Lecture-23
Prepared under QIP-CD Cell Project
Internal Combustion Engines
Ujjw jjwal K Saha aha, Ph.D. Depar partment of Mec Mec hanic hanica al Engineer ngineering ing
Indian Indian Ins Institut itute e of Tec hno hnolog logy Guw G uwa ahat hati 1
Pur urp pose of Lubr ubric ica atio ion n Reduce uce the fr frict ic tiona ionall re resist istance nce of the engine to a minimum to ensure maximum mech ec hanic al eff effic ienc enc y.
Protec t the engine agains instt wear.
Serve as a cooling agent by picking up heat.
Remove all impurities lub lubrica icated re region.
from
the
Form a seal between piston rings and the c ylin ylinde der r wall lls s to pr preve ven nt blowby.
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Lubrication Systems Tw o Stro k e En g in e s Mist lubrication system Wet sump lubrication system Fo u r Stro k e Engines Dry sump lubrication system
Mist lubrication system is mainly employed in twostroke cycle engines, whereas wet and dry sump systems are used in four-stroke cycle engines. The wet sump system is employed in relatively small engines, such as automobile engines, while the dry sump system is used in large stationary, marine and aircraft engines. 3
Mist Lubrication Systems In two-stroke engines, the charge is compressed in the crankcase, and as such it is not suitable to have the lubricating oil in the sump. Therefore, such engines are lubricated by adding 3 % to 6 % oil in the fuel tank itself. The oil and fuel mixture is inducted through the carburetor. The fuel gets vaporized and the oil, in the form of mist, goes into the cylinder through the crankcase. The oil that impinges the crankcase walls lubricates the main and connecting rod bearings, and the rest of the oil lubricates the piston, piston rings and cylinder. The main advantage with this system lies in the simplicity and low cost as the system does not require any oil pump, filter etc.
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Wet Sump Lubrication Systems In the wet sump system, the bottom of the crankcase contains an oil sump (or pan) that serves as the oil supply reservoir. Oil dripping from the cylinders and bearings flows by gravity back into the wet sump where it is picked up by a pump and re-circulated through the engine lubricating system. The types of wet sump systems used are:
the splash and circulating pump system the splash and pressure system the full force-feed system
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Splash and Circulating Pump System C a m s h a ft
C o n n e c t in g r o d bearings M a in b e a rin g
m a in bearing Lower o il p a n
O il troughs
O il p u m p O il S t r a i n e r 6
Splash and Pressure System C a m s h a ft
C o n n e c t in g r o d bearings M a in b e a rin g
m a in bearing
O il p u m p O il S t r a i n e r 7
Full Force Feed System
Camshaft
End leakage from rod
conn ecting rod bea ring Main bearing
Header main bearing line
Oil pum p Oil Strainer 8
Dry Sump Lubricating System To Bearings O il Cooler
Vent Pressure release valve Supply Tank
Oil pum p E ngine crankcase Dry sum p
Strainer
Filter B y-P ass Presuure relief va lve
Filter
S c a v e n g in g p u m p 9
Properties of Lubricating Oil The oil used in an engine must serve as a lubricant, a coolant and an agent for removing impurities.
It must be able to withstand high temperatures without breaking down. The oil must operate over a good range of temperature.
They must not oxidize on the chamber walls, piston crown or at the piston rings. Oil should have high film strength to prevent metal-to-metal contact even under extreme loads.
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Properties of Lubricating Oil (a) Viscosity: Viscosity: The lubricating oil should have the correct viscosity so that it flows easily to all the moving parts. If oil viscosity is too high, more work is required to pump it and to shear it between the moving parts. This results in greater friction work, reduced brake work and hence reduced power output. Viscosity is highly dependent on temperature, increasing with decreasing temperature. The fuel consumption may increase by as much as 15 %.
