Internal Combustion Engines and Reciprocating machines

1. Engine Friction and Lubrication
System
By : Yitagesu Tesfaye
student at Debre Berhan University
Mechanical Engineering Department

2. Introduction
 Friction generally refers to forces acting between surfaces in relative
motion.
 In engines frictional losses are mainly due to sliding as well as rotating parts.
 Usually engine friction is expressed in terms of frictional power (𝑓𝑝).
Frictional loss is mainly attributed to the following mechanical losses.
I. Direct frictional losses
II. pumping losses
III. power loss to drive the components to charge and scavenge
IV. power loss to drive other auxiliary components

3. I. Direct frictional losses
 It is the power absorbed due to the relative motion of different bearing surfaces such as piston rings,
main bearings, cam shaft bearings etc.
II. Pumping losses
 The pumping loss is the net power spent by the engine (piston) on the working medium
(gases) during intake and exhaust strokes.
 In the case of two-stroke this is negligible since the incoming fresh mixture is used to scavenge the
exhaust gases.
III. power loss to drive the components to charge and
scavenge
 This loss is considered as negative friction loss.
 (For example supercharged and turbocharged for four stroke engines takes power from
the engine output.)
 In case of two-stroke engines with a scavenging Pump the power to drive the pump is
supplied by the engine.

4. IV. power loss to drive other auxiliary-
components
 A good percentage of the generated power output is spent to drive auxiliaries such as water pump ,lubricating oil pump, fuel pump, cooling fan,
generator etc. This is considered a loss because the presence of each of these components reduces the net output of the engine.
Mechanical Efficiency
 Mechanical losses can be written in terms of mean effective pressure that is frictional torque divided by engine displacement volume per unit time.
Therefore, frictional mean effective pressure, 𝑓𝑚𝑒𝑝 can be expressed as
𝑓𝑚𝑒𝑝 = 𝑚𝑚𝑒𝑝 + 𝑝𝑚𝑒𝑝 + 𝑎𝑚𝑒𝑝 + 𝑐𝑚𝑒𝑝
𝑤ℎ𝑒𝑟𝑒 𝑚𝑚𝑒𝑝 = 𝑚𝑒𝑝 𝑟𝑒𝑞𝑢𝑖𝑟𝑒𝑑 𝑡𝑜 𝑜𝑣𝑒𝑟𝑐𝑜𝑚𝑒 𝑚𝑒𝑐ℎ𝑎𝑛𝑖𝑐𝑎𝑙 𝑓𝑟𝑖𝑐𝑡𝑖𝑜𝑛
𝑝𝑒𝑚𝑝 = 𝑚𝑒𝑝 𝑟𝑒𝑞𝑢𝑖𝑟𝑒𝑑 𝑓𝑜𝑟 𝑐ℎ𝑎𝑟𝑔𝑖𝑛𝑔 𝑎𝑛𝑑 𝑠𝑐𝑎𝑣𝑒𝑛𝑔𝑖𝑛𝑔
𝑎𝑚𝑒𝑝 = 𝑚𝑒𝑝 𝑟𝑒𝑞𝑢𝑖𝑟𝑒𝑑 𝑡𝑜 𝑑𝑟𝑖𝑣𝑒 𝑡ℎ𝑒 𝑎𝑢𝑥𝑖𝑙𝑖𝑎𝑟𝑦 𝑐𝑜𝑚𝑝𝑜𝑛𝑒𝑛𝑡𝑠
𝑐𝑚𝑒𝑝 = 𝑚𝑒𝑝 𝑟𝑒𝑞𝑢𝑖𝑟𝑒𝑑 𝑡𝑜 𝑑𝑟𝑖𝑣𝑒 𝑡ℎ𝑒 𝑐𝑜𝑚𝑝𝑟𝑒𝑠𝑠𝑜𝑟 𝑜𝑟 𝑠𝑐𝑎𝑣𝑒𝑛𝑔𝑖𝑛𝑔 𝑝𝑢𝑚𝑝
 A knowledge of engine friction is essential for calculating the mechanical efficiency of the engine.
Remember: mechanical efficiency is the ratio of brake power to indicated power.

