LUBRICATION AND COOLING

FUNCTIONS OF ENGINE OIL• Lubrication (Primary function)• Wear-reduction• Protection• Cooling• Cleaning• Corrosion protection• Hydraulic action

Lubricating Oil Properties• Gravity• Flash Point• Viscosity• Cloud Point• Pour Point• Carbon-Residue Test• Ash Test• Precipitation number

Gravity• The gravity of petroleum oil is a numerical value

which serves as an index of the weight of a measured volume of this product

Flash Point• The flash point of an oil is the temperature to which the oil must

be heated in order to give off enough vapor to form a combustible mixture above the surface that will momentarily flash or burn when the vapor is brought into contact with a very small flame.

Viscosity• Viscosity is technically defined as the fluid friction of an oil• To put it more simply, it is the resistance an oil offers to

flowing• Heavy-bodied oil is high in viscosity and pours or flows

slowly• Oils are typically classified by viscosity. SAE 30 is a

reflection of the oils viscosity. The higher the number the higher the viscosity.

Multiviscosity Oils• When you see a W on a viscosity rating it means that this oil viscosity has been

tested at a Colder temperature. The numbers without the W are all tested at 210° F or 100° C which is considered an approximation of engine operating temperature. In other words, a SAE 30 motor oil is the same viscosity as a 10w-30 or 5W-30 at 210° (100° C).

• The difference is when the viscosity is tested at a much colder temperature. For example, a 5W-30 motor oil performs like a SAE 5 motor oil would perform at the cold temperature specified, but still has the SAE 30 viscosity at 210° F (100° C) which is engine operating temperature. This allows the engine to get quick oil flow when it is started cold verses dry running until lubricant either warms up sufficiently or is finally forced through the engine oil system. The advantages of a low W viscosity number is obvious. The quicker the oil flows cold, the less dry running. Less dry running means much less engine wear.

Cloud Point• The cloud point is the temperature at which the separation of wax

becomes visible in certain oils under prescribed testing conditions

• When such oils are tested, the cloud point is slightly above the solidification point

Pour Point• The pour point of an oil is the temperature at

which the oil will just flow without disturbance when chilled

Carbon-Residue Test• The purpose of the carbon-residue test is to

study the carbon-forming properties of a lubricating oil.

Ash Test• The ash test is an extension of the carbon-

residue test• If an unused oil leaves almost no ash, it is

regarded as pure• The ash content is a percentage (by weight) of

the residue after all carbon and all carbonaceous matter have been evaporated and burned

Precipitation number• The precipitation number recommended by the

ASTM (American Society for Testing and Materials) is the number of milliliters of precipitate formed when 10 mL of lubricating oil is mixed with 90 mL of petroleum naphtha under specific conditions and then centrifuged

Lubricating Oil Types• Straight Mineral Oil• Ashless Dispersant Mineral Oil• Synthetic Oil• Mineral/Synthetic Blends

Straight Mineral Oil• Straight mineral oil is one of many types of oil used in

aircraft reciprocating engines• It is blended from selected high-viscosity-index base

stocks• These oils do not contain additives, except for a small

amount of pour-point depressant for improved fluidity at cold temperatures

• Often used after engine overhaul or when new to facilitate seating of the piston rings (wear-in).

Ashless Dispersant Oil• Most aircraft oils other than straight mineral oils

contain a dispersant that suspends contamination such as carbon, lead compound and dirt

• The dispersant helps prevent these contaminants from gathering into clumps and forming sludge or plugging oil passageways

Synthetic Oil• Because of the high operating temperatures of

gas-turbine engines, it became necessary to develop lubricants which would retain their characteristics at temperatures that cause petroleum lubricants to evaporate and break down

• Synthetic lubricants do not break down easily and do not produce coke or other deposits

Multiviscosity Oil• In certain circumstances, all single-grade oils

have short comings• In cold-weather starts, single grade oil generally

flows slowly to the upper reaches and vital parts of the engine

• Multigrade oils have viscosity characteristics that allow for better flow characteristics at engine start

Characteristics of Aircraft Lubricating Oil• It should have the proper body (viscosity)• High antifriction characteristics• Maximum fluidity at low temperatures• Minimum changes in viscosity with changes in temperature• High antiwear properties• Maximum cooling abilities• Maximum resistance to oxidation• Noncorrosive

Characteristics of Lubrication Systems• Pressure Lubrication• Splash Lubrication and Combination Systems• Principal Components of a Lubrication System• Oil Capacity

Pressure Lubrication• In a pressure lubrication system, a mechanical

pump supplies oil under pressure to the bearings• Oil flows into the inlet of the pump through the

pump and into an oil manifold which distributes it to the crankshaft bearings

Splash Lubrication and Combination Systems• Although pressure lubrication is the principle

method of lubrication on all aircraft engines, some engines use splash lubrication also

