UNIT-IV IC ENGINES AIR CONDITIONING

Transcription

1 UNIT-IV IC ENGINES & AIR CONDITIONING

2 UNIT-IV INTERNAL COMBUSTION ENGINES&AIR CONDITIONING SYSTEMS 1. Explain Psychometric Properties. The properties of dry air and water vapor mixture are known as psychometric properties. Some of the more important properties are given below. 1. Dry air: When there is absence of water vapour in the atmosphere, it is called dry air. The pure dry air is a mixture of number of gases such as nitrogen, oxygen, carbon dioxide, hydrogen etc., Among these except nitrogen and oxygen other gases present only in negligible quantity, So the volumetric composition of dry air is 77% of nitrogen and 23% if oxygen. 2. Moist air: It is a mixture of dry air and water vapour. The amount of water vapour present in the air varies with temperature. 3. Saturated air: The maximum quantity of water vapour present in air at particular air temperature is known as saturated air. 4. Dry bulb temperature (DBT) (t d ): The temperature which is measured by an ordinary thermometer is known as dry bulb temperature. It is denoted by td. 5. Wet Bulb temperature (WBT)( t w ): It is the temperature of air measured by a thermometer when its bulb is covered with wet cloth and is exposed to a current rapidly moving air. It is denoted by tw. 6. Wet Bulb depression (WBD): It is the difference between dry bulb temperature and wet bulb temperature WBD = DBT WBT 7. Dew point temperature (DPT) (t dp ): The temperature at which the water vapour present in air begins to condense when the air is cooled, is known as dew point temperature 8. Dew point depression (DPD): It is the difference between dry bulb temperature and dew point temperature DPD = DBT DPT 9. Specific Humidity (or) Humidity ratio (or) moisture content (w): It is defined as, the mass of water vapour present in one kg of dry air. Specific Humidity, w = Mass of water vapour / Mass if dry air w mv ma

3 10. Degree of saturation or percentage humidity (H) (or) Saturation ratio: It is defined as the ratio of specific humidity of the most air to the specific humidity of saturated air at the same temperature. Degree of saturation (or) percentage humidity (or) saturation ratio Specific Humidity of moist air Specific humidity of saturated air 11. Relative humidity ( ):It is defined as the ratio between mass of water vapour in a given volume and saturated mass of water in same volume and temperature Relative humidity, ( ) = _ Mass of water vapour in a given volume Saturated mass of water vapour in same volume and temperature 2. Explain winter and Summer Air conditioning systems Air conditioning is the process of supplying sufficient volume of clean air containing a specific amount of water vapor and maintaining the predetermined atmospheric condition with in an enclosed space. The air conditioning system is broadly classified into two groups. 1. Comfort air conditioning 2. Industrial air conditioning SUMMER AIR CONDITIONING SYSTEM The schematic arrangement of a typical summer air conditioning system is shown in fig. The air from outside is sucked into the system through the damper and gets mixed with re circulated air.

4 The mixed air passes through a filter. The filter removes the dust particles and harmful bacteria s from the air and the filtered air is passed through the cooling coil. The cooling coil is kept at temperature below the dew point temperature of air. So, it cools the air. When the cooled air passes through membrane, moisture in the cooled air gets condensed and collected in a sump. Finally the cooled conditioned air is supplied to the space which is to be air conditioned. From the conditioned space a part of the used air is exhausted to the atmosphere and the remaining part of the air is again recirculated. WINTER AIR CONDITONING SYSTEM Winter Air Conditioning System The schematic arrangement of a typical winter air conditioning system is shown in fig. The air from outside is sucked into the system through the damper and get mixed with recirculated air. The mixed air passes through a filter. The filter removes the dust particles and harmful bacteria s from the air and the filtered air is passed through pre-heater. Pre- heater is used to heat the air for better humidification during winter. After that it passes through a heating coil. The heating coil is used to heat the air to the required temperature. Finally the conditioned air is supplied to the space which is to be air conditioned. From the conditioned space a part of the used air is exhausted to the atmosphere and the remaining part of the air is again recirculated.

5 3. Explain automobile air conditioning systems. Automobile air conditioning system Most of us own a automobile which we use to move around and travel over distances. Summer days can get surprisingly hot. As a automobile is a closed compartment and the engine running up front, the heat in the contraption can get exceptionally high. When this happens, even opening a window can prove to be rather unhelpful. That is why the invention of the automobile AC air conditioning is one of the best additions to any vehicles. The AC is a cooling system that helps in controlling the temperatures within a vehicle by ensuring that the driver and his passengers can get cool air on hot days or warmer temperatures on cold days. How an air conditioner in the car works is relatively and is slightly similar to the workings of a refrigerator. For you to have an easier time with your conditioning system, understanding how it works can be beneficial to you. Let us have a look at its working process until it gets to offering warm or cold air. Most AC s use a Refrigerant; gas that has a low boiling point and a freezing point lower than zero degrees. There are two sides of a conditioner: The high pressure side The low pressure side

