Cooling
Content
2.11 Cooling System for I.C. Engines Internal combustion engines at best can transform about 25 to 35 percentage of the chemical energy in the fuel in to mechanical energy. About 35 percentage of the heat generated is lost in to the surroundings of combustion space, remainder being dissipated through exhaust¶ and radiation from the engine. 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 state below. 2.11.1 Necessity for Engine Cooling 1) 2) 3) Engine valves warp (twist) due to overheating. Damage to the materials of cylinder body and piston. Lubricating oil decomposes to form gummy and carbon particles.
4) Thermal stresses are set up in the engine parts and causes distortion (twist or change shape) and cracking of components. 5) Pre ± ignition occurs (i.e. ignition occurs before it is required to igniter due to the overheating of spark plug. 6) 7) 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 every cool the efficiency reduces, because starting the engine from cold requires more fuel. 2.11.2 Requirements of a good Cooling System (i) It should remove only about 30% 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, loss of power, high fuel consumption, higher emissions, starting troubles, excessive formation of sludge, lower thermal efficiency and greater wear and tear of parts.
Methods of cooling There are two methods of cooling an l.C. engine. They are: 1. Air cooling or Direct cooling. 2. Water cooling or Indirect cooling
air cooling system 2.11.3 Air cooling (Direct cooling) Inair 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 radiation from the surface also increases. More air passes over the fins and comes 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 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. These types of cooling are 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 1. Air cooling is not as effective as water cooling and efficiency of the engine is reduced. 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 a separate fan for circulation. 4. Fans used for a stationary engine consumes 5% of engine power. 5. Such engines are suitable only for low horsepower engines. 2.11.4 Water cooling (Indirect cooling)
Water cooling (Indirect cooling) In this system, water is circulated around the cylinder and cylinder heat to carry away the heat. The water passes through a passage called ³water jacket´ There are two methods of water cooling; (i) Natural circulation of water. (ii) Forced circulation of water. 2.11.5 Cooling by Natural Circulation of Water The cooling by natural circulation of water is also known thermo-syphon cooling. The principle that water becomes less dense µ heating is the basis of this method of cooling. The radiator is connection the water jacket at the top and bottom ends. As the water gets heat 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. In a radiator the transfer of heat from coolant to the air is by conduction and forced convention and not radiation. A drain tap is provided for removing water periodically. 2.11.6 Forced Circulation of Water
Cooling by Forced Circulation of Water This system has a centrifugal water pump. The water pump gets the power from the rotating engine crankshaft. 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 also be provided near the radiator for rapid transfer of heat from radiator to the outside air. Since water is circulated by a pump it may become very cold. The water temperature should not go below 75°C, since this will cause corrosion and acid formation which attack the cylinder barrel. A temperature controller thermostat valve is provided to control the cooling of water. If the temperature below 75°C, water is bypassed and when temperature reaches 75°C or above thermostat valve opens, hot water flows through the radiator and get cooled. 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 general in automobile engines such as buses, lorries, cars and trucks. Advantages of water cooling 1. 2. 3. 4. 5. Cooling is more efficient, thus engine efficiency is more. Uniform cooling is obtained. Water cooled engines can be installed anywhere. Chances of engine overheating are greatly reduced. 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 materials, hence greater maintenance is required.
4) Thermal stresses are set up in the engine parts and causes distortion (twist or change shape) and cracking of components. 5) Pre ± ignition occurs (i.e. ignition occurs before it is required to igniter due to the overheating of spark plug. 6) 7) 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 every cool the efficiency reduces, because starting the engine from cold requires more fuel. 2.11.2 Requirements of a good Cooling System (i) It should remove only about 30% 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, loss of power, high fuel consumption, higher emissions, starting troubles, excessive formation of sludge, lower thermal efficiency and greater wear and tear of parts.
Methods of cooling There are two methods of cooling an l.C. engine. They are: 1. Air cooling or Direct cooling. 2. Water cooling or Indirect cooling
air cooling system 2.11.3 Air cooling (Direct cooling) Inair 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 radiation from the surface also increases. More air passes over the fins and comes 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 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. These types of cooling are 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 1. Air cooling is not as effective as water cooling and efficiency of the engine is reduced. 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 a separate fan for circulation. 4. Fans used for a stationary engine consumes 5% of engine power. 5. Such engines are suitable only for low horsepower engines. 2.11.4 Water cooling (Indirect cooling)
Water cooling (Indirect cooling) In this system, water is circulated around the cylinder and cylinder heat to carry away the heat. The water passes through a passage called ³water jacket´ There are two methods of water cooling; (i) Natural circulation of water. (ii) Forced circulation of water. 2.11.5 Cooling by Natural Circulation of Water The cooling by natural circulation of water is also known thermo-syphon cooling. The principle that water becomes less dense µ heating is the basis of this method of cooling. The radiator is connection the water jacket at the top and bottom ends. As the water gets heat 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. In a radiator the transfer of heat from coolant to the air is by conduction and forced convention and not radiation. A drain tap is provided for removing water periodically. 2.11.6 Forced Circulation of Water
Cooling by Forced Circulation of Water This system has a centrifugal water pump. The water pump gets the power from the rotating engine crankshaft. 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 also be provided near the radiator for rapid transfer of heat from radiator to the outside air. Since water is circulated by a pump it may become very cold. The water temperature should not go below 75°C, since this will cause corrosion and acid formation which attack the cylinder barrel. A temperature controller thermostat valve is provided to control the cooling of water. If the temperature below 75°C, water is bypassed and when temperature reaches 75°C or above thermostat valve opens, hot water flows through the radiator and get cooled. 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 general in automobile engines such as buses, lorries, cars and trucks. Advantages of water cooling 1. 2. 3. 4. 5. Cooling is more efficient, thus engine efficiency is more. Uniform cooling is obtained. Water cooled engines can be installed anywhere. Chances of engine overheating are greatly reduced. 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 materials, hence greater maintenance is required.
