Download - 10 Friction
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Friction
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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
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Engine Friction
Friction losses for
various engine
components
measured by
motoring tests
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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
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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
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Engine Friction
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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
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Engine Friction
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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
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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
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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
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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
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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
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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
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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
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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
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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
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Engine Lubricating System
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Engine Lubricating System
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Engine Lubricating System
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Engine Lubricating System
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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
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Oil-Pressure Relief Valve
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Schematic of Full-flow Filter
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Schematic of By-pass Filter
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Lubricating Oil Filter
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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
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PCV System
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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
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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
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Oil Loss from Engine