6 The high pressure side of the air conditioning system As you turn on your car AC, the process starts in the high pressure side of the system which is found in the engine compartment of your car. Here it goes through: 1. The compressor The refrigerant enters the compressor as a low pressure gas. It is then compressed and pushed out of the unit under pressure. As it is forced out, this exerts a lot of force on the gas creating heat. 2. Condenser When pushed out of the compressor, the refrigerant requires to be cooled down. The condenser unit in the car consists of coils and tubes where the heated and pressurized gas passes through. As it goes through the condenser, the refrigerant loses most of the heat and is cooled into a liquid. 3. Receiver dryer This is a small reservoir type dryer installed in the AC system to take up any form of water that may be in the system. This reservoir contains desiccants to absorb all water droplets to prevent the formation of ice crystals. From there, the refrigerant is ready to cool your car. It now enters the low pressure side of your car s Air conditioning system. The low pressure side of the air conditioning system 4. Controlled restriction The controlled restriction area is where the liquid gets to expand and comes from high to low pressure. This enables the refrigerant to enter the evaporator. The controlled restriction is essential to ensure that the whole system works continually and steadily. 5. Evaporator The evaporator part of the AC is situated in the car s cabin area. The evaporator consists of coils and tubes which work at absorbing heat from the surrounding area rather than dissipating it. The refrigerant enters the evaporator at about 0⁰ Celsius taking up the heat in the car s cabin. It then turns into a gas and heads out towards the compressor- starting the process all over again. As it does this, the air surrounding the evaporator becomes cooler. Fans blow this air to the passenger s area of the car offering the cooling sensation from the cold air. An AC system goes through this procedure every time it s switched on as it is a circulating process. Basically, this is how you get cold air being blown out of your AC.

7 4. Explain Constructional details and Main components of an internal combustion engine. CONSTRUCTIONAL DETAILS OF AN INTERNAL COMBUSTION ENGINE: Fig shows the constructional details of an internal combustion engine (petrol engine). The main components of a four stoke cycle engine are cylinder, piston, connecting rod, crank, crankshaft, cylinder head, crankcase, inlet valve, exhaust valve. Spark plug (in case of petrol engines) or injector (in case of diesel engines), cam, camshaft, push rod, piston rings, gudgeon pin, rocker arm, big end bearing, cam follower, valve springs, inlet port etc. Construction: The piston reciprocates inside the cylinder. Piston rings are inserted in the circumferential grooves of the piston. The cylinder and cylinder head are bolted together. The reciprocating motion of the piston is converted into rotary motion of the crank. The small end of the connecting rod is connected to the piston by a gudgeon pin. The big end of the connecting rod is connected to the crank pin. Crank pin is a bearing surface and it is rigidly fixed to the crankshaft. The crankshaft is mounted on the main bearings. The main bearings are housed in the main bearings. The main bearings are housed in the crankcase. Camshaft is driven by the crankshaft through timing gears (not shown in figure). The camshaft actuates the inlet and exhaust valves. The valve springs are provided to bring back the valves in the closed position. The oil sump containing lubricating oil is provided at the bottom of the crankcase. Lubricating oil is circulated to the various parts of the engine from the oil sump. A spark plug is provided in petrol engines to ignite the air-fuel mixture in the engine cylinder. An injector is provided in diesel engines to inject the fuel into the hot compressed air during power stroke. MAJOR COMPONENTS OF AN INTERNAL COMBUSTION ENGINE: 1. Cylinder block: It is the main block of the engine. It contains cylinders. The cylinders are accurately finished to accommodate pistons. The cylinder block houses crank, camshaft, piston

8 and other engine parts, In water cooled engines, the cylinder block is provided with water jackets for the circulation of cooling water. Side valve engines contain openings for the valves and valve ports. The materials used for cylinder block are grey cast iron, aluminium alloys etc. It is usually made of a single casting. 2. Cylinder head: The cylinder head is bolted to the cylinder block by means of studs. Water jackets are provided for cooling water circulation. The materials used for cylinder head are cast iron, aluminium alloy etc. This is also generally made of single casting. 3. Cylinder liners: The liner is a sleeve which is fitted into the cylinder bore. It provides wear resisting surface for the cylinder bores. Liners may be of two types. (a) Dry liners and (b) Wet liners (a) Dry liners have metal-to-metal contact with the cylinder block. They are not directly in touch with cooling water. (b) Wet liners: These liners are surrounded or wetted by cooling water. It provides wear resisting surface for the piston to reciprocate. Also it acts as a seal for the water jacket. Liner martial should withstand withstand abrasive wear and corrosive wear. Chromium plated mild steel tubes are used as liners. Good quality cast iron liners produced by centrifugal casting is also commonly used in engines. 4. Crankcase: It may be cast integral with the cylinder block. Sometimes, it is cast separately and then attached to the block. The materials used for crankcase are cast iron, aluminium alloys or alloy steels. 5. Oil pan or oil sump: Oil sump is the bottom part of the engine. It contains lubricating oil. A drain plug is provided in the oil sump to drain out the oil. It is made of pressed steel sheet. 6. Piston: The piston serves the following purposes: 1. It acts as a movable gas tight seal to keep the gases inside the cylinder. 2. It transmits the force of explosion in the cylinder to the crankshaft through connecting rod. The piston is a cylindrical part. It is opened at the bottom and closed at the top. The top of the piston is called crown. The bottom portion is called skirt. Crown is made thicker than the skirt. There are three grooves on the circumference of the piston.

