Engine Cooling And Lubrication System
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Cooling System: Despite the vast improvement in the basic internal combustion engines, around 70% of the energy from the gasoline is converted to heat. As it is not dissipated to the atmosphere on its own, a cooling system is employed for this purpose.
Several purposes of the cooling system which it serves by cooling the engine include cooling the engine to keep it from overheating by transferring the heat to the air. This helps avoid the excessive wear and tear at high temperatures, auto-ignition due to hot cylinder which may result in knocking and hence, piston/cylinder failure. It may also incorporate thermal stresses which is not good for the engine itself.
Figure . Cooling system and plumbing connection
While it serves the purpose of cooling the engine, it also helps the engine to warm up quickly during cold start ups and then maintaining a constant temperature. When the engine is cold, components wear out faster too and the engine is less efficient, emitting more pollution.
Types of Cooling System:
There are two types of cooling systems found in cars:
Many small and the medium-sized engines are air-cooled. This category includes most small engines like lawn mowers, chain saws, model airplanes etc. Using the air-cooled system allows both the weight and price of the engine to be kept low, along with reduced complexity of the machine.
The air-cooled system is still widely used on most of the motorcycles in use these days. This system utilizes the concept of heat transfer through fins to cool the engine. The cross-sectional area of the fin being larger closer to the head and a reduction in the area as we move further from the engine block.
The basic principle on which the air-cooled engines rely on is the flow of air across their external surfaces to remove the excess heat to keep the engine from overheating. The airflow on machines like motorcycles and aircrafts is provided across the surface when the vehicle moves forward. Deflectors and ductwork is incorporated to direct the airflow to the critical locations where more cooling is required. The outer surface of the engine is made from a good conductor of heat and the surface is finned to promote maximum heat transfer, along with which an extra fan is used to increase the air-flow rate; whereas others use the concept of free-convection. These fins are to be properly designed for appropriate cooling effect which is required.
Some automobile engines also use exposed flywheels with air-deflectors fastened to the surface. When the engine is in operation, these deflectors create air motion which increases the heat transfer on the finned surface.
Even after considering and applying all the measures, the uniform cooling of cylinders is still difficult to achieve on air-cooled engines as compared to the liquid-cooled engines. The figure below shows that the cooling needs are not the same at all the locations.
Figure . Variation of heat losses from the fins of an air-cooled aircraft engine. Seventy-one percent of the heat losses occur on the hotter side of the cylinder, containing the exhaust valve. The engine shown was used on a number of different aircrafts.
Hotter areas, such as the ones around the exhaust valve and manifold need greater cooling and hence larger finned surface area. Cooling the front of an air-cooled engine which faces the forward motion of the vehicle is much easier and efficient as compared to the back surface of the engine. This may result in temperature differences and thermal expansion problems.
Disadvantages of air-cooled engines are that they:
Are less efficient,
Are noisier, with greater air flow requirements and no water jacket to dampen the noise,
Need a directed air flow and finned surfaces.
When compared with liquid-cooled engines, air-cooled engines have the following advantages:
They are lighter in weight,
They cost less,
No coolant system failures (e.g., water pump, hoses),
No engine freeze-ups, and
Faster engine warmup.
In a liquid or water-cooled engine, the engine block is surrounded by a water jacket through which the coolant flows. This allows for a better control of the heat removal from the engine, just by added weight and a more complex system.
Very few water-cooled engines use just water as the cooling fluid in the water jackets; this is because the water has a freezing temperature of 0°C which is unacceptable as coolant in colder regions, so additives are usually used for better performance. Although water has very good heat transfer properties, but when used alone, it causes rust and corrosion in many of the pipes of the cooling system.
Ethylene glycol (C2H6O2) is the antifreeze agent which acts as a rust inhibitor and a lubricant for the water pump. When added to water, it lowers the freezing temperature and raises the boiling temperature of the coolant. The properties of the mixture depend on the ratio in which water and the antifreeze agent are mixed. Pure ethylene glycol should not be used, and even at high concentrations the heat transfer properties of the water are lost as well. The properties of the ethylene glycol - water mixture are shown in the table below.
