Mechinopedia For Mechanical Engineers

The steam flow through the nozzle may be assumed as adiabatic flow. Since no heat is supplied or rejected by the steam during flow through a nozzle and there is no work done during the flow of steam.   i.e,  Q = 0 and W = 0 Velocity Of Steam : Steam enters the nozzle with high pressure and low velocity, and leaves with high velocity and low  pressure. The Outlet Velocity (V2) of steam can be found as follows. Consider a unit mass flow of steam through a nozzle. Let, V 1  = Velocity of Steam at the entrance of nozzle - m/s V2 = Velocity of Steam at any section considered - m/s  h1  =  Enthalpy of steam entering the nozzle - m/s h2  =  Enthalpy of steam at any section considered - m/s  We get from steady flow equation, V2 = 44.72 ( h1-h2)^(1/2)

 NOZZLE :  Nozzle is a duct varying cross-sectional area in which the velocity increases with the corresponding drop in pressure. It is main function is to produce a jet of steam with high velocity. For example, nozzles are used in steam turbines, gas turbines, in jet engines, rocket motors, flow measurement and many other applicants. SHAPES OF NOZZLES : Following 3 types of  Nozzles are important from the subject point of  view :  1. CONVERGENT NOZZLE : In Convergent nozzle, the cross-sectional area decreases from the inlet section to the outlet section. 2. DIVERGENT NOZZLE : In Divergent nozzle, the cross sectional area increases from the inlet section to the outlet section. 3. CONVERGENT - DIVERGENT NOZZLE : When the cross section of a nozzle first decreases form the inlet section to throat and then increases from its throat to outlet section, it is called a convergent - divergent nozzle.

In most of the engines, a crankcase ventilation is produced by the positive crank case ventilating system. In this system, filtered air from the carburetor air cleaner is introduced in the crank case. The air carries away the blow-by gases and petrol through a special P.C.V. Valve. Thus, the blow by gases and vapors enter the engine combustion chamber along with fresh charge and are burnt there. The consists of a spring-loaded tapered valve for flow-control. The crank case pressure and the manifold vacuum act together with the aid of spring to close or open the valve. At idle and low speed crank case emissions are very less due to lower cylinder pressure. Moreover, manifold vacuum is high. It permits the valve to open slightly and allows small flow through PCV valve. This flow would be sufficient to keep the crank case clean. At normal speeds, blow by increases and manifold vacuum decreases due to which the valve moves slightly and increases the flow. The engine may backfire during cra

 Engines have some blow. It is mostly got past the piston ring and into the crank case. To remove these, air is circulated through crank case when the engine is running. This is called crank case ventilation. The crank case ventilation system prevents build up and oil leakage. In addition, it removes blow by gas to prevent the formation of corrosion and sludge (sludge is thick , creamy , black substances that can form in the engine. It clogs oil screens and lines thereby preventing oil circulation)

The fuel injection system of  IC engine has no float bowl. Therefore, the evaporative control system is used only fuel vapor from the fuel tank. In this, the canister has two connections. One is the connection to fuel tank, the other is the purge line to the throttle body. Instead of vacuum operated purge valve, an electric purge control solenoid may be used. It is mounted on the canister or in the purge line.  

Petrol Vapors from fuel tank escape into atmosphere by evaporation  ( Or breathing ). This Vapor comes through filter cap vent or the tank vent tube. When the engine is not running , petrol will evaporate in the carburetor float chamber. It is estimated that the loses from the fuel tank are about 10% of the total Hydrocarbon Emission. In this , fuel tank and carburetor float chamber are vented to a carbon canister. The canister absorbs the vapor and stores it. "Absorb" means the petrol vapors are trapped by stricking to the outside of the charcoal particles. A vapor-liquid separator is provided on the top of fuel tank. Vapors go to the tap of the separator is filled with filter material that allows vapor, but not liquid to pure. A float type separator in which the float goes up and closes the orifice to the canister hose. Carburetor float bowl has two vents. One is connected to the air cleaner and helps to compensate a clogged air cleaner. The other vents connect to charcoal

