10 Friction

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


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


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


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


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


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


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


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

10 Friction

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