5. Mechanical Friction
 mainly mechanical friction may have six classes. Which are the following
1. Fluid-film or Hydrodynamic friction
 It is associated with the phenomenon when a complete film of lubricant exists between the two bearing surfaces.
 In this case the friction force entirely depends on the lubricant viscosity.
 This type of friction is the main mechanical friction loss in the engine.
2. Partial-film Friction
 When rubbing (metal) surfaces are not sufficiently lubricated, there is a contact between the rubbing surfaces in some regions.
 During normal engine operation there is almost no metallic contact except
between the compression (top) piston ring and cylinder walls.
 This is mainly at the end of each stroke where the piston velocity is nearly zero.
 During starting of the engine, the journal bearings operate in partial-film friction
 Thus partial-film friction contributes very little to total engine friction and hence, it may be neglected.

6. 3. Rolling friction
 The rolling friction is due to rolling motion between the two surfaces. Ball and roller bearings and tappet
rollers are subjected to rolling friction.
 This friction is partly due to local rubbing from distortion under local and partly due to continues calming of
roller.
 Rolling friction coefficient is lower than journal bearing friction coefficient during starting and initial running
engine because the oil viscosity is high and moreover, partial friction exists in journal bearing during starting
where engine uses plain journal bearings on the crankshaft.
 Rolling friction is negligible compared to total friction.
4. Dry Friction
 Even when an engine is not operated for a long time there is little possibility for direct metal to metal
contact.
 It is safely neglected while considering engine friction.
5. Journal Bearing friction
 A circular cylindrical shaft called journal rotates against a cylindrical surface called the bearing.
 Journal bearings are called partial when the bearing surface is less than full circumference.
 Engine journal bearing operates under load which varies in magnitude and direction with time.

7. 6. Friction due to Piston Motion
 Friction due to the motion of piston can be divided into
i. viscous friction to piston
ii. non-viscous friction due to piston ring
The non-viscous piston ring friction can be further subdivided into
i. friction due to tension.
ii. friction due to gas pressure behind the ring.
 Average oil film thickness between the piston and the cylinder wall varies with load and speed.
 piston friction also depends upon the viscosity of the oil and the temperature at the various points on
the piston.
 Piston rings are categorized into compression rings and oil rings

10. Factors affecting the mechanical friction
 Various factors affect mechanical friction.
A. Engine Design
The design parameters which influence the friction Losses are:
I. Stroke-bore Ratio: Lower stroke-bore ratio may tend to slightly decrease the 𝑓𝑚𝑒𝑝. It is
mainly due to less frictional area in case of lower stroke to bore ratio.
II. Effect of engine size: Larger engines have more frictional surface, therefore requires high
lubrication.
III. Piston rings: Reducing the number of piston rings and reducing the contacting surface of
the ring with cylinder wall reduces the friction.
IV. Compression Ratio: the 𝑓𝑚𝑒𝑝 increases with increase in compression ratio.
V. Journal Bearings: Reducing journal diameter/ diametrical clearance ratio in journal
bearing reduces the 𝑓𝑚𝑒𝑝.
B. Engine Speed
 Friction increases rapidly with increasing speed.
C. Engine Load
 Increasing the load increases the maximum pressure in the cylinder which results in slight increase in friction
values.

11. D. Cooling Water Temperature
 A rise in cooling water temperature slightly reduces engine friction by reducing oil viscosity.
 Friction losses are high during starting since temperature of water and oil are low and viscosity is high.
E. Oil Viscosity
 viscosity and friction loss are (directly) proportional to each other.
 The viscosity can be reduced by increasing the temperature of the oil.
 But beyond a certain value of oil temperature, failure of local film may occur resulting in partial fluid film
friction or even metal to metal contact which is very harmful to the engine.
F. LUBRICATION
 FUNCTIONS OF THE LUBRICATION SYSTEM
 Lubrication: is essential to reduce friction and wear between the components in an
engine.
 COOLING: The lubricating oil carries away the heat from the component, which is lubricated.
 CLEANING: The engine oil has an ability to clean all the engine components, which are in contact
with it.
 Sealing: The engine oil helps the piston rings to form a tight seal between the rings and the
cylinder walls.

