ENGINE COOLING SYSTEM

Transcription

1 ENGINE COOLING SYSTEM 3.1 Necessity for cooling In an internal combustion engine, the fuel is burned within the engine cylinder. During combustion, high temperatures are reached within the cylinder, for example in a compression ignition engine as high as C is reached. About one third of the heat energy liberated b the burning fuel is converted into power. Another one third goes out through the exhaust pipe unused. The remaining one third flows into the various components of the engine, namely, cylinder, cylinder head, valves, spark plug (in SI engines), fuel injector (in CI engines) and pistons. This heat flow takes place during combustion and expansion processes. 3.2 Effects of over heating of the engine components: 1. Evaporation of lubricating oil that lubricates the piston and cylinder wall. This will result in metal to metal contact of the piston and cylinder wall leading to piston scuffing and piston seizure. 2. Setting up of thermal stresses in the cylinder, cylinder head and piston. This may lead to cracking of them. 3. Sticking of piston rings in the ring grooves, due to carbonization of the oil. Ring sticking will result in inefficient sealing of the cylinder, increased blow by of gases and loss of thermal efficiency. 4. Burning of piston crown. 5. Burning and warping of exhaust valves. 6. Reduction in volumetric efficient i.e. reduced weight of charge retained in the cylinder.

2 3.3 Optimum cooling To avoid overheating, and the consequent ill effects mentioned above, the heat transferred to an engine component (after a certain level) must be removed as quickly as possible and be conveyed to the atmosphere. It will be proper to say the cooling system as a temperature regulation system. It should be remembered that abstraction of heat from the working medium by way of cooling the engine components is a direct thermodynamic loss. Effects of excessive cooling: 1. Reduction in thermal efficiency. 2. Increased corrosion of engine parts. 3. Reduced mechanical efficiency. 4. Improper vaporization of fuel. Hence, if high thermal efficiency is desired the quantity of heat abstracted shall not be more than that necessary to prevent overheating. 3.4 Air cooling In air cooling, large quantities of air is circulated around the hot engine components. In two wheeler engines, air flows over the engine components due to the movement of the vehicles. In other types, where the engine is totally enclosed by the vehicle body, air is forced by a fan or blower of generous capacity. The fan or blower is fixed to the flywheel.

3 The air is delivered through the ducts and is directed along the spaces between the fins. Typical blower cooling system of an inline engine can be seen in figure below. Baffles or cowling surround the cylinder and direct air well over the cylinders and thereby improve cooling.

4 3.5 Advantages of air cooling: 1. Cylinder and cylinder head casting are less complicated. 2. Engine weight is reduced, because engine jackets are not there. 3. Cheaper to manufacture- both labor and material. 4. Volume or size (overall) may be reduced, as no device such as radiator is required for re-cooling the coolant. 5. Engine warms up more quickly, and delivers its full power in lesser time than the liquid cooled engines. 6. Quick starting of the engine is possible even in frosty weather. 7. Rate of coolant frost (ice) damage of the engine (jackets) is not there.

5 8. Risk of coolant frost (ice) damage of the engine (jackets) is not there. 9. Engine is less liable to break down. Radiator unit and joints tubing and other sources of coolant leakage. Disadvantages of air cooling: 1. Greater mechanical noise, particularly because of the fan. 2. Not suitable for multi-cylinder engines, unless a fan (which absorbs some power) and suitable cowling are used. 3. Not suitable for engines to be mounted on vehicles meant for agricultural and construction applications. This is because the space between the cooling fins are likely to be blocked, either partly or fully with vegetable matter i.e. grass, chaff, straw and weeds, and mud and other dust particles. 3.6 Liquid Cooling: In the present day automotive engines, liquid cooling is widely used for the following reason: The resistance to heat transfer from the cylinder wall to a liquid in contact with it is low. When the velocities of the liquid are fairly high, this resistance is very low. Due to this, the film heat transfer coefficients are high. Also heat transfer is greater. This fact results in low temperature differences between the cylinder wall and the coolant when liquid cooling is used instead of air cooling. The most commonly used substances for cooling is water. Other liquids are ethylene glycol or Preston or glycerin. These have boiling points much higher than water. But they have corrosive effects on engine parts. In liquid cooled engines, spaces are provided around the cylinders and on the cylinder heads. These spaces are called water jackets. The coolant is circulated