9 Two grooves are of smaller width and they are meant for compression rings. Third groove is slightly of larger width to accommodate oil ring. In split skirt pistons, slots (T or I or C slots) are cut to allow expansion. Some of the materials used for piston are cost iron, aluminium alloy, chrome nickeliron alloy and cast steel. They are manufactured by casting or forging method. 7. Piston rings: Piston rings are inserted in the grooves provided in the piston. Two types of piston rings are used in a piston. 1. Compression rings and 2. Oil rings or oil control rings compression rings provide an effective seal for the high pressure gases inside the cylinder. They Prevent the leakage of high pressure gases from the combustion chamber into the crankcase. Each piston is provided with at least two compression rings. Oil rings wipe off the excess oil from the cylinder walls. It also returns this excess oil sump through the slots provided in the rings. The materials used for piston rings should be wear resistant. Normally piston rings are made of alloy cost iron containing silicon and manganese, alloy steels etc. piston rings are usually plated with chromium or cadmium. 8. Connecting rod: It connects the piston and crankshaft. It transmits the force of explosion during power stroke to the crankshaft. The connecting rod has bearings at both ends. The small end of the connecting rod has a solid eye or split eye and contains a bush. This end is connected to the piston by means of a gudgeon pin. The other end is called as big end of the connecting rod. This is always split and connected to the crank. Some connecting rods contain a straight drilled hole from big end to small end for the circulation of lubricating oil.

10 9. Crankshaft: It is the main rotating shaft of the engine. Power (useful work) is obtained from the crankshaft. The crankshaft in combination with connecting rod converts reciprocating motion of the piston into rotary motion (of the crankshaft). The crankshaft is held in position by the main bearings. There The counter weights are provided to keep the system in perfect balance. Front end of the crankshaft is provided with the following: (a) A gear or sprocket: It drives the camshaft at half the speed of the crankshaft. (b) Vibration damper: It protects the crankshaft from the torsional vibrations set up during power strokes. (c) Pulley: The pulley is fitted with V-belts. It drives engine fan, water pump, dynamo etc. 10. Camshaft: Camshaft contains number of cams. It is used to convert rotary motion into linear or straight line motion. It has so many cams as the number of valves in an engine. An additional cam is also provided to drive the fuel pump. A gear is provided in the cam shaft to drive the distributor or oil pump. The opening and closing of the engine valves are controlled by the cams provided on the cam shaft. 4. EXPLAIN THE FUEL SUPPLY SYSTEM IN IC ENGINES? The fuel supply system supplies fuel into the cylinder at correct proportion for combustion, the method by which the fuel enters the cylinder in a petrol engine (spark ignition engine) and diesel engine (compression ignition engine) are different. In a petrol engine a device called "carburetor" is used to send the correct proportion of airpetrol mixture into the cylinder. In a diesel engine a device called "fuel pump" and "fuel injector" is used to inject the correct quantity of fuel (diesel) into the cylinder.

11 FUEL FEED SYSTEM OF A PETROL ENGINE To run an automobile engine, the fuel (petrol or diesel) from the fuel tank must reach by some means to the engine cylinder. The fuels such as petrol, benzol and alcohol used in ST engines, vaporize easily at atmospheric conditions, therefore the engine suction is sufficient to vaporize these fuels and no preheating is required. The fuels such as light oils and paraffin oils used in diesel engines do not vaporize easily and so the engine suction is not sufficient to vaporize these fuels. The fuel injection system is used in oil engines. In petrol engine, the petrol from the fuel tank reaches through the fuel pump, filter and carburetor to the engine cylinder. Thus, the fuel feed system of a petrol engine consists of the following components: Fuel tank, Fuel pump, Fuel filter, Carburetor, Intake manifold, Fuel tubes for necessary connections, Gauge to indicate to the driver the fuel level in the fuel tank. The functions of the fuel feed system are: To store fuel in the fuel tank, To supply fuel to the engine to the required amount and proper condition. To indicate to the driver the fuel level in the fuel tank. ENGINE) CARBURETTOR (FUEL SUPPLY SYSTEM FOR PETROL

12 Carburetor is a device for mixing of fuel (petrol, also known as gasoline) and air in correct proportion. In a carburetor, petrol which is a volatile liquid is atomized (made into tiny spray particles) and is vaporized and mixed along with air at a proportion to suit the engine condition. The function of a Carburetor is: To atomize and vaporize the petrol. To supply the required quantity of air and fuel mixture at correct proportion under all conditions of load, temperature and speed of the engine. To maintain a small reserve of petrol at a constant level. Main components of a simple Carburetor The figure shows the various parts of a simple carburetor. It is also called a single jet carburetor. (i) Float and float chamber: As the name suggests a float is one that continues to stay up even if the level of petrol rises in the float chamber. One end of the float is linked to the wall of the float chamber to avoid any sway or movements to the sides. The float chamber is open to the atmosphere by an opening. In the float chamber, a constant level of petrol is maintained by the float and a needle valve. (ii) Needle valve: When the amount of petrol in the float chamber falls below the required level (this happens when the load on the engine increases, i.e. when the supply from the fuel tank to the float chamber is less than the fuel supplied through the main jet) the float comes down, needle valve opens and fuel from fuel tank through the fuel pump reaches the float chamber, When the required float level is reached, the needle valve closes the opening through which fuel enters. Thus the needle valve opens and closes the opening in order to maintain a constant level of petrol in the float chamber. (iii) Venturi throat: The Venturi throat is the region in the mixing chamber where the cross-section is minimum. (iv) Throttle valve: The throttle valve is linked to the accelerator pedal; The vehicle driver by pressing the accelerator pedal can open the throttle valve fully or partially. (v) Choke or Strangler: It is a valve which chamber. Working principle of a simple Carburettor During the suction stroke in an engine, vacuum is created inside t cylinder. This causes a pressure difference between the cylinder and outside the carburettor. Due to this, atmospheric air rushes into the carburettor, the air passes through the venturi. At the venturi the velocity of air increases and pressure reduces. (This is because, at the venturi the cross section is less, and air passing