In addition to good thermal properties, a coolant should satisfy the following requirements:
1. Chemically stable under conditions of use
4. Low toxicity
6. Low cost
Most commercial antifreezes satisfy these requirements. Many of them are basically ethylene glycol with small amounts of additives. Some commercial engine coolants use propylene glycol as the base ingredient. It is argued that when coolant systems leak or when the coolant becomes aged and is discarded, these products are less harmful to the environment than ethylene glycol.
The basic components of a liquid-cooled system is shown below.
Figure . Basic liquid-cooled system
radiator top hose
radiator bottom hose
electric cooling fan
The radiator is the part of the cooling system which is responsible for the heat rejection from the coolant and into the atmosphere. The radiator core is usually made up of flattened tubes with aluminum strips (fins) that zigzag between the tubes. These fins effectively transfer the heat contained in the coolant into the air stream to be lost into the atmosphere. On each end of the radiator is a tank made up of plastic to cover the ends. The tubes either run horizontally or vertically between the two tanks. The aluminum-plastic system is more efficient and cost effective.
On radiators with plastic end caps, there are gaskets between the aluminum core and the plastic tanks to seal the system and keep the fluid from leaking out. The tanks have a large hose connection, one mounted towards the top of the radiator to let the coolant in, the other mounted at the bottom of the radiator on the other tank to let the coolant back out. On the top of the radiator is an additional opening that is capped off by the radiator cap.
Another component in the radiator for vehicles with an automatic transmission is a separate tank mounted inside one of the tanks. Fittings connect this inner tank through steel tubes to the automatic transmission. Transmission fluid is piped through this tank inside a tank to be cooled by the coolant flowing past it before returning to the transmission.
One or two electric fans are mounted on the back of the radiator close to the engine. These fans used the concept of forced convection to cool the heated coolant going through the pipes in the radiator core.
If noticed, this fan starts working once the engine reaches a predefined temperature, after which the cooling by just natural convection during the forward motion of the car cannot be achieved. In the cars with air conditioning, there is an additional radiator mounted in front of the normal radiator. This radiator is called the air conditioner condenser, which also needs to be cooled by the air flow entering the engine compartment. As long as the air conditioning is turned on, the system will keep the fan running, even if the engine is not running hot. This is because if there is no air flow through the air conditioning condenser, the air conditioner will not be able to cool the air entering the interior.
Pressure cap & reserve tank:
The pressure cap is simply a cap which maintains the pressure in the cooling system up to a certain point. If the pressure builds up higher than the set pressure point, the spring loaded valve releases the pressure.
Figure . Pressure cap
When the pressure in the cooling system reaches the point when the cap needs to release this excess pressure, some amount of coolant is bled off. The coolant which is bled off goes into the reserve tank which is not pressurized, which causes a partial vacuum in the cooling system.
The radiator cap on these closed systems has a secondary valve which allows the vacuum in the cooling system to draw the coolant back from the reserve tank into the radiator.
It is a simple pump which helps in circulation of the coolant around the system. This pump is run using one of the following:
A fan belt that will also be responsible for driving an additional component like an alternator or power steering pump
A serpentine belt, which also drives the alternator, power steering pump and AC compressor among other things.
The timing belt that is also responsible for driving one or more camshafts.
The impeller of the pump uses centrifugal force to draw the coolant in from the lower radiator hose and send it under pressure to the engine block. A gasket seals the water pump to the engine block and prevents the flowing coolant from leaking out where the pump is attached to the block.
The thermostat is simply a valve that measures the temperature of the coolant, and if the coolant is hot enough it opens to allow the coolant to flow through the radiator otherwise the flow to the radiator is blocked and the fluid is directed to a bypass system that returns the coolant to the engine.
Figure . Thermostat
The engine is at times allowed to run at higher temperatures of 190-195°C; this reduces emissions, moisture condensation inside the engine is quickly burned off improving engine life, and a more complete combustion improving fuel economy.