6 through these jackets. In most of the automotive engines, the coolant jacket is cast integral Thermosyphon cooling system: When a vessel of cold water is heated, the hot water will tend to rise (by virtue of its lower density) and its place will be taken over by cold water. This causes a definite circulation within the water mass from top to bottom and vice versa. This is called natural convection. The Thermosyphon, circulation cooling system shown in fig.1, works on this principle. In the thermosyphon circulation cooling system, when the engine is cold the whole water is at the same temperature and is at rest. When the engine is

7 operating, the water around the cylinder heads and cylinder walls gets heated and flow up to the top header tank of the radiator. Now the cold water from the lower part of the radiator flows and fills the coolant spaces in the cylinder head and the cylinder block. The hot water flows downward through the radiator tubes. This water is cooled by the stream that flows past the tubes. Air is sucked by the fan which is driven by the engine crankshaft. In this system, the water circulation through the cylinder, cylinder head and the radiator is by natural means. For success in operation, the passages through the jackets and radiator should be free and the connecting pipes large. Further the jackets should be placed as low as possible relatively to the radiator, in order that the hot leg shall have as great a height as possible. During operation the water level must on no account be allowed to fall below the level of the delivery pipe to the radiator top. If this happens, water circulation will cease.

8 In general the thermo-syphon system requires a larger radiator and carries a greater body of water than the pump circulation system. Further, a somewhat excessive temperature difference is necessary to produce the requisite circulation. On the other hand, to some extent it automatically prevents the engine being run too cold. The thermo-syphon circulation cooling system is simpler in construction and operation. This system is used in some motor cars Forced circulation cooling system (or Pump circulation cooling system): Present day automotive engines are of larger power output and operate at higher speeds. As such they tend to operate at higher temperatures. The rate of coolant circulation in these engines must be greater in order to increase the cooling effect. This necessitates the use of a water pump in these engines. The pump ensures forced circulation of the coolant. The pump draws cold water from the radiator bottom tank, circulates it through the engine coolant jacket and pushes it back to the hot water tank (top tank of the radiator). The pump assisted forced circulation system can be seen in figure 2.

9 1. Radiator: In mobile units, the hot water let out by the engine must be cooled by some method and then re-circulated by the cooling system. Radiator is widely used for this purpose in transport vehicles. The radiator presents a large amount of cooling surface to the air so that the water passing downward through it in thin steams is cooled efficiently. The practical factors that govern the choice of the available radiator are available space, air flow resistance and cost. The details of the radiator are seen in the figure.

10 Details of the radiator and thermostat Radiators constructed at present are mainly of two types: 1. Vertical water tube type (Down flow) and, 2. Cellular type (Cross flow). In the vertical water tube type, the water falls through a nest of tubes from an upper header tank to a lower tank and air flows horizontally between the tubes. In the cellular type, the air flows horizontally through short tubes assembled honeycomb fashion with small clearances between them through which the water circulates vertically from top to bottom. The amount of heat that can be dissipated by a radiator depends upon the following: 1. Relative wind velocity, 2. Air density and humidity,

11 3. Water and air temperature, 4. Cooling (or heat transfer) surface provided, 5. Ratio of tube depth to diameter, 6. Conductivity of the metal used, 7. Design of radiator and its disposition. 3. Thermostat Types of Thermostat 1. Bellow type (Figure 1), 2. Wax type (Figure 2) A Pump, B - Thermostat

