6/26/2013

1

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

6/26/2013

2

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

6/26/2013

3

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

6/26/2013

4

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

6/26/2013

5

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

6/26/2013

6

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

6/26/2013

7

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

6/26/2013

8

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

6/26/2013

9

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

6/26/2013

10

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

6/26/2013

11

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