Oil as a Coolant:
The oil when used to lubricate the engine also helps to cool the engine. The piston for example gets very little cooling from the coolant in the water jacket or the externally finned surface, so when the back surface of the piston crown is subjected to the oil splash or flow the piston is cooled to some extent. This is very necessary as the piston is one of the hottest elements in the engine. Usually, the oil is sprayed in pressurized systems, and splashed in non-pressurized systems. The oil acts as the coolant on the back face of the piston crown as it absorbs energy and then runs back into the larger reservoir where it mixes with the cooler oil and dissipates this energy into the other engine parts. This splash cooling of the piston is extremely important in small air-cooled engines as well as in automobile engines.
A few other engine components other than the piston are also cooled by oil circulation, either by splash or by the pressurized flow from the oil pump. Oil passages through internal components like the camshaft and connecting rods offer the only major cooling these parts are subjected to. As the oil cools the various components, it absorbs energy and its temperature rises. This energy is then dissipated to the rest of the engine by circulation and eventually gets absorbed in the engine coolant flow. Some high-performance engines have an oil cooler in their lubricant circulation system. The energy absorbed by the oil as it cools the engine components is dissipated in the oil cooler, which is a heat exchanger cooled by either engine coolant flow or external air flow.
Oil Pump: The gear-type oil pump has a pair of meshing gears. The spaces between the teeth are filled with oil when the gears unmesh. The oil pump obtains oil from the oil pan and sends oil through the oil filter to the oil galleries and main bearings. Some oil passes from the holes in the crankshaft to the rod bearings. Main bearings and rod bearings are lubricated adequately to achieve their desired objectives. In the rotor type oil pump, the inner rotor is driven and drives the outer rotor. As the rotor revolves, the gaps between the lobes are filled with oil. When the lobes of the inner rotor move into the gaps in the outer rotor, oil is forced out through the outlet of pump. An oil pump can also be driven by a camshaft gear that drives the ignition distributor or by the crankshaft.
Oil Pan: Oil also flows to the cylinder head through drilled passages that make up the oil gallery, lubricates camshaft bearings and valves, and then returns to oil pan. Some engines have grooves or holes in connecting rods, which provide extra lubrication to pistons and walls of cylinders.
Oil Cooler: Oil cooler prevents overheating of oil, by flow of engine coolant past tubes carrying hot oil. The coolant picks excess heat and carries it to the radiator.
Oil Filter: The oil from oil pump flows through oil filter before reaching the engine bearings. The oil filter retains the dirt particles and allows only clean filtered oil to pass.
The Lubrication system and its types:
There are three basic types of oil distribution systems used in engines:
A combination of these.
The crankcase is used as the oil sump (reservoir) in a splash system, and the crankshaft rotating at high speed in the oil distributes it to the various moving parts by splash; no oil pump is used. All components, including the valve train and camshaft, must be open to the crankcase. Oil is splashed into the cylinders behind the pistons and onto the back of the piston crowns, acting both as a lubricant and a coolant. Many small four-stroke cycle engines (lawn mowers, golf carts, etc.) use splash distribution of oil.
An engine with a pressurized oil distribution system uses an oil pump to supply lubrication to the moving parts through passages built into the components. A typical automobile engine has oil passages built into the connecting rods, valve stems, push rods, rocker arms, valve seats, engine block, and many other moving components. These make up a circulation network through which oil is distributed by the oil pump. In addition, oil is sprayed under pressure onto the cylinder walls and onto the back of the piston crowns. Most automobiles actually use dual distribution systems, relying on splash within the crankcase in addition to the pressurized flow from the oil pump. Most large stationary engines also use this kind of dual system. Most aircraft engines and a few automobile engines use a total pressurized system with the oil reservoir located separate from the crankcase. These are often called dry sump systems (i.e., the crankcase sump is dry of excess oil). Aircraft do not always fly level, and uncontrolled oil in the crankcase may not supply proper lubrication or oil pump input when the plane banks or turns. A diaphragm controls the oil level in the reservoir of a dry sump system, assuring a continuous flow into the oil pump and throughout the engine.
