COOLING SYSTEM

Introduction

I.C Engines at best can transform about 25-30 % of the chemicalenergy in to the fuel in to mechanical energy.

About 35 % of the heat generated is lost to the cooling mediumremainder being dissipated through exhaust and lubricating oil.

1/3 Heat to Useful Work

1/3 Heat to cooling System

1/3 Heat to exhaust system

Introduction

The Purpose of Cooling System

Prevent Overheating

Excess Heat generated in engine

Peak temperatures exceed melting point of metal

Regulate the most efficient Temperature

Regulate Temperature

Allow engine to warm up in cool weather

Maintain engine in optimum range.

Introduction

Consequence of engine running too hot:

Pre-ignition

Detonation/Knock

– All result in very high pressure rise and possible damage to engine

Heat Fatigue of components

Burnt Pistons

Burnt Valves

Failure of lubrication system

Oil breakdown

Oil film at cylinder destroyed at 200C Scoring of piston & sleeves

Warping & fracture of components

Introduction

Consequence of engine running too cold:

Unnecessary Wear

Poor Fuel Economy

Incomplete Combustion

Lower coolant Temperature More energy transferred out of cylinder

Energy from is wasted & not available for work (Power)

Promotes corrosive conditions in engine

Water of combustion reacts with sulfur oxides in exhaust Forms acids

Allows water & sludge to accumulate in crankcase

Over time, fuel diluting the oil will accumulate Normally lighter volatile fuel will evaporate as temp. rises

Critical engine components that need to be maintained at optimal designed temperature

Combustion chamber walls

Cylinder wall

Cylinder head

Piston

Exhaust valve

Spark plug

Gasoline / Diesel injector

Engine lubricants

Introduction

Centrifugal pump

Cooling fluid

Radiator

Fan

Thermostat

Cooling System components Function

Cooling fluid circulation

Heat transfer

Heat exchange with the ambient

Air through the radiator at low vehicle speed

Engine temperature stabilization

Cooling system

Expansion tank

Filler pressure cap

Passenger compartment radiator

Lubricant radiator

EGR cooling radiator (Diesel)

Fluid expansion and gas release

Cooling circuit pressure

Passenger compartment heating

Engine lubricant cooling

Exhaust gas cooling

Cooling System components Function

Cooling system

Heat transfer fluid Etilene glycol mixture in water (30 –60% concentration)

Water

High specific heat

Low viscosity

High heat of vaporization

Constant characteristics vs time and temperature

Etilene glycol

High thermal capacity

Low pressure drop

Low gas formation

Low freezing point

Cooling system

EG Weight Percent (%)

Freezing Point (deg F)

Freezing Point (deg C)

0 32 010 25 -420 20 -730 5 -1540 -10 -2350 -30 -3460 -55 -4870 -60 -5180 -50 -4590 -20 -29100 10 -12

Ethylene glycol freezing point vs concentration in water

Cooling system

( )hkJigwuf QQQQQ +++=

fQ

uQ

wQ

gQ

iQ

= heat introduced into the engine through the fuel combustion

= work equivalent heat at the engine shaft

= heat realeased to the engine cooling system

= heat rejected to the exhaust gases

= lost heat for radiance

where:

General equation for engine thermal balance

Cooling system

Introduction

Variation of Gas Temperature Piston Temperature Distribution

Introduction

Heat Transfer As a result of combustion, high temperature are produced, inside

the engine cylinder

Considerable heat flow from the gases to the surrounding metalwalls

Shearing of the oil film

Heat transfer from gases to the cylinder walls may occurpredominantly by Convection & Radiation

Heat transfer through the cylinder wall occurs only by conduction

The temperature profiles across the cylinder barrel wall For water cooled engine

Air cooled engine

Introduction

Cylinder wall temp profile

Parameters Affecting Engine Heat Transfer

Engine heat transfer depends upon many parameters, unless the effect ofthese parameters is known, the design of a proper cooling system will bedifficult.

Fuel-Air Ratio

A change in fuel-air ratio will change the temperature of the cylinder gases andaffect the flame speed.

