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Cooling and Lubrication SystemsChapter 8

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Cooling Systems

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Introduction

• Internal combustion engines use heat to convert the energy of fuel to power.

• Not all of the fuel energy is converted to power.

• Excess heat must be removed from the engine.

• In engines, heat is moved to the atmosphere by fluids--water and air.

• If excess heat is not removed, engine components fail due to excessive temperature.

• Engine temperature is not consistent throughout the cycle.

• Heat moves from areas of high temperature to areas of low temperature.

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Engine Construction

• Engine components are constructed of aluminum and cast iron alloys, steel and non-metallic materials.

• The physical properties of these alloys determines the physical characteristics of the engine components.

Desirable cast iron properties.– Greater density

– Structural integrity when mechanically or thermally stressed.

– Porous surface--improved lubrication.

– Graphite enhanced lubrication.

– Less dimensional changes when heated.Undesirable properties of cast

iron: Increased weightPropensity for oxidation/corrosionDifficulty machiningRelative poor heat conductivity.

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Engine Construction-cont.

Desirable properties of cast aluminum:1. High strength to weight

ratio.

2. Lower production cost

3. Excellent heat dissipation

Undesirable properties of cast aluminum:

1. Higher raw material costs

2. Less resistant to wear.

3. Greater thermal expansion

Aluminum is the material of choice for modern small gas engines.

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Cooling Terms

• Thermal Conductivity– Ability of a material to conduct

and transfer heat

• Thermal expansion– Expansion of a material when it

is heated.

• Thermal growth– Increase in size caused by

heating.

– When cooled does not return to normal size.

• Thermal distortion– Asymmetrical or nonlinear

thermal expansion.

Three means of heat transfer:

1. Conduction

2. Convection

3. Radiation

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Heat Movement

• Conduction– Movement of heat through materials.

• Convection– Movement of heat by fluids.

• Radiation– Heat movement by transfer from one body to another.

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Two Cooling Systems

• Small engines use two cooling systems;– Air– Liquid

• Both systems have two common features.– Heat is transferred from the combustion chamber to

the crankcase by the oil.– A large portion of the excess heat is removed with

the exhaust gases.

• The difference is in the medium used to move the heat from the engine to the atmosphere.

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Air Cooled Heat Movement

• In air cooled engines the excess heat in the combustion chamber moves through the cylinder walls by conduction.

• The heat transfers from the engine parts to the air at the exterior surfaces and into the atmosphere by convection.

• The air fins increase the surface area between the engine and the air--increasing heat transfer.

• The heart of the system is the fins on the flywheel which pumps the air around the engine.

• The air flow is directed by the air shrouds.

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Water Cooled Heat Movement

• Water cooled engines transfer the excess heat from the combustion chamber through the cylinder walls by conduction.

• Water flowing past the exterior cylinder walls absorbs the heat and transfers it to the radiator.

• Air flowing through the radiator absorbs the heat and transfers it to the atmosphere.

• The system relies on a water pump to circulate the water through the system and a fan to move air through the radiator.

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Lubrication System

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Introduction

• The lubrication system provides oil to appropriate areas of the engine to maintain a film of oil to separate bearing surfaces.

• Oil also transfers combustion heat to the crankcase.

• An important characteristic of oil is the viscosity.

• Viscosity is the internal resistance to flow of a fluid.

• The SAE rates the viscosity of oils.

• Low viscosity means a high volume of oil flows through a specific orifice at a specified temperature, atmospheric pressure, and time period.

• Multi-viscosity oils are popular because they offer low viscosity characteristics in low temperatures and high viscosity characteristics with higher temperatures.

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Oil Standards

• Service Categories--Gasoline Engines– SA, SB, SC, SD, SE,

SF, SG, SH are obsolete.

– SJ: 2001 and older automotive engines.

– SL: All automotive engines manufactured after July 2001.

• Service Categories--Diesel Engines– CA, CB, CD, CE are obsolete.– CF: Off road, indirect-injected

manufactured after 1994 and with fuels over 0.5% sulfur.

– CF-2: Severe duty, two cycle engines.– CF-4: High speed, four cycle engines.– CG-4: Severe duty, high speed, four

stroke, with fuels less than 0.5% sulfur.– CH-4: High speed, four stroke engines

designed to meet 1998 exhaust standards.

– CI-4: High speed, four stroke engines designed to meet 2002 exhaust standards.

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Lubrication Systems

• Small engines use one of three lubrication systems.– Splash

– Pressure filtration

– Pressure