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Rating of Lubricating Oil Lubricating oil is generally rated using a viscosity scale established by the SAE. Commonly used viscosity grades are:
SAE 5 SAE 10 SAE 20 SAE 30 SAE 40 SAE 45 SAE 50
The oil with lower viscosity grades is less viscous and is used in cold-weather operation. Modern high temperature, high speed, close tolerance engines use high viscosity grades oil.
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Rating of Lubricating Oil When certain polymers are added to oil, the temperature dependency of oil viscosity is reduced. These oils have low viscosity grades when they are cold and higher as they become hot.
As for example, SAE 10W-30 means that the oil has a grade 10 when it is cold (W stands for winter) and 30 when it is hot. Commonly used oils in this category are:
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 13
Rating of Lubricating Oil (b) Specific Gravity: This property is of little importance except as an indicator of weight and volume. The specific gravity of oil varies from 0.85 to 0.96.
(c) Pour Point: It indicates the temperature below which the lubricating oil loses its fluidity and will not flow or circulate in the system. This characteristics of the oil is important at low temperature Pour point must be at least 150F lower than the operating temperature to ensure maximum circulation. 14
Rating of Lubricating Oil (d) Oxidation Stability: Oxidation stability of an oil is its resistance to oxidation. Due to oxidation, oil forms deposits on the piston rings, and thereby loses its lubricating property. Some inhibitors are used to counteract these tendencies. (e) Acidity and Neutralization Number: The lubricating oil should have low acidity. The neutralization number is a measure of acidic or alkaline contents of oil. (f) Oiliness: Oiliness is responsible for the boundary layer of molecules that can adhere or cling to a metal surface and provide lubrication after most of the oil gets displaced or squeezed out. 15
Types of Lubricating Oil Vegetable oils have been used in the past, especially for racing car engines. The main advantages of these oils are their high film strength, and they have a good lubricity.
Later, specially formulated mineral oils have replaced their use in high performance engines. Mineral oils are most readily available and cost effective. They readily respond to additives, and can be produced in a wide range of viscosities. The main disadvantage lies with its wax content that affects cold performance and can clog filters.
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Classification of Lubricants •
•
•
•
Animal Vegetable Mineral Synthetic
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Animal Lubricants •
Lubricants with animal origin: Tallow Tallow oil Lard oil Neat’s foot oil Sperm oil Porpoise oil – – – – – –
•
•
These are highly temperatures
stable
at
normal
Animal lubricants may not be used for internal combustion because they produce fatty acids. 18
Vegetable Lubricants •
Examples of vegetable lubricants are: Castor oil, Olive oil, Cottonseed oil Animal and vegetable oils have a lower coefficient of friction than most mineral oils but they rapidly wear away steel. –
•
Mineral Lubricants •
•
These lubricants are used to a large extent in the lubrication of aircraft internal combustion engines. There are three classifications of mineral lubricants: Solid, Semisolid, Fluid –
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Synthetic L ubricants Lubricants •
•
Because of the high operating temperatures of gas-turbine engines, it became necessary to develop lubricants which would retain their characteristics at temperatures that cause petroleum lubricants to evaporate and break down. Synthetic lubricants do not break down easily and do not produce coke or other deposits.
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Additives
Corrosion and Rust Inhibitors
Anti-foam Agents
Detergent-Dispersants
Pour Point Improvers
Oiliness and Film-strength Agents
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Conclusions
In order to minimize friction between the moving parts and hence wear, lubrication system in an engine plays a significant role. The lubrication system is designed to deliver clean oil at the correct temperature and pressure to every part of the engine. Lubricating oil can be supplied to the various engine components by a splash system or by a pressurized system or a by a combination of both. In two-stroke engines, oil is mixed with the fuel itself. 22
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Wisley.
13. Srinivasan S, (2001), Automotive Engines , Tata McGraw Hill. 14. Stone R, (1992), Internal Combustion Engines , The Macmillan Press Limited, London. 15. Taylor CF, (1985), The Internal-Combustion Engine in Theory and Practice , Vol. 1 & 2, The MIT Press, Cambridge, Massachusetts. 23
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