Figure . Lubrication of an engine consisting of a combination of a pressurized system and splash system
Oil pumps can be electric or mechanically driven off the engine. Pressure at the pump exit is typically about 300 to 400 kPa. If an oil pump is driven directly off the engine, some means should be built into the system to keep the exit pressure and flow rate from becoming excessive at high engine speeds.
A time of excess wear is at engine startup before the oil pump can distribute proper lubrication. It takes a few engine cycles before the flow of oil is fully established, and during this time, many parts are not properly lubricated. Adding to the problem is the fact that often the oil is cold at engine startup. Cold oil has much higher viscosity, which further delays proper circulation. A few engines have oil preheaters which electrically heat the oil before startup. Some engines have pre-oilers that heat and circulate the oil before engine startup. An electric pump lubricates all components by distributing oil throughout the engine.
It is recommended that turbocharged engines be allowed to idle for a few seconds before they are turned off. This is because of the very high speeds at which the turbocharger operates. When the engine is turned off, oil circulation stops and lubricated surfaces begin to lose oil. Stopping the oil supply to a turbocharger operating at high speed invites poor lubrication and high wear. To minimize this problem, the engine and turbocharger should be allowed to return to low speed (idle) before the lubrication supply is stopped.
Lubrication system in 2-stroke engines:
Many small engines and some experimental two-stroke cycle automobile engines use the crankcase as a compressor for the inlet air. Automobile engines which do this generally have the crankcase divided into several compartments, with each cylinder having its own separate compressor. These engines cannot use the crankcase as an oil sump, and an alternate method must be used to lubricate the crankshaft and other components in the crankcase. In these engines, oil is carried into the engine with the inlet air in much the same way as the fuel. When the fuel is added to the inlet air, usually with a carburetor, oil particles as well as fuel particles are distributed into the flow. The air flow then enters the crankcase, where it is compressed. Oil particles carried with the air lubricate the surfaces they come in contact with, first in the crankcase and then in the intake runner and cylinder.
In some systems (model airplane engines, marine outboard motors, etc.), the oil is premixed with the fuel in the fuel tank. In other engines (automobiles, some golf carts, etc.), there is a separate oil reservoir that feeds a metered flow of oil into the fuel supply line or directly into the inlet air flow. Fuel-to-oil ratio ranges from 30:1 to 400:1, depending on the engine. Some modern high-performance engines have controls which regulate the fuel-oil ratio, depending on engine speed and load. Under conditions of high oil input, oil sometimes condenses in the crankcase. Up to 30% of the oil is recirculated from the crankcase in some automobile engines. It is desirable to get at least 3000 miles per liter of oil used. Most small lower cost engines have a single average oil input setting. If too much oil is supplied, deposits form on the combustion chamber walls and valves will stick (if there are valves). If too little oil is supplied, excess wear will occur and the piston can freeze in the cylinder.
Engines that add oil to the inlet fuel obviously are designed to use up oil during operation. This oil also contributes to HC emissions in the exhaust due to valve overlap and poor combustion of the oil vapor in the cylinders. New oils that also burn better as fuel are being developed for two-stroke cycle engines. Some two-stroke cycle automobile engines and other medium- and large-size engines use an external supercharger to compress inlet air. These engines use pressurized/ splash lubrication systems similar to those on four-stroke cycle engines with the crankcase also serving as the oil sump.
The oil used in an engine must serve as a lubricant, a coolant, and a vehicle for removing impurities. It must be able to withstand high temperatures without breaking down and must have a long working life. The development trend in engines is toward higher operating temperatures, higher speeds, closer tolerances, and smaller oil sump capacity. All of these require improved oils compared to those used just a few years ago. Certainly, the technology of the oil industry has to continue to improve along with the technology growth of engines and fuel. Early engines and other mechanical systems were often designed to use up the lubricating oil as it was used, requiring a continuous input of fresh oil. The used oil was either burned up in the combustion chamber or allowed to fall to the ground. Just a couple of decades back, the tolerances between pistons and cylinder walls was such that engines burned some oil that seeped past the pistons from the crankcase. This required a periodic need to add oil and a frequent oil change due to blowby contamination of the remaining oil. HC levels in the exhaust were high because of the oil in the combustion chamber.