13 through the small throat section gets accelerated, there by the pressure drops). Due to the reduction in pressure at the throat created by the fast moving air, the fuel from the main jet is sucked out. The fuel (petrol) gets split into fine particles (This is called atomization) and mixes along with the fast moving air to form air-fuel mixture. This air-fuel mixture is supplied to the engine through a throttle valve. The throttle valve (controlled by the accelerator pedal) controls the air - fuel mixture supplied to the engine. As the throttle valve is closed, less air flows through the venturi and less quantity of air-fuel mixture is delivered to the cylinder and hence less power is developed. As the throttle valve is opened, more quantity of air-fuel mixture is delivered to the cylinder. At normal speed, the carburettor delivers a normal mixture of air fuel ratio 15:1 (15 parts of air by weight is mixed with 1 part of fuel by weight). When cold starting the engine during cold weather conditions, the choke valve is completely closed. This causes a greater pressure difference and more fuel is drawn from the main jet, thus a more rich mixture is obtained. FUEL SUPPLY SYSTEM OF A DIESEL ENGINE The fuel supply system of a diesel engine consists of (a) fuel tank, (b) fuel filter, (c) injection pump, (d) injector, (e) fuel lines for necessary connections, and (f) fuel gauge. The main difference between the fuel feed system of a diesel engine and that of petrol engine is that the fuel feed system of diesel engine consists of an injector instead of a carburetor, remaining elements being the same. The fuel from the fuel tank flows direct to a hand lever type of fuel priming pump from which it then passes out to the inlet side of the main fuel filter. The fuel after being filtered proceeds to the inlet side of the fuel injection pump. From the injection pump it flows under pressure in the feed pipes leading to the fuel injectors. Each injector has a leave-off fuel pipe to carry off any back leakage fuel. This back leakage fuel is returned to the fuel tank Requirements of Fuel Injection System (a) The fuel injection should occur at the correct moment, rate and quantity as required by the engine at different load conditions. (b) The fuel should be injected in a finely atomized condition. (c) The fuel should be distributed uniformly inside the combustion chamber. (d) The beginning and end of injection should take place sharply. FUEL INJECTION PUMP AND FUEL INJECTOR (FUEL SUPPL SYSTEM FOR DIESEL ENGINE) Fuel Injection Pump

14 The figure shows the working principle of a Bosch fuel injection pump. This is most commonly used in diesel engines. The function of the fuel injection pump is to meter out (measure deliver) the correct quantity of fuel required for the working stroke at pressure. Construction (i) Plunger: The plunger reciprocat2s inside a pipe called barrel. Plunger can also be rotated by the rack and pinion arrangements. The line movement of the rack causes the pinion to rotate. At the same time plunger can reciprocate (move up and down) inside the barrel by means of cam. The plunger has a vertical rectangular slot (see fig.). The plunger also has a deep helical (spiral) groove on its outer surface. (ii) Barrel: The barrel has two parts, the supply port and the spill port. These ports are opened and closed by the moving plunger. It is through the supply port that the diesel enters, and any fuel that is not delivered through the - delivery valve is rejected through the spill port and is re-circulated back into the system. (iii) Delivery value: The fuel delivery valve is seated by the force of the spring, It is through this valve that the diesel is sent out through the fuel passage and finally into the fuel injector. Working The figure shows the three important positions of the plunger. It is important to understand that the plunger can be rotated (by rack and pinion mechanism) and can also be reciprocated (by the cam mechanism) both simultaneously. The rack is connected to the accelerator, the vehicle driver can press the accelerator in order to move the rack and there by rotate the plunger.

15 In the above fig. when the plunger is at BDC the fuel enters through the supply port, it occupies the space in the slot and also the helical groove. The spill port remains closed. The pressure is built up above the plunger. The accumulated pressure lifts the delivery valve supplying fuel through the fuel passage. When the rack is moved further, the plunger moves slightly up and the supply port gets closed fig. Some of the fuel that is in the slot and above the plunger escapes through the deep helical groove out of the spill port which has just opened. The pressure above the plunger slightly drops, In fig., the plunger turns further and also moves up, at this time the other side of the helical groove directly connects the supply port and the spill port. Now the fuel entering the supply port, leaves through the spill port freely. The pressure above the plunger drops further, as a result of which the delivery valve gets partially closed (i.e. less fuel is pumped through fuel passage). In fig., the plunger turns further and almost reaches the top dead center (TDC). The fuel from the supply ports flows out more freely through the spill port. The pressure above the plunger drops further and the delivery valve closes due to the greater spring pressure. Now no fuel is delivered through the fuel passage. Thus by controlling the movement of the rack which is attached to the accelerator pedal of the driver, the quantity of fuel delivered can be controlled. Fuel Injector (Atomizer) The function of a fuel injector is to deliver finely, divided particles of fuel in the form of spray (atomised fuel) into the combustion chamber. Construction: (i) Nozzle: The nozzle is attached to the body of the injector. It is the bottom most part of the fuel injector. (ii) Nozzle valve: It is seated in its seat by the force of the spring. The spring can be loosened or tightened by adjusting the screw. The spring is connected to a spindle which trys to keep the nozzle valve down. (iii) Fuel passage:

