Air Cooling System
Content
AIR COOLING SYSTEM
In air cooling system air is used as cooling medium the air is made to flow past the hot object The flowing air takes away the heat generated in the system. The heat transfer coefficient obtained by flow of air across system is order 0.8w/m0c which is very less in sense of amount of heat generated in a system. To increase the heat flow the various systems are used like Forcing air to flow past the cylinder by various means Providing increase in surface area of object to increase area available for heat flow.
The heat transfer which is obtained by providing forced air flow across the cylinder heat transfer coefficient is of order 4 to 5 w/m0c.this is also not as per requirement .hence FINS or EXTENDED surfaces are provided in order to increase the heat transfer coefficient. The heat transfer coefficient increases by 15 to 20 times as compared to cylinders without fins . Industrial engines are kept in closed room rooms hence blowers or fans are to be
provided to cool the engine . in automobiles air cooling is used in motorbikes & other small application . There is no blower provided to force air as bike moves pressure
difference is created which causes air to flow past the cylinder. There is no need to provide blower to induce air with velocity onto cylinder, this is achieved by the motion of vehicle.
PART AIR COOLING SYSTEM ARE
y y BAFFLES COOLING FINS
BAFFLES
The heat generated in engine is not uniform more amount of heat is generated in exhaust port; hence more cooling air has to be provided in order to cool the engine. The air is guided into various portion of ic engine by means of baffles these baffles guide air to the specific location for cooling . the path of air is controlled by the baffles
(Figure shows baffles)
The various baffle arrangement are shown in figure above. The figure no 1 shows baffle arrangement for maximum pressure drop. The baffle is not used due more power is required by blower. The figure 2 shows baffle for petrol engine the rounded edges reduce entry & exit loses. The figure shows combination of baffle arrangement for petrol engine. The fig 4 baffle is used for cooling in diesel engine. The amount of cooling required in diesel is more due to higher compression ratio.
COOLING FINS
Figure shows the two types of fins used to cover the cylinder. The length of the fins and the spacing between them is quite important for efficient cooling. Larger inter-spacing between the fins offers larger area for cooling air but heating of air is less, so more cooling air is required. Smaller inter-spacing in fins results in smaller flow area for cooling air and hence air is heated more and input to the cooling air is less. However, the pressure drop required for flow is more. If the distance between to fins is quite small the laminar layers of the opposite areas come in contact with each other and the efficiency of the fins reduced very much. Therefore, a spacing less than 2.5 mm is not used.
The height of the fins controls the distance between the two cylinder and hence, the size of engine. Usually the fin height varies from 15 to 25 mm.
While fixing the fins care must be taken that the most heavily thermally stressed parts such as exhaust valves and exhaust manifolds, etc, are adequately finned. Air cooling result in higher engine temperature. This necessitates the provision of bigger clearance between various parts, especially between piston and arm. The upper part of the cylinder barrel is most critical part of the air cooled engine and usually limits the engine rating.
Applications of air cooling---- Air cooling is usually used for small engines and for engines whose application gives extreme importance to weight such as aircraft engines. Other areas for air cooled engines are industrial and agricultural engines where there can be strong objection to use water as coolant. MERITS OF AIR COOLING SYSTEM
i. ii. iii. iv. v. vi. vii. DIRECT TRANSFER OF HEAT ENGINE DESIGN BECOMES EASY ECONOMICAL NO POSSIBLITY OF COOLANT LEAKAGE OVERALL WEIGHT OF ENGINE REDUCES SOME AMOUNT OF DAMAGE CAN BE SUSTAINED LESS SENSITIVE TO EXTERNAL CONDITIONS
DEMERITS OF AIR COOLING SYSTEM
i. ii. iii. iv. v. vi. COOLING IS NOT UNIFORM AIR COOLING IS APPLIED ONLY TO SMALL AND MEDIUM SIZED ENGINES VOLUMETRIC EFFICIENCY REDUCES SPECIFIC FUEL CONSUMPTION INCREASES FAN IF USED USES 5% OF ENGINE POWER COOLING FINS USED MAY TEND TO VIBRATE AND MAY RAISES NOISE
In air cooling system air is used as cooling medium the air is made to flow past the hot object The flowing air takes away the heat generated in the system. The heat transfer coefficient obtained by flow of air across system is order 0.8w/m0c which is very less in sense of amount of heat generated in a system. To increase the heat flow the various systems are used like Forcing air to flow past the cylinder by various means Providing increase in surface area of object to increase area available for heat flow.
