Download - Friction, Lubrication, Cooling_Joy.ok
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Engine Friction, Lubrication, and
Engine Cooling
Engine Friction
• Friction between two mutually contacting sliding surfaces
may fall into any of three categories: dry, boundary or viscous friction
– Dry friction occurs when there is no lubricant between the
surfaces
– In boundary friction, some lubricant remains between the
surfaces but as molecules bonded or keyed to the surface,
rather than as a continuous film, so some metal-to- metal
contact occurs
– In viscous friction, the resistance to motion is solely that
due to the shearing of the oil, and is therefore a function of
its viscosity
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
Engine Friction
Friction losses for various engine components
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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
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
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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
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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
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
Additives for Lubricating Oils 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
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
Lubricating Oil Pump
Eccentric-rotor-type pumpGear-type pump
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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
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Oil-Pressure Relief Valve Schematic of Full-flow and Full/Bypass Filter
Lubricating Oil Filter Oil Pressure Indicators
– 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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Other Engine Lubricants
• 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
Oil Loss from Engine
• 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 Engine Cooling
• The maximum temperature during combustion is
approximately the melting point of platinum and the temperature even of the exhaust gases is above that of
aluminium
Energy balance in a water cooled, 1.9 L SIE
at 90 km/h at constant speed in 4th gear
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Engine Cooling
• The two methods of engine cooling are: air cooling, and
water cooling
– Direct dissipation of heat from the cylinder barrel and head
to the surrounding air, or air-cooling as it is called, is
feasible with small engines and is practically universal with
motor-cycles
• Natural flow air-cooling is very difficult to regulate, and as the
size of the cylinder increases the difficulties become very
serious, for the heat developed increases as the cube of the linear dimensions, while the radiating surface increases only
as the square
• Air-cooled engines have the reputation of being noisier than the water-cooled form
Engine Cooling
– For the normal medium and large engines water cooling is
practically universal
• Heat is far more readily transferred from metal to water than
from metal to air
• There are broadly two methods of temperature control in
use:
– Control over water flow by means of a thermostat valve
– Control over air flow by means of radiator shutters
actuated either manually or automatically by a thermostatic
device
• Thermostats can be either of bellows-type or wax-element
type
Engine Cooling
Simple engine cooling
system
Bellows-type
thermostat (top);
bellows contain
low-boiling point
liquids like acetone
or alcohol (60-80C
bp)
Wax-element type
thermostat (bottom)
Engine Cooling
Radiator expansion tank
Working principle of
Radiator pressure cap
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Engine Cooling – Radiators
•
Rib and pipe systems for radiators
with round and flattened oval pipes
Soldered flat tube system for radiators
Engine Cooling – Radiators
Radiators for cars with cross-flow
arrangement and commercial vehicle
cooling module with radiator of downdraft
design
Engine Cooling – Radiators Cooling Fans
• Aerodynamically more efficient moulded plastics – nylon
or polypropylene – fans are used in modern engines, which are lighter, easier to balance, look better, and
safer
• Basically, two types of fans are in use today
– One, by far the simplest, is to incorporate in the hub of the
fan a clutch that slips at a certain torque, thus limiting the
amount of power that can be transmitted to, and therefore
absorbed by, the fan
– The second approach is to introduce a thermostatic control
either to limit the torque transmitted to the fan, or to cut it
out altogether when the coolant temperature falls below a
predetermined level
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Cooling Fans
Cooling module for use in cars with
radiator, expansion tank, A/C condenser,
refrigerant receiver, and electric fan with
housing
Cooling Fans
• Most of these fans electrically driven and are
thermostatically controlled
– Variable speed fans and flexible blade fans reduce the
power required and also reduce fan noise
Typical characteristics of a fan drive
Cooling Fans Cooling Fans