16 The fuel from the fuel injection pump enters the fuel injector through the fuel passage. The fuel Passage is cut in the body of the injector. Working The high pressure fuel from the fuel injection pump passes into the injector through the fuel passage. This fuel is collected in the pressure chamber of the nozzle. Due to the high pressure of the fuel, the nozzle valve is lifted off its seat against the spring force. The fuel is injected into the combustion chamber in the form of fine spray. When the pressure of the fuel flowing through the fuel passage falls, the nozzle valve is brought back to its seat (closing the hole of the nozzle) by the spring force. At the tip of the nozzle there may be a single hole or multiple hole for spraying the fuel. The fuel injection nozzle helps to deliver the fuel into the combustion chamber and it also helps to atomize the fuel. 5. Explain Ignition System Of I.C. Engines. In internal combustion engines, the ignition of fuel-air mixture should take place at the end of compression stroke. This ensures efficient and smooth running of an engine. There are mainly two different types of ignition used in internal combustion engines. 1. Compression ignition and 2. Spark ignition. Compression ignition: This system is used in heavy oil engines working on diesel cycle. In these engines, fresh air is drawn into the engine cylinder during suction stroke. This air is compressed to a high compression ratio (12 to 18) during compression stroke. The air pressure at the end of compression is about 3500 kn / m 2. The temperature at the end of compression is about 600 C. At the end of compression stroke, the fuel is injected into the engine cylinder in the form of fine spray. The

17 temperature of the air is higher than the self-ignition temperature of the fuel injected. Hence the fuelair mixture is ignited simultaneously. In diesel engines, the ignition of fuel-air mixture takes place due to high pressure and temperature of the air. Hence they are known as compression ignition (C.I.) engines. Spark ignition: Spark ignition is mostly used in gas engines, petrol engines, and light oil engines working on Otto cycle. In this system the fuel-air mixture is ignited by a high tension electric spark. Hence they are known as spark ignition (S.I.) engines. There are different types of spark ignition systems used in Otto cycle engines. The commonly used ignition systems are given below. (a) Coil ignition system; (b) Magneto ignition system (c) Electronic ignition system etc. (a) Coil ignition system: It is employed in petrol engines. Fig. shows the wiring diagram of a simple coil ignition system of a four cylinder engine. This system is used in automobiles. Construction: It consists of a battery, ignition coil, condenser, contact breaker, distributor and spark plugs. Generally 6 or 12 volts batter is used. The ignition coil consists of two windings primary and secondary. The primary winding consists of thick wire with less number of turns. The secondary winding consists of more number of turns of thin wire. The condenser is connected across the contact breaker. It prevents excess arcing and pitting of contact breaker points. The contact breaker is housed in the distributor itself. It makes and breaks the primary ignition circuit. The distributor distributes the high voltage to respective spark plugs at regular intervals in the sequence of firing order of the engine. (The sequence in which the firing or power occurs in a multi cylinder engine is known as firing order. The fring order of a 4-cylinder in-line engine is or The firing order of a 6-cylinder inline engine is ). The spark plug is fitted on the combustion chamber of the engine. It produces spark to ignite the fuel-air mixture. The rotor of the distributor and contact breaker cam are driven by the engine. There are two circuits in this system. One is the primary circuit. It consists of

18 battery, primary coil of the ignition coil, condenser and contact breaker. The other circuit is the secondary circuit. It consists of secondary coil, distributor and spark plugs. Working: The ignition switch is switched on and the engine is cranked. The cranking of the engine opens and closes the contact breaker points through a cam. When the contact breaker points are closed: 1. The current flows from the battery to the contact breaker points through the switch and primary winding and then returns to battery through the earth. 2. This current builds up a magnetic field in the primary winding of the ignition coil. 3. When the primary current is at the highest peak, the contact breaker points are opened by the cam. 1. The magnetic field set up in the primary winding is suddenly collapsed. 2. A high voltage (15000 volts) is generated in the secondary winding of the ignition coil. 3. This high voltage is directed to the rotor of the distributor. 4. The rotor directs this high voltage to the individual spark plugs in the sequence of the firing order of the engine. 5. This high voltage tries to cross the spark plug gap (0.45 to 0.6 mm) and the spark is produced. This spark ignites the fuel-air mixture. (b) Magneto ignition system: In this system the battery is replaced with a magneto. Fig shows the wiring diagram of a magneto ignition system. It consists of a switch, magneto, contact breaker, condenser, distributor and spark plugs. This system is used in two wheelers like motor cycles, scooters etc. Magneto ignition system