13. Lubrication System
 The function of a lubrication system is to provide sufficient quantity of cool, filtered oil to give
positive and adequate lubrication to all the moving parts of an engine.
 The various lubrication system used in IC engines are classified as
I. mist lubrication system
II. wet sump lubrication system
III. dry sump lubrication system
I. Mist Lubrication System
 It is Used in 2 Stroke engines. The oil is mixed in the fuel by 2 to 3% in the fuel tank. The oil
and the fuel mixture is inducted through the carburetor. The gasoline is vaporized and the
oil in its form of mist goes to the cylinder walls.
Advantage
 Simplicity and low cost
 No oil pump or filters are required

14. II. Wet Sump Lubrication System
 In the wet sump system, the bottom of the crankcase contains an oil pan or sump
from which the lubricating oi1 is pumped to various engine components by a
pump .After lubricating these parts, the oil flows back to the sump by gravity.
Again it is picked up by a pump and recirculated through the engine lubricating
system. There are three varieties in the wet sump lubrication system. They are:
a) Splash system
b) Splash and pressure system
c) Pressure feed system

16. a) Splash system
 Used in light duty , slow speed engines(<205 rpm)
 Lubricating oil is stored at the bottom of engine crankcase and maintained at a
predetermined level.
 The oil is drawn by the pump and delivered through a distributing pipe into the
splash trough located under the big end of all the connecting rods.
 These troughs are provided with overflows and oil in the trough is therefore kept at
a constant level.
 A splasher or dipper is provided under each connecting rod cap which dips into oil
in the trough at every revolution of the crankshaft and the oil is splashed all over
the interior of crankcase, into the pistons and onto the exposed portion of cylinder
walls.
 The oil dripping from the cylinder is collected in the sump where it is cooled by the
air flowing around. The cooled oil is then recirculated.

18. b. Splash and pressure lubrication system
 The lubricating oil is supplied under pressure to main and camshaft bearings.
 Oil is also supplied under pressure to pipes which direct a stream of oil against the
dippers on the big end connecting rod bearing cup and thus the crankpin bearings are
lubricated by the splash or spray of oil thrown up by the dipper.
C. Pressure feed lubrication system
 Lubricating oil is drawn in from the sump and forced to all the main bearings of the
crankshaft through distributing channels.
 Drilled passages are used to lubricate connecting rod bearings.

21. III. Dry sump lubrication system
 The supply of oil is carried in an external tank.
 An oil pump draws oil from the supply tank and circulates it under pressure to the various
bearings of the engine.
 Oil dripping from the cylinders and bearings into the sump is removed by a scavenging pump which
in turn the oil is passed through a filter, and is fed back to the supply tank. Thus, oil is prevented
from accumulating in the base of the engine.
 The capacity of the scavenging pump is always greater than the oil pump.
 In this system a filter with a bypass valve is placed in between the scavenge pump and the supply
tank.
 If the filter is clogged, the pressure relief valve opens permitting oil to by-pass the filter and
reaches the supply tank.
 A separate oil cooler with either water or air as the cooling medium, is usually provided in the dry
sump system to remove heat from the oil.

23. Lubrication oil requirements
 Should not react with the lubricating surfaces
 A low pour point to allow flow of the lubricant at low temperatures to the
oil pump
 No tendency to form deposits by reacting with the air, water, fuel or the
product of combustion
 Cleaning ability
 Non foaming characteristics
 Non – toxic and inflammable
 Low cost

24. Properties of lubricating oil
 Viscosity: Viscosity is a measure of oil’s resistance to flow. It must be compatible with the load and
speed of the engine. A low viscosity oil is thin and flows easily whereas, a high viscosity oil is thick and
flows slowly.
 Viscosity Index: it is the measure of how much the viscosity of an oil changes with
temperature. Viscosity number is set by SAE (Society of Automotive Engineers).
• Single viscosity oils SAE 5W, SAE 10W (Winter) and
SAE 20, SAE30 … (Summer)
• Multiple viscosity oils SAE 10W-30. This means that the oil is
same as SAE 10W when cold and SAE30 when hot.
• The higher the number the higher the
viscosity(thickness) of oil

25.  Corrosion and Rust Inhibitor: Displaces water from metal surfaces,
to prevent corrosion.
 Foaming Resistance: Rotating crankshaft tends to cause bubbles (Foam) in the oil and bubbles in
oil will reduce the effectiveness of oil to lubricate.
 Stability: The ability of oil to resist oxidation that would yield acids, lacquers and sludge is called
stability.
 Flash and Fire Points
 Cloud and Pour Points
 Oiliness or Film Strength
 Corrosiveness
 Detergency