Lubrication and CoolingChapter 5

Aim

To understand principals of operation of aircraft lubrication and cooling systems

Objectives1.Describe the function of oil in combustion

engines2.Describe the properties of oil3.State the types of oil and how they are classified4.Explain the design of typical oil systems5.State operational aspects associated with oil

system operation6.Describe the design of typical engine cooling

systems7.State operational aspects associated with cooling

system operation

1. Function of oilOil must perform a number of functions in the engine including• Lubrication• Cooling• Cleaning• Protection• Sealing and shock absorption

1. Function of oilLubrication

The primary job of the oil is to interpose a film of oil between the moving parts of the engine to reduce frictionWithout this power losses due to heat would be great and the engine would seize up resulting in complete engine failure

1. Function of oilCooling

Oil is pumped in and around the moving parts of the engine where high temperatures existAs it is circulated through the sump and the engine cooler the heat is dissipated before entering the hot areas of the engine again

1. Function of oilCleaning

Over time dirt, grit, combustion bi-products and metal flakes will end up in and around the moving parts of the engineIn order to prevent friction losses and engine damage that these contaminants could cause oil is used to remove themAn oil filter is used to prevent these contaminants from continually circulating through the engineWhen the filter is replaced the cartridge is inspected for metal contaminants as this may be an indication of high engine wear

1. Function of oilProtection

When exposed to moisture, salt or chemical products rust or pitting can form on internal engine partsCylinder walls and parts of the crank shaft that have been hardened are particularly susceptible to corrosion

1. Function of oilSealing and Shock Absorption

Oil provides a seal between the piston and the cylinder wall preventing the charge or exhaust gases from escaping into the crank caseOil around the crankshaft, bearings, connecting rods, etc helps cushion the shocks produced by reciprocating forces

2. Properties of oilViscosity

Viscosity is a resistance to flow of any liquidA liquid with high viscosity will be thick and not flow easily,As a liquid is heated it becomes less viscousPractically when an engine is cold, such as just after start up, the oil will not be performing its functions to the best of its abilityWe must allow the oil to reach working temperatures (inside the green range) before applying high power settings. On cold days this may mean keeping the power in the low range before running it upConversely if the oil temperature is too high oil will have too low viscosity to be able to perform its functionsThe type of oil used is determined by the outside air temperatures in which the aircraft will be operating

2. Properties of oilIgnition Point

Oil must have a sufficiently high flashpoint to ensure it does not vaporise or catch fire within the normal operating temperatures of the engine

StabilityOil must be chemically stable and not change its characteristics over the operating range of the engine

3. Classification of oilStraight Mineral Oil

In modern aircraft straight mineral oil is used in a new engine to encourage the moving parts to rub against each other and bed-inIt is typically only used up to the first oil change (10-25 hours depending on the engine), a placard will be placed in the engine bay and the maintenance release will be endorsedMineral oil has a tendency to oxidise when exposed to high temperatures. It also has a higher tendency to pick up accumulated carbon deposits associated with the normal combustion process, clogging the filter

3. Classification of oilAshless Dispersant Oil

AD oils are the most common in general aviationDoes not have the carbon forming properties of straight mineral oil, the dispersant additives prevent the collected carbon deposits from forming into larger massesThe oil will begin to darken soon after an oil change due to the suspended carbon particles

3. Classification of oilSynthetic Oil

Synthetic oils are used on aircraft operating over a wide temperature rangeTypically only found in turbine enginesUnless specified in the flight manual do not use synthetic oil in piston engines as they may destroy seals

3. Classification of oilOil Grade

The correct grade of oil must be matched to the engine in line with its operational requirements, manufacturers recommendations and maintenance cycleOils are graded in line with the society of automotive engineers rating system and are assigned an SAE number. Aviation oil is given a commercial aviation number which is double the SAE ratingThe oil grades used in UniSA aircraft are:• 100 (SAE 50) – A straight mineral oil used to wear in new engines,

typically for the first 50 hours• W 100 (SAE 50) – Ashless dispersant

mineral oil used during normal operations for C172SP and BE55 aircraft

• W 100 Plus (SAE 50) - Ashless dispersant mineral oil with anti wear and corrosion additives. Used in the older aircraft in the fleet C172RG, C172N and PA28