19 Construction: The magneto consists of a rotating magnet assembly and a fixed armature. The armature contains primary and secondary windings. The magnet is located on the outer rim of the flywheel. This is revolved around the stationary ignition coil. There are two basic circuits. The primary circuit consists of primary winding, condenser and contact breaker points. The secondary circuit consists of secondary coil, distributor and spark plugs. Working: The ignition switch is switched on and the engine is cranked. The engine rotates the magnetic assembly and also opens and closes the contact breaker points. When the contact breaker points are closed: 1. The current flows in the primary circuit. 2. This produces a magnetic field in the primary winding. 3. When the primary current is at the highest peak the contact breaker points are opened by the cam. When the contact breaker points are opened: 1. There is a break in the primary circuit. 2. The magnetic field in the primary winding is suddenly collapsed. 3. A high voltage (1500 volts) is generated in the secondary winding. 4. This high voltage is distributed to the respective spark plugs through the rotor of the distributor. 5. The high voltage tries to cross the spark plug gap and a spark is produced in the gap. This spark ignites fuel-air mixture in the engine cylinder. (c) Electronic ignition system: In this system contact breaker points are replaced by the magnetic pickup and reluctor. This eliminates the defects due to contact breaker points. Fig. shows a schematic diagram of an electronic ignition system using magnetic pick up.

20 Construction: It consists of a battery, ignition switch, electronic control module, magnetic pickup, reluctor or armature, ignition coil, distributor and spark plugs. Metal tips are provided on the reluctor. The number of metal tips on the reluctor are same as the number of cylinders of the engine. A permanent magnet is carried by the distributor. The reluctor provides a path for the magnetic lines from this magnet. Working: The engine is cranked by switching on the ignition switch. The reluctor passes the up coil. Every time the reluctor passes the pick up coil, an electric pulse is generated and magnetic field is passed on to the magnetic pick up. This causes magnetic field in the primary winding. Then this small current triggers the electronic control module. This control Module stops the flow of battery current to the ignition coil. The magnetic field in the primary winding is collapsed. This high voltage is directed to the spark plug through distributor. Because of high voltage, the spark is produced across the spark is produced across the spark plug gap. This ignites the air-fuel mixture. In this system the reluctor does the work of contact breaker cam. Advantages: 1. The parts such as reluctor, magnetic pick up and electronic control module are not subjected to wear and tear as a mechanical contact breaker. 2. Periodic adjustment of engine timing is not necessary. 3. It gives very accurate control of timing. 6. Explain Governing Of Internal Combustion Engine. Governing is the process of varying the fuel supply according to the load to run the engine Practically at constant speed. The device used for achieving this is known as governor. The different methods of governing internal combustion engines are given below. 1. Quantity governing. 2. Quality governing. 3. Combined quantity and quality governing. 4. Hit and miss governing. 1. Quantity governing: In this method, the quantity or air-fuel mixture entering into the engine cylinder is varied. The composition of the mixture remains constant when the load varies.

21 In petrol engines, the control is obtained by means of a throttle valve in the carburettor (Fig.). In automobiles, the throttle valve is operated by the accelerator pedal through links. In gas engines, the lift of the inlet valve is reduced and thus the quantity of air-fuel mixture entering the engine cylinder is varied. 2. Quality governing: In quality governing, the composition of air-fuel mixture is changed. The composition is changed by admitting is changed. The composition is changed by admitting more or less fuel according to the variation in load. The air flow rate remains constant. The variation in fuel is obtained by. (i) Altering the effective stroke of the fuel injection pump. It is done by changing the angular position of helical Groove of the pump plunger. (or) (II) by-passing a part of the fuel to the reservoir. (or) (III) Delaying the closure of the suction valve of the fuel injection pump. Quality governing is widely adopted in high speed diesel engines. 3. Combined quantity and quality governing: This system makes use of quantity as well as quality governing. Both the quantity and quality of the mixture are varied. This system is flexible and economical. 4. Hit and miss governing: In this method, an explosion is omitted when the speed increases above the normal speed. i.e., one power stroke is missed. The charge is of normal strength and it is not varied. The centrifugal governor closes the inlet valve in the case of gas engines. In diesel engines, the governor makes the fuel pump out of action. No fuel will be supplied and hence there will be no power stroke. Hit and miss governing used in small gas engines is given in fig.. It consists of a cam, rocker, roller, pecker, pecker block, a lever (connecting pecker block to governor) etc. the valve is seated in its seat by the spring force.