Modern engines run hotter, have closer tolerances which keep oil consumption down, and have smaller oil sumps due to space limitations. They generate more power with smaller engines by running faster and with higher compression ratios. This means higher forces and a greater need for good lubrication. At the same time, many manufacturers now suggest changing the oil every 6000 miles. Not only must the oil last longer under much more severe conditions, but new oil is not added between oil changes. Engines of the past that consumed some oil required periodic makeup oil to be added. This makeup oil mixed with the remaining used oil and improved the overall lubrication properties within the engine.
The oils in modern engines must operate over an extreme temperature range. They must lubricate properly from the starting temperature of a cold engine to beyond the extreme steady-state temperatures that occur within the engine cylinders. They must not oxidize on the combustion chamber walls or at other hot spots such as the center crown of the piston or at the top piston ring. Oil should adhere to surfaces so that they always lubricate and provide a protective covering against corrosion. This is often called oiliness. Oil should have high film strength to assure no metal-to-metal contact even under extreme loads. Oils should be non-toxic and non-explosive.
Some desired qualities of Lubrication oil:
Lubricating oil must satisfy the following needs:
Lubrication. It must reduce friction and wear within the engine. It improves efficiency by reducing the friction forces between moving parts.
Removal of contaminants
Enhancement of ring seal and reduction of blowby
Stability over a large temperature range
Long life span
Hydrocarbon Components in Lubricating oil:
The basic ingredients in most lubricating oils are hydrocarbon components made from crude oil. These are larger molecular weight species obtained from the distillation process.
Various other components are added to create a lubricant that will allow for the maximum performance and life span of the engine. These additives include:
These reduce the foaming that would result when the crankshaft and other components rotate at high speed in the crankcase oil sump.
Oxygen is trapped in the oil when foaming occurs, and this leads to possible oxidation of engine components. One such additive is zinc dithiophosphate
These are made from organic salts and metallic salts. They help keep deposits and impurities in suspension and stop reactions that form varnish and other surface deposits. They help neutralize acid formed from sulfur in the fuel.
Viscosity index improvers
Rating of Lubricating Oils and grades:
Lubricating oils are generally rated using a viscosity scale established by the Society of Automotive Engineering (SAE).
The higher the viscosity value, the greater is the force needed to move adjacent surfaces or to pump oil through a passage. Viscosity is highly dependent on temperature, increasing with decreasing temperature. In the temperature range of engine operation, the dynamic viscosity of the oil can change by more than an order of magnitude. Oil viscosity also changes with shear, decreasing with increasing shear. Shear rates within an engine range from very low values to extremely high values in the bearings and between piston and cylinder walls. The change of viscosity over these extremes can be several orders of magnitude. Common viscosity grades used in engines are:
Common oils available include:
SAE 5W-20 SAE 10W-40
SAE 5W-30 SAE 10W-50
SAE 5W-40 SAE 15W-40
SAE 5W-50 SAE 15W-50
SAE 10W-30 SAE 20W-50
A number of synthetically made oils are available that give better performance than those made from crude oil. They are better at reducing friction and engine wear, have good detergency properties which keep the engine cleaner, offer less resistance for moving parts, and require less pumping power for distribution. With good thermal properties, they provide better engine cooling and less variation in viscosity. Because of this, they contribute to better cold-weather starting and can reduce fuel consumption by as much as 15%. These oils cost several times as much as those made from crude oil. However, they can be used longer in an engine, with 24,000 km (15,000 miles) being the oil change period suggested by most manufacturers.
Available on the market are various oil additives and special oils that can be added in small quantities to standard oils in the engine. These claim, with some justification, to improve the viscous and wear resistance properties of normal oils. One major improvement that some of them provide is that they stick to metal surfaces and do not drain off when the engine is stopped, as most standard oils do. The surfaces are thus lubricated immediately when the engine is next started. With standard oils it takes several engine rotations before proper lubrication occurs, a major source of wear.
