Post on 10-Dec-2015
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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
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
• 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
• 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
– 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
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
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