22 During normal running: The cam on the camshaft is rotated by the engine. It operates the rockerarm through the cam roller. The rocker carries a pecker. The pecker strikes the pecker block. The pecker block lifts (opens) the valve against the spring forces. The valve is opened once in two revolutions of the crankshaft. When the speed exceeds the normal: The governor weights fly further outwards. The sleeve of the governor rises up. The pecker block is slightly lifted by the lever. Now the pecker is unable to hit (strike) the pecker block. Thus opening of the inlet valve is missed. In the case of oil engines, this governor operates the plunger of the fuel pump to stop the supply of fuel. 7. EXPLAIN THE COOLING SYSTEM FOR IC ENGINES? The temperature of the burning gases in the engine cylinder is about 2000 to 2500 C. The engine components like cylinder head, cylinder wall, piston and the valve absorb this heat. Such high temperatures are objectionable for various reasons stated below. Necessity for engine cooling Engine values warp (twist) due to overheating. Damage to the materials of cylinder body and piston. Lubricating oil decomposes to form gummy and carbon particles. Thermal stresses are set up in the engine parts and causes distortion (twist or change shape) and cracking of components. Pre - ignition occurs (i.e. ignition occurs before it is required to ignite) due to the overheating of spark plug. Reduces the strength of the materials used for piston and piston rings. Overheating also reduces the efficiency of the engine. To avoid the above difficulties, some form of cooling is provided to keep the temperature of engine at the desired level. It should be noted that if the engine becomes very cool the efficiency reduces, because starting the engine from cold requires more fuel. Requirements of a good cooling system (i) It should remove only about 30 Io of the heat generated in the combustion chamber. Too much cooling reduces the thermal efficiency of the engine. (ii) A good cooling system should remove heat at a faster rate when the engine is hot. During starting, the cooling should be very slow. The components in the cylinder must be reasonably hot (250 C).Over-cooling of the engine results in insufficient vaporization of fuel causing loss of power, high fuel consumption, higher emissions, starting troubles, excessive formation of sludge, lower thermal efficiency and greater wear and tear of parts.

23 Methods of cooling There are two methods of cooling an IC engine. They are: (1) Air cooling or Direct cooling. (2) Water cooling or Indirect cooling. (1) Air cooling (Direct cooling) In air cooling, air is circulated around the cylinder block and cylinder head, fins are provided outside the cylinder and on the cylinder head. Fins increase the surface area exposed to the atmosphere and the heat radiated from the surface also increases. More air passes over the fins and comes in contact with the cylinder, thus the engine heat is removed efficiently. The use of fins increases the heat transfer surface by 5 to 10 times of its original value (i.e. without the use of fins). The high velocity of air required for cooling is obtained by the forward motion of the engine (vehicle) itself. In stationary engine, air circulating fan is provided. Application The engine design is much simpler and lighter in weight than water cooling engine. This types of cooling is used for small industrial engines and small capacity engines such as motor cycle engine and scooter engine. Advantages of air cooling 1. The engine design is much simpler. 2. Lighter in weight than water cooled engines since there is no water jacket, radiator, pump etc. It also minimizes the maintenance and operating cost. 3. There is no danger from freezing water in cold climates. 4. This is very much useful in water scarcity areas and desert. 5. Less space is required. 6. Warming up the engine is faster than water cooled engine. Disadvantages of air cooling l. Air cooling is not as effective as water cooling and efficiency of the engine is reduced.

24 2. Engine parts are not uniformly cooled. The front portion of the engine which faces the air is cooled more than the rear portion. This results in slight distortion. 3. Not suitable for multi cylinder engines. This requires set for circulation. 4. Fans used for a stationary engine consumes 5% of engine power. 5. Such engines are suitable only for low horse power engines. 2. Water cooling (Indirect cooling) In this system, water is circulated around the cylinder and cylinder head to carry away the heat. The water passes through a passage called "water jacket". There are two methods of water cooling; (i) Natural circulation of water. (ii) Forced circulation of water. (i) Natural circulation of water The cooling by natural circulation of water is also known as thermo-syphon cooling. The principle that water becomes less dense while heating is the basis of this method of cooling. The radiator is connected to the water jacket at the top and bottom ends. As the water gets heated it moves up and travels through the radiator. There it gets cooled by the radiator fins and travels downwards. The word "radiator" is not the correct word (a misnomer). In a radiator the transfer of heat from coolant to the air is by conduction and forced convection and not radiation. A drain tap is provided for removing water periodically. (ii) Forced circulation of water This system has a water pump. The water pump gets the power from the rotating engine crank shaft. The water pump draws cold water from the radiator. This cold water is forced into the water jackets of the cylinder. The rate of circulation of water is increased. Thus the engine parts are cooled efficiently. After circulating, the hot water enters the radiator top. The hot water in the radiator flows from top to bottom. The heat from the water is cooled by the radiator fins and it gets circulated again by the water pump. A fan may also be provided near the radiator for rapid transfer of heat from radiator to the outside air.

25 Since water is circulated by a pump it may become very cold. The water temperature should not go below 75, since this will cause corrosion and acid formation which attack the cylinder barrel. A temperature controller is provided to control the cooling of water. A drain tap is provided at the bottom of the radiator for the removal of water periodically. Application This system is used in light and heavy duty vehicles. It is generally used in automobile engines such as buses, Lorries, cars and trucks. Advantages of water cooling 1. Cooling is more efficient, thus engine efficiency is more. 2. Uniform cooling is obtained. 3. Water cooled engines can be installed anywhere in the vehicle. 4. Chances of engine overheating is greatly reduced. 5. Engine temperature can be controlled. Disadvantages of water cooling 1. More weight, since it uses radiator, pump, fan etc. 2. Requires more maintenance. The engine may have to be stopped even if a small leakage of water is detected in the radiator. 3. In cold weather, freezing of water causes trouble. An electric heater may be required to heat the radiator. 4. Water circulating pump consumes more power. 5. Water causes scale formation in the water circulating jacket and corrosion of metals, hence greater maintenance is required