The Pollution may be controlled by the following two ways.  1. The formation of  Pollutants is prevented as far as possible. 2. The pollutants are destroyed after they are formed. Control Of  Hydrocarbons : Formation Of  Hydrocarbon may be reduced by the following methods, 1. Reducing the compression ratio. 2. Changing the design of Combustion Chamber. 3. Changing The design of piston. 4. By supplying lean mixture. 5. By maintenance of piston and piston ring  Destroying the Hydrocarbons can be done by following Methods, 1. By supplying air to the inlet manifold. 2. By using after burner. 3. By using catalytic converter. Control Of CO : Methods of reducing CO are, 1. By using closed loop control. 2. By supplying lean mixture. 3. Providing suitable overlap of valves. Methods of destroying CO are : 1. By using reactor in exhaust manifold. 2. By using after burner. 3. By using catalytic converter. Control of Oxides Of Nitrogen : Methods of  reducing oxides of nitrogen are as follows : 1. B

 There are three main sources of air pollution due to petrol engine. 1. Exhaust emissions through tail pipe, 2. Evaporative losses through carburetor, and  3. Crankcase blow by. I) EXHAUST EMISSIONS THROUGH TAIL PIPE :    The Exhaust Emission contains HC, CC and NO2 ( Oxides Of Nitrogen). HC Occurs in exhaust gases due to incomplete combustion. The emission of HC is closely related to many designing and operating factors like induction system , combustion chamber design, air fuel ratio, speed, load and mode of operation. Lean mixture gives lower HC emission. CO Occurs due to insufficient amount of air in the air fuel mixture or insufficient time for complete combustion. The combustion of nitric oxide (NO) and Nitrogen Dioxide (NO2) ( Called Oxides of Nitrogen No2) occurs only in the engine exhaust. High Temperatures and availability of oxygen are the two main reasons for the formation of No2. The Spark advance and air-fuel ratio are the two important factors which affect the formation

 A well - maintained diesel engine emits negligible amount of carbon monoxide and hydrocarbons though considerable amount of nitrogen oxides are emitted. Diesel smoke is another pollutant incase of diesel engines. The diesel smokes are of two types  (1) White smoke, and  (2) Black smoke .  The white smoke normally arises due to  ( i ) Too low operating temperature. (ii) Too long delay between the start of the fuel injection and beginning of combustion.  White smoke appears during starting and warming up. The black smoke appears  after the engine has fully warmed up and accelerating or pulling under load. The black smoke is a suspending of soot particles in the exhaust gases. It results incomplete combustion of fuel. Blue smoke occurs due to excessive lubricating oil consumption. Its emission indicates a very poor condition of the engine, such as worn-out piston rings or valve guide etc. The blue smoke is not considered as a serious pollution.

  1.) Carbon monoxide :      CO is produced because of insufficient supply of air for combustion. CO has more affinity than oxygen for hemoglobin to carry O2 to the body tissues. Hence, it will affect the nervous system and vision, if the percentage of CO is more. Finally, it affects hearts. 2.) Oxides Of  Nitrogen :      In high temperature, nitrogen reacts with oxygen and produces nitric oxide and nitrogen di oxide. They affect living organisms. They affect blood purification system. It may be mixed with moisture and produces dilute nitric acid in the heart and affects hearts. 3.) Hydrocarbons :    Hydrocarbon is produced due to incomplete combustion Hydrocarbon produces smog. This affects vision. Smog is the mixture on fog and smoke. 4.) Photo Chemical smog :    Some hydrocarbons and oxides of nitrogen in the exhausts reacts with atmospheric air in the presence of sunlight and produce photochemical smog. It damages the plant's life. It reduces the visibility. It produces eye irr