• Splash lubrication is never used by itself• All lubrication systems are pressure systems or

combination pressure/splash systems

Components of Lubrication Systems• Plumbing for Lubrication Systems• Temperature Regulator (Oil Cooler)• Oil Viscosity Valve• Oil Pressure Relief Valves• Oil Separator• Oil Pressure Gauge• Oil Temperature Gauge• Oil Pressure Pumps• Scavenge Pumps• Oil Dilution System

Plumbing for Lubrication Systems• Oil plumbing is essentially the same as is used

in oil and hydraulic systems• When the lines will not be subject to bending,

aluminum tubing is used• Synthetic hose is often used near the engine

and other places on the aircraft that are subject to vibration or other movement

Temperature Regulator (Oil Cooler)• An oil temperature regulator is designed to

maintain the temperature of the oil for an operating engine at the correct level

• These regulators are often called oil coolers since cooling of engine oil is one of their main functions

Oil Viscosity Valve• The oil viscosity valve is generally considered a

part of the oil temperature regulator unit and is employed in some oil systems

• The viscosity valve consists essentially of an aluminum alloy housing and a thermostatic control element

• The oil viscosity valve works with the oil cooler valve to maintain a desired temperature and keep the viscosity within required limits

Oil Pressure Relief Valves• The purpose of the oil pressure relief valve is to

control and limit the lubricating pressure in the oil system

• This is necessary to prevent damage caused by excessive system pressure and to ensure that engine parts are not deprived of fuel due to a system failure

Oil Separator• Air systems where oil or oil mist is present may

require the use of an oil separator• These are often used on vacuum pump outlets• The oil separator contains baffle plates which

cause the air to swirl and it deposits on the baffles

Oil Pressure Gauge• An oil pressure gauge is an essential component

of any engine oil system• These gauges generally use a bourdon tube to

measure the pressure• They are designed to measure a wide range of

pressures

Oil Temperature Gauge• The temperature probe for the oil temperature

gauge in the oil inlet line or passage between the pressure pump and the engine system

• On some installations the temperature probe is located in the oil filter housing

• These are normally electric or electronic

Oil Pressure Pumps• Oil pressure pumps may either be of the gear

type or vane type• The gear type pump is used in the majority of

reciprocating engines and uses close fitting gears that rotate and push the oil through the system

Scavenge Pumps• Scavenge pumps are driven in the same manner

as the pressure pumps but have a greater capacity

• This higher capacity is because the oil in the sump is foamy which means it has a much greater volume than air-free oil

Oil Dilution System• The purpose of the oil dilution system is to

provide thinner oil during engine start• This allows faster lubrication of engine

components• Oil dilution is accomplished by pumping a small

amount of fuel into the oil

Sludge Chambers• Some reciprocating engines have sludge

chambers which are in the hollowed out connecting-rod journals

• These journals accumulate carbon sludge and dirt particles as they are designed to

• During engine overhaul these must be replaced

Typical Lubrication Systems• Oil System for Wet-Sump Engine• Oil System for Dry-Sump Engine• Oil Tanks

Wet Sump System• In a wet sump system the oil is housed within

the crankcase.• This is not possible in a turbine engine

application because of the high operating temperatures.

Oil Tanks• Dry sump engine lubrication systems require a separate tank for

each engine system• These tanks can be constructed in three different ways:

• Welded sheet aluminum• Riveted aluminum• Stainless steel

• Some aircraft are equipped with synthetic rubber tanks

Troubleshooting• Symptom: Oil pressure decrease, oil temp.

increase.• Cause: loss of oil causing temp. to rise, engine

failure imminent.• Action: reduce power to maximize engine life.

Troubleshooting• Symptom: Oil pressure decrease, oil temp.

steady• Cause: Oil pressure gauge malfunction.• Action: monitor engine instruments.

Troubleshooting• Symptom: Slight drop in oil pressure, steady or

slight rise in oil temp.• Cause: possible filter blockage, by-pass valve

restricts flow.• Action: inform maintenance.

Usage Monitoring• It is important to monitor oil usage trends in

order to detect problems before they become critical.

• Each company will have specific trend monitoring procedures.

Engine Cooling Systems• Cowling• Baffles• Cooling fins• Cowl flaps• Augmenters

Liquid Cooled Cylinder

Air Cooled Engine

Engine Cooling Airflow

Pilot Handling • It is the pilots responsibility to ensure engine

operating temperatures remain in the normal operating range.

• Temperature can be controlled by adjusting:• Cowl flaps• Power setting• Airspeed• Fuel mixture

Thermal Shock• Thermal shock occurs when an engine operating

at high temperatures is allowed to cool quickly.• Some parts cool more rapidly than others and

causes stress cracks in the cylinder head.• Pilot handling can prevent thermal shock by

avoiding rapid power reductions, especially in cold outside air temps.