26 8. EXPLAIN THE LUBRICATION SYSTEMS IN I.C. ENGINES? Need for lubrication In an IC engine, moving parts rub against each other causing frictional force. Due to the frictional force, heat is generated and the engine parts wear easily. Power is also lost due to friction, since more power is required to drive an engine having more friction between rubbing surfaces. To reduce the power lost and also wear and tear of the moving parts a substance called lubricant is introduced between the rubbing surfaces. Function of lubrication (or) purpose of lubrication (a) Lubricant reduces friction between moving parts. (b) It reduces wear and tear of the moving parts. (c) It minimizes power loss due to friction. (d) It provides cooling effect. While lubricating it also carries some heat from the moving parts and delivers it to the surroundings through the bottom of the engine (crank case). (e) It helps reduce noise created by the moving parts. Engine parts which are lubricated: The following are some engine parts that require adequate lubrication. 1. Crank shaft 2. Crank pin 3. Big and small end of the connecting rod 4. Piston pin 5. Internal surfaces of cylinder walls 6. Piston rings 7. Valve mechanisms 8. Cam shaft etc. Types of lubricants 1. lubricants like mineral oil, vegetable oil etc., 2. Semi liquid like grease 3. solid lubricant like graphite powder either alone or mixed with oil or grease. Requirement of good lubricating oil 1. high viscosity index (if the change o viscosity with temperature is less, the oil is rated to have a high viscosity index) 2. high flash point and low pour point temperature 3. non corrosive. 4. good detergent quality to keep the rubbing surface clean 5. high film strength (ability to maintain a thin film of oil even at high load).

27 6. the quality of lubricating oil is improved adding different types of additives such as the viscosity index improvers, the corrosion inhibitors, the detergent additives, and the film strength additives. Lubrication systems The main lubrication systems are : l. Petrol lubrication system or Mist lubrication system. 2. Wet sump lubrication system. (1) Petrol lubrication system or Mist lubrication system This system of lubrication is used in scooters and motor cycles. About 3 to 6% of lubricating oil is added with petrol in the petrol tank. The petrol evaporates when the engine is working. The lubricating oil is left behind in the form of mist. The parts of the engine such as piston, cylinder walls, and connecting rod are lubricated by being wetted with the oil mist left behind. Disadvantages (i) If the added oil is less, there will be insufficient lubrication and even result in seizure of the engine. (ii) If the added oil is more, it will lead to excess exhaust smoke and carbon deposits in the cylinder, exhaust parts and spark plugs. (2) Wet sump lubrication system In this system, oil "sump" ;sump is the bottom part of the engine, which is the crank case) contains lubricating oil. From the sump, oil is splashed or pumped to the different parts of the engine to be lubricated. The two important types of wet sump lubrication system are : (i) Splash lubrication system. (ii) Pressure lubrication system. (iii) Partial pressure lubrication system. (i) Splash lubrication system

28 In this system, the lubricating oil is filled in the sump at the bottom of the crank case. Scoops (it is like a spoon) are attached to the big end of the connecting rod During each rotation of the crank shaft, when the piston reaches the bottom dead centre (BDC) the scoops (dippers) dip into the oil present in the sump. This oil gets splashed in all directions due to the centrifugal force created by the rotating crank shaft. Splashed oil droplets settle on the piston, cylinder walls, crank shaft etc. The splashed oil then drips back into the sump. (ii) Pressure lubrication system In this system, lubricating oil is forced under pressure by a pump for effective lubrication. The figure shows the working of the system. It consists of oil sump, oil pump, oil gallery, oil filter and oil pressure gauge. Working The oil pump is submerged in the oil and it is driven. The oil is drawn from the sump by the pump and delivered to the oil gallery at a pressure of 0.35 bar to 2 bar. From the oil gallery, the oil is distributed under pressure to the different parts of the engine through oil tubes. Separate oil tubes carry oil to the bearing. From the bearing, the oil flows to the connecting rod (there are oil holes drilled through the connecting rod and crank shaft) through oil hole which leads to the piston end and other engine parts. How does oil from the oil hole enter the bearing? is may be understood from the figure. Figure (a) oil flows under pressure through oil tube and through oil holes. Figure (b) oil flow is blocked.

29 In fig (a) oil from the oil gallery flows through the oil tube under pressure. The oil tube is in permanent contact with the bearing. (Note: the bearing is not eccentric or off axis, so it rotates about its own axis). When the opening in the oil hole of the crank shaft contacts the opening in the oil pipe, the oil rushes through the oil hole and is distributed by the other oil holes drilled diagonally in the crank shaft. It thus reaches the other parts of the engine like connecting rod where such holes are drilled. As the crank shaft rotates fig(b) the oil hole in the crank shaft does not contact the oil flowing through the oil tube and there is no flow. However since the crank shaft rotates at high speed, the oil tube and the oil hole frequently comes in contact with the oil hole opening and thus gets distributed to all the parts which have a oil hole. The excess oil supplied drips back into the oil sump. The pressure gauge records the oil pressure in the system. Advantages 1. All the parts of the engine are effectively lubricated 2. Even small gaps between sliding surfaces can be lubricated since the oil is supplied under pressure. (iii) Partial pressure lubrication system It is also called as semi-pressure lubrication system. It is the combination of splash system and pressure system. Some parts are lubricated by splash system and some parts by pressure system. Almost all the four stroke engines are lubricated by this system.