The heat transfer which is obtained by providing forced air flow across the cylinder heat transfer coefficient is of order 4 to 5 w/m0c.this is also not as per requirement .hence FINS or EXTENDED surfaces are provided in order to increase the heat transfer coefficient. The heat transfer coefficient increases by 15 to 20 times as compared to cylinders without fins . Industrial engines are kept in closed room rooms hence blowers or fans are to be
provided to cool the engine . in automobiles air cooling is used in motorbikes & other small application . There is no blower provided to force air as bike moves pressure
difference is created which causes air to flow past the cylinder. There is no need to provide blower to induce air with velocity onto cylinder, this is achieved by the motion of vehicle.
PART AIR COOLING SYSTEM ARE
y y BAFFLES COOLING FINS
BAFFLES
The heat generated in engine is not uniform more amount of heat is generated in exhaust port; hence more cooling air has to be provided in order to cool the engine. The air is guided into various portion of ic engine by means of baffles these baffles guide air to the specific location for cooling . the path of air is controlled by the baffles
(Figure shows baffles)
The various baffle arrangement are shown in figure above. The figure no 1 shows baffle arrangement for maximum pressure drop. The baffle is not used due more power is required by blower. The figure 2 shows baffle for petrol engine the rounded edges reduce entry & exit loses. The figure shows combination of baffle arrangement for petrol engine. The fig 4 baffle is used for cooling in diesel engine. The amount of cooling required in diesel is more due to higher compression ratio.
COOLING FINS
Figure shows the two types of fins used to cover the cylinder. The length of the fins and the spacing between them is quite important for efficient cooling. Larger inter-spacing between the fins offers larger area for cooling air but heating of air is less, so more cooling air is required. Smaller inter-spacing in fins results in smaller flow area for cooling air and hence air is heated more and input to the cooling air is less. However, the pressure drop required for flow is more. If the distance between to fins is quite small the laminar layers of the opposite areas come in contact with each other and the efficiency of the fins reduced very much. Therefore, a spacing less than 2.5 mm is not used.
The height of the fins controls the distance between the two cylinder and hence, the size of engine. Usually the fin height varies from 15 to 25 mm.
While fixing the fins care must be taken that the most heavily thermally stressed parts such as exhaust valves and exhaust manifolds, etc, are adequately finned. Air cooling result in higher engine temperature. This necessitates the provision of bigger clearance between various parts, especially between piston and arm. The upper part of the cylinder barrel is most critical part of the air cooled engine and usually limits the engine rating.
Applications of air cooling---- Air cooling is usually used for small engines and for engines whose application gives extreme importance to weight such as aircraft engines. Other areas for air cooled engines are industrial and agricultural engines where there can be strong objection to use water as coolant. MERITS OF AIR COOLING SYSTEM
i. ii. iii. iv. v. vi. vii. DIRECT TRANSFER OF HEAT ENGINE DESIGN BECOMES EASY ECONOMICAL NO POSSIBLITY OF COOLANT LEAKAGE OVERALL WEIGHT OF ENGINE REDUCES SOME AMOUNT OF DAMAGE CAN BE SUSTAINED LESS SENSITIVE TO EXTERNAL CONDITIONS
DEMERITS OF AIR COOLING SYSTEM
i. ii. iii. iv. v. vi. COOLING IS NOT UNIFORM AIR COOLING IS APPLIED ONLY TO SMALL AND MEDIUM SIZED ENGINES VOLUMETRIC EFFICIENCY REDUCES SPECIFIC FUEL CONSUMPTION INCREASES FAN IF USED USES 5% OF ENGINE POWER COOLING FINS USED MAY TEND TO VIBRATE AND MAY RAISES NOISE
