汽车专业英语电子课件Unit5

1、Unit 5Cooling System and Lubrication Cooling System and Lubrication SystemSystemText AText A Cooling SystemCombustion temperatures sometimes reach as high as 4500; average temperature of combustion is close to 2000. Aluminum piston melt at about 1225, so this heat must be carried off rapidly to prev

2、ent engine damage. Most of this heat is not used to propel the piston and must be carried off by the cooling system.The purpose of the cooling system is to keep the engine at its most efficient operating temperature under all operating conditions and during all seasons of the year.There are two type

3、s of cooling systems used by automobile manufacturers, the air cooling system and the liquid cooling system.Text AText A Cooling System Air CoolingSome smaller engines are only cooled by air. Instead of circulating fluid through the engine, the engine block is covered in aluminum fins that conduct t

4、he heat away from the cylinder. A powerful fan forces air over these fins, which cools the engine by transferring the heat to the air.Text AText A Cooling System Liquid CoolingThe liquid cooling system (Fig.5.1) is more common. The cooling medium in them is either water or some low-freezing liquid,

5、called antifreeze. A liquid cooling system consists of shutter, radiator, radiator cap, cooling fan, necessary hoses, water pump, thermostat, water jacket and so on.In this system, the liquid coolant is circulated through the cylinder block and cylinder head. The coolant absorbs heat and circulates

6、back to the radiator where the heat is expelled or radiated to the atmosphere. The coolant is then returned to the engine to repeat this heat transfer process. Text AText A Cooling SystemText AText A Cooling System Water PumpThe coolant pump, usually called a water pump centrifugal impeller-type pum

7、p. It is mounted on the front of the engine and is belt driven by the crankshaft. Usually, the alternator uses the same belt and has a provision for adjusting the belt tension. If the belt slips, the pump will not pump as much coolant.An impeller fits in the circular pump housing. As it turns, centr

8、ifugal force draws coolant into the pump inlet near the center of the pump. It exits from the outlet at the outside of the pump. Text AText A Cooling System RadiatorThe radiator is mounted in front of the engine compartment just behind the grill. Air in front of the car is directed into the radiator

9、. The job of the radiator is to transfer the heat in the coolant to the outside air. A typical radiator consists of two tanks and a center core. The coolant pump circulates hot coolant out of the engine block and through a large radiator hose into the right side tank. The hot coolant then flows acro

10、ss the core of the radiator through long, narrow tubes. These tubes are made from aluminum or copper for good heat transfer.Text AText A Cooling System Radiator CapTodays liquid cooling system operates in an enclosed environment under a certain pressure. The amount of pressure is regulated by the ra

11、diator cap.The radiator cap serves not only preventing the coolant from splashing out the center core radiator but also preventing evaporation of coolant. The two valves in the radiator cap are the pressure valve and the vacuum valve.Text AText A Cooling System ThermostatThe temperature of the coola

12、nt must be regulated to be neither too hot nor too cold. A thermostat regulates the temperature of coolant leaving the block. When the coolant temperature is too low, it is trapped in the block until it heats up sufficiently to cause the thermostat to open.Text AText A Cooling System Water JacketsTh

13、e heat generated by the mixture burned in the engine must be transferred from the iron or aluminum cylinder to the water in the water jackets. Hollow water jackets are cast into all of the areas of the block and heads that are subjected to high temperatures. Coolant is circulated through these passa

14、ges to carry away excess heat. Text B Lubrication System Lubrication SystemInternal combustion engines require lubrication in operation to allow moving parts to slide smoothly over each other. Insufficient lubrication will subject the engine to rapid wear and ultimately, it may even seize up entirel

15、y. The lubrication system makes sure that every moving part in the engine gets oil so that it can move easily. The two main parts needing oil are the pistons (so they can slide easily in their cylinders) and bearings that allow things like the crankshaft and camshafts to rotate freely.A lubrication

16、system (Fig.5.2) consists of oil pan, gauze screen filter, oil pump, relief valve, oil filter, by-pass valve, main oil gallery and so on.Text B Lubrication System Lubrication SystemText B Lubrication System Lubrication SystemIn most cars, oil is sucked out of the oil pan by the oil pump, run through

17、 the oil filter to remove any grit, and then squirted under high pressure onto bearings and the cylinder walls. The oil then trickles down into the sump, where it is collected again and the cycle repeats. Two kinds of lubricating system have been used on modern automotive engines, splash and combina

18、tion splash and pressure feed. In the second one, oil pump takes oil from the oil pan and forces it through holes drilled in cylinder block and crankshaft.Text B Lubrication System Lubrication SystemThe lubricating oil circulating through the engine to all moving parts also performs other jobs. (1)

19、Lubricate surfaces between the moving parts to minimize wear and power loss from friction. (2) Remove heat of the moving parts by acting as a cooling agent.(3) Absorb shocks between bearings and other moving parts, to reduce engine noise and to extend engine life. (4) Form a good seal between piston

20、 rings and cylinder walls. (5) Be as a cleaning agent, washing the working surfaces free of chemical deposits, dirt and dust to protect them from corrosion.Text B Lubrication System Lubrication System Oil PumpIn most cars, the oil pump is in the crankcase above the sump. It draws oil through a tube

21、that extends downward into the sump. This tube has a filter screen over its bottom end. The screen keeps large pieces of sludge and dirt from being drawn into the pump. The oil pump usually has a drive shaft that is driven by a gear on the camshaft, although some oil pumps are driven by the cranksha

22、ft. Pumps that are driven by the cam gear are usually connected to the bottom of the distributor by a driveshaft with a tang or hex on its end. On the bottom of most distributors, there is a gear that meshes with the gear on the camshaft.Text B Lubrication System Lubrication SystemThere are three ty

23、pes of oil pumps(Fig.5.3). They are the external gearFig.5.3(a), the rotorFig.5.3(b) and the internal/external gear (which is similar to a rotor pump) Fig.5.3(c).Internal/external pumps save space. They are driven by the crankshaft. External gear pumps and rotor pumps are the most common types. Cast-iron pumps are considered to be better