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13 3.7 LUBRICATION SYSTEM Lubrication circuit is one of the most important ones in the engine. The engine cannot run smoothly for more than a few minutes without the lubricating oil. Whenever two metallic surfaces move over each other under direct contact, dry or solid friction is produced. This is due to the irregularities on the two surfaces interlocking each other. The dry friction thus. created produces a lot of heat and results in wear of the metal surface. 3.8 Purpose of lubrication 1. To reduce friction between the moving parts. 2. To reduce wear of the moving parts by way of lubricating the surfaces and keeping them apart. 3. Remove heat from engine parts by acting as a cooling agent. To keep down the temperature of the moving parts and thus prevent scuffing and seizure. 4. To act as a seal and prevent leakage between the parts such as pistons, piston rings and cylinders. 5. Absorb shocks between bearings and other engine parts, thus reducing engine noise and extending engine life. 6. Acts as a cleaning agent. To wash away acidic accumulation and abrasive metal worn from the friction surfaces. 3.9 Crankcase ventilation In the case of four stroke engines, air must circulate through the crankcase, when the engine is running. This removes the hot lubricating oil vapors and blow by from the crankcase. For entry of air and exit of the gases, the crankcase is provided with a breather tube. The air entry into the crankcase also cools the oil to some extent. However, discharging the crankcase vapors into the atmosphere

14 causes air pollution. To prevent atmosphere pollution, modern engines have a positive crankcase ventilation (PCV) system. In this case, the crankcase is connected by a pipe to the inlet manifold. This enables the oil vapors to be sucked into the inlet manifold during engine suction and burned Methods of lubrication: The different parts of an engine are lubricated by anyone combinations of the following methods. 1. Mechanical system Splash lubrication 2. Pressure lubrication system. a. Wet sump lubrication system b. Dry sump lubrication system. 3. Semi pressure lubrication system Splash lubrication: In the splash lubrication system, the oil retained in the oil pan is churned and splashed up by the internal parts of the engine (connecting rod big end and crankshaft) into a combination of liquid and mist. This oil mist is sprayed over the interior of the engine i.e. on the cylinder walls and on the underside of the piston crown. Some of the oil splashed gets collected in pockets over the main bearings. The collected oils then flow through the bearings by means of oil hole and grooves in the bearing surfaces. In one design, the connecting rod big end caps have scoops. These scoops pick up oil from the oil pan, during the lower ends of the connecting rod travel. The connecting rod caps have drilled holes. Through these holes, part of the oil picked up by the scoop reaches the end bearings. The remaining oil lubricates the rest of the engine parts by splash i.e. the throwing of oil by the connecting rods, crankshaft and other oiled moving parts.

15 In another design, the connecting rod big end cap has thin projections. These projections improve oil splashing PRESSURE LUBRICATION SYSTEM In the pressure lubrication system, the lubricating oil is pumped under pressure to the various engine bearings. The oil is delivered by an oil pump under a pressure of 1.30 to 1.40 kscm into an oil gallery or distributor duct. This gallery distributes oil to the various engine parts and bearings. The oil pump is driven by the engine crankshaft or camshaft. This system can be seen in figure. The crankshaft main bearing (each one) is pressure fed from the main oil gallery. Internal drilled holes (or ducts) in the webs of the crankshaft conduct oil from the main bearings to the connecting rod big end bearings. The oil flows from the big end bearing through the long hole drilled in the shank of the connecting rod to the wrist. pin. The lubricating oil flow through a bearing is controlled by maintaining limited clearance all around between a round bearing and a round shaft. The clearance between a round bearing and a round shaft. The clearance between the two is called oil clearance. Proper clearance must exist and this varies with different engines, but mm is a typical clearance.

