I.C. ENGINES

LECTURE NO: 14 (5 May 2014)

Engine Heat

   Combustion can reach 4500ºF (2500ºC) This is hot enough to melt metal parts The cooling system removes excess heat

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Importance of Heat Transfer

Peak burn gas temperature leads to heat fluxes to the chamber walls as high as 10 M/m 2 During other part of the operating cycle heat flux is essentially zero Flux varies with location High heat flux zone – thermal stresses are more results in Fatigue cracking Temp should be less then   Cast Iron Al 400˚C 300 ˚C

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Importance of Heat Transfer

Gas side surface of the cylinder wall must be below 180˚C to prevent deterioration of lubrication oil Spark plug and valves must be cool to avoid Knocking and pre ignition problems

Heat Transfer Affects

   Engine Performance Efficiency Emission

Modes Heat Transfer

   Conduction Convection Radiation

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Conduction

Conduction

      Cylinder head Cylinder walls Pistons Through pistons rings Engine block Manifolds

Convection

   Through fluid in motion Between fluid and solid surface in relative motion Heat is transferred by FORCE CONVECTION between the in- cylinder gases and the  Cylinder head       Valves Cylinder Walls Piston during induction Compression process Expansion process Exhaust process

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Convection

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Radiation

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Overall Heat Transfer Process

Maintain Operating Temperature    180ºF to 210ºF (80ºC to 100ºC) Ensures that clearances are correct  when an engine warms to operating temperature, parts expand Ensures proper combustion, minimum emissions, and maximum performance

Reach Operating Temperature Quickly  This minimizes several conditions:      poor combustion (poor fuel vaporization) part wear oil contamination reduced fuel economy increased emissions

Heater Operation   The cooling system circulates coolant to the vehicle’s heater Engine heat is used to warm the passenger compartment

Cooling System

Cooling System Operation   The water pump forces coolant through the engine water jackets The pump is belt or gear driven off the crankshaft

Cold Engine Operation    The thermostat is closed The coolant circulates inside the engine The engine warms quickly

Hot Engine Operation    At operating temperature, the thermostat opens Heated coolant then flows through the radiator Excess heat is transferred from the coolant to the air flowing through the radiator

Cooling System Types  Two common types:   air cooling liquid cooling

Air Cooling Systems     Large cylinder cooling fins and outside air remove excess heat The cooling fins increase the surface area of the metal around the cylinder This allows enough heat to transfer to the outside air Plastic or metal shrouds direct air over the cylinder fins

Liquid Cooling Systems    Circulate coolant through the water jackets Combustion heat is transferred to the coolant The cooling system carries it out of the engine

Liquid Cooling Advantages     Precise temperature control Less temperature variation Reduced emissions Improved heater operation

Air Cooling versus Liquid Cooling

Liquid Cooling Heat is transferred to cylinder wall and then into the coolant, where it is carried away

Conventional Coolant Flow   Hot coolant flows from the cylinder head to the radiator After being cooled in the radiator, the coolant flows back into the engine block

Reverse Flow Cooling    Cool coolant enters the head and hot coolant exits the block to return to the radiator Helps keep a more uniform temperature throughout the engine Found on high-performance engines

 Components:      water pump radiator hoses radiator fan thermostat

Water Pump A ribbed belt powers this pump

Crank pulley Impeller Ribbed belt Water pump pulley

Impeller Pump Coolant is thrown outward by centrifugal force, producing suction in the center of the pump housing

Water Pump Cutaway Seal leakage will drip from the vent hole

Coolant Flow

Coolant Flow (Conventional)   Coolant flows out of the radiator, through the lower hose, into the pump It then flows through the pump, around the cylinders, through the heads, up through the thermostat, and back into the radiator

Hoses   Radiator hoses   carry coolant between the engine water jackets and the radiator the lower hose is exposed to water pump suction, so a spring may needed to prevent collapse Heater hoses   carry hot coolant to the heater core smaller diameter than radiator hoses