I.C engines convert heat energy into mechanical energy by burning the fuel in its combustion chamber called cylinder. Mostly, petrol and diesel oil are used as fuels for I.C engines. These fuels contain Hydrogen and Carbon in Various Combinations. During Combustion, Oxygen Combines with Hydrogen and Carbon to form water (H20), Carbon Monoxide (CO) and Carbon dioxide (Co2). The Nitrogen in the fuel combines with oxygen and forms Nitrogen oxide ( No2). Remaining fuel goes unburnt resulting in smoke and ash. Exhaust gas constituents consist of partly burned petrol, carbon monoxide , nitrogen oxides and if Sulphur in petrol, Sulphur oxides, Pollute the air.  The oxides of  Nitrogen together with hydrocarbons react in the presence of sunlight and form petrochemical smog. As a result, smog is created, the atmosphere becomes dirty and breathing becomes difficult. Its bad effect includes crop damage, eye irritation, objectionable odor, decrease of visibility, cracking in rubber etc. Smog is a

The Catalytic converter converts the pollutants like HC, CO and No2 into harmless gases. It is placed between exhaust manifold and silencer. It contains the plastic pallets coated with the catalyst. All exhaust gas must flow through it. The catalyst causes a chemical change without being a part of the chemical reaction. The catalytic converter may have two different catalysts. One catalyst treats the HC and CO. The other treats No2. The catalyst encourages the HC and CO to unit with oxygen to become H2O and Co2. This type of converter is known as oxidizing converter because it oxidizes the HC and CO. The metal platinum and palladium are used as oxidizing catalysts. The Catalyst for NO2 splits the oxygen from the NO2. The No2 becomes harmless nitrogen and oxygen. This type of converter is known as reducing converter. The metal rhodium is used as  reducing catalyst Vehicles with catalyst and makes it ineffective. For the catalytic converter to the most effective, the air fuel mixture mus

Treating the exhaust gas means some cleaning or reducing the percentage of pollutants in it. It takes place after leaving the exhaust gas from the engine cylinders and before it exists the tail pipe and enters the atmosphere. It reduces the amount of HC, CO and No2 in the exhaust gas. The exhaust gas is treated in two ways. One is by injecting fresh air into the exhaust system. The other is by sending the exhaust gas through catalytic converter. 1) Air Injection System :    The main parts of an air-injection system are the air pump, one way check valve and piping into the exhaust main fold. The air pump is driven by a belt from the crank shaft pulley. When the engine is cold, air is supplied to exhaust manifold. This fresh air supplies excess oxygen to oxide of HC and CO into CO2 and water vapor.  The diverter valve presents back-firing in the exhaust system. The diverter valve is operated by a vacuum diaphragm. When the fuel Injection system is in idling or in part or full throttle co

Excessive Nitrogen Oxides (No2) form when peak combustion temperature exceeds 1950*C. To lower the combustion temperature, many engines have EGR system. It recirculates about 10% of the inert gas back into the intake manifold. The cooler exhaust gas absorbs heat from the much hotter combustion process. It reduces peak combustion temperature and lowers the formation of  No2. The EGR system provides a passage between exhaust manifold and inlet manifold. An EGR valve provided on this line opens and closes the passage. It consists of a spring-loaded vacuum diaphragm linked to a tapered valve. This valve controls the passage for the exhaust gas. The chamber is connected by a tube to a vacuum port in the throttle body. When there is no vacuum at this port, the spring will push the diaphragm down and keep the passage closed. Therefore, no exhaust gas recirculates. It happens during the idle, when No2 formation is at a minimum. Also EGR could stall an idling engine.  When the throttle is opene

A Thermostat valve is used in the water cooling system to regulate the circulation of water in system to maintain the normal working temperature of the engine parts during the different operating conditions. The thermostat valve automatically works in the cooling system. When the engine is started from cold, the thermostat valve prevents the flow of water from engine to radiator so that the engine readily reaches to its normal working temperature after which it automatically comes into action. Generally, the thermostat valve does not permit the water below 70*C.