16 When there is excessive clearance, the oil pump cannot build up normal pressure. Excessive clearance can also cause some bearings to fail due to oil starvation. On the other hand, if the oil clearances are insufficient, then there will be metal contact between the bearing and the shaft journal. Extremely rapid wear and quick failure will occur. Further, there will not be enough oil throw off for adequate lubrication of the cylinder walls, pistons and rings. In 1 head engines oil is fed under pressure to the valve mechanisms in the head. For example, some engines have the rocker arms mounted on hollow shafts. These shafts feed oil to the rocker arms. Some engines with independently mounted rocker arms have hollow mounting studs. These studs feed oil gallery in the hed to the rocker arm ball pivots. On other engines, the oil flows up through hollow push rods to lubricate the valve stems and other valve train parts. The oil spills off the rocker arms and provides lubrication for the valve stems and other valve train parts, the oil spills off the rocker arms and provides

17 lubrication for the valve stems and push rod and valve stem tips. Therefore, all valve mechanism parts are adequately lubricated. Cylinder walls are lubricated by splashing oil thrown off from the connecting rod bearings. Some engines have oil spit holes or groove in the connecting rods that index with drilled holes in the crankpin journals with each revolution. As this happens a stream of oil is spit, or thrown, onto the cylinder walls. When oil spit that holes are not used, the engine relies on oil that flows through the side clearance between the side of the connecting rod and the crankshaft for cylinder wall lubrication. In many engines, the piston pins are lubricated with oil scraped off the cylinder walls. The pistons have grooves, holes, or slots to feed oil from the oil ring groove to the piston pin bosses. Lubrication of piston pin, camshaft bearings, rocker arm shaft, rocker arm, cam and tappet surfaces can be seen in figure.

18 Semi pressure lubrication system The semipressure lubrication system is a combination of splash and pressure lubrication system. Most automotive engines use this system. This is more simple and less costly than the complete pressure lubrication system. This system also enables bearing loads and engine speeds than for the splash system Wet sump lubrication In the wet sump lubrication system, the main oil supply is kept in the sump which is below the engine cylinder in the crankcase. The oil pump draws oil from the crankcase and forces it through the lubricating oil filters to the various parts of the engine. This system is widely used incars and trucks.

19 In some engines, the oil pump is submerged in the sump oil. Some designs use tiling intake pipe and strainer unit. Where there is no oil hole in the connecting rod shank, the cylinder walls and piston pins are lubricated by the spray of oil that is forced out of the connecting rod bearings and thrown by the revolving cranks. This spray also lubricates many exposed internal parts. A separate line supplies lubricant to the accessories and their drive shafts, valves and rocker arms. Used oil drains back into the crankcase by gravity for recirculation Dry sump lubrication The dry sump lubrication is used in more expensive cars. In the dry sump pressure lubrication system, there is no oil sump in the crankshaft chamber. In this system, the oil is kept either in a separate tank or reservoir, provided with cooling fins. Two pumps are used in this system. One pump sucks the oil from the reservoir and forces it under pressure to the various bearings of the engine, as in the wet sump system. The other pump (also called scavenger oil pump) is of large capacity. This pump sucks oil which drains down to the bottom of the crankshaft chamber, and returns it to the oil reservoir.

20 The main advantage of the dry sump lubrication system is that the oil is cooled during its circulation. As such, the oil has better lubricating value. Dry sump system is suitable for engines fitted to vehicles which may have to work in inclined positions.

21 Full flow and partial or by-pass flow pressure system The full flow system and the partial or bypass flow system can be seen in figure. The full flow lubrication system is one in which the entire quantity of oil delivered by the pump is forced through the oil filter before the oil filter reaches the various parts of the engine. It is the most common and widely used type. The bypass lubrication system does not filter all of the oil that enters the engine bearings. It filters some of the extra oil not needed by the bearings.

22 ******************* Exercise 3 1. What is the necessity of Cooling? 2. What are the effects of engine overheating? 3. What are the effects of Over cooling and under cooling? 4. What are the advantages and disadvantages of air cooling? 5. What is thermosyphon cooling? Explain with a neat sketch working themosyphon cooling? 6. Write a short note on with a neat sketch working of i) Forced circulation cooling system ii) Thermostat cooling 7. What is the purpose of lubrication? 8. What are the methods of lubrication? Explain with a neat sketch working of splash lubrication. 9. With a neat sketch explain working of a Pressure lubrication system 10. Explain briefly with a neat sketch working of Dry sump lubrication