4. Design of lubrication systemWet sump

Most light training aircraft use a wet sump type of oil systemIn this system a mechanically driven pump pumps oil out of the sump and though the engine via the oil filter and coolerThe filter and cooler have bypass valves incorporated into them in order to bypass the system should either component failAfter the oil has done its work it sinks back down to the sump via gravityThe disadvantage of this system is that inverted flight is limited (10-30 seconds)

4. Design of lubrication systemOil System

The dry sump oil system is mostly used on aerobatic aircraft where extended periods of inverted flight and unusual attitudes mean if the wet sump type were used, oil starvation could potentially occurThe major difference is that a scavenger pump is utilized in the engine to take the used oil to an external tankAs with the wet sump system, oil temperature and pressure is read just before the oil enters the engine

5. Operation of lubrication systemPre-flight

During pre-flight it is important to check for any leaking fluid or blocked air intakes (in front of the oil cooler)Oil quantity must be within manufacturers limitationsOnly use the correct grade of oil if refilling and do not mix oil typesIf the oil is over-filled, excess will escape though the breather valve once the engine has started

5. Operation of lubrication systemMalfunctions – Low oil pressure

Can be caused by a number of factors including:• Low oil quantity due to poor pre-flight, leaking tank or broken supply

line• Loss of oil pressure due to pump failure, failure of pressure line or

bearing failure, etc• High oil temperature causes the oil to become less viscous, reducing

pressure• Faulty gauge, if a corresponding rise in oil temperature is not observed

gauge failure is the likely cause

5. Operation of lubrication systemMalfunctions – High oil pressure

Most likely caused by a faulty pressure relief valveIf the pressure is excessively high damage to the system such as seal failures may occur resulting in complete engine failure

Malfunctions – Fluctuating oil pressureA fluctuating gauge is an indication that the oil quantity is dangerously low and the pump is drawing in air from either the sump or storage tankIn the case of a dry sump system it may indicate a failure of the scavenge pump

5. Operation of lubrication systemMalfunctions – High oil temperature

High oil temperature may be the result of general overheating of the engineLow oil content results in less cooling, giving a higher oil temperatureIt could also indicate an issue in the oil cooler (bypass valve stuck open)If a corresponding decrease in oil pressure is noted it may indicate immanent failure of the system

6. Cooling System DesignCooling Systems

During the combustion process, high temperatures are generated within the engine. The majority of this heat is dissipated through the exhaust system however up to 33% remains in the engine and must be dissipated by the cooling systemSome older, large piston engines use a liquid cooling system (radiator). These can be heavy and expensive to maintainMost modern piston engines use air cooling systems where air is taken in through ducting and directed around the engine with baffles before spilling overboardSome aircraft are fitted with adjustable cowl flaps to enable the pilot more control of engine cooling

6. Cooling System DesignCooling Fins

In order to help dissipate the heat in the engine, cylinders are cast with cooling fins around the cylinder heads and barrelThese work by increasing the surface area of the cylinder

6. Cooling System DesignPropellor

Slipstream from the propeller will aid in coolingMost aircraft are fitted with spinners on the propeller hub to help direct airflow into the engine

7. Operating the Cooling SystemOperational aspects

Monitor the cylinder head temperature where possibleIn general terms, to avoid overheating you should avoid:• Running the engine on the ground for extended periods with power at idle,

where the slipstream from the prop can not aid in cooling. Park into wind if possible to aid cooling

• Climbing the aircraft with high power settings and low airspeed for extended periods

• Descending with the throttle closed for extended periods (shock cooling)• Sudden, large decreases in throttle/manifold pressureProper use of cowl flaps and carburettor heat should ensure the engine is maintained within the correct operating temprature rangeRefer where possible to the aircraft flight manual

Questions?