the evaporator

Work done by pump + Heat supplied to the generator

DOMESTIC REFRIGERATOR

The domestic refrigerator works under the principle of vapour compression system. It consists

of the following four basic components:

Evaporator: It is the equipment in which liquid refrigerant evaporates. In a typical

refrigerator, it is located in the freezer compartment of the refrigerator. The refrigerant flows

through the tubes surrounding the evaporator absorbs heat and becomes dry vapour.

Compressor: The function of the compressor is to compress and increase the pressure of dry

vapour refrigerant coming out of the evaporator.

Condenser: The equipment used to convert the refrigerant vapour to liquid is calledcondenser. It is usually a wire and tube mounted at the back of the refrigerator. The heat




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absorbed by the refrigerant while passing through the evaporator is released in the

condenser.

Capillary Tube (Expansion device): The liquid refrigerant leaving the condenser enters the

capillary tube. It is a pressure reducing device. When liquid refrigerant flows through the

capillary tube, pressure is reduced suddenly and as a result some part of the refrigerant

converts to liquid. This wet vapour coming out of the capillary tube is fed into the evaporator

and the cycle is repeated.




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MODULE IV

MULTI POINT FUEL INJECTION (MPFI)

MPFI is a new technology used in Petrol engines. In ordinary petrol vehicles, a carburetor is

used to mix fuel and air in required ratio. This fuel-air mixture is fed to the engine cylindersthrough inlet valves. In MPFI engines, the use of carburetor is eliminated. The fuel is mixed

with fresh air just before reaching the inlet valve using a fuel injector. Fuel injectors are placed

near the inlet valves of each cylinders. Fuel in required quantity is injected using this injectors

before the air enters the engine cylinders. The quantity of fuel injected and the timing of

injection is controlled by an Engine Control Unit (ECU). It is a microprocessor used in modern

vehicles. Using different sensors, the information regarding the engine temperature, engine

speed, coolant temperature, inlet air temperature, exhaust air temperature, oxygen content

in the inlet air, speed and acceleration of the vehicle etc. are collected and this data is

processed by the ECU for deciding the quantity and timing of fuel injection.

Advantages:

• More power is produced

• Complete combustion of fuel

• Better Efficiency

• Better Mileage

• Less Pollution

COMMON RAIL DIRECTION INJECTION (CRDI)

CRDI is a new technology used in Diesel engines. In ordinary diesel engines, a fuel injecton pump is

used to distribute fuel to fuel injectors. In CRDI engines, a closed tube known as Common Rail is usedand fuel is maintained at a pressure of 200 MPa inside this rail using a high pressure pump. This rail

has a number of solenoid or piezoelectric valves which are controlled by ECU. From this valve, fuel at

high pressure is distributed to fuel injectors. The quantity of fuel and the timing of injection of fuel is

controlled using the solenoid values by the ECU. Using different sensors, the information regarding

the engine temperature, engine speed, coolant temperature, inlet air temperature, exhaust air

temperature, oxygen content in the inlet air, speed and acceleration of the vehicle etc. are collected

and this data is processed by the ECU for controlling the solenoid valve.

Advantages:

• More power is produced

•

Complete combustion of fuel

• Better Efficiency

• Better Mileage

• Less Pollution

• Less vibration of engine

TYPES OF CHAINS

ROLLER CHAIN

Roller chain is the most commonly used type of chain in chain drives. Compared to the other type,

Roller chains are less cost and maintenance is easy. But these type requires more lubrication and noisy

also.




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SILENT OR INVERTED TOOTH CHAIN

These types of chains are used in special equipment where more power has to be transmitted which

is not possible using ordinary chains. These chains are silent in operation but more costly.

SINGLE PLATE CLUTCH

Clutch is a device for connecting and disconnecting the engine power from reaching the gear

box. It is situated between the engine and the gear box. When the clutch is engaged, power

from the engine shaft reaches the gear box. When clutch is disengaged, the connection

between the engine shaft (driver shaft) and the gear box shaft (driven shaft) is separated.

Single plate clutch is a friction clutch, which transmits power by friction. There are mainly 3

components in a single plate clutch. A flywheel which is attached to the end of the driver

shaft, a pressure plate which is actuated with a spring and a clutch plate which has a friction

lining on both the sides. The clutch plate is free to slide over the driven shaft, in between the

flywheel and the pressure plate. When the clutch plate rotates, the driven shaft is alsorotated.

When the clutch engages, the spring in the pressure plate is expanded and the pressure plate

is moved towards the clutch plate. The pressure plate pushes the clutch plate to the flywheel

and due to the friction between the surface of the flywheel and clutch plate, the rotation of

the flywheel is transferred to the clutch plate and consequently the driven shaft starts

rotating.

When the clutch disengages, the spring in the pressure plate compresses and the pressure

plate is pulled back. This separates the flywheel and the clutch plate. And then the flywheelrotates without driving the clutch plate and the driven shaft.




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TYPES OF BRAKES

DRUM BRAKE

A Drum brake contains a brake shoe with a friction lining on their surface. It has lever and

spring mechanisms for actuating the brake. These components are housed in a casing calleddrum which is fixed to the wheel. When brake is applied, the brake shoe is expanded outwards

and it touches the inside surface of the brake drum. Due to friction, the rotation of drum is

reduced causing the vehicle to slow down or stop.

DISK BRAKE

Disk brakes are more efficient than drum brakes and the maintenance is less. These usually

works by means of hydraulic mechanisms. It contains a disk which is fixed to the wheel of the

vehicle. A brake pad with friction lining is equipped to stop the rotation of the disk when the

brake is applied. The brake is actuated by a small piston cylinder arrangement which is

controlled using fluid pressure.

Documents

BME notes (Module 3 & 4)