In Water Cooling , Water is used for cooling the engine by circulating it through water jackets around each combustion chamber cylinder, cylinder head, valve and valve sheet. By absorbing heat, water will become hot. When it is again passed through radiator, it will be cooled by air blast due to forward motion of the vehicle as well as of this engine to absorb heat.  These are two systems of water-cooling. 1. Thermosyphon System. 2. Pump Circulation System. I) THERMOSYPHON SYSTEM :  The principle of hot water going up and cold water coming down due to difference in density is used here. There is no pump to circulate water. The light hot water from the engine goes to the top of the radiator by itself and gets cooled by the surrounding air and hence goes down to bottom of  radiator and again goes to engine cylinder. It is simple, cheap but cooling is slow. Water should be maintained to correct level at all time. II) PUMP CIRCULATION SYSTEM : To make Thermosyphon system more effective and

 Method of cooling an engine by the use of  atmosphere is called air-cooling. Generally the two stroke engines are air-cooled. The heat from inside the cylinder is spread over a large area of the outer surface of the cylinder heads and cylinder by providing fins. If a fan is used to supply a continuous air over the large finned surface, heat can be quickly removed. More Temperature difference between air and cylinder is due to good heat conductivity of metal help in air-cooling. The use of copper and steel alloys improves heat transfer. Advantages : 1. Light in weight since there are no radiators, cooling water and pipelines.  2. No coolant is used and so no leak and no anti freeze required. 3. Warming up is faster. 4. Maintenance is easy and hence cheaper. Disadvantages : 1. Less efficient since air is poor conductor of  heat compared with water. 2. Since it is not possible to maintain even cooling some time distortion may take place. 3. Noisy operation. 4. It can be used only in Smal

 When the air fuel mixture is ignited and combustion takes place at 2500*C for producing power inside an engine the temperature of the cylinder, cylinder head , piston and valve, continuous to raise when the engine runs. It these parts are not cooled by some means then they are likely to get damaged and even melted. The piston may cease inside the cylinder. To prevent this, the temperature of the parts around the combustion chamber is maintained as 200*C to 250*C. Too much of cooling will lower the thermal efficiency of the engine. Hence, the purpose of the cooling is to keep the engine at its most efficient operating temperature at the engine speeds and all driving conditions. The cooling system is so designed that it prevents cooling until the engine reaches to its normal operating temperature. When the engine warms up the cooling system will begin to function. It cools rapidly when the engine is too hot and it cools slowly or not at all when the engine is cold or warming up. Thus, t

It is also called as partial pressure lubrication system. This is a modification of splash lubrication system. This system is used if the bearing loads are heavy and splash lubrication is not sufficient. It is a combination of splash and pressure lubrication. This system consists of  oil pump, oil gallery , oil filter , oil pressure gauge and scoops attached to connecting rod big end. The pump pumps the oil to the main gallery. From the gallery, oil is forced under pressure to the engine parts to be lubricated. Scoops or dippers are attached to the big end of the connecting rod.  The lubricating oil is directed to the scoops through oil jets from gallery. The scoops splash this oil in all directions to lubricate the engine parts such as piston , cylinder walls etc. Engine parts lubricated by splash : 'Piston, cylinder walls , cams , piston pin and rings , spring and guides of valve stems , oil pump drive gear 

In this system, lubricating oil id forced under pressure by a pump at a pressure of 2 to 4 MPa. It consists of oil sump, oil pump, oil gallery , pressure release valve, oil filter, oil pressure gauge and oil dipstick. The lubricating oil from the sump or oil pan is sucked by the oil pump and lifted to oil main gallery through oil filter and strainer. The oil pump is driven by camshaft. Oil pump and filter are always immersed in the oil. From the oil gallery, the oil is distributed under pressure to various parts of the engine to be lubricated by the oil tubes. Oil from gallery enters the crank pin bearing through a taper hole in the crank shaft. A through hole is provided at the center of  the connecting rod. Separate oil tubes carry oil for lubricating timing gears, rocker arm assembly, cam shaft etc. Another oil line is connected to the pressure gauge to show the pressure of the oil. The excess supplied oil drips back into the oil sump. A pressure relief valve is provided to avoid an