Radiator and Heater Hoses

Radiator Hoses Two basic types of radiator hoses

Hose Clamps Three basic types of hose clamps

Radiator Transfers coolant heat to the outside air

Radiator Types

Downflow Crossflow

Transmission Oil Cooler   Often placed in the radiator on cars with automatic transmissions Prevents the transmission fluid from overheating

Transmission Oil Cooler Small tank inside one of the radiator tanks

Oil Cooler System

Radiator and A/C Condenser   The condenser is usually mounted in front of the radiator  in this arrangement, heat from the condenser flows through the radiator, reducing efficiency Side-by-side mounting is sometimes used

Radiator and A/C Condenser This vehicle has side-by-side mounting

Radiator Cap     Seals the radiator Pressurizes the system Relieves excess pressure Allows coolant flow between the radiator and the coolant reservoir

Radiator Cap

Radiator Cap Pressure Valve     Spring-loaded disk Normally, water boils at 212ºF (100ºC) For each pound of pressure increase, the boiling point goes up about 3ºF (1.7ºC) Typical pressure:   12–16 psi (83–110 kPa) raises the boiling 250–260ºF (121–127ºC) point to

Radiator Cap Vacuum Valve Opens to allow flow back into the radiator when the coolant temperature drops

  Closed cooling system   uses an expansion tank overflow tube is routed into reservoir tank Open cooling system  allows excess coolant to leak onto the ground

Pressure Cap Operation Hot engine

Pressure Cap Operation Cold engine

Cooling System Fans    Pull air through the core of the radiator Increase the volume of air flowing through the radiator Driven by fan belt or electric motor

Flex Fan High engine speed causes the blades to flex, reducing the blowing action

Fluid Coupling Fan Clutch Filled with silicone-based oil The clutch slips at higher rpm

 Thermostatic Fan Clutch Bimetal spring controls clutching action   cold—clutch slips hot—clutch locks

Electric Cooling Fans     An electric motor and a thermostatic switch provide cooling Common engines on transverse-mounted Save energy and increase cooling efficiency Fans only function when needed

Electric Fan Operation Cold engine

Electric Fan Operation Hot engine

PCM-Controlled Fans    When cold, the ECM does not energize the fan relays After warm-up, the ECM feeds current to the fan relay coils, closing the relay contacts High current flows to fans

Electronic Controlled Fans

Radiator Shroud Ensures that the fan pulls air through the radiator core

   Thermostat Senses the coolant temperature and controls coolant flow through the radiator Reduces coolant flow in a cold engine Increases coolant flow in a hot engine

Thermostat A temperature-sensitive valve

  Thermostat Operation Cold engine   wax-filled pellet has contracted spring holds valve closed Hot engine    when heated, pellet expands spring tension is overcome valve opens

Thermostat Operation A. Cold engine B. Hot engine

Thermostat Operation Cold engine

Thermostat Operation Hot engine

Bypass Valve Permits coolant circulation through the engine when the thermostat is closed

Bypass Thermostat Blocks off the bypass at operating temperature

Impeller Flow to radiator Water pump drive pulley Bypass spring Water pump housing Bypass flow Thermostat Main spring Main flow

 Two common types:   temperature warning light engine temperature gauge

Temperature Warning Light   When the coolant becomes too hot, a temperature sending unit (switch) in the block closes, completing a light circuit The warning light glows

Warning Light Circuit

Engine Temperature Gauge   Shows the exact operating temperature Components:   gauge variable resistance sending unit

Engine Temperature Gauge Operation      When cold, the sending unit has a high resistance Current flow through the gauge is low The gauge reads cold When hot, the sending unit resistance lowers Current right flow through the gauge increases, the needle deflects to the

     Composed of ethylene glycol mixed with water Prevents winter freeze-up Prevents rust and corrosion Lubricates the water pump Cools the engine

Corrosion Protection

Protected with antifreeze Water only

Antifreeze/Water Mixture

50% ANTIFREEZE 50% WATER

Lowers the coolant freezing point to about –34 ºF (–37 ºC)

   Aids engine starting in cold weather 120-volt heating element mounted in the block water jacket Common on diesel engines

Block Heater Installation

Engine and Cooling System

Engine and Cooling System