ENGINE LUBRICATION SYSTEMS There are three basic types of oil distribution systems used in engines: splash, pressurized, or 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 (see the above figure). 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 cylin-der 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. 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 pre-heaters 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. LUBRICATING OIL
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 blow-by contamination of the remaining oil. HC levels in the exhaust were high because of the oil in the combustion chamber. A rule in the 1950s and 1960s was to have an oil change in an automobile every 1000 miles. 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. Lubricating oil must satisfy the following needs:
lubricant that will allow for maximum performance and life span of the engine. These additives include: 1. Antifoam agents These reduce the foaming that would result when the crankshaft and other components rotate at high speed in the crankcase oil sump. 2. Oxidation inhibitors Oxygen is trapped in the oil when foaming occurs, and this leads to possible oxidation of engine components. One such additive is zinc dithiophosphate. 3. Pour-point depressant 4. Antirust agents 5. Detergents
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. 6. Antiwear agents 7. Friction reducers 8. Viscosity index improvers Viscositv. Lubricating oils are generally rated using a viscosity scale established by the Society of Automotive Engineering (SAE). Dynamic viscosity is defined from the equation Ts = /-t(dUjdy) where: Ts = shear force per unit area /-t = dynamic viscosity (dUjdy) = velocity gradient 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 (Fig. 11-11). 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, duj dy, 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: SAE 5 SAE 10 SAE 20 SAE 30
SAE 40 SAE 45 SAE 50 The oils with lower numbers are less viscous and are used in coldweather operation. Those with higher numbers are more viscous and are used in modern hightemperature,
high-speed,
close-
tolerance engines.
If oil viscosity is too high, more work is required to pump it and to shear it between moving parts. These results in greater friction work and reduced brake work and power output. Fuel consumption can be increased by as much as 15%.
Starting
a
cold
engine
lubricated with high-viscosity oil is very difficult (e.g., an automobile at -20°C or a lawn mower at 100e). Multi-grade oil was developed so that viscosity would be more constant over the operating temperature range of an engine. When certain polymers are added to oil, the temperature dependency of the oil viscosity is reduced, as shown in Fig. 11-12. These oils have low-number viscosity values when they are cold and higher numbers when they are hot. A value such as SAE lOW-30 means that the oil has properties of 10 viscosity when it is cold (W = winter) and 30 viscosity when it is hot. This gives a more constant viscosity over the operating temperature
range (Fig.11-12). This is extremely important when starting a cold engine. When the engine and oil are cold, the viscosity must be low enough so that the engine can be started without too much difficulty. The oil flows with less resistance and the engine gets proper lubrication. It would be very difficult to start a cold engine with high-viscosity oil, because the oil would resist engine rotation and poor lubrication would result because of the difficulty in pumping the oil. On the other hand, when the engine gets up to operating temperature, it is desirable to have a higher viscosity oil. High temperature reduces the viscosity, and oil with a low viscosity number would not give adequate lubrication. Some studies show that polymers added to modify viscosity do not lubricate as well as the base hydrocarbon oils. At cold temperatures SAE 5 oil lubricates better than SAE 5W-30, and at high temperatures SAE 30 oil lubricates better. However, if SAE 30 oil is used, starting a cold engine will be very difficult, and poor lubrication and very high wear will result before the engine warms up. Common oils available include: SAE 5W-20 SAE 10W-40 SAE 5W-30 SAE lOW-50 SAE 5W-40 SAE 15W-40 SAE 5W-50 SAE 15W-50 SAE lOW-30 SAE 20W-50
Synthetic Oils 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. This 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. 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.
