Piston Engine lubrication and cooling Flashcards
Why does an engine need lubrication
To reduce friction and wear in moving engine parts. Achieved by injecting/lubricating a thin film of oil between them.
What are other functions of engine oil.
Cooling by absorbing and dispersing heat to atmosphere.
Cleaning-removing deposits and metal flakes which are trapped in oil filter.
Protection and hydraulic operations.
What is a useful indicator of engine condition
Engine oil temperature and pressure important indicators
What characteristics do Aero Engine oils need
Inhibit corrosion, Maintain a suitable viscosity over a range of temps, low evap rate,
Not react with materials it comes in contact with.
To meet these requirements additives are added.
Viscosity Grading
Important characteristic of oil is its viscosity.
Aero oils graded to viscosity e.g SAE30(low viscosity)
Different types of engine and operating air temperatures determine grade of oil used.
Types of Oil
Mineral oil, synthetic oil, compound oils
Lubrication Systems
Engines may be classified to the way in which engine oil is stored.
Dry sump system- oil held in tank mounted remotely from engine.
Wet sump system- oil held in bottom of crankcase-the sump
How Dry Sump system works
Oil is drawn from tank by Engine drive pump. Pump provides more oil pressure than is normally necessary to ensure sufficient pressure even under extreme conditions. Pressure valve regulates system pressure.
Highly loaded points are with high pressure oil
Lightly loaded components such as camshaft are fed with low pressure oil.
Spent oil falls back to pump where it is collected by a scavenge pump.
Dry Sump Components
Oil tank-Reserve of oil
Coarse filter
Magnetic plugs,
Pressure relief valve, pressure filter oil temp gauge, oil pressure gauge
Wet pump system
Uses same components as dry but no oil tank or scavenge line. Oil is stored in sump which is reservoir formed by part of crank case.
How wet sump works
Splash lubrication- action of crankshaft splashing through oil helps to lubricate surfaces.
Also pump submerged in wet sump sends pressurised oil to bearings.
Comparison of wet and dry sump systems
Principal advantage of dry sump is oil is contained in separate oil tank. Ensures supply of cooled and filtered oil under all flight conditions. For wet sump splash lubrication oil is not filtered or cooled. Depending on shape of crank cases may be necessary to carry a greater quantity of oil than necessary .
Checking oil level
Dry sump-Check immediately as oil starts to drain back into engine and give false low reading.
Wet-20 minutes to allow oil to drain back to sump.
What factors affect oil consumption.
Oil is steadily lost during normal operation. Primary cause is oil leaking past piston rings into cylinder where it combusts with mixture and is expelled as exhaust. Worn piston rings result in greater oil consumption.
Relationship between oil temp and pressure
Increase in temp results in decrease in oil.
Engine cooling systems
Oil alone not enough to dissipate all heat. Piston engines can either be air or liquid cooled.. Liquid is more efficient and produces less drag but carries more weight.
How do liquid cooling systems work
Extract heat from engine by running mixture of water and glycol through galleries built in to crank cases. Hot fluid then piped to radiators which uses ram air to cool fluid before returning to engine.
Air cooling systems
Aero-engines are usually cooled to save weight and reduce working components. Air inlet directs air over engine. Fins on cylinders increase external surface area available for cooling.
What is cylinder head temp affected by
Amount of power produced, temperature of cooling air, aircraft speed and ratio of mixture. Pilot can control 3 of theses
When is engine overheat most likely to occur
Low speed high power- extended climb.
Lean weak mixture will cause and increase in CHT
Engine can over cool which can result in thermal shock which can cause cylinders to crack. Most likely to occur at high speeds and lower power settings.