Included in most pressurized oil systems is a filtration system to remove impurities from the engine oil. One of the duties of engine oil is to clean the engine by carrying contaminant impurities in suspension as it circulates. As the oil passes through filters that are part of the flow passage system these impurities are removed, cleaning the oil and allowing it to be used for a greater length of time. Contaminants get into an engine in the incoming air or fuel or can be generated within the combustion chamber when other than ideal stoichiometric combustion occurs. Dust and other impurities are carried by the incoming air. Some, but not all, of these are removed by an air filter.
Fuels have trace amounts of impurities like sulfur, which create contaminants during the combustion process. Even pure fuel components form some contaminants, like solid carbon in some engines under some conditions. Many engine impurities are carried away with the engine exhaust, but some get into the interior of the engine, mainly in the blowby process. During blowby, fuel, air, and combustion products are forced past the pistons into the crankcase, where they mix with the engine oil. Some of the water vapor in the exhaust products condenses in the crankcase, and the resulting liquid water adds to the contaminants. The gases of blowby pass through the crankcase and are routed back into the air intake. Ideally, most of the contaminants are trapped in the oil, which then contains dust, carbon, fuel particles, sulfur, water droplets, and many other impurities. If these were not filtered out of the oil, they would be spread throughout the engine by the oil distribution system. Also, the oil would quickly become dirty and lose its lubricating properties, resulting in greater engine wear.
Figure . Oil Filter
Flow passages in a filter are not all the same size but usually exist in a normal bell-shaped size distribution. This means that most larger particles will be filtered out as the oil passes through the filter, but a few as large as the largest passages will get through. The choice of filter pore size is a compromise. Better filtration will be obtained with smaller filter pores, but this requires a much greater flow pressure to push the oil through the filter. This also results in the filter becoming clogged quicker and requiring earlier filter cartridge change. Some filter materials and/or material of too small a pore size can even remove some additives from the oil. Filters are made from cotton, paper, cellulose, and a number of different synthetic materials. Filters are usually located just downstream from the oil pump exit. As a filter is used, it slowly becomes saturated with trapped impurities. As these impurities fill the filter pores, a greater pressure differential is needed to keep the same flow rate. When this needed pressure differential gets too high, the oil pump limit is reached and oil flow through the engine is slowed. The filter cartridge should be replaced before this happens.
Figure . Exploded view of an Oil Filter
Figure . Pore size distribution for common filters
Sometimes, when the pressure differential across a filter gets high enough, the cartridge structure will collapse and a hole will develop through the cartridge wall. Most of the oil pumped through the filter will then follow the path of least resistance and flow through the hole. This short circuit will reduce the pressure drop across the filter, but the oil does not get filtered.
There are several ways in which the oil circulation system can be filtered:
1. Full-flow oil filtration. All oil flows through the filter. The filter pore size must be fairly large to avoid extreme pressures in the resulting large flow rate. This results in some larger impurities in the oil.
2. Bypass oil filtration. Only part of the oil leaving the pump flows through the filter, the rest bypassing it without being filtered. This system allows the use of a much finer filter, but only a percentage of the oil gets filtered during each circulation loop.
3. Combination. Some systems use a combination of full-flow and bypass. All the oil first flows through a filter with large pores and then some of it flows through a second filter with small pores.
4. Shunt filtration. This is a system using a full-flow filter and a bypass valve. All oil at first flows through the filter. As the filter cartridge dirties with age, the pressure differential across it needed to keep the oil flowing increases. When this pressure differential gets above a predetermined value, the bypass valve opens and the oil flows around the filter. The filter cartridge must then be replaced before filtering will again occur.
Solid lubricants, such as powdered graphite, have been developed and tested in some engines. These are attractive for adiabatic engines and engines using ceramic components, which generally operate at much higher temperatures. Solid lubricants remain functional at high temperatures that would break down and destroy more conventional oils. Distribution is a major difficulty when using solid lubricants.
Figure . Lubrication System Block diagram
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