Friction

Friction and Lubrication

• A percentage of the power generated within the engine

cylinders is lost to friction, with a reduction in the

resulting brake power obtained off the crankshaft

• Friction mean effective pressure (fmep) is defined as

fmep = (imep)net – bmep

– (imep)net can be obtained by integrating a p-v diagram and

bmep can be determined by a dynamometer

– Alternately, fmep can be approximated by a motoring test

– Another approximate equivalent of direct motoring test for

diesel engines is the Willans line method – a plot of fuel

consumption versus brake output obtained from engine

tests at a fixed speed is extrapolated back to zero fuel

consumption

– Morse test can be used for multi-cylinder engines

Engine Friction

Friction losses for

various engine

components

measured by

motoring tests

Engine Friction

• The piston assemblies of most engines contribute about

half of the total friction and can contribute as much as

75% at light loads

– The piston rings alone contribute about 20% of total friction

• The valve train of an engine contributes about 25% of

total friction, crankshaft bearings about 10% of total, and

engine-driven accessories about 15% of total

• Motoring test results indicate that

– Friction forces occurring during expansion are about twice

as large as those occurring during any other stroke

– Friction forces tend to be high just after TC and BC,

probably because there is metallic contact between the

rings and the cylinder wall

Engine Friction

The results indicate that the

piston and ring fmep

increase with oil viscosity,

piston speed, and imep

Modern piston design has

been changed significantly

from previous design

considerations – piston skirt

areas and weights have

been greatly reduced

Inertia load (side thrust) has

also been lowered with the

introduction of offset wrist

pins

Engine Friction

Engine Friction

• Other changes have been made to reduce piston and

ring friction

– The number and width of piston rings has been reduced

– Piston ring cross-sections have changed from being more

or less square to having barrel face

• The stroke to bore ratios have been reduced, resulting in

a lower piston speed at the same rpm

• Valve train is another place where friction occurs

– The major losses in the valve train occur at the cam-lifter

interface and in the rocker-arm pivot

– Overhead cam, direct acting system having only one

sliding contact, the torque required to drive the valve train

is the lowest

Engine Friction

Engine Friction

• Journal bearings on the crank, both ends of the

connecting rod, and the camshaft, all add to friction

• The remaining friction in an engine, after accounting for

the aforementioned ones, is primarily caused by the

pumps employed to circulate the oil, water, and fuel

Engine Lubricating System

• Purposes of the lubricating system are -

– supplies lubricating oil to all moving parts in the engine

– lub oil picks up engine heat and dissipates it through the

oil pan

– oil fills the clearances between bearings and rotating

journals

– lub oil forms a seal between piston rings and cylinder walls

– oil acts as a cleaning agent

Lubricating Oil

• Lubricating oil is required to have the following properties

– proper viscosity - high viscosity oil flows too slowly and low

viscosity oil has a reduced ability to stay in place - both of

them may cause rapid engine wear

– viscosity index (VI) - is a measure of how much the

viscosity of oil changes with temperature

– viscosity numbers - single-viscosity oil has several grades

- winter grade or other than winter grade

– corrosion and rust inhibitors

– detergent-dispersants

– extreme pressure resistance

Lubricating Oil

– energy-conserving oil - has friction modifiers - a chemical

dissolved completely in oil or suspended carbon or

molybdenum

• two types of EC oils are EC I and EC II - EC II provides

better fuel-economy than an EC I oil

– synthetic oil - made from carbon compounds and alcohols,

or from coal and crude oil - better than petroleum based

oils

• The base ingredients in most lubricating oils, however,

hydrocarbon components made from crude oil

– These are large molecular weight species

Lubricating Oil

• Service ratings of oil - a designation by the API

– there are 8 service ratings for SI engine oils - SA, SB,..,SH

– there are 6 service ratings for CI engine oils - CA, CB,..,CF

– the ratings are open-ended

Engine Lubricating System

• 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

– 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

Engine Lubricating System

• A dry sump system (i.e., the crankcase sump is dry of

excess oil) is a total pressurized system with the oil

reservoir located separate from the crankcase

– 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

• Note that 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

– Also that often the oil is cold at engine startup which has a

much higher viscosity, which further delays proper

circulation

Lubrication in Two-Stroke Cycle Engine

• In a two-stroke cycle SI engine, crankcase cannot be

used as an oil sump

– Lubricating oil is carried into the engine with the intake air

• 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, the oil is premixed with the fuel in the

fuel tank

– In other engines, 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

Engine Lubricating System

• Lubrication system components

– Oil pump - two types of oil pumps are used

• gear-type pump

• rotor-type pump

– Drive arrangement of oil pumps are

• in camshaft-in-block engines, the camshaft spiral gear that

drives the ignition distributor usually drives the oil pump

• in OHC engines, the oil pump is driven by separate drive

shaft - 'jackshaft'

• in distributorless engines, oil pumps are driven by crankshaft

Engine Lubricating System

Engine Lubricating System

Engine Lubricating System

Engine Lubricating System

Engine Lubricating System

– Oil pressure relief (regulator) valve

• used to prevent excessive oil pressure

– Oil pumps can deliver more oil than the engine requires

– Some engines use oil cooler

• increases the cooling efficiency of the engine

– Oil filters

• oil from the pump flows through the filter

• the filter has a pleated paper filtering element

• it has a spring-loaded bypass valve

• some have anti-drainback valve - helps prevent oil from

draining out while the engine is off

• some engines use internal oil filters - attached directly to the

oil pump

Oil-Pressure Relief Valve

Schematic of Full-flow Filter

Schematic of By-pass Filter

Lubricating Oil Filter

Engine Lubricating System

– Oil pressure indicators warn the driver if engine pressure is

too low

– There are four types

• indicator light - connected through an oil pressure switch -

very common

• electric gauge - balancing coil type - the engine unit has a

diaphragm connected to a sliding contact

• electronic gauge - bar graph display made up of a series of

segments

• digital gauge

PCV System

Engine Lubricating System

• In addition to oil, other lubricants and special fluids are

used in engines

– Grease - a semi-solid fluid is very common

• made from petroleum and thickened with metallic soaps such

as, Li, Ca, Na, Al, and Ba or non-metallic substance like clay

• a good grease must have consistency, stability, oxidation

resistance, ability to protect against friction, wear and

corrosion, and feedability

Engine Lubricating System

• The engine loses oil by burning or by leaking

– Three main factors resulting in 'more than normal' oil

consumption are

• engine speed - high speed produces high temperature and

lowers oil viscosity - oil can get into the combustion chamber

and get burnt, oil-control ring can flutter or float, crankcase

ventilation system takes some oil with it in the form of mist

• engine wear - such as, bearing wear, cylinder wear, piston

ring wear, valve guide wear - causes more oil consumption

• engine oil can leak past the gaskets (sealing), from loose

fittings, or filter

• Oil changes

– Change oil when it gets dirty or contaminated - a result of

wearing out of additives

Oil Loss from Engine