Otto four stroke cycle

• A four stroke petrol engine is shown in Fig The valve operating the inlet is called inlet valve and the valve operating the exhaust is called exhaust valve. The spark plug fitted at the top of cylinder head initiates the ignition of the air fuel mixture.
• The piston performs four strokes to complete one working cycle. The four different strokes are: (i) Suction stroke (ii) Compression stroke (iii) Power stroke (iv) Exhaust stroke.

(i) Suction stroke 

• During this stroke, inlet valve opens and exhaust valve is closed, the pressure in the cylinder will be atmospheric.As the piston moves from TDC to BDC, the volume in the cylinder increases, while simultaneously the pressure decreases.This creates a pressure difference between the atmosphere and inside of the cylinder.Due to this pressure difference the petrol and air mixture will enter into the cylinder through carburetor.This stroke is represented by the horizontal line 1-2 on the p-V diagram.The crankshaft has now made hall rotation i.e. 180 of crank angle. 

Diesel four stroke cycle

• The diesel engines work on the principle of Diesel cycle, also called constant pressiure SUction, compression, power, and exhaust strokes. Fig.  shows the working and construction of a four stroke diesel engine. heat addition cycle shown in Fig. 
• The four stroke diesel engine cycle also consists of petrol engine, except instead of spark plug, a fuel injector is mounted in its place as shown in Fig. 7.7. A fuel pump supplies the fuel oil to the injector at higher pressure The basic construction of a four stroke diesel engine is same as that of four stroke

(i) Suction Stroke :

• During this stroke, inlet valve opens and exhaust valve is closed, the pressure in the cylinder will be atmospheric. As the piston moves from TDC to BDC, the volume in the cylinder increases, while simultaneously the pressure decreases. This creates a pressure difference between the atmosplhere and inside of the cylinder. Due to this pressure difference only the atmospheric air will enter into the cylinder through air filter and inlet. This stroke is represented by horizontal line 1-2 on p-V diagram shown in Fig. 7.6. erature 
• At the end of this stroke, the cylinder will be filled completely with air and inlet valve will be closed.

(ii) Compression stroke: 

• During this stroke, both inlet valve and exhaust valve remain closed. The piston moves from BDC to TDC. As this stroke is being performed, the airin the cylinder will be compressed, so pressure and temperature of air increases. The process ol compression is shown in Fig. 7.6 by the curve 2-3. 
• The compression ratio of this engine is higher than petrol engine. Due to higher compression ratio, air will have attained a higher temperature than self ignition temperature of diesel fuel. 
• Near the end this stroke, a metered quantity of the diesel fuel is injected into the cylinder. As the diesel fuel particles come in contact with high temperature air, it will ignite automatically. This is called auto-ignition or self-ignition. In this engine compressed airignites the diesel fuel, this type of engine is also called as compression Ignition engine or C.I. eugine

Bucket or Barrel Calorimeter 

(a) Construction: 

• The apparatus shown in Fig. 4.4 consists of a copper calorimeter which is placed on wooden blocks ina vessel. The vessel is large enough to provide an air space around the calorimeter. This air space provides insulation to prevent heat loss. The top cover is made of wood and it closes both the calorimeter and the vessel. This cover has two holes. Through one hole, the steam pipe is led into the calorimeter. The steam is distributed in the water in the calorimeter by the holes in the bottom ring which is connected to the end of the steam pipe. The thermometer is inserted from the second hole to measure the temperature of waterin the calorimeter.

(b) Working:

• The first step of the experiment is to measure the weight of calorimeter. With the known value of specific heat of copper, the water equivalent of calorimeter is calculated. Then necessary amount of water is taken, in the calorimeter and its initial temperature is noted. 
• The calorimeter is placed in the vessel. The top cover is placed in position and the steam pipe is connected to main steam pipe.
• The steam comes in contact with water in the calorimeter when steam is passed through the water. It condenses and gives out its entire enthalpy of evaporation (latent heat) and part of ts sensible heat. Due to heat transfer from steam to water in the calorimeter.

(c) Calculation of dryness fraction of steam

• Let, p= Pressure of steam in a steam pipe, bar =Enthalpy of liquid at p.kJ/kg 4=Temperature of water and vessel before experiment, "C =Temperature of water and vessel afterexperiment, °C =Enthalpy of water after mixing at , kJ/kg SEnthalpy of evaporation of steam, kJ/kg m =Mass of steam condensed, kg =Mass of water in calorimeter, kg mt=Mass of Calorimeter,kg Cpw= Specificheat of water, kJ/kg K Cne Specific heat of calorimeter, kJ/kg KX = Dryness fraction of steam Heat lost by the steam = Heat gained by water and calorimeter.

Ozone Depletion 

• The atmosphere of the Earth is divided into 5 laverrs, From closest to farthest layers are troposphere.stratosphere, mesosphere, thermosphere and exosphere. 
• The majority of dhs msphere's pzone remains stratosphere, whichextends from 10 kms above the Eath's surface to 50kms. The earth's stratospheric ozone layer plays a critical roll in absorbing ultra violet radiation emitted by our sun and protects the Earth from the harmful effect of ulraviolet rays: otherwise it causes skin cancer of human and can lead to genetic damage 
• (A)5smcs Hence, the ozone layer is essential to life earth, as it absorbs harmful ultraviolet-B radiation Irom the sun. In recent years the thickness of this layer has been decreasing, leading to create holes in the layer is called ozone depletion. In the last thirty years, it has been discovered that stratospheric ozone is depleting as a result of chlorine and bromine based pollutants 
• Every chlorine atom can destroy up to 100 000 azone molecules Oone depletion occurs when the nafural halance between the production and Jestruction of striatosphenic ozone is stopped The main ozone depleting substances are 
• (i) Chlorofluorocarbons-it is used as coolants in refrigerators,freezers and air conditioners in buildings and cars 
• (li) Halons-it is used in some fire extinguishers, in cases where materials and equipment would be destroyed by water or other fire extinguisher chemicals.
• (iiMethyl Chloroform-it is used mainly in industry for reducing vapour, some aerosols,cold cleaning,adhesives and chemical processing 
• (iv) Carbon Tetrachloride-it is used in solvents and some fire extinguishers. 
• Chlorofluorocarbons (CFCS)are main substance causing ozone depletion, accounting for over 80% of total stratospheric ozone depletion CFCS are not destroyed reactions with ocher chemicals or washed back to Earth by rain. They simply do not break down in the lower atmosphere and they can remain in the atmosphere 120 Finally, they are transported into the stratosphere where they broken down by ultrayiolet (UV) rays from the Sun,releasing free chlorine. The chlorine becomes actively involved in the process of destruction of ozone. The net result is that two molecules of ozone are replaced by three of molecular oxygen, leaving the chlorine Ozone is converted to ygen. leaving the chlorine atom free to repeal the process up to 100,000 times, resulting in areduced level of ozone alin increase mo w hum. 
• CI+O,CIO+0, 
• CIO+OCI+0 
• There are a number of things that we as individuals can do to protect the ozone layer These include proper disposal of old refrigerators,the use of halon-free fire extinguishers and the recycling of foum and other non-disposable packagingAlso, if emissions of ozone depleting substances are now being controlled. 
• the ozone layer is not likely to fully repair itself for everal decades. Consequently, we should take precautions when exposing ourselves to the San.