Spark Advance

More or less spark advance from the optimum value will result in increased heatrejection to the cooling system

Pre-ignition and knocking

Engine output

Engines which are designed for high M.E.P or high piston speeds, heat rejection willbe less

Cylinder wall temperature

Characteristics of an efficient cooling system

The two main characteristics desired of an efficientcooling system It should capable of removing about 30 % of the heat

generated in the combustion chamber while maintaining theoptimum temperature of the engine under all operatingconditions of the engine

It should remove heat at a faster rate when engine is hot.However during starting of the engine the cooling should beminimum, so that the working parts of the engine reach theiroperating temperatures in a short time

Types of cooling System

There are two types of system in general Liquid or indirect cooling system

Air or direct cooling system

Liquid cooling system Mainly water is used and made to circulate through the cooling

jackets provided around the cylinder, cylinder head, valve portsand seats where it extracts most of the heat

The heat transferred from the cylinder wall and other parts byconvection and conduction

The heat from liquid in turn is transferred to air. Hence it is calledthe indirect cooling system

Methods of water-cooling System

Water cooling can be carried out by any one of the

following five methods

Direct or non return system

Thermosyphon system

Forced circulation cooling system

Evaporation cooling system

Pressure cooling system

Direct or Non return System

This system is useful for large installations where plenty ofwater is available.

The water from a storage tank is directly supplied through aninlet valve to the engine cooling water jacket

The hot water is not cooled for reuse but simply discharged.

Thermosyphon System

Heat is supplied to the fluid in the tank A

Because of relatively lower density, the hot fluid travels up

Its place being taken up by comparatively cold fluid from the Tank Bthrough the pipe p2

The hot fluid flows through the pipe P1 to the tank B where it gets cooled

The fluid circulates through the system in the form of convection current

Thermosyphon System

Tank A –represents the cylinder jackets

Tank-B- represents a radiator and

water acts as the circulating fluid.

The advantages are

Its simplicity

Automatic circulation of cooling water

The man limitation of the system is its inability to meet the requirementof large flow rate of water, particularly for high output engines.

Forced circulation Cooling System

This system is added in a large number of Automobiles.

Here the flow of water from radiators to water jackets is byconvection assisted by a pump

Forced circulation Cooling System

Water or coolant is circulated with through jackets around the parts ofthe engine to be cooled is kept in motion by a centrifugal pump which isdriven by the engine

The water is passed through the radiator where it is cooled by theforward motion of the vehicle

A thermostat is used to control the water temperature required forcooling

This system consists of 4 components

Radiator water pump

Fan thermostat

Basic Schematic Layout

Radiator

WaterPump

Engine Block &Cylinder head Water Jacket

Thermo-stat

Heater Control

Cab heatexchanger

Fan

Parts of Engine Cooling System

Cooling System Construction

Radiator

Fan

Water Pump

Top Hose

Thermostat

Radiator Cap

Engine Block & Water Jacket

Cold Engine

When an engine is cold, thethermostat is cold. Coolant flowis through the bypass hose andthe water jackets. This allows theengine to warm up evenly.

Warm Engine

The thermostat opens whenthe engine warms up. Thisallows coolant to circulatethrough the radiator andthe water jackets.

Water jackets

Defined as the open space within in the cylinder block andcylinder head where coolant flows

Water jackets are designed to allow coolant flow to the rightspots flow so that maximum cooling can be obtained

Water Pump

The purpose

Is to circulate the water through the cooling system

Located on the front part of the engine

In most of the vehicles it is driven by a belt is attached to the crankshaft

As the crankshaft turns the fan belt turns

31

The water pump provides circulation of the engine coolant(antifreeze) through the cooling system: it pushes the coolantthrough the passages (water jackets) in the engine cylinder blockand cylinder head and then out into the radiator. The hot coolantpasses through the radiator where it cools down and then returnsback to the engine.

Centrifugal pump is the most used:it is a rotodynamic pumpthat uses a rotating impeller to increase the pressure and flow rateof a fluid. The fluid enters the pump impeller along or near to therotating axis and is accelerated by the impeller, flowing radiallyoutward or axially into a diffuser or volute chamber, from where itexits into the downstream piping system.

A water pump is usually driven by the engine through the drivingbelt and only sometimes by a timing belt. A water pump consists ofthe housing with the shaft rotating on the bearing pressed inside.At the outer side there is a pulley mounted on the shaft. At theinner side there is a seal to keep the coolant from leaking out andthe impeller.

Cooling system

• Impeller diameter (∅ = 60 - 75 mm)

• Impeller height (h = 12 - 20 mm)

• Paddles number and design (z = 5 - 10)

• Axial and radial impeller clearance

• Drive ratio

engine

pump

nn

=τ = 1.3 - 1.6

Main design characteristics

Cooling system

32

Thermostat

One of the most important parts of the cooling system

Purpose

Is to keep the engine coolant at most efficient temperature

The thermostat is used to bringthe coolant temperature up tooperating as quickly aspossible

It is designed to sense thetemperature of the coolant

Thermostat

35

Target - In internal combustion engines a thermostat is used to maintain the engine at itsoptimum operating temperature by regulating the flow of coolant to the external air cooledradiator. It must balance the heat rejected from the engine to the coolant and the heat rejectedfrom the radiator to the ambient in any operating vehicle mode.

This type of thermostat operates mechanically: it makes use of a wax pellet inside a sealedchamber. The wax is solid at low temperatures but as the engine heats up the wax melts andexpands. The sealed chamber has an expansion provision that operates a rod which opens avalve when the operating temperature is exceeded. The operating temperature is fixed, but isdetermined by the specific composition of the wax, so thermostats of this type are available tomaintain different temperatures, typically in the range of 70 to 90°C. Modern engines run hot,that is, over 80°C, in order to run more efficiently and to reduce the emission of pollutants.Most thermostats have a small bypass hole to vent any gas that might get into the system, e.g.,air introduced during coolant replacement, which also allows a small flow of coolant past thethermostat when it is closed. This bypass flow ensures that the thermostat experiences thetemperature change in the coolant as the engine heats up; without it a stagnant region ofcoolant around the thermostat could shield it from temperature changes in the coolant adjacentto the combustion chambers and cylinder bores.

Wax thermostatic elements permit the transforming of thermal energy into mechanical energy.Their working principle is based on the large increase in the thermal expansion of waxes whenthey pass from the solid to the liquid state

Cooling system

Radiator

Purpose Is to allow fresh air to reduce the temperature of the coolant

As the coolant passes through the tubes air is forcedaround the tube

This causes a transfer of heat from the hot coolant to thecooler air. This is called Heat exchanged

In this case, heat is exchanged from the liquid coolant toair. This is called a liquid-to air heat exchanger

Radiator

Coolant flows through the air fins The fins and tubes are cooled core

Radiator Parts

Radiator Hose

Purpose Transport coolant from engine to radiator and back

Radiator pressure cap

Pressure caps are designed to

Increase the pressure on the cooing system

Reduce cavitations

Protecting the radiator hose

Prevent or reduce surging

It is important to put pressure on the cooling system.Radiator pressure caps are typically near 15 psi

As the pressure increases the boiling point of the coolantalso increases (about 3 degrees for each 1 psi increase

Pressure Cap

Pressure cap

Pressure cap

Expansion tank

Expansion tank

Fan

Electric Fan Relay

Coolant Temperature Indicator

Coolant Temperature Indicator

Types of Coolant

Coolant

Coolant

Coolant

Anti-freeze

IAT: inorganic additive technology

OAT: organic acid technology

HOAT: hybrid organic acid technology

Extended and conventional anti-freezes cannot bemixed

EG: ethylene glycol

PG: propylene glycol

Anti-freeze

Maximum concentration: 67% anti-freeze Minimum concentration: 50% for corrosion

prevention Pure anti-freeze has higher viscosity and does not

flow well Does not transfer heat well