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Engine friction and lubrication

Dr. Primal Fernandoprimal@eng.fsu.edu Ph: (081) 2393608

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Background

• Friction work is varies 10% at full load to 100% at idle (no load)

• Good engine design has lower frictional loss• Large part of frictional losses appear as heat and

removed in the oil cooler and radiator systems, thus influence the size of the cooling system.

• Friction work = Work delivered to the piston while the working fluid is contained within (compression and expansion stroke) - usable work delivered to the drive shaft

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Work dissipated as heat

• Pumping work (Wp): to draw the fresh mixture to the cylinder and to expel the burn gas from the cylinder

• To overcome the resistance to relative motion (rubbing friction work- Wrf): friction between piston rings, piston skirt, and cylinder wall; friction in the wrist pin, big end, crankshaft and camshaft bearings; friction in the valve mechanism; friction in the gears, or pulleys and belts

• To drive the engine accessories (accessory work-Wa): fan, water pump, oil pump, fuel pump, the generator, secondary air pump for emission control, power-steering pump, and air-conditioner Note: the absolute value of friction work varies with load, and increases as speed increases

Wtf= Wp+ Wrf+ Wa

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Equivalent

v

P

Wnet

vmin vmax

Actual Processes

Wwork per cycle

v

P

vmin vmax

MEP

Equivalent by MEP

Mean Effective Pressure, MEP Concept

TDC BDC

Wwork per cycle = (MEP) x (Displacement Volume)

= (MEP) x (Vmax-Vmin)

= (MEP) x Vd

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Power and Mean Effective Pressure Wwork per cycle = (mep) x (Displacement Volume)

= (mep) x (Vmax-Vmin)

= (mep) x Vd

cycle strokefour for 2 and cycle stroke-for two 1

cycle,per srevolution ofnumber

R

Rd

Rpercycle

n

n

NVmep

n

NWP

Therefore, Wtf , Wp, Wrf, Wa can be define as, tfmep & Ptf, pmep & Pp, rfmep & Prf, amep & Pa

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Frictional losses can be classified in to two groups depending on dissipation

1. Friction between two metals in relative motion, with a lubricant in between

2. Turbulent dissipation-part of the total friction work is spent in pumping fluids through flow restrictions P v2.

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Operation conditions of two common geometries for lubricated parts

load

oil

supporting oil film

bearingjournal

Lubricated journal bearing

Lubricated slider bearing

Note: A primary problem in understanding friction between lubricated surfaces in engines is the wide variation in the magnitude of the forces involved.

load

bearingsupporting oil film

oil

pad

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Coefficient of friction f (Tangential force/ Normal force)

The coefficient of friction can be expressed as

Ls fff )1(

fs is metal-to-metal coefficient of dry friction, friction is called boundary, i.e., close to solid friction. Lubricating film is reduced to one or a few molecular layers and can not prevent metal-to-metal contact between surface asperities

fL is hydrodynamic coefficient of friction or viscous or thick film friction. Lubrication film completely separates the surfaces of relative motion.

is metal-to-metal contact constant, varies between 0 and 1

Between fs and fL is mixed or partial lubrication regime

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Coefficient of friction f (Tangential force/ Normal force) for a journal bearing is plotted against dimensionless duty parameter

0.001

0.01

0.1

Coe

ffici

ent o

f fric

tion,

f

Solid friction, fs

Hydrodynamic friction, (1-)fL

Ls fff )1(

Boundary lubrication

Mixed lubrication

Hydrodynamic lubrication

mN/s

m-Lubricant dynamic viscosity

s-Loading force per unit area

N-rotational speed

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Coefficient of friction f (Tangential force/ Normal force) for a sliding surface is plotted against dimensionless duty parameter

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Boundary lubrication

• Surface properties in relative motion– Roughness, hardness,

elasticity, plasticity, shearing strength, thermal conductivity and wetability with respect to the lubricant

• Lubricant properties– Surface properties or

chemical properties, which governs the lubricants to attach themselves to the to the solid surfaces

(during start and stopping process, between slow moving parts - valve stem-rocker arms, heavily loaded parts, crank shaft, timing gears, chains)

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Real contact area

m

nr

F

stressyield

loadNormalA

s

mrt AstrengthsheararearealF

m

m

N

t

F

Ff

s

Note: for dissimilar materials, the properties of weaker materials dominates the friction behavior

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Hydrodynamic lubrication

• Hydrodynamic lubrication conditions occur when the shape and relative motion of the sliding surfaces form a liquid film in which there is a sufficient pressure to keep the surfaces separated. Resistance to motion results from the shear forces within the liquid film

dy

dvm

(bearings, piston skirt and cylinder liner , at high speed, piston rings and liners at high speeds)

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Lubrication system• The lubrication system

provides oil to appropriate areas of the engine to maintain a film of oil to separate bearing surfaces.

• Oil also transfers combustion heat to the crankcase.

• An important characteristic of oil is the viscosity.

• Viscosity is the internal resistance to flow of a fluid.

• The SAE rates the viscosity of oils.

• Low viscosity means a high volume of oil flows through a specific orifice at a specified temperature, atmospheric pressure, and time period.

• Multi-viscosity oils are popular because they offer low viscosity characteristics in low temperatures and high viscosity characteristics with higher temperatures.

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Oil Standards

• Service Categories--Gasoline Engines–SA, SB, SC, SD, SE, SF, SG, SH are obsolete.–SJ: 2001 and older automotive engines.–SL: All automotive engines manufactured after July 2001.

• Service Categories--Diesel Engines–CA, CB, CD, CE are obsolete.–CF: Off road, indirect-injected manufactured after 1994 and with fuels over 0.5% sulfur.–CF-2: Severe duty, two cycle engines.–CF-4: High speed, four cycle engines.–CG-4: Severe duty, high speed, four stroke, with fuels less than 0.5% sulfur.–CH-4: High speed, four stroke engines designed to meet 1998 exhaust standards.–CI-4: High speed, four stroke engines designed